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Photoinduced Magnetism in Nanostructures of Prussian Blue Analogues

Permanent Link: http://ufdc.ufl.edu/UFE0041849/00001

Material Information

Title: Photoinduced Magnetism in Nanostructures of Prussian Blue Analogues
Physical Description: 1 online resource (324 p.)
Language: english
Creator: Pajerowski, Daniel
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: complex, heterostructure, hexacyanochromate, hexacyanoferrate, nanoparticle, photoinduced, prussian, solid, thin
Physics -- Dissertations, Academic -- UF
Genre: Physics thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: A wide range of experimental and theoretical investigations have been made on nanostructures of Prussian blue analogues, and a variety of results have been obtained. Most notably, a novel photomagnetic effect has been observed in Co-Fe-(CN)6/Ni-Cr-(CN)6 Prussian blue analogue heterostructured films, and this effect persists near liquid nitrogen temperatures, which is an unprecedented high temperature for photocontrol in this class of compounds. Magnetic memory, such as that present in personal computers and music players, is based on the ability to change long lived magnetic states with external stimuli. Memory density is paramount in the quest to store ever increasing amounts of information, and this goal is a driving force of nanoscience. Photocontrol of magnetic states has the potential to boost both speed and density. The work in this dissertation has increased the understanding of photomagnetism at the nanoscale, as well as provided progress towards designer magnets at technologically viable temperatures.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Daniel Pajerowski.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Meisel, Mark W.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041849:00001

Permanent Link: http://ufdc.ufl.edu/UFE0041849/00001

Material Information

Title: Photoinduced Magnetism in Nanostructures of Prussian Blue Analogues
Physical Description: 1 online resource (324 p.)
Language: english
Creator: Pajerowski, Daniel
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: complex, heterostructure, hexacyanochromate, hexacyanoferrate, nanoparticle, photoinduced, prussian, solid, thin
Physics -- Dissertations, Academic -- UF
Genre: Physics thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: A wide range of experimental and theoretical investigations have been made on nanostructures of Prussian blue analogues, and a variety of results have been obtained. Most notably, a novel photomagnetic effect has been observed in Co-Fe-(CN)6/Ni-Cr-(CN)6 Prussian blue analogue heterostructured films, and this effect persists near liquid nitrogen temperatures, which is an unprecedented high temperature for photocontrol in this class of compounds. Magnetic memory, such as that present in personal computers and music players, is based on the ability to change long lived magnetic states with external stimuli. Memory density is paramount in the quest to store ever increasing amounts of information, and this goal is a driving force of nanoscience. Photocontrol of magnetic states has the potential to boost both speed and density. The work in this dissertation has increased the understanding of photomagnetism at the nanoscale, as well as provided progress towards designer magnets at technologically viable temperatures.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Daniel Pajerowski.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Meisel, Mark W.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041849:00001


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PHOTOINDUCED MAGNETISM IN NANOSTRUCTURES OF PRUSSIAN BLUE
ANALOGUES



















By

DANIEL MATTHEW PAJEROWSKI


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2010



























2010 Daniel Matthew Pajerowski



























To all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed
bun









ACKNOWLEDGMENTS

My sincere thanks go out to those who helped me during the course of my PhD

studies at the University of Florida. First, I would like to thank the talented chemists

Dr. Franz A. Frye, Dr. Justin E. Gardner, Matthieu F. Dumont, and Matthew J. Andrus

for generously providing valuable chemistry insights and samples. I appreciate the help

afforded by my predecessor in Professor Meisel's lab, Dr. Ju-Hyun Park, as well as

students in the neighboring low temperature labs, Dr. Byoung-Hee Moon and

Dr. Pradeep Bhupathi from Professor Lee's lab, and Kyle Thompson from Professor

Ihas' lab. I would like to thank cryogenic engineers Greg Labbe and John Graham for

running the helium liquefier and providing time and plumbing for trouble-shooting

cryogenic issues. Thanks also go out to electrical engineers Larry Phelps, Pete Axson,

and Rob Hamersma of the UF Physics electronics shop for valuable insight, electronic

components in a pinch and for fixing broken equipment. I would like to thank Brent

Nelson and David Hansen from UF Physics Information Technology. I would like to

thank Don Brennan, Tim Noland, and Jay Horton for help with technical issues in the

New Physics Building and around lab B133. I would like to thank program assistants

Dori Faust, Darlene Latimer, Kristin Nichola, Denise Carlton, and Carolyn Grider. I

would like to thank Mark Link, Ed Storch, Bill Malphurs, and Mike Herlevich of the UF

Physics instrument shop for incisive design critiques and superior craftsmanship when

building custom apparatus. I would like to thank Ben Pletcher of the UF Major

Analytical Instrumentation Center for the TEM images and EDS data. I would like to

thank Professor Steve Hill, Dr. Andrew Ozarowski, and Dr. Saitti Datta of the NHMFL for

help with the magnetic resonance studies. I would like to thank Dr. Steve Nagler,

Dr. Jerel Zaretsky, Dr. V. Ovidiu Garlea, Dr. Lou Santodonato, Chris Redmon, and the









entire sample environment team of the ORNL for help with the neutron scattering

experiments. I would like to thank the teachers of courses I took, as well as fellow

students who were essential to the learning process. I would like to thank my thesis

committee members, Professors Alan T. Dorsey, Hai-Ping Cheng, Yoonseok Lee,

Daniel R. Talham and Mark W. Meisel. I would like to thank Professor Talham for his

time and insight, specifically during Chem-Phys group meetings. Finally, I would like to

thank Professor Mark W. Meisel for his advice and his dedication to providing me with

research and learning opportunities.

This work was supported, in part, by the National Science Foundation (NSF)

through DMR-0701400 and the NHMFL via cooperative agreement with NSF

DMR-0654118 and the State of Florida. The research at Oak Ridge National

Laboratory's High Flux Isotope Reactor was sponsored by the Scientific User Facilities

Division, Office of Basic Energy Sciences, U. S. Department of Energy.









TABLE OF CONTENTS

page

A C K N O W LE D G M E N T S .................................................................................. .... ....

LIST OF TABLES ...................................................................... ......... 11

LIST O F FIG URES........................................... ............... 12

LIST OF ABBREVIATIONS..................... .......... .............................. 19

A B S T R A C T ...................................................... 2 2

1 IN T R O D U C T IO N .................................... .................................................................... 24

1.1 E xpe rim e nta l T echn iq ues ...................................................... ... .. ............... 2 5
1.2 Theoretical Methods................................. ...................... 26
1.3 Quantitative Analysis of Magnetization in Prussian Blue Analogues.............. 27
1.4 Cobalt Hexacyanoferrate Nanoparticles....................... ................... 28
1.5 Thin Films of Prussian Blue Analogues............... ...................... 28
1.6 Heterostructures of Prussian Blue Analogues........... ................................ 29

2 EXPERIMENTAL TECHNIQUES............................................ 31

2.1 Sample Environment ........................................... .. .............. 32
2.1.1 Vacuum Equipment ...................................................... ........ ............... 32
2.1.1.1 Pumps ............... ......... ........... .............. 32
2.1.1.2 Pum ping lines ............... ..... ................................... 34
2.1.1.3 Vacuum gauges............................... ...... ............... 34
2.1.1.4 Oil m ist filters and fore-line traps ................ ........... ........ 35
2 .1.1.5 O -ring s ........................... ....... .. .............. .... ........ 3 5
2.1.2 Cryostats ......... .... .. ......... ............... ..... .......... 36
2.1.2.1 Bath cryostats ......................................................... ......... 36
2.1.2.2 Continuous flow cryostats and cryogenic inserts................... 36
2.1.2.3 C losed cycle refrigerators .................................................... 37
2.1.3 Superconducting Magnets..... ........... ........ ........................... 38
2.1.3.1 M agnet construction ....... ........ ................................... 38
2.1.3.2 M agnet operation................ ................................. ....... 39
2 .1.4 L ig ht G u id e s ................................................ .................... .... 4 0
2.2 D election M ethods .............................................................. .............. 41
2.2.1 SQUID magnetometer....................................................................... .41
2.2.1.1 Superconducting quantum interference devices..................... 41
2.2.1.2 Quantum Design MPMS XL magnetometer............................ 42
2.2.1.3 Remnant fields and degaussing the MPMS......................... 43
2.2.2 Additional Methods Performed at UF.......................... ........... .... 44
2.2.2.1 Atomic force microscopy............ ...................... 44
2.2.2.2 Carbon, hydrogen, and nitrogen combustion....................... 44









2.2.2.3 Energy dispersive x-ray spectroscopy................................ 45
2.2.2.4 Fourier transform infrared spectroscopy.............................. 45
2.2.2.5 Inductively coupled mass spectrometry.............. .......... 46
2.2.2.6 Transmission electron microscopy ..................................... 47
2.2.2.7 Ultraviolet and visible spectroscopy.................. .......... 48
2.2.2.8 X-ray powder diffraction ............ ............. ....................... 48
2.2.3 National Laboratories ............................... ....... ....... ............... 49
2.2.3.1 Electron magnetic resonance at the NHMFL....................... 50
2.2.3.2 HB1A neutron triple-axis spectrometer at HFIR..................... 51
2.2.3.3 HB2A neutron powder diffractometer at HFIR..................... 52
2.2.3.4 Inelastic neutron scattering on SEQUOIA at SNS .................. 52
2.3 C custom A pparatus................ .......... ....................... ....... ......... 53
2.3.1 SQUID Probe with Low Temperature Rotation and Optical Fibers...... 53
2.3.1.1 Probe specifications and design ........... ... .......... ......... 54
2.3.1.2 Probe material properties ......... .... .... ..................... 56
2 .3 .1 .3 O p e ra tio n ............................................ ............... 5 6
2 .3 .1.4 C o nclusio ns ................................................. .............. .......... 5 8
2.3.2 Neutron Scattering Probe for Photoinducing Opaque Powders......... 59

3 THEO RETICA L M ETHO DS ............................................... ............................. 74

3.1 Quantum Mechanics of Transition Metal Ions .......... .................................. 75
3.1.1 Coulomb Interaction and the Multi-Electron Ion................................ 76
3 .1.2 Ligand F ie ld T heory ............................................................. 79
3.1.3 Spin-O rbit Coupling ...................................................... ........ ............... 81
3.1.4 Zeeman Splitting........................................ .. ............... 82
3.1.5 Superexchange Interaction............... ......................... 82
3.1.6 Mean-Field Theory ........................................ ................. 83
3.2 Tight-Binding Approximations ...... ..................... ................. 85
3.2.1 Extended Huckel Theory ...... ...................... .............. 86
3.2 .2 Ligand Field T heory ............................................................. 88
3.2.3 Superexchange Interaction............... ......................... 88
3.2.4 Infrared Vibrational Spectroscopy........... .... .... ..................... 88
3.3 Fitting A lgorithm s ........... ................. ............... .......................... 89
3.3.1 Least Squares .... ................................... ... .... .......... 89
3.3.2 Levenberg-M arquardt ............ ...... ............. ...................... 91
3.3.3 R ietveld Refinem ent ............... ..... ........................ ............... 92

4 QUANTITATIVE ANALYSIS OF MAGNETIZATION IN COMPLEX CYANIDES... 101

4.1 Synthesis and Chemical Composition........................ .... ........... .... 102
4.2 Spectroscopy ................................... ............................ ........... 102
4.3 M agnetic Susceptibility .................................... ........................... .... 103
4.4 M icroscopic Probe of Magnetization .................................. ....................... 104
4.5 Magnetization Fitting.. .. ......................... ........ ........... 105
4.5.1 K3Cr(CN)6 .................. .......... ..................................... 105
4.5.2 K3Fe(CN)6................................................................. 106









4.5.3 Cs2.8Ni4[Cr(CN)6]4 nH20 ......... ......................... ........... .... 107
4.5.4 Co4[Fe(CN)6]3 nH20 ................ ............... 108

5 COBALT HEXACYANOFERRATE NANOPARTICLES............... ................ 114

5.1 Nanoparticles of Rubidium Cobalt Hexacyanoferrate................................. 116
5 .1.1 Introduction ................................ ................. ...... ... ........ 117
5.1.2 Synthesis and Chemical Composition ......... ... .... ........... ... 117
5.1.3 Structure ................................................ ............. ............... 118
5.1.3.1 Transmission electron microscopy................ .................. 118
5.1.3.2 Fourier transform infrared spectroscopy ............................. 119
5.1.4 Magnetization ......................................... .................. 119
5.1.5.1 DC susceptibility .................................. ........ ............. 119
5.1.5.2 DC m agnetization ............... ............................. ............... 120
5.1.5.3 AC susceptibility ................................... ..... .... ............ 120
5.1.5 Discussion ..................... ........ ................. 121
5 .1.6 C o nclusio ns ......... ............. ........................... ......... .......... .............. 12 3
5.2 Nanoparticles of Potassium Cobalt Hexacyanoferrate.............................. 124
5.2 .1 Introduction ................................ ................. ...... ... ....... 124
5.2.2 Synthesis and Chemical Composition ............ ............................ 126
5.2.2.1 Fourier transform infrared spectroscopy............................. 126
5 .2 .3 S tru c tu re ......... ....... .................................................. .. ............... 1 2 6
5.2.3.1 Transmission electron microscopy................. ..... ............. 127
5.2.3.2 X-ray diffraction ................ ............................... 127
5.2.3.3 Neutron diffraction ............... ............................ ................. 129
5.2.4 Magnetization ................ ..... ......... .... .. ................ 130
5.2.4.1 Quenched high temperature DC susceptibility..................... 131
5.2.4.2 Quenched low temperature DC susceptibility...................... 131
5.2.4.3 Quenched low temperature magnetization ....................... 132
5.2.4.4 Isotherm al relaxation ....................................................... 132
5.2.4.5 Photoinduced low temperature DC susceptibility................. 132
5.2.4.6 Photoinduced low temperature magnetization................... 133
5.2.5 D discussion .............. .... ...... ............. ......... .............. 133
5.2.5.1 Details of modeling .... .. .......... ... ... .......... ....... 133
5.2.5.2 Size dependence of thermal quenching............................... 136
5.2.5.3 Photoinduced versus quenched states.............................. 137
5.2.5.4 Resulting schema of bulk and nanoparticles ..................... 138
5.2.6 Conclusions .............................................................. 138

6 THIN FILMS OF PRUSSIAN BLUE ANALOGUES.......................................... 159

6.1 Introduction ............................ 1.... .......... 159
6.2 Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 thin films......... ............. ......................... 160
6.2.1 Sam ple C haracterization .............. .............. .............. .................. 161
6.2.1.1 Chem ical composition...................................... 161
6 .2 .1 .2 A F M ........................... ......... ............... ........... 16 1
6.2.1.3 FT-IR ............... ......... ........... .............. 162









6.2.1.4 UV-Vis .............. ........ ..........._. ......... .... ............... 162
6.2.2 Magnetization .............. .......... .............. ..... 163
6.2.2.1 DC susceptibility in 100 G ............................ .... ...... ..... 163
6.2.2.2 DC m agnetization in 40 kG ........................... ......... ...... 163
6.2.2.3 DC magnetization field dependence.................. ......... 164
6.2.2.4 Magnetization Process ................. .............. ............... 164
6.2.2.5 DC magnetization angular dependence............................... 165
6.2.3 Electron Magnetic Resonance................... .................. 165
6.2.3.1 EMR temperature dependence............................. 165
6.2.3.2 EMR angular dependence ....... .... ................................... 167
6.2.3.3 EM R frequency dependence ............. ............ .................. 168
6.2.4 X-ray Diffraction .................. ........ .. ......... .. .. .......... .... 168
6.3 Additional Prussian Blue Analogue Thin Films................................. 168
6.3.1 Rbo.6Co4.o[Cr(CN)6]2.9-nH20 Thin Films ................................ ...... 168
6.3.2 Rbo.7Cu4.o[Cr(CN)6]2.9-nH20 Thin Films ................................ ...... 169
6.3.3 Rbo.3Zn4.o[Cr(CN)6]2.8nH20 Thin Films......... .... ....... ........... 170
6.3.4 Rbo.9Ni4.o[Fe(CN)6]2.8.nH20 Thin Films ................ ........ .......... 171
6.3.5 Rbo.7Co4.o[Fe(CN)6]2.8-nH20 Thin Films ................ ......... ......... 172
6.3.6 Rbo.5Cu4.o[Fe(CN)6]2.7-nH20 Thin Films ................ ......... ......... 172
6.3.7 Rbo.sZn4.o[Fe(CN)6]2.8-nH20 Thin Films ................................ ...... 173
6.4 Discussion ....................... ................... ............ .......... 173
6.4.1 Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 Thin Films .................. ........ .......... 174
6.4.2 Additional Prussian Blue Analogue Thin Films................................ 183
6.5 Conclusions ....................................... ........... 185

7 HETEROSTRUCTURES OF PRUSSIAN BLUE ANALOGUES........................... 212

7.1 Solid Solutions of Cobalt Hexacyanoferrate ............. .......... .................. 212
7 .1.1 Intro d uctio n ................................ ................. ........ .... ....... 2 12
7.1.2 Synthesis and Chemical Composition ............ ............................ 215
7.1.3 Transm mission Electron Microscopy ....... .... .................................... 216
7.1.4 Infrared Spectroscopy .... ............... ................. ... ............... 217
7.1.5 X-Ray Diffraction ............... ............... ......... ...... ........... 217
7.1.6 Mean-Field Calculations ........................... ............... 218
7.1.6.1 Low temperature magnetization in the mean-field................ 219
7.1.6.2 Mean-field magnetic susceptibility and spin-crossover......... 221
7.1.7 M agnetic M easurem ents ............................................. ... ............... 222
7.1.7.1 Low temperature DC susceptibility ............... ... ............ 223
7.1.7.2 DC magnetization ..................... ................... 224
7.1.7.3 High temperature DC susceptibility.............. ................ 224
7.1.7.4 Physically mixed x = 0.66 compound................. ............... 225
7.1.8 Discussion ...... .... .......... .... ... .. ... .. .......... ....... 225
7.1.8.1 Photoinduced decrease in magnetization........................... 226
7.1.8.2 Scaling of magnetic properties ................................ ...... 228
7.1.8.3 Spin-crossover dilution ................................. ... .......... .. 230
7.1.8.4 Mean-field predictions versus observations....................... 233
7.1.9 Conclusions ............. ................................... ............... 234









7.2 Heterostructured Films Containing Cobalt Hexacyanoferrate ..................... 235
7.2 .1 Introduction ............................................................... 235
7 .2 .2 S ynthesis................................................................ ... ............ 2 36
7.2.3 Magnetization of Nickel Hexacyanochromate and Cobalt
Hexacyanoferrate Heterostructures.............. ................... 237
7.2.3.1 Slow deposition multilayer films ................ ........... ........ 237
7.2.3.2 Stacked films .................... ............................. 238
7.2.3.3 Thin sandwiched films ........ ............ ...................... 239
7.2.3.4 Thick sandw iched film s..................................... ............... 239
7.2.4 40/40/40 Heterostructure .............. ............... ................................ 241
7.2.4.1 40/40/40 film, 10 kG temperature sweeps.......................... 242
7.2.4.2 40/40/40 film, transmission electron microscopy .................. 242
7.2.4.3 40/40/40 film, energy dispersive x-ray spectroscopy ........... 242
7.2.4.4 40/40/40 film, x-ray powder diffraction............................... 243
7.2.4.5 40/40/40 film, infrared spectroscopy.................................. 243
7.2.5 Capping Layers of Cobalt and Chromium Hexacyanochromates...... 243
7.2.5.1 Magnetization of cobalt hexacyanochromate and cobalt
hexacyanoferrate sandwich heterostructures ................... 244
7.2.5.2 Magnetization of chromium hexacyanochromate and cobalt
hexacyanoferrate sandwich heterostructures ................... 244
7.2.6 Discussion .......... .......... ......... ................ ........ ....... 245
7.2.7 Conclusion..................................... ........... 248

8 SUMMARY AND CONCLUSIONS................ ............................ 285

APPENDIX

A U N IT S ......... ...... ............ .................................. ............................ 2 9 0

B LOW TEMPERATURE ROTATION PROBE DRAWINGS........ ............... 291

C COPYRIGHT RELEASE FORMS ................................ ............... 306



LIST OF REFERENCES .......... ............ ......... ................ ............... 314

BIOGRAPHICAL SKETCH .............. ........... ............................. 324









LIST OF TABLES


Table page

2-1 Magnetic response of candidate probe materials for optical rotator
magnetization probe ........... ........... .......................... ............... 69

4-1 Parameters to be used in modeling magnetization data for
Cs2.8Ni4[Cr(CN)6]4nH20, Co4[Fe(CN)6]3nH20, K3Cr(CN)6,and K3Fe(CN)6......... 111

5-1 Synthesis and chemical composition of rubidium cobalt hexacyanoferrate
nanoparticles. ........... ......... ......... ...... ............... .... ........... 140

5-2 Magnetic properties of rubidium cobalt hexacyanoferrate nanoparticles ........ 144

5-3 The microscopic states relevant to KjCok[Fe(CN)6]r-nH20...... ........................ 145

5-4 The macroscopic states relevant to KjCok[Fe(CN)6]r nH20............................. 145

5-5 Metal oxidation states of bulk powder and nanoparticles, in addition to fitting
parameters used for ......... ........................................................... 146

5-6 Chemical composition and characteristic sizes of potassium cobalt
hexacyanoferrate nanoparticles and bulk powder ........................................... 146

5-7 Magnetic properties of quenched potassium cobalt hexacyanoferrate
nanoparticles and bulk powder............................... ............... 157

5-8 Magnetic properties of photoinduced potassium cobalt hexacyanoferrate
nanoparticles and bulk powder............................... ............... 157

6-1 Molecular formulas of films measured and techniques used............................ 201

7-1 Molecular formulas and unit cell parameters for NaaCoxNil-x[Fe(CN)6]p. nH20. 249

7-2 Nickel hexacyanochromate and cobalt hexacyanoferrate heterostructures
studied ............... ..... .. ........... ....... ........... ....... ......... 267









LIST OF FIGURES


Figure page

1-1 Illustration of constrained geometries in nanostructures........................... 30

1-2 Illustration of designer heterogeneous geometries in nanostructures............... 30

2-1 Illustrations showing operation of standard pumps that can be found in a
cryogenic laboratory ............ .......... ........................ .............. .............. 60

2-2 Illustration of a superconducting solenoid magnet................ .... ............... 61

2-3 Illustration of a SQUID magnetometer circuit ................ .............. ............... 61

2-4 Illustration of SQUID magnetometer pickup coils ................... .. .. ........... .. 62

2-5 Remnant fields and degaussing the MPMS.................................... .......... 62

2-6 AFM schematic......................................... .......... 63

2-7 A schematic of a combustion train for CHN analysis............... ................. 63

2-8 E D S scheme atic ............. .............. .......... .... ................. .............. 63

2-9 FT-IR schematic .......... ......... ... ......... ................ ... ............. 64

2-10 ICP-MS schematic........................................ .......... 64

2-11 TEM schematic......................................... .......... 65

2-12 UV-Vis spectrometer schematic ............. .. ..... ......... .... ................ ... 65

2-13 XRD schematic......................................... .......... 66

2-14 T heoretical E M R schem atic............................................................ ............... 66

2-15 Experimental EMR schematic ............... ........... ... .. .................. 67

2-16 The triple-axis spectrometer at HFIR on beamline HB1A................................. 67

2-17 The neutron powder diffractometer at HFIR on beamline HB2A......................... 68

2-18 The SEQUOIA inelastic time-of-flight spectrometer at SNS ............................. 68

2-19 Photographs of the optical rotation probe for use in a SQUID magnetometer.... 70

2-20 Magnetization versus field for potential optical rotation probe materials............. 71









2-21 A circuit diagram of the control board for the automated operation of the
custom probe using a stepper motor ......... ........ ....... .......................... 71

2-22 Magnetization versus rotation angle measured with the custom probe for two
different magnetite samples at 300 K (a) and 2 K (b)................................ 72

2-23 Photoirradiation of powdered neutron scattering samples.............. .......... 72

2-24 Photoirradiation of powdered neutron scattering samples using tumbler probe. 73

3-1 The energy differences between different d5 Fe3+ free-ion terms arising from
electron-electron repulsion as a function of the Racah repulsion parameter B... 95

3-2 The octahedral coordination geometry ............... ..... .... ......... ......... 95

3-3 The energy of a molecular term as a function of the octahedral splitting
parameter, A, for a d5 ion, such as Fe3 .............. ..................................... 96

3-4 Energy shift plotted versus the tetragonal distortion parameter, 6................... 97

3-5 Energy splitting of the octahedral hexacyanoferrate 2T2g ground state............... 98

3-6 The Zeeman splitting versus applied magnetic field for the spin-orbit split
2T2g-like ground state of hexacyanoferrate ....... .... ...................................... 99

3-7 The effect of superexchange on magnetization.......................... ... ............... 99

3-8 The cyanide m olecule.............................................. .............. 100

4-1 The cubic complex cyanide Prussian blue analogue structure...................... 110

4-2 UV-Vis of Rbo.7Ni4.o[Cr(CN)6]2.9 nH20 .................................... .............. 110

4-3 Measurement of magnetic ordering temperature in Cs2.8Ni4[Cr(CN)6]4-nH20,
Co4[Fe(CN)6]3.nH20, K3Cr(CN)6,and K3Fe(CN)6 ........................................... 111

4-4 Magnetic properties of K3Cr(CN)6................................ 112

4-5 Magnetic properties of K3Fe(CN)6 .......... ............. ........... ................ 112

4-6 Magnetic properties of Cs2.8Ni4[Cr(CN)6]4-nH20....... ..... ........... ............ 113

4-7 Magnetic properties of Co4[Fe(CN)6]3 nH20 ....................... ....................... 113

5-1 Prussian blue analogue structure ...................... ........ ............... 139

5-2 A detail of the photoexcitation processes in Ko.2Co1.4[Fe(CN)6]-6.9H20 ......... 139

5-3 TEM of RbjCok[Fe(CN)6]r nH20 nanoparticles ................................... ......... 140









5-4 FT-IR absorption spectra of RbjCok[Fe(CN)6]r nH20 nanoparticles ................. 141

5-5 The temperature dependence of the low field, 100 G, susceptibilities of
RbjCok[Fe(CN)6]r nH20 nanoparticles ................................ .... ........... ..... 142

5-6 The T= 2 K magnetization versus magnetic field sweeps of
RbjCok[Fe(CN)6]r nH20 nanoparticles ................................ .... ........... ..... 143

5-7 The temperature dependence of the real (X') and imaginary (X") AC
susceptibilities of RbjCok[Fe(CN)6]r nH20 nanoparticles............................. 144

5-8 Ordered magnetic components of the smaller batches ............................... 145

5-9 FT-IR spectra of bulk and nanoparticles of K-Co-Fe .................... ........ 146

5-10 TEM of K-Co-Fe. (left) Typical TEM images are shown .............................. 147

5-11 XRD of K-Co-Fe ...................... ........ ......................... ............... 148

5-12 Neutron scattering of K-Co-Fe............................... ............... 149

5-13 Neutron diffraction as a function of temperature for K-Co-Fe........................ 150

5-14 Magnetic neutron scattering in K-Co-Fe....... ... ......................... ............. 151

5-15 Temperature dependent magnetic moment of quenched states for K-Co-Fe... 151

5-16 Magnetic ordering of quenched states in K-Co-Fe ................................. 152

5-17 Magnetization versus field of quenched states for K-Co-Fe ........................... 153

5-18 Relaxation of magnetization in quenched states of K-Co-Fe.......................... 154

5-19 Magnetic ordering of photoinduced states in K-Co-Fe................ ............... 155

5-20 Magnetization versus field of photoinduced states for K-Co-Fe ................... 156

5-21 Linearization of modeling................................ ............... 157

5-22 Ordering temperatures and coercive fields of batches in different
macroscopic states ........... ........... ......... ................ ....... ........ 158

5-23 Microscopic schema based upon all data........ ..... ... ................... 158

6-1 Prussian blue analogue structure ...... .... ............................. 186

6-2 The multiple sequential adsorption method that can be used for generating
thin films of Prussian blue analogues ....................................................... 187









6-3 Different orientations of the magnetic thin films with respect to the applied
magnetic field are expected to have different behavior .............. ............... 187

6-4 AFM of thin films.... ............................................................. 188

6-5 Room temperature FT-IR spectroscopy measurements of the cyanide
stretches present in Ni-Cr materials ..... ................................ 188

6-6 UV-Vis spectroscopy of Ni-Cr materials ................... .................... 189

6-7 Temperature dependent magnetization of Ni-Cr materials............................. 189

6-8 Temperature dependent magnetization of Ni-Cr materials at high fields......... 190

6-9 Field dependent magnetization of Ni-Cr materials...................................... 190

6-10 Magnetizing process of thin Ni-Cr film ....... ... ...................... ......... ....... 191

6-11 Magnetizing process of thick Ni-Cr film ........ ... ................................. ...... 191

6-12 Magnetizing process of Ni-Cr powder................ .......... .. .......... ........ 192

6-13 Angle dependence of magnetization in Ni-Cr materials ............................. 192

6-14 EM R lines of N i-C r pow der ........... ............... ......................... ............... 193

6-15 Results of fitting EM R lines of Ni-Cr powder............................................... 193

6-16 EMR lines of Ni-Cr thin film perpendicular..................................... 194

6-17 Results of fitting EMR lines of Ni-Cr thin film perpendicular ........................... 194

6-18 EM R lines of Ni-Cr thin film parallel ............... .... ........... ............. ............ 195

6-19 Results of fitting EMR lines of Ni-Cr thin film parallel...................................... 195

6-20 EMR lines of Ni-Cr thick film perpendicular .............................................. 196

6-21 Results of fitting EMR lines of Ni-Cr thick film perpendicular.......................... 196

6-22 EM R lines of Ni-Cr thick film parallel .......................... ................. ......... .. 197

6-23 Results of fitting EMR lines of Ni-Cr thick film parallel................................... 197

6-24 EMR lines of Ni-Cr thin film as a function of angle....................................... 198

6-25 Results of fitting EMR lines of Ni-Cr thin film as a function of angle................. 198

6-26 EMR lines of Ni-Cr thick film as a function of angle................................... 199









6-27 Results of fitting EMR lines of Ni-Cr thick film as a function of angle.............. 199

6-28 EMR lines of Ni-Cr thick film as a function of angle in lower field ................ 200

6-29 Results of fitting EMR lines of Ni-Cr thick film as a function of angle in lower
f ie ld ............. ................. ............... .................................................. 2 0 0

6-30 Ligand field levels of Co-Cr ................................ ......... .. ..... .......... 201

6-31 Magnetic susceptibility of Co-Cr thin film ............... .............. ... ............ 202

6-32 Ligand field energies of Cu-Cr................................. ............... 202

6-33 Magnetic susceptibility of Cu-Cr thin film ............... .............. ... ............ 203

6-34 UV-Vis of Cu-Cr thin film ............ ... ..... .. ......... ......................... 203

6-35 Ligand field energies of Zn-Cr ... ................................ ........ ... ............... 204

6-36 Magnetization of Zn-Cr versus field ........... ........................... ...... ............ 204

6-37 Ligand field energies of Ni-Fe................................. ............... 205

6-38 Magnetic susceptibility of Ni-Fe thin films................................................... 205

6-39 Ligand field energy levels and rotational magnetism of Co-Fe...................... 206

6-40 Ligand field energies of Cu-Fe................................. ............... 207

6-41 Magnetic susceptibility of Cu-Fe thin films.................................................. 207

6-42 Demagnetizing fields in films uniformly magnetized perpendicular to the
surface.... .......... ......... ......................................... ............... 208

6-43 Demagnetizing fields in films uniformly magnetized parallel to the surface...... 208

6-44 Fitting magnetization of Ni-Cr thin films in low field ................................. 209

6-45 Fitting magnetization of Ni-Cr thin films in high field................................. 210

6-46 Thickness dependence of thin films................................ 210

6-47 EMR lines of Ni-Cr thin films and powder........... ..................................... 211

7-1 The NaaNil-xCox[Fe(CN)6]- nH20 material........... ..................................... 249

7-2 Typical TEM micrographs for samples reported in Table 7-1 for different
values of x ............... ................................................................. 250









7-3 FT-IR spectra and fitting parameters of NaaNil-xCox[Fe(CN)6]p. nH20 as a
function of x .............. ............................... ............ ....... 251

7-4 Full XRD diffractograms of NaaNil-xCox[Fe(CN)6]- nH20............................... 252

7-5 XRD of the x = 1 NaaNil-xCox[Fe(CN)6]-. nH20 ....... .................................... 253

7-6 Room temperature XRD reflection with background subtracted and intensity
normalized to show the continuous evolution with x ............. .............. 253

7-7 Photoinduced magnetization of NaaNil-xCox[Fe(CN)6].pnH20.................... 254

7-8 Molar magnetic susceptibility of NaaNil-xCox[Fe(CN)6]p- nH20 as a function of
time irradiated at 5 K and 10 G, measured in a SQUID................................. 255

7-9 Magnetization versus field for NaaNil-xCox[Fe(CN)6]p-nH20 ........................... 256

7-10 Checking for asymmetry in the hysteresis loop as a possible explanation of
the reduction in He for the x = 0.66 sample ................................................ 257

7-11 Thermal induced changes in magnetization of NaaNil-xCox[Fe(CN)6]p-nH20.... 258

7-12 Microscopic versus macroscopic mixing.............................. 259

7-13 Magnetization of macroscopically mixed NaaNil-xCox[Fe(CN)6]p-nH20............. 259

7-14 Field dependence of photoinduced magnetization for
N a N il-xC ox[Fe (C N )6] n H 20 ........................................................ ............... 26 0

7-15 Scaling of magnetic properties of NaaNil-xCox[Fe(CN)6]p-nH20..................... 261

7-16 Superexchange in NaaNil-xCox[Fe(CN)6]p-nH20............... .............. 262

7-17 Charge transfer induced spin transition parameters for
N a N ilxC ox[Fe (C N )6] n H 20 ........................................................ ............... 262

7-18 Amount of CTIST materials in NaaNil-xCox[Fe(CN)6]p-nH20........................... 263

7-19 FT-IR parameters in NaaNil-xCox[Fe(CN)6]p-nH20............................ .......... 263

7-20 Comparison of ferromagnetic versus antiferromagnetic Co-Fe components... 264

7-21 Modification of superexchange energy in NaaNil-xCox[Fe(CN)6]p.nH20............ 265

7-22 A scheme showing synthesis of a heterostructured thin film using multiple
sequential adsorption cycles. ............................. ............. ............... 266

7-23 Magnetization of slow deposition multilayer films..................... .......... 267









7-24 Magnetization of 10/10 Co-Fe/Ni-Cr thin film oriented parallel .................... 268

7-25 Magnetization of 10/10 Co-Fe/Ni-Cr thin film oriented perpendicular .............. 268

7-26 Magnetization of 10/10 Ni-Cr/Co-Fe thin film oriented parallel ...................... 269

7-27 Magnetization of 10/10 Ni-Cr/Co-Fe thin film oriented perpendicular............... 269

7-28 Magnetization of 10/5/10 Ni-Cr/Co-Fe/Ni-Cr thin film oriented parallel......... 270

7-29 Magnetization of 10/5/10 Ni-Cr/Co-Fe/Ni-Cr thin film oriented perpendicular... 270

7-30 Magnetization of 10/10/10 sandwich film versus temperature....................... 271

7-31 Photoinduced magnetization of 10/10/10 film oriented perpendicular .............. 272

7-32 Photoinduced magnetization of 10/10/10 film oriented perpendicular .............. 272

7-33 Magnetization of 40/40/40 Ni-Cr/Co-Fe/Ni-Cr heterostructure........................ 273

7-34 Magnetization of 20/40/20 Ni-Cr/Co-Fe/Ni-Cr heterostructure........................ 274

7-35 Magnetization of 40/20/40 Ni-Cr/Co-Fe/Ni-Cr heterostructure........................ 275

7-36 Magnetization data of the 40/40/40 sandwich Ni-Cr/Co-Fe/Ni-Cr PBA film ...... 276

7-37 High magnetic field, 10 kG temperature sweeps of 40/40/40
N i-C r/C o-Fe/N i-C r ................................................ .......... 277

7-38 Transmission electron microscopy of the 40/40/40 Ni-Cr/Co-Fe/Ni-Cr
heterostructure ......... ........... ................................................ ........... 277

7-39 An energy dispersive x-ray line scan of the 40/40/40 heterostructure.............. 278

7-40 X-ray powder diffraction of a 40/40/40 heterostructure................................ 279

7-41 Cyanide stretching energies in the infrared ............ .. .... .................. 280

7-42 Magnetization data of Co-Cr/Co-Fe/Co-Cr heterostructures ......................... 281

7-43 Magnetization data of Cr-Cr/Co-Fe/Cr-Cr heterostructures........................... 282

7-44 Photoexcitation of Co-Fe ......... .... .......... ................... 283

7-45 Distortions in Ni-Cr ....... .................... ...... ......... 283

7-46 Anisotropy in Ni-Cr ....... .................... ...... ......... 284









LIST OF ABBREVIATIONS


A Alkali cation

AC alternating current

AFM Atomic force microscopy

AQ Aqueous solution

B Magnetic field

CHN Carbon hydrogen nitrogen

CLB Chemistry Lab Building

CN Cyanide

CTIST Charge transfer induced spin transition

DC Direct current

EDS Energy dispersive spectroscopy

EMF Electromagnetic field

emu Electromagnetic unit

EMR Electron magnetic resonance

EXAFS Extended x-ray fine structure

FC Field cooled

FCC Face centered cubic

FT-IR Fourier transform infrared

FWHM Full-width-half-maximum

G Gauss

H Magnetizing field

HB HFIR Beamline

Hc Coercive field

HFIR High Flux Isotope Reactor









HS High spin

HT High temperature

ICP-MS Inductively coupled mass spectrometry

K Kelvin

Ib Pound

LCAO Linear combination of atomic orbitals

LS Low spin

LT Low temperature

m meter

M Transition metal

M' Transition metal

MAIC Major Analytical Instrumentation Center

MPMS Magnetic Properties Measurement System

NHMFL National High Magnetic Field Laboratory

NMR Nuclear magnetic resonance

NPB New Physics Building

OD Outer diameter

ORNL Oakridge National Laboratory

Pa Pascale

PBA Prussian blue analog

PVP Polyvinylpyrrolidone

RMS Root mean squared

S Spin quantum number

SCO Spin crossover

SEM Scanning electron microscopy









SEQUOIA

SNS

SQUID

T

Tc

TCTIST


Tf

Tup


Tdown


TEM

UF

UV-Vis

XRD

ZFC

X

XRD


an inelastic diffractometer at SNS, after the Native American chief

Spallation Neutron Source

Superconducting quantum interference device

Tesla or Temperature (depending upon context)

Magnetic ordering temperature

Temperature around which the thermal spin crossover centers,
(Tup+Tdown)/2

Freezing temperature associated with spin-glass materials

Temperature at which half of the spin crossover material has
transitioned in the heating cycle

Temperature at which half of the spin crossover material has
transitioned in the cooling cycle

Transmission electron microscopy

University of Florida

Ultraviolet and Visible (spectroscopy)

X-ray diffraction

Zero field cooled

Magnetic susceptibility

X-ray diffraction









Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

PHOTOINDUCED MAGNETISM IN NANOSTRUCTURES OF PRUSSIAN BLUE
ANALOGUES

By

Daniel Matthew Pajerowski

August 2010

Chair: Mark W. Meisel
Major: Physics

A wide range of experimental and theoretical investigations have been made on

nanostructures of Prussian blue analogues, and a variety of results have been obtained.

Most notably, a novel photoeffect has been observed in RbjCok[Fe(CN)6]1-nH20 /

RbjNik[Cr(CN)6]r nH20 Prussian blue analogue heterostructured films, and this effect

persists up to ~ 75 K, which is an unprecedented high temperature for photocontrol in

this class of compounds. This engineering of the high-Tc heterostructures was made

possible by insight gained from studying other nanostructured Prussian blue analogues.

Specifically, solid solutions of NaaNil-xCox[Fe(CN)6]p-nH20 proved that the sign and the

magnitude of photoinduced magnetization can be tuned with chemical formula. In

addition, these solid solutions elucidated the effect that diluting the lattice has on the

magnetic properties of the photomagnet. By studying nanoparticles of

RbjCok[Fe(CN)6]1-nH20, a size-dependent scaling of magnetic ordering temperatures

and coercive fields was established, with "bulk" long-range magnetic order occurring in

particles larger than ~ 25 nm. In parallel work, nanoparticles of KjCok[Fe(CN)6]1-nH20

showed a reduction in bistable material with decreasing particle size. The magnetic

anisotropy of sequential adsorption Prussian blue analogue films proved to be









complicated, and required a plethora of probes to tease out the inherent properties.

Many analogues were studied, and magnetostatic, g-factor, and magnetocrystalline

effects were identified in different samples, depending upon the magnetic ions. In

addition to a discussion about characterization of these nanostructured Prussian blue

analogues, a review of the relevant theoretical and experimental techniques is

presented. For example, the design and implementation of a custom magnetometer

probe with fiber optics and automatic sample rotation is noteworthy and is discussed.

Furthermore, the design of a custom optic probe for use with photoinducible opaque

powders in neutron scattering experiments is included. Theoretical tools and numerical

investigations that were employed to understand the experimental results are

overviewed, with specific attention given to properly parameterizing the photoinduced

systems. Detailed models of Prussian blue analogue materials are presented. Specific

examples showing the potential ambiguity of assigning superexchange constants based

on magnetization in the presence of first order orbital angular momentum are also

discussed, and one example is the photomagnetic RbjCok[Fe(CN)6]1-nH20. Finally,

experimental techniques, which include AFM, CHN, EDS, EMR, FT-IR, ICP-MS,

neutron scattering, SQUID magnetometry, TEM, UV-Vis spectroscopy, and XRD, are

reviewed for the purpose of understanding the data presented.









CHAPTER 1
INTRODUCTION

To begin, magnetism is central to all discussion in this thesis. From a purely

scientific standpoint, magnetism fascinates because it is an inescapable manifestation

of quantum mechanics. Among fundamental physical concepts, magnetism is of

particular importance technologically, most especially for the storage of information in

memory devices. Modern memory storage is done with oxides or alloys that require

high temperatures for synthesis, and memory is written using other magnetic materials.

Currently, potential alternative materials are being investigated with much vigor, with the

hopes of either having similar performance using cheaper manufacturing than industry

standards or improved performance with limited increase in expense. Of the many

alternative possibilities, molecule based complex cyanides are the focus of this work.

Compared to metallurgical synthesis, the room temperature and pressure wet chemistry

required for complex cyanides is cheap and user-friendly. In addition, while standard

methods may be used to write to these materials, the magnetization of the complex

cyanides can also be changed by the application of external light, heat, and

pressure [1-2]. Possible benefits of the complex cyanides include potentially storing bits

in individual nanometer sized molecules, as well as the fast, three dimensional write

ability that would be awarded by using photons, instead of magnetic field induced torque,

to write. The problem can then be stated as the search to understand the

magnetization of the complex cyanides and to see how practical the goals of an

optically controlled cyanometallate memory storage device may be.

More specifically for this dissertation, the main thrust of the research has been

studying the effects of incorporation of complex cyanides into nanostructures. The









nanostructures studied include nanoparticles, thin films, and heterostructures of thin

films, along with atomically mixed solid solutions of bulk materials. Novel

photomagnetic effects of the nanostructures and phenomena related to the effects will

be discussed in detail in the main chapters of the thesis.

On a practical note, this thesis is written at a level so as to be accessible to

someone with an undergraduate education in a physical science, with the possible need

for additional reading to understand the details of certain subsections. No details will be

given on the experimental and theoretical topics covered in typical curricula, and the

interested reader is directed to standard texts on the subjects [3-4]. In addition, the

units will generally be c.g.s., except in cases where precedent dictates otherwise for

ease of comparison. When plotting magnetic fields, to will be suppressed for readability.

Appendix A addresses the units used herein.

The structure of the dissertation is such that Chapters 2 and 3 provide specific,

pertinent background information that may be helpful to fully understand the new

measurements and materials that are presented in Chapters 5, 6, and 7. Chapter 4 is

somewhat between the experimental chapters and the background chapters, as it

illustrates essential concepts and treats data already present in the literature to a

quantitative analysis based upon tools outlined in the background sections.

1.1 Experimental Techniques

Experimental methods are presented in Chapter 2. While DC magnetization

measurements make up the majority of data presented in this thesis, complementary

techniques are essential to achieve in depth understanding of magnetization results. In

the modern world, many apparatus for probing solids have been honed to a high level of









sophistication, and an adequately educated scientist can either participate in

collaborations or simply step in as a guest to use the desired equipment. Therefore,

part of the experimental methods chapter consists of an encyclopedia of the

experimental procedures utilized. As external lab facilities with well supported user

programs were essential to some investigations, including national laboratories and

other labs at the University of Florida, capabilities as well as the locations of the

equipment have been documented. Furthermore, more substantial space is allotted for

procedures where the author has made significant contributions. In particular, an

artisanal rotation probe to study the angular dependence of magnetization in thin films is

detailed. Also, as photoinduced magnetization has been one of the most exciting topics

studied, advancements on magnetization optic probes and neutron scattering optic

probes are also presented.

1.2 Theoretical Methods

Physics is an experimental science. On the other hand, theoretical work is

essential for potential predictive power and insight into experimental results. What can

provide greater joy than finding the additional term necessary to not only accurately fit a

puzzling data set but to actually open up a whole new avenue of experiments? While

complicated first principles theories can dazzle with their power, well parameterized

Hamiltonians tractable to the average physicist are light and elegant at their best (but

misleading and confusing at their worst). In Chapter 3, starting with a bare atom, the

relevant interactions will be introduced using the magnetically ubiquitous iron as a

bellwether. In vacuum, the first-order energy of an atom is determined by

electron-electron Coulomb repulsion and the Pauli exclusion principle. Additional

corrections, such as relatiVistic effects and spin-orbit coupling can provide additional









structure to the energy spectrum. Next, if a molecule forms with a single magnetic ion,

the bare ionic energies will be affected by the presence of the additional non-magnetic

atoms, the so-called ligands. Dubbed ligand field theory, the changes in the electronic

energy levels of the ions in the molecule have been found to be caused by both

electrostatic and covalent interactions [ 5]. The situation becomes increasingly complex

as more magnetic ions are introduced to molecules and allowed to interact with one

another. Magnetic interactions, mediated through the non-magnetic ligands in a

coordination network, further perturb the energy levels and can even lead to

macroscopic correlation of magnetic moments throughout a compound. This

long-range magnetic order can have many exotic properties of its own, as well as

technological implications for information storage.

1.3 Quantitative Analysis of Magnetization in Prussian Blue Analogues

To provide a strong foundation for the chapters discussing even more complicated

nanostructured materials, the magnetic properties of nickel hexacyanochromate and

cobalt hexacyanoferrate, the two compounds most thoroughly studied, will be described

in Chapter 4 using machinery presented in the theoretical methods chapter. The

detailing of two materials of nearly identical structures having different magnetic ions will

hopefully reinforce the need to understand the quantum mechanical theories that can

accurately describe this type of magnetism. To begin, the paramagnetic precursors,

K3[Cr(CN)]6 and K3[Fe(CN)]6, will be analyzed, stressing the single-ion properties of the

molecules, which remain relevant in the more complicated energy spectra of the full

complexes. Subsequently, an analysis of the Ni4.0[Cr(CN)6]3.0.nH20 and

Co4.o[Fe(CN)6]3.o nH20 materials will be presented along with how magnetization as a









function of temperature and magnetic field may be explained as resulting from the

magnetic energy levels of the ions.

1.4 Cobalt Hexacyanoferrate Nanoparticles

The first set of new materials studied consist of nanoparticles of the

photomagnetic RbjCok[Fe(CN)6]r nH20 Prussian blue analogue. Partially motivated by

the "room at the bottom" approach to science, magnetic nanoparticles (Figure 1-1) are

relevant to the development of memory storage devices as memory media are made

increasingly dense and finite size effects may help or hinder device performance. The

photoinduced magnetism of Ko.2Coi.4[Fe(CN)6]-6.9H20 was first discovered in a bulk

powder, showing long-range magnetic order that was modified with the application of

light [1]. More recently, researchers have been synthesizing photomagnetic

nanoparticles, however, no long-range order was observed [6] [7]. It was not until

nanoparticles of RbjCok[ [Fe(CN)6]r nH20 were synthesized with fine size control that a

size dependent study of photomagnetic nanoparticle magnetic properties was

performed. This work showed modifications of the coercive fields and the ordering

temperatures as a function of size, spanning the regimes from bulk-like to

superparamagnetic properties [8]. Further size-dependent studies were performed on

KjCok[Fe(CN)6]r nH20 magnets, which can be trapped into different magnetic states by

varying the cooling rates [9]. Details of the experiments, including magnetization, x-ray

diffraction, neutron diffraction, AC-susceptibility, infrared spectroscopy and energy

dispersive x-ray spectroscopy, will be presented in Chapter 5.

1.5 Thin Films of Prussian Blue Analogues

While nanoparticles limit the size of a material in all three spatial dimensions, thin

films are the result of limiting the size of only one spatial dimension. In effect, the thin









film shape breaks symmetry along the shortened axis (Figure 1-1), and one might

expect this broken symmetry to be reflected in the material properties. For both optical

applications and memory device applications, thin films are important. Previously, the

photoinduced magnetism of RbjCok[Fe(CN)6]r nH20 thin films were studied, finding an

anisotropy of the photoinduced magnetization [10 ]. This chapter focuses mainly on thin

films of RbjNik[Cr(CN)6]r-nH20 that do not possess photoinduced magnetism, but do

have a high ordering temperature, between 60 and 90 K, and a simpler magnetic

ground state. Additional films were also studied, substituting different transition metals

and studying how the magnetic anisotropy is affected. The goal of the study was not

only to characterize the specific films in question, but to provide insight into the general

issue of the anisotropy in complex cyanide thin films. The current understanding of this

phenomenon, as well as detailed experimental studies, including magnetization,

magnetic microwave resonance, UV-Vis spectroscopy, infrared spectroscopy, x-ray

diffraction, scanning electron microscopy and atomic force microscopy, will be

presented in Chapter 6.

1.6 Heterostructures of Prussian Blue Analogues

Using the knowledge base compiled during the photomagnetic nanoparticle and

thin film studies, an entirely different class of heterostructured materials were

synthesized and investigated. The main idea was to take the useful properties from two

different materials and to put them together in a new meta-material that possess both of

the desirable properties of the constituents. It is interesting that when materials are

combined, new unexpected properties can evolve, not native to either parent compound.

Two different types of heterostructures were studied, solid solutions and multi-layered

thin films, Figure 1-2. Two exciting results were a NaaCoxNil-x[Fe(CN)6]- nH20 powder,









in which the sign of the photoinduced magnetism can be tuned with chemical formula,

and a RbjCok[Fe(CN)6]1-nH20 / RbjNik[Cr(CN)61]nH20 heterostructured film, which has

photomagnetic effects at unprecedented temperatures for Prussian blue analogues.

The experimental magnetization, x-ray diffraction, infrared spectroscopy, transmission

electron microscopy, and energy dispersive x-ray spectroscopy measurements will be

presented in Chapter 7, along with descriptions of the present understanding of the

underlying heterostructure properties.



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heterostructure are shown.









CHAPTER 2
EXPERIMENTAL TECHNIQUES

Perhaps experimental physics may be understood as the harnessing of well

documented physical phenomenon to further investigate the less well known properties

of matter. A certain level of understanding of the apparatus used is necessary for a

rigorous analysis of results. This understanding not only allows the experimentalist to

identify the correct piece of equipment to probe the property of interest, but immensely

aids in the ability to recognize and avoid spurious results in the forthcoming data. This

chapter seeks to survey the experimental techniques utilized in taking the data shown in

later chapters.

In Section 2.1, sample environments will be discussed, followed by a summary of

the detection methods in Section 2.2. Methods are sorted according to the physical

location where the apparatus is located, with those performed in Professor Meisel's lab

in the New Physics Building (NPB) Room B133 outlined in Subsection 2.2.1, those

performed in Professor Talham's lab in the Chemistry Lab Building (CLB) Room 404,

techniques at the Major Analytical Instrumentation Center (MAIC) at the University of

Florida or other external labs in Subsection 2.2.2, and, finally, probes located at national

labs in Subsection 2.2.3. Within these detection sections, methods are ordered

alphabetically. Finally, Section 2.3 will be devoted to custom probes designed by the

author and used to collect data presented in the dissertation. Detailed machine

drawings and additional photographs of the custom equipment are relegated to the

appendices.









2.1 Sample Environment

Sample environment is a crucial aspect of experimental physics [11-14]. For the

data to be presented in the following chapters, the three most relevant parameters are

temperature, magnetic field, and the application of light to the sample. Specifically,

relevant vacuum technology, cooling schemes, superconducting magnets, and

photoirradiation methods will be described in the following subsections.

2.1.1 Vacuum Equipment

Vacuum equipment is necessary in cryogenic applications for many reasons. The

two most relevant examples are the need to evacuate sample and insulation spaces,

and to reduce the vapor pressure over liquid helium in order to reach temperatures

below 4.2 K. Proper pumping of all cold spaces is important because an atmosphere

with significant proportions of gas will impede cooling power.

2.1.1.1 Pumps

Rotary pumps are the workhorses of the cryogenic laboratory. The pumping

mechanism is purely mechanical in nature, consisting of a series of vanes that force air

from the inlet to the exhaust as the pump turns, Figure 2-1 (a). Routine base pressures

of 10-2 or 10-3 mbar can be reached, with maximum working pressures of a few hundred

mbar, or 1 bar in spurts. Rotary pumps are available in many different sizes, with

throughputs ranging from less than 1 m3/hour to a few tens of m3/hour.

Rotary pumps are often used as roughing pumps before more sophisticated

vacuum technology is recruited, backing pumps for diffusion or turbo pumps, or for

reducing the vapor pressure over a liquid helium bath. Roughing pumps are required in

many instances as technologies with lower base pressures often have lower maximum

operating pressures. Diffusion pumps must have a back-pressure below 0.1 mbar or oil









can back-stream into the vacuum system ruining expensive equipment, while turbo

pumps with too large of a pressure differential can bend a fin, permanently ruining the

pump. Sometimes it is desirable to arrange two rotary pumps of different sizes in

stages in order to achieve base pressures down to 10-4 mbar.

Most rotary pumps have a gas-ballast. When opened, a gas-ballast causes the

pump to work extra hard, thereby heating the oil. This heat helps remove any water that

may have condensed in the pumping system when pumping a cryostat that has been

exposed to air or has been left unused for a long time. Most surfaces release adsorbed

water vapor when the pressure is reduced, and without the proper use of a gas-ballast,

the long term base pressure of the system will suffer.

Oftentimes, it is necessary to reach pressures lower than those achievable by

rotary pumping technology for proper thermal insulation of a cryostat. Oil diffusion

pumps are capable of reaching pressures down to 10-7 mbar, but, as mentioned

previously, have maximum working pressures of about 0.1 mbar and must be backed

with a rotary pump. Air is moved by the use of heated oil vapors that create a high-

velocity stream to guide air from the inlet to the exhaust. This stream is created in

practice by use of a resistive heating element at the base, and either air, water, or liquid

nitrogen cooling of the walls, Figure 2-1 (b). Pumps are generally fitted with a cold trap

to help remove water from the vacuum space while simultaneously preventing pump oil

contamination of the cryostat or sample space.

The other main pumping technology for reaching high vacuum is the

turbomolecular pump, Figure 2-1 (c). Turbomolecular pumps are capable of reaching

pressures down to 10-8 mbar, and must also be backed with a rotary pump. One









important aspect of the turbo pump is the dependence of pumping speed and base

pressure upon the mass of the gas being pumped. Therefore, heavy oil molecules are

pumped especially well, but helium actually has one of the poorest ratios for turbo

pumps and it is due to this mass effect that diffusion pumps can outperform turbo

pumps in helium applications.

2.1.1.2 Pumping lines

If pumps are analogous to the voltage source in a pumping circuit, then pumping

lines are the resistive wires. In order to achieve the maximum pumping power at the

vacuum space, lines with minimal impedance are of the highest importance. Ideally,

lines are metal for high vacuum, as plastics can be permeable to helium gas. Pumping

lines should be free of adsorbed impurities, and therefore it is ideal to clean lines with a

volatile substance, like acetone. Finally, the cross-sectional area of the lines should be

large enough to ensure that the displacement throughput of the lines is larger than that

of the pump being used.

2.1.1.3 Vacuum gauges

Vacuum in the range of 1 to 1000 mbar can easily be measured with a simple

spring loaded dial gauge. However, for higher vacuum, pressure is generally measured

with a Pirani gauge in the range of 10 mbar to 10-3 mbar, and a Penning gauge in the

range 10-2 to 10-7 mbar. Pirani gauges consist of a wire filament in contact with the

atmosphere, and depending upon the gas concentration, different thermal conductivity

in the gauge is measured. Penning gauges measure the ion conductivity across a

large voltage drop. It is important to remember that the calibration of these high

vacuum gauges is dependent on the type of gas used in the system.









2.1.1.4 Oil mist filters and fore-line traps

In order to remove the oil vapors present in the exhaust gas of a rotary pump, oil

mist filters are used. This filtration is for three important reasons: first, there is a health

hazard associated with the inhalation of oil mist; second, oil mist may contaminate

plumbing, pressure gauges, and flow-meters behind the pump; and third, since UF has

a helium recovery system, it is desirable to limit the amount of oil that must be removed

in the recovery laboratory. Filters can either be coalescing or centrifugal. Centrifugal

filters are mainly utilized for price reasons but have the downside that they must be

drained occasionally. Fore-line traps are used on the inlet of a rotary pump to reduce

the amount of oil back-streaming up the pumping line. These must also be changed

regularly as traps saturate with oil over time.

2.1.1.5 O-rings

Any time two pieces of vacuum equipment are joined together, a seal must be

made. Ideally, joints would be soldered or welded, but oftentimes setups are dynamic,

so temporary seals are used. These may be o-ring seals, or different varieties of a flare

fitting.

For o-rings, the material to be used is the most important aspect of the seal.

Nitrile rubber is the ubiquitous o-ring material used in modern day vacuum technology,

most simply because it is the best seal for the price. Nitrile has a working temperature

range from -400 C to 1200 C. Silicone rubber is has a higher temperature range of

workability, -1000 C to 2500 C, however it is not used in the cryogenic apparatus

because it is permeable to helium gas. Butyl rubber is another o-ring material that has

a low gas permeability. It can be used down to -600 C, but is a little more expensive









than nitrile. Fluoroelastomers, such as Viton, are used for higher temperatures but is

more expensive and less ductile than nitrile. It has a working temperature range from -

200 C to 2000 C. Teflon can also be used to make seals, but one has to always be wary

of the dodgy mechanical properties and large thermal contractions. For low

temperature o-rings, indium seals are ideal because of the similar thermal expansion

coefficient of the indium and the metal cryogenic apparatus.

2.1.2 Cryostats

Helium is the mainstay refrigerant for any researcher looking to reach

temperatures below 70 K. Specifically, the more common 4He isotope is capable of

reaching temperatures down to 1 K, due to its thermodynamic phase diagram.

Depending upon needs and availability, liquid helium cryostats or closed cycle cryostats

are used.

2.1.2.1 Bath cryostats

The simplest cryostat configuration is a bath cryostat, consisting of a large volume

of cryogen (tens of liters) thermally insulated from ambient temperature. More

complicated setups may include internal structure, such as a continuous flow cryostat,

to allow for greater control of sample temperature. Regardless of the internal structure,

bath cryostats require substantial shielding to achieve the necessary thermal insulation

for economical experiments. Shielding generally consists of a radiative shield thermally

anchored to either a liquid nitrogen or helium gas cooled shield. Currently, the boil-off

rates are comparable for nitrogen or helium gas shielded systems.

2.1.2.2 Continuous flow cryostats and cryogenic inserts

While it is possible to pump on the entire bath to reduce the temperature below

4.2 K, it is common practice to thermally isolate a smaller volume, which has been









equipped with radiation shielding, to be pumped on while leaving the bath at 4.2 K.

These volumes are a type of continuous flow cryostat called variable temperature

inserts that allow for separate temperature control of the bath and the sample space.

This separation of sample and bath spaces allows for cooling without the need for large

displacements of gas and reduces the ~35% boil-off of liquid necessary to reach the

lambda point (2.17 K) to a small fraction of the bath [13]. Of course, this convenience

requires an additional level of complexity, in which a stream of super-cooled gas in

conjunction with a resistive heater are able to achieve stability over wide ranges of

temperature. Helium is drawn from the bath through an impedance line (which is

sometimes variable), and the temperature of the insert is controlled by using a resistive

heater and pump to control the pressure within the insert. Cooling is achieved as the

cold gas flows over the sample and out through the exhaust to the pump. Additional

stability can be achieved at the expense of a longer relaxation time constant when

changing temperatures by putting a heat exchanger between the cold gas and the

sample.

2.1.2.3 Closed cycle refrigerators

While bath cryostats require an external liquefier and the transfer of liquid helium

into the apparatus, closed cycle refrigerators offer an increasingly popular alternative.

The most common type of refrigeration cycle used is one of the Gifford-McMahon type.

Grossly, they consist of a closed circuit of helium gas, a probe, and a compressor.

During operation, helium is alternatively compressed and allowed to expand at tens of

Hertz, using the entropy of the expansion to cool the sample. Depending upon the type

of shielding employed and the power of the compressor, temperatures as low as 5 K

can be routinely reached in these instruments. Potential drawbacks of closed cycle









refrigerators are the coupling of the compressor vibrations to the experiment,

maintenance of the compressor, and the comparatively high initial cost of the setup

when compared to a continuous flow cryostat.

2.1.3 Superconducting Magnets

Perhaps the highest impact application for superconductors is in the wire of

superconducting magnets. At the expense of refrigeration, magnets with

superconducting wire are capable of magnetic fields in the neighborhood of 20 T without

the need for high voltage power supplies. The most common winding geometries for

superconducting lab magnets are a continuous solenoid or split pair, Figure 2-2. While

solenoids allow for the best field homogeneity, for neutron scattering it is necessary to

have access to the sample perpendicular to the magnetic field.

2.1.3.1 Magnet construction

Due to the need for high current densities, most commercial magnets utilize type I

superconductors. In order to increase the critical field, alloying is employed, as impurity

sites act as local flux pinning minima. In addition, the wire is generally multi-filamentary

to further prevent dissipative flux jumping. The most common superconducting wire is

the NbTi alloy, because it is cheap, ductile, and has a high supercurrent density even in

strong fields. Using only NbTi, fields up to 9 T at 4.2 K or 11 T at 2.2 K can be routinely

reached. To get in the neighborhood of 20 T, the magnet wire must operate at or below

the lambda point (2.17 K), and the inner windings must be made from the more

expensive and brittle Nb3Sn alloy.

Magnetic field homogeneity is always a concern, and standard superconducting

solenoids have flat magnetic field profiles over a centimeter at the center field up to one

part per thousand. To achieve higher homogeneity, many modern systems utilize









compensating coils to combat the field gradients produced from the finite nature of the

wound solenoids. For finer adjustment, shim coils can tune the homogeneity of the

center field to 1 part in 105 for series shims and as good as 1 part in 107 for tunable

shims. Finally, counter-wound cancellation coils may be fit on the ends of a magnet to

help cancel stray fields at distances away from the center of the magnet. Overall, the

case is slightly worse for split pair magnets, where field homogeneity is typically an

order of magnitude lower for analogous setups.

2.1.3.2 Magnet operation

Perhaps the most important thing to remember when working around and using a

superconducting magnet is the huge amount of energy held within the structure, of the

order of a megajoule. This enhanced awareness of magnetic forces is especially true in

split pair setups, where large mechanical reinforcements are needed to stop the pairs

from joining together. Due to their small resistances, superconducting coils are able to

produce huge back EMF's when the current is ramped up or down. This inductance

forces users to ramp at sufficiently slow rates to avoid transitioning the wires to the

normal state, thereby quenching the magnet and potentially damaging expensive

equipment. If the wires do phase transition to the normal state, the large amount of

energy stored in the circuit must then be dissipated. It is not uncommon to see a plume

of helium from the magnet bath vent during a quench. All modern magnets have

measures in place to avoid the most destructive consequences of a quench. The so-

called superconducting winding protection circuit is, most simply, resistive elements put

in parallel with the magnet windings, thereby shorting the magnet once a quench has

occurred.









Happily, unlike resistive magnets, superconducting magnets do not require the

application of an external voltage to maintain current flow. Practically, this aspect is

often exploited in superconducting magnets to limit helium boil-off during operation by

the use of a persistent magnet mode. The persistence mode is when there is still

supercurrent in the coils, but the external power supply has been turned off. To allow

for persistence as well as ramping of the field, a superconducting switch is wired in

parallel to the main coil windings, and a small heater is placed near the switch. When

charging the magnet, heat is applied, causing the switch to go normal and therefore

acting effectively as a broken wire in the circuit, dropping all applied voltage over the

magnet coils. For the persistent mode, heat is removed from the superconducting

switch, thereby isolating the coils, so the external power supply can then be slowly

ramped down and turned off, leaving a persistent supercurrent in the windings. It is

important to remember that if the field is to be changed after entering the persistent

mode, the power supply must be ramped to the correct voltage corresponding to the

current in the magnet, and the superconducting switch must then be activated before

any changes in the voltage across the magnet circuit can be made. Typical decay rates

in persistent mode are 100 iT/hour.

2.1.4 Light Guides

For photoinduced studies of magnetization and structural changes, it is necessary

to have a way to get light from a room temperature halogen light source, with typical

powers of 1 or 2 mW, down into the cold space of a cryostat. For small scale probes,

optical fibers from Ocean Optics, Model 200 UV-Vis, OD ~ 270 pm were used. These

fibers are ideal because they are flexible, are thermally insulating, and are easily









arranged to direct light onto small samples. For larger scale probes, fiber optics are no

longer ideal or even economically feasible. In these cases, solid quartz rods were used

as light guides; although they are not flexible, much larger amounts of light can be

guided down to the sample space.

2.2 Detection Methods

With sample environment considerations in one hand, the other obvious element

to an experimental study is the detection methodology itself. With the large amount of

scientific infrastructure already in place, experimentation often comes down to keen

identification of the correct probe and the conditions to extract the material properties of

interest. In the following subsections, the Superconducting QUantum Interference

Device (SQUID) magnetometer that was used most heavily, auxiliary methods

performed in other labs, and probes located at national lab facilities will be overviewed.

2.2.1 SQUID magnetometer

2.2.1.1 Superconducting quantum interference devices

Superconducting QUantum Interference Devices (SQUIDs) are highly precise

amplifiers, often utilized in magnetometers due to their sensitivity to magnetic fields

weaker than 10-14 T [15] [16]. A clear example of the precision of SQUIDs is their ability

to detect, and actually discover, that flux is quantized in units of

h 15 2.1
(o = 2.0678 x 10-15 Tm2 2.1
o 2e


where h is Planck's constant, and e is the charge quanta. SQUID devices are based

upon Josephson junctions, which consist of two superconducting regions connected by

a weak link that allows quantum tunneling between the two regions without bulk

transport.









Josephson junctions are generally made from niobium or niobium alloy

superconductors, such as NbSe2 or NbTi. While originally a point contact using a

sharp-ended screw was used, modern day junctions are microbridges using patterned

lithography. Current and voltage can then be measured across the weak link created at

the point contact.

The Josephson junction alone does not act as a magnetometer, so it must be

included in a larger SQUID circuit, Figure 2-3. The basic aspects of a SQUID circuit are

a transformer coil, which is large enough to interact with the sample, coupled to a signal

coil, which is manufactured symmetrically with a radio-frequency detector coil. The

signal coil and detector coils are connected through a weak link, allowing flux coupling.

Magnetic flux changes arising from a magnetic sample induce current in the transformer,

and these changes couple directly to the detector coil via mutual inductance. Finally,

the radio-frequency voltage across the detector coil can be measured, after going

through a conditioning circuit, and fit to extract the field associated with the sample.

2.2.1.2 Quantum Design MPMS XL magnetometer

Two different commercial magnetometers from Quantum Design were used for the

DC- and AC-SQUID measurements, an MPMS-5S and an MPMS-XL [17-19]. The

MPMS-5S, located in the New Physics Building Room B20, is the older of the two and is

equipped with an AC detection board, a 5 T superconducting magnet equipped with a

field reset, magnetic shielding, and a pumped 4He cryostat. The MPMS-XL, located in

the New Physics Building Room B133, is newer and has the added benefit of a 7 T

superconducting magnet. An additional advantage of the MPMS-XL is a low

temperature impedance allowing for the continuous operation at base temperatures









lower than 2 K, but the XL is not equipped with any of the other additional features of

the 5S model.

Powder samples were mounted in either diamagnetic gelcaps or on sticky tape to

increase the optical cross-section for photoinduced experiments. Commercial straws

were used as a diamagnetic sample rod to allow translation of the sample through the

SQUID magnetometer detector coils. In general, backgrounds were subtracted based

upon the known mass susceptibility of the sample holders, but in many cases,

background contributions were insignificant.

The Quantum Design magnetometers utilized a second derivative transformer coil

to measure sample flux and to couple to the SQUID, Figure 2-4. The advantage of the

multiple coil setup is from the inherent background subtraction of any signal that is a

longer wavelength than the ~ 4 cm long transformer. These noise sources will simply

cause, for example, a positive voltage in the top coil, a negative voltage in the second, a

negative voltage in the third, and a positive voltage in the fourth coil, summing to

zero [18].

2.2.1.3 Remnant fields and degaussing the MPMS

For probing magnetic systems with weak anisotropy, the use of small fields may

be necessary. The fields can be measured using a custom Hall probe for the SQUID

magnetometer (based on a Toshiba THS118E chip) developed by the author. The use

of the standard "oscillate" option on the MPMS provides "zero field" of ~ 10 G, Figure

2-5 (a). However, this level of error uncertainty can be undesirable at times, when a

manual degaussing sequence can be employed instead. The specific degaussing

method depends on the recent history of the magnet. An example of a degauss

sequence would consist of high resolution charging between -40 kG, 30 kG, -20 kG,









10 kG, -5 kG, 2.5 kG, 1 kG, 500 G, -250 G, 100 G, -75 G, 50 G, -25 G, 10 G, -10 G, 0,

5 G, -5 G, 0. The results of this sequence give a different field profile with a smaller

magnitude of ~ 1 G, Figure 2-5 (b). It is also important to wait a sufficient amount of

time for eddy currents to dissipate, since superconducting magnets have long time

constants because of a large inductance combined with a small resistance.

2.2.2 Additional Methods Performed at UF

2.2.2.1 Atomic force microscopy

Atomic Force Microscopy (AFM) is a highly sensitive scanning probe microscopy

that was used to characterize the surface morphology of thin films [20]. All AFM studies

were performed on a Digital Instruments multimode scanning probe microscope in

Professor Talham's Chemistry Lab in CLB Room 404. Roughly a square centimeter of

film was cut and placed under the scanning tip. The scanning tip is a sharp point

attached to the end of a cantilever, which in turn is connected to the feedback

electronics that monitor the height of the tip over the surface to help prevent tip crashes

onto the sample, Figure 2-6. Nanometer scale changes in the deflection of the tip are

detected by a laser coupled to a photodiode.

2.2.2.2 Carbon, hydrogen, and nitrogen combustion

Carbon, Hydrogen, and Nitrogen combustion analysis (CHN) was utilized to

determine light atom (with 2p electrons) concentrations for selected samples [21]. All

CHN was performed at the University of Florida Spectroscopic Services laboratory.

Roughly 3-5 milligrams of sample are used and destroyed in the measurement process.

To achieve controlled combustion, the sample is sealed in an oxygen atmosphere and

external heat is applied. As atoms are released, they flow through a series of columns

containing a water trap, a carbon dioxide trap, and a nitric oxide trap, Figure 2-7. By









measuring the masses of the different traps after combustion of the sample is complete,

the analytical determination of the chemical make-up is possible with straightforward

calculations.

2.2.2.3 Energy dispersive x-ray spectroscopy

Energy Dispersive X-ray Spectroscopy (EDS or EDX) was the primary analytical

technique used for determining relative concentrations of "heavy" atoms (containing 3d

electrons) for selected samples [22]. All results reported herein were recorded on a

JOEL 201 OF Super-Probe, housed at the Major Analytical Instrumentation Center

(MAIC) at the University of Florida (UF). Only micrograms of samples are necessary to

perform the measurements, with mounting achieved by deposition of microliter

quantities of sample containing solution on holey-carbon TEM grids purchased from Ted

Pella, Inc. Simplistically, the experimental apparatus required are an electron gun with

proper magnetic lenses and an inelastic x-ray detector, Figure 2-8 (a). In this method, a

beam of electrons is focused on the sample, with a finite probability of incident electrons

ejecting bound electrons. If the ejected electron was from an inner shell, the atom will

seek the new ground state, emitting energy in the form of a photon in the process,

Figure 2-8 (b). These photons can then be collected and analyzed by a detector,

revealing the electronic transitions present. A typical spectrum consists of x-ray counts

as a function of energy. As each atom has a unique electronic structure, the electronic

transitions present in the experimental x-ray spectrum are diagnostic of the chemical

composition of the sample.

2.2.2.4 Fourier transform infrared spectroscopy

Fourier Transform InfraRed (FT-IR) spectroscopy, in the middle of the spectrum, is

the study of how light with wavelengths from 4,000 cm-1 to around 400 cm- interacts









with matter [23]. All measurements were performed on a mid infrared Nicolet 6700

spectrometer located in CLB Room 411. Powder samples were either mounted in

pressed KBr pellets or sandwiched between two salt-plates for the study of materials

that were sensitive to pressure. Thin film samples were run with no additional

modifications. As with all spectroscopic methods, infrared spectroscopy is sensitive to

transitions between discrete energy levels for the material being irradiated. These

energy levels can be either vibrational or electronic in origin, and for this work,

vibrational modes were of primary accessibility and interest. Interferometers are the

basis of an infrared spectrometer, which consists of a broad-band source, a

beam-splitter, a fixed mirror, a movable mirror, a sample, and a detector, Figure 2-9 (a).

Spectra are obtained as a function of moveable mirror position. Fourier transform

spectroscopy is so called because it consists of sending a pulse of radiation with many

frequency components through the sample to the detector, which registers the signal in

the time domain (technically the moveable mirror position domain). Finally, a Fourier

transform is performed to obtain the spectra in the frequency domain, Figure 2-9 (b).

2.2.2.5 Inductively coupled mass spectrometry

Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is an analytical

chemical technique used to determine the concentrations of metals, and some

non-metals, with a high sensitivity. For example, detection limits are less than a

picogram per second for transition metals [24]. ICP-MS results were obtained using a

Thermo-Finnigan Element-2 spectrometer located at the UF Department of Geology.

Samples were prepared for ICP-MS by dissolving them in trace metal grade nitric acid.

This chemical dissolution is necessary in order to aerosolize the sample for when it is

introduced into the plasma chamber. This aerosol enters an argon environment of the









chamber and is subsequently exposed to powerful radio frequency radiation, converting

the gas into a plasma, Figure 2-10. Argon is chosen because of the much higher first

ionization potential, compared to all elements except He, F, and Ne. It is the dynamics

of the argon and sample plasma that provide for sample explosion and subsequent

ionization. Once ionization is complete, the RF field also serves to delineate ions

having different charge to mass ratios, and thus determine the elemental content of the

sample.

2.2.2.6 Transmission electron microscopy

Patterned after the more traditional light transmission microscopy, Transmission

Electron Microscopy (TEM) allows for resolution of a few A's, owing to the short

deBroglie wavelength of the electrons [25]. All results reported herein were recorded on

a JOEL 2010F Super-Probe, housed at the Major Analytical Instrumentation Center

(MAIC) at the University of Florida (UF). Only micrograms of samples are necessary to

perform the measurements, with mounting achieved by the deposition of microliter

quantities of sample containing solution on various holey-carbon TEM grids purchased

from Ted Pella, Inc. A representative setup consists of an electron gun, conditioning

lenses, and an imaging screen, Figure 2-11. Electrons are generated at a thermionic

electron gun. This source is then focused by the use of magnetic lenses, in which the

trajectory of the charged particles is bent by the presence of the applied field. Electrons

next travel through an aperture to avoid background from large angle particles.

Subsequently, the focused electrons are scattered by the matter present in the sample,

creating a negative image of the sample in the electron beam. The objective optic

serves to focus the image, and the objective aperture again cuts off high-angle

scatterers. The image is then enlarged by the intermediate and projector lenses onto









the imaging screen, where electrons interact with phosphor to produce light that can be

recorded with standard camera techniques. These methods have been utilized to

obtain light-field, dark-field, and diffraction data on the samples discussed in the

following chapters.

2.2.2.7 Ultraviolet and visible spectroscopy

Spectroscopy in the UltraViolet and Visible range (UV-Vis) is useful for studying

coordination networks because the wavelengths of approximately 200 to 800 nm probe

energy scales of 6.21 to 1.24 eV, which are comparable to the energies separating

different multi-electron magnetic energy levels [26]. A typical spectrometer consists of a

source, a monochromator, the sample space, a photodetector, and a computer interface,

Figure 2-12. Two different machines located in the Chemistry Lab Building were used,

a room temperature device, and a spectrometer equipped with a close-cycle cryostat for

temperature dependent studies. The source consists of two elements, a tungsten

halogen for wavelengths above 320 nm and a deuterium arc lamp for wavelengths

below 320 nm. Monochromators are made up of a diffraction grating and a series of

filters to remove higher order diffractions. Samples are mounted using quartz slides for

thin films and quartz cuvettes for solutions. The detector is a silicon photodiode.

Spectra are then recorded as ASCII delimited files.

2.2.2.8 X-ray powder diffraction

X-Ray Diffraction (XRD) is used to find the average positions of heavy atoms in a

wide range of samples [27]. All samples studied were polycrystalline, so the XRD

Philips APD 3720 20 powder diffractometer located in MAIC Room 117 was used. A

standard Cu K, source is used, producing a dominant wavelength of 1.54 A (933 eV)









x-rays by applying voltages near the ionization energy of the K (1s) to L3 (1 P3/2)

transition in Cu. For good signal to noise on organometallic compounds, tens of

milligrams are desirable, but samples on the order of one milligram show clear, refinable

Bragg peaks. Samples were mounted on a 25 mm x 47 mm glass slide purchased from

Ted Pella Inc., and immobilized using a 1 cm2 piece of double-sided sticky tape in the

center of the slide. In XRD, the lattice planes within ordered crystals satisfy the

condition of constructive interference when the path length difference between x-rays

scattering from different lattice layers is an integer multiple of the incident wavelength,

Figure 2-13. For randomly oriented polycrystalline samples, all lattice planes are

effectively probed at the same time, the disadvantage is that the experimental

integration over crystalline angles reduces the amount of structural information.


2.2.3 National Laboratories

National Laboratory user facilities are an important resource for the modern

research scientist and the author feels fortunate to have been able to Visit a few

different facilities. For the materials studied in this thesis, measurements were carried

out at the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida,

and the Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee. High field

Electron Magnetic Resonance (EMR) studies were performed at NHMFL, Neutron

Diffraction (ND) studies were performed at the High-Flux Isotope Reactor (HFIR) at

ORNL on the triple-axis spectrometer on beamline HB1A and the neutron powder

diffractometer on beamline HB2A, and Inelastic Neutron Scattering (INS) was

performed at the Spallation Neutron Source (SNS) on the fine-resolution Fermi chopper

spectrometer at beamline 17. Online at full-power since 1966, the 85 megawatt HFIR









source is unique because it has the highest flux, of the order of a million neutrons/cm2s

on most beamlines, of any reactor based source of neutrons for condensed matter

research in the United States. Neutrons at the HFIR are in the thermal spectrum, with a

small portion of cold neutrons available. On the other hand, the SNS is still in the

process of being commissioned as of this writing, but is expected to have record

neutron fluxes available for studying materials. Unlike the HFIR, the SNS uses

spallation to create bursts of neutrons with a wide range of energies, and as such, the

spectrometers must operate in time-of-flight mode.

2.2.3.1 Electron magnetic resonance at the NHMFL

Electron Magnetic Resonance (EMR) experiments were performed in Professor

Stephen Hill's lab at the NHMFL, using a resonant cavity insert to a Quantum Design

Physical Property Measurement System (PPMS) equipped with a 5 T magnet. For

approximately a square centimeter of sample, Prussian blue analogue films require

about 400 cycles, which is an arrangement that may also be achieved by using multiple

films of less than 400 cycles. For powder resonance, approximately a milligram of a

Prussian blue analogue magnet is required. Resonant absorption of external

microwave radiation for electron systems can be easily understood in the context of

energy splitting to be presented in the next chapter. Briefly, if an electron is placed in a

magnetic field, there is a difference in energy between the parallel and antiparallel

orientations of the spin vector with respect to the field vector, Figure 2-14 (a). When the

energy difference between the two orientations is in resonance with external radiation,

spins may be excited from the lower energy level to the upper one, resulting in the

absorption of the incident radiation, which may be measured experimentally,

Figure 2-14 (b).









The apparatus necessary for performing resonant absorption experiments is

generalized in Figure 2-15. The first essential ingredient is a tunable external magnetic

field that is large enough to split the energy levels of the system to separations excitable

by an external radiation source, which is the second essential ingredient. Furthermore,

the sample is often placed in a resonant cavity to further amplify the effects of

absorption. To measure absorption, the ratio of the microwave intensity before the

sample and after the sample can be compared.

2.2.3.2 HB1A neutron triple-axis spectrometer at HFIR

The Fixed-Incident Energy Triple-Axis Spectrometer on beamline HB1A at HFIR

(Figure 2-16) is ideal if the exact energies and moment of interest in a sample are

known. Approximately five grams of deuterated Prussian blue analogue powder are

required for HB1A. Momentum transfers from 0.2 to 4.9 A- can be measured in elastic

mode, and energy transfers from roughly -35 meV to 11 meV at q = 3 A- can be

measured in inelastic mode. In the best case, energy resolutions of ~0.5 meV are

possible. This beamline has one of the most intense, with a flux at the sample of

~2 x 107 neutrons/cm2s, and cleanest beams at the reactor, due in part to the pyrolitic

graphite monochromator system, which fixes the incident energy at 14.6 eV. In addition

to the monochromator, HB1A has a variety of analyzers to help condition the beam, with

analyzer angles able to be ranged from -600 to 1200, as well as the option to place a

sapphire filter in the beam before the monochromator. A variety of collimators are

available at different portions of the beam, before the monochromator the collimation is

48', between the monochromator and the sample, collimators of 10', 20', 30' or 40' may

be used, between the sample and the analyzer, collimators of 10', 20', 30', or 40' may

be used, and finally between the analyzer and the detector, collimators of 70' or 140' are









available. The fixed detectors give access to scattering angles from -5 to 1200.

Finally, the maximum beam size on HB1A is 40 mm x 150 mm.

2.2.3.3 HB2A neutron powder diffractometer at HFIR

The Neutron Powder Diffractometer (NPD) on beamline HB2A at HFIR

(Figure 2-17) is optimized to take powder patterns, as opposed to the triple-axis

machine. Approximately three grams of deuterated Prussian blue analogue powder are

required for HB2A. This spectrometer is useful to refine crystal and magnetic

structures. The detectors are 44 movable 3He tubes setup to detect intensities in a

Debye-Scherrer geometry. As the detectors are situated, they give access to 0 to 1500

scattering angles, although the lower end may be plagued by air-scattering and the high

end is weak due to the structure factors of samples. The monochromator is germanium

and is capable of providing three different incident wavelengths, 1.54 A, 2.41 A, or 1.12

A. The collimator may be left out of the beam, giving 12' collimation, or 16', 21' or 31'

collimators may be used, at the expense of intensity. The beam is optimized for

samples with a 25 x 25 mm2 cross-sectional area, but it may be masked with borated

plastics if smaller samples are necessary. Finally, the maximum resolution is about 0.20

or 2 x 10-3 Ad/d, where d is the real space distance between lattice planes.

2.2.3.4 Inelastic neutron scattering on SEQUOIA at SNS

Inelastic Neutron Scattering (INS) was performed on the SEQUOIA spectrometer

(Figure 2-18) at SNS. Approximately five grams of deuterated Prussian blue analogue

powder are required for SEQUOIA. Operating in time-of-flight mode, two choppers are

utilized, a To chopper to block the unwanted high-energy neutrons resulting from

spallation, and a Fermi chopper to choose the incident energy range. This setup allows

for an incident energy range from 10-2000 meV, but it may be possible to increase the









upper limit. The energy resolution varies from 1% to 5%, getting worse in the higher

energy limit. This fine energy resolution is due to the 5.5-6.3 m sample to detector

distance.

2.3 Custom Apparatus

2.3.1 SQUID Probe with Low Temperature Rotation and Optical Fibers

In order to study materials with magnetic anisotropy as well as photo induced

magnetism, a new probe has been developed [28]. In situ sample rotation is a valuable

tool to measure the angular dependence of the magnetization. Additionally, there is an

ongoing research effort to study materials that show changes in magnetization with

applied light. Specifically, samples showing photoinduced magnetization as well as

magnetic anisotropy have been identified [10] [29]. Therefore, an experimental setup

that is able to measure magnetizations down to low temperatures, while affording in situ

sample irradiation and rotation, is beneficial.

There are a few inherent difficulties one has to be aware of when setting up such a

system. A probe that is suitable for use with commercial SQUID magnetometers was

designed because signals can be quite small, i.e. 10-6 emu and less. Although using a

commercial setup was considered most promising, it puts significant size and weight

constraints on the probe and, most importantly, on the sample space. Also, because of

the small signals involved, a minimization of the background signal from the probe is

sought. Finally, care has to be taken that the system can operate at temperatures

below the boiling point of 4He. Thermal contraction and expansion of parts must be

taken into account, as well the need to keep parts movable while minimizing heating.

As an important part of developing the experimental setup, possible construction

materials have been characterized. A probe that meets the desired specifications has









been built and tested. Probe specifications, design, materials data, and operation will

be presented and discussed.

2.3.1.1 Probe specifications and design

The probe setup is shown in Figure 2-19. Detailed machine drawings of the probe

can be found in Appendix B. Sample rotation is uniaxial about an axis perpendicular to

the applied magnetic field. Rotation is done with a line connecting the low temperature

sample holder to a cylinder at ambient temperature, with an additional line for resetting.

Operation can be completely manual, manually controlled by a stepper motor, or

automated by using computer controls for the stepper motor. Commands can be

initiated in commercial software by using control data bits that are insignificant. For

example, scan length parameters ranging from 4.00001 cm to 4.001 cm can be mapped

onto 100 different commands and subsequently read by the stepper motor control

program. A fiber optic cable allows for irradiation.

The head of the probe must be light enough to accommodate the servo that

translates the entire probe vertically to move the sample through the SQUID coils. The

housing is aluminum because of its density, strength, and machinability. The drive shaft

is beared by a slide-seal assembly consisting of o-rings and plastic spacers. Vacuum is

achieved by the use of rubber o-rings for the drive shaft seal, the connection of the

probe head to the shaft, and for the two additional access ports, one on the top and one

on the side. The o-rings themselves to not keep the vertical position of the probe head

static, so a quick connect of Teflon ferrules is attached to the probe head and can be

tightened at the desired height; this variability in height can be quite useful to

accommodate additional slack that may be introduced by the different thermal









contraction of the long sections between room temperature and low temperature.

Stycast 2850GT black epoxy seals the clear holes made for the optical fiber.

The shaft of the probe is constrained to be 0.12" (0.305 cm) OD for use in a

commercial QD-MPMS SQUID magnetometer, allowing the commercial shaft seal

assembly to be used. Most of the shaft is stainless steel for strength, but the bottom

portion is quantalloy to minimize the background signal. The shaft is attached to the low

temperature end with epoxy. For the drive lines, fishing materials were of prime

consideration because they are non-metallic, thin, and strong. To decide between

monofilament or braided lines, two exemplary products were studied: Spiderwire 8 Ib

monofilament (mono-line) and Spectra PowerPro 15 Ib (braided-line). The braided-line

was chosen for its larger Young's modulus, since stretching of the lines can lead to

errors in sample angles. Although there is a larger magnetic signal associated with the

braided-line, the amount near the SQUID coils is only &10 milligrams. Magnetic and

mechanical properties of the lines are summarized in the next subsection.

For the low temperature end of the probe, the main pieces are a yoke and a

rotatable sample stage. Delrin was chosen for its compromise of strength and small

magnetic signal. The yoke is long enough to be locally symmetric with respect to

translations vertically through the detector coils, minimizing its flux contributions. The

rotation stage is a hollow cylinder beared by plastic on plastic, with all but 90 open for

accessibility during irradiation. The drive strings are attached to the rotation stage via

nylon set screws. The magnetic properties of black DelrinTM acetal polymer, brown

VespelTM polyimide, and white nylon are summarized in the next subsection.









2.3.1.2 Probe material properties

During the design process, several candidate materials were investigated.

Magnetic properties were measured in a Quantum Design MPMS-XL SQUID

magnetometer, and the results are summarized in Table 2-1 and Figure 2-20. All

samples were mounted in uniform straws using press fits, so no background signal was

subtracted. Annealing of the Delrin was attempted in case additional magnetism was

coming from free bonds [30], but no obvious change in the susceptibility was observed.

Temperature sweeps were done at 100 G, 1 kG, and 1 T between 2 K and 300 K. Field

sweeps were done at 2 K, 10 K, and 100 K for fields up to 7 T. The magnetic

susceptibility results could be well fit to a semi-empirical formula,

c 2.2
X = -+ L.T+D


Additionally, for the drive lines, some mechanical properties were investigated.

Force constants were examined at room temperature to test the line deformation in

response to an applied force. The mono-line has a diameter of 0.010 in (0.254 mm)

and a measured Young's modulus of 1.4 GPa, which is lower than 2.3 GPa reported to

us by Berkley Fishing in a private communication. The braided-line has a diameter of

0.007 in (0.1778 mm) and a measured Young's modulus of 68 GPa, which is close to

the 73 124 GPa range reported by Honeywell for different Spectra fibers [31].

2.3.1.3 Operation

Use of the custom rotation probe is more complicated than simply using a

standard MPMS sample rod. First, the user must mount the sample at the bottom of the

probe, while being careful not to let grease touch the axles of the rotation cell. Next, if

the fiber optic cable is desired, the side head having the fiber optic cable must be in









place rather than a blank. The user must decide what form of angular control is to be

used, manual, manual with the stepper motor, or automated with the stepper motor. For

simple manual control, the dial will be mounted on the probe head, but for stepper motor

control, the dial is removed and the stepper motor mounted in its stead. Good practice

consists of testing rotation on the bench at room temperature before operation.

Extreme care must be taken as to the extent to which the probe is rotated in one

direction or the other, as without such care, the user may over rotate the probe and

cause damage. Generally, one should completely load one end of the spool to prepare

for rotation in the opposite direction. Insertion of the probe into the SQUID should utilize

the custom counterbalance and preset weight, at which point standard practices should

be followed. Once the probe has been cooled to the desired temperatures, careful

tensioning of the control line should be checked, since slack may be introduced upon

cooling. Tensioning may be adjusted by the set of ferrules attaching the probe head to

the shaft. By the same token, if slack was taken up at cold temperatures, the user must

remember to add slack to the probe before warming. If only one direction of rotation is

desired, the issue of slack is less important.

If manual operation is to be used, the probe has now been completely prepared for

use. If stepper motor control is to be utilized, additional steps are necessary. The

driver board (Figure 2-21) must be set-up, consisting of the 12 V power supply for the

motor, the 5 V power supply for the logic, and cables connecting to the probe and (if

desired) the computer parallel port. To begin controlling the probe via board-only

(i.e. no computer control) automated operation, make sure that switch SW3 is up. Set

SB1-1, SB1-3 and SB1-5 to "up"; SB1-2 and SB1-4 can be changed at the discretion of









the user. Whenever changes are made to the board's inputs, depressing SW1 (the

master reset) may be necessary. To begin controlling the probe with computer

automated operation, make sure switch SW3 is in the down position. Set SB1-1, SB1-2,

and SB1-5 to "down;" SB1-3 and SB1-4 can be changed at the discretion of the user.

Connect the computer controller cable to CN3, making sure that position 1 is placed in

the GND terminal and position 4 is placed on the CLK terminal. The bits can then be

written to by the parallel port using the computer's logic power. The "Stepper Motor

Control.vi" on the SQUID computer can now be used for automated control of the motor

and simultaneous data acquisition with MPMS MultiVu.

To verify successful probe operation, a piece of magnetite with the magnetic axis

aligned perpendicular to the axis of rotation was measured without any applied field.

Sample rotation was tested through greater than 3600 and at temperatures down to 2 K,

Figure 2-22 (a). Sample irradiation was also tested using thin films of

AiCok[Fe(CN)6]/nH20 oriented parallel to an applied magnetic field of 100 G at 5 K, as

seen in Figure 2-22 (b).

2.3.1.4 Conclusions

The ability to photoirradiate and rotate samples in situ while using the convenient

setup of a commercial magnetometer has been demonstrated and represents a

combination of previously unreported features. Probe materials and design have been

presented with the hopes of providing insight to others who are investigating the

photomagnetic properties of new materials. Future improvements may be made to the

probe by more carefully etching the materials to remove possible magnetic impurities

introduced by the machining process.









2.3.2 Neutron Scattering Probe for Photoinducing Opaque Powders

One final project is the photoinducing of opaque powders for neutron scattering.

As this endeavor is a new experimental undertaking, a probe must be developed to

provide the necessary sample environment. As of this writing, the development of a

second generation of prototypes for the neutron light experiments are underway. The

novel part of the sample design consists of a low temperature tumbler that allows for

opaque particles to be exposed to light as a function of time, without the problem of

surface particles blocking light from the rest of the sample. A schematic of the problem

and proposed solution can be seen in Figure 2-23 and Figure 2-24. The expertise

gained in rotating samples at low temperature with the custom SQUID probe described

in the above section, as well as more standard photoinduced magnetism probes for the

SQUID is invaluable in the development of such a probe.











d) Ju) 3)
(iv) (v) ( v)v: .

S(iii 3 (i


(vi (i


(i)~ i)




Figure 2-1. Illustrations showing operation of standard pumps that can be found in a
cryogenic laboratory. (a) A schematic cutaway showing the main design
elements of a rotary pump, (i) the rotor, (ii) the stator, (iii) the exhaust valve,
(iv) the exhaust outlet, (v) the pump inlet, (vi) the vanes, and (vii) the pump oil
bath. (b) A schematic cutaway showing the main design elements of an oil
diffusion pump, (i) the heater, (ii) the hot oil, (iii) cooling elements, (iv) the
low-pressure inlet, (v) different compression stages of the oil vapor jets, and
(vi) the exhaust. Oil is represented by dashed lines and the pumped
molecules by small circles. A schematic cutaway showing the main design
elements of a turbomolecular pump, including the (i) exhaust, (ii) motor, (iii)
turbines, and (iv) low-pressure intake. These images were generated by the
author and inspired by standard texts on the subject [11-14].





























Figure 2-2. Illustration of a superconducting solenoid magnet. A schematic cutaway
showing the main design elements of a superconducting solenoid, including
(a) a nitrogen jacket, (b) the magnetic coils, (c) the helium bath, (d) the
magnet bore, and (e) the vacuum isolation space. This image was generated
by the author and inspired by standard texts on the subject [11] [14].


SQUID


Josephson junction sample


twisted pair


RF detector coil signal coil transformer coil

Figure 2-3. Illustration of a SQUID magnetometer circuit. A circuit set up to detect flux
changes resulting from a magnetic sample, utilizing the high sensitivity
afforded by a SQUID amplifier.











sample rod
\


superconducting
second-derivative
pickup coil


sample


Induced Voltage


0


measured
fit


Figure 2-4. Illustration of SQUID magnetometer pickup coils. The superconducting
second derivative pickup coil used in the MPMS magnetometer and the
voltage induced as a function of the position of a magnetic sample within the
coils.


IV.

8- -. *. (a)

6 -



2 -
a
0 -

0 5 10 15 20
distance in magnet (cm)


(b)


*
- -







0 5 10 15 20 21
distance in magnet (cm)


Figure 2-5. Remnant fields and degaussing the MPMS. (a) The field in the magnet
after a standard oscillate to zero protocol, with center field ~ 8 cm. (b) The
field in the magnet after a manual degaussing sequence, with center field
~ 10 cm.


""'""



















Sample Surface


! Laser


Cantilever and Tip


PZT Scanner


Figure 2-6. AFM schematic. The sample tip scans over a material, revealing details of
the surface morphology.


Excess oxygen
*GA


Figure 2-7. A schematic of a combustion train for CHN analysis.


(a)


^Cc


sample


Figure 2-8. EDS schematic. (a) Experimental setup of EDS. (b) Microscopic effect
showing incident electron (green square) hitting bound electron (red triangle)
causing it to be ejected. The atom then relaxes down to the ground state by
filling in electrons, one example is displayed (yellow circle), and emitting
x-rays to conserve energy.


^.^














-I
moveable
mirror


Figure 2-9. FT-IR schematic. (a) A schematic of a typical Fourier transform infrared
spectrometer. (b) A schematic showing the how the high-spectral-content
pulse is modified after passing through the sample, and subsequently is
Fourier transformed to give a spectrum in frequency space.


Ar


... ,.aerosol Ij ,i-ma


detet
detecor


nebulizer


Figure 2-10. ICP-MS schematic. The sample dissolved in acid is aerosolized in a
nebulizer and introduced into an argon environment, in which it flows into an
ionizing RF field, and the ions are subsequently detected via mass
spectrometry.


RF coil


la -Ic


L











electron g'un


con densi g i ltics X


coC, de isli g a[ Perture

sample
oljejct;e lens rCl C E
objective ap:,e t i e t

intermediateoptics [] [


projectoi lens c < 0i

imagingscreen


Figure 2-11. TEM schematic. Electrons travel from the top to the bottom, with the waist
of the beam represented by the solid lines, and magnets represented by
boxes with exes in them.





mirror
radiation
source /
monochromator

L-: filter




ete or sample beam splitter Z


Figure 2-12. UV-Vis spectrometer schematic. More complicated setups may include
conditioning optics, multiple sources and references beams, for example.









(a)






x-ray tube


(b)






d
d


Figure 2-13. XRD schematic. A schematic showing how incident x-rays gain an extra
path length of 2dsin(O) when scattering off of evenly spaced planes a distance
d apart.





(a) (b)


c) I





magnetic field magnetic field

Figure 2-14. Theoretical EMR schematic. (a) The simplest energy level splitting of an
S = 1/2 spin species as a function of magnetic field. The horizontal lines
indicate the microwave energy, and the dashed red line indicates the
resonance condition. (b) The increase in cavity absorbance when the
resonance condition is met.






Reference arm with


Frequency counter/
power meter
Microwave source


Figure 2-15. Experimental EMR schematic. The key features are a microwave source,
a resonant cavity, a magnet, and a detector.


Figure 2-16. The triple-axis spectrometer at HFIR on beamline HB1A. Here, the red
lines show the neutron beam.
























Figure 2-17. The neutron powder diffractometer at HFIR on beamline HB2A. Here, the
red lines show the neutron beam.


Figure 2-18. The SEQUOIA inelastic time-of-flight spectrometer at SNS. Here, the red
line is the neutron beam.









Table 2-1. Magnetic response of candidate probe materials for the optical rotator
magnetization probe. Remnant magnetizations (MREM) are in emuG/gram, C
is emuK/gram, L is emu/gramK, and D is emu/gram.


mono-line braided-line


T (K)
2-300
2-300
2-300
2-300
2-300
2-300
2-300
2-300
2-300
2
10
100


H (T)
1e-2
1e-2
1e-2
le-1
le-1
le-1
1
1
1
7 0
7 0
7 0


Vespel
5.3498e-07
-5.9675e-12
8.2524e-07
5.1355e-07
1.1519e-10
6.0252e-08
4.8105e-07
-8.5564e-11
-3.8356e-07
1.5669e-04
1.0041e-04
3.3839e-06


Delrin
7.2737e-07
8.7623e-11-
8.2636e-08
2.6522e-07
-6.5593e-12
-3.8065e-07
2.3244e-07
-3.8665e-11
-5.0794e-07
4.7316e-05
2.1461e-05
6.8093e-06


nylon
1.5198e-07
-8.5243e-11
-3.3978e-07
9.1798e-08
-1.1522e-11
-3.9787e-07
8.1238e-08
-1.5503e-11
-4.0962e-07
6.2121e-06
1.3669e-05
4.1010e-06


C
L
D
C
L
D
C
L
D

MREM
MREM
MREM


9.1068e-07
-4.2817e-10-
-1.3229e-07
1.8156e-07
-1.0635e-10-
-4.6009e-07
1.6334e-07
-9.9114e-11-
-6.0505e-07-
5.4541e-05
4.3157e-05
-1.5032e-05


1.0671e-05
-4.1188e-09
2.5228e-06
6.9705e-06
-2.6259e-10
3.0526e-07
6.2278e-06
-5.5029e-10
-4.6172e-07
2.3989e-04
2.2723e-04
1.0743e-04








(a) (b) e (c)
(iii)



(iii)



((iii)i)






(i)i






Figure 2-19. Photographs of the optical rotation probe for use in a SQUID
magnetometer. (a) There are three main sections of the probe: (i) a low
temperature end that sits within the SQUID coils and magnet bore and
houses the rotating sample stage, (ii) a shaft that seats within an o-ring for
movement of the probe through the SQUID coils that connects the high and
low temperature spaces, and (iii) a head that contains the active rotation
element and other probe elements. (b) A photograph of the low temperature
end of the probe displays the (i) drive lines, (ii) the optic fiber, and (iii) the
sample rotation cell. (c) A photograph of the top end of the probe shows (i)
the drive spool, clearly Visible through a clear plastic seal, and (ii) the manual
dial for angular control.











4 -
4 -




(a) (b) (c) (d) n (e)
I IE i i i i
0 2 4 6 0 2 4 6 0 2 4 6 0 2 4 6 0 2 4 6
H (T)

Figure 2-20. Magnetization versus field for potential optical rotation probe materials.
Magnetization as a function of field measured at 100 K (green A), 10 K (red)
and 2 K (black ) for the mono-line (a), braided-line (b), Vespel (c), Delrin (d),
and nylon (e).


Figure 2-21. The schematic of the circuit control board for the automated operation of
the custom probe using a stepper motor. Circuit elements use standard
shorthand notation for, "C" for capacitors, "D" for diodes, "R" for resistors, "CN"
for plugs, "SB" for switch banks, "SW" for switches.














= I I I I


- -








ST= 300K "
* *, .


' I II


--

- T= 2 K '*'
'l'l'l'l'l'


1.90

- 1.89
E
- 1.88
E
0 1.87

2 1.86


0 180 360 540 720 0 60 120 180 240 300 360
angle (degrees) angle (degrees)


Figure 2-22. Magnetization versus rotation angle measured with the custom probe for
two different magnetite samples at 300 K (a) and 2 K (b). (b) Magnetization
versus time for thin films of A.Cok[Fe(CN)6], nH20. Light was introduced to the

sample at t = 0 minutes and turned off at t = 90 minutes.

















Figure 2-23. Photoirradiation of powdered neutron scattering samples. Because the
powders to be photoinduced are opaque, the top layer of the powder may be
photoexcited, but the majority of the sample does not become photoexcited
because it does not receive any radiation. The dark state of the sample is
represented by dark blue and the photoexcited state is represented by yellow.


0 30 60 90
Time (minutes)



















Figure 2-24. Photoirradiation of powdered neutron scattering samples using tumbler
probe. To mitigate the problem of opacity, powders are instead mounted in a
quartz tumbler cell that may be rotated about one axis. As irradiation starts,
the same problem as a fixed cell is encountered, with the top layer blocking
light from reaching the majority of the sample. However, the cell can rotate
and tumble the previous top layer to be on the bottom. After a sufficiently
long time, all powder within the cell will be photoexcited and measured. The
dark state of the sample is represented by dark blue and the photoexcited
state is represented by yellow.









CHAPTER 3
THEORETICAL METHODS

While this thesis is experimentally driven, the interpretation of experimental results

is eternally intertwined with the theoretical methods that seek to explain them. This

desire for more fundamental explanations of data is the main reason that an

experimentalist must delve into the realm of theory, where even simple models can be

predictive and further drive the experimental research. The ideology behind the

theoretical applications employed in this work is not always to obtain precise

quantitative explanations of results, but often to glean information from results that is not

otherwise obvious using semi-empirical, transparent methods. The different

experimental techniques introduced in Chapter 2 require varying degrees of post

processing in order to extract the desired information, and while methods such as

microscopy provide information even to the untrained eye, spectroscopy and

magnetization data can be exceedingly complex and can require detailed modeling. As

the studies undertaken are explicitly of the photoinduced magnetism of a coordination

network, the theories presented seek to provide additional insight into this problem.

First, in Section 3.1, the general machinery of the quantum mechanical

interpretation of transition metal ions within a localized picture is presented, with the

different relevant energies in the system introduced one at a time. Next, Section 3.2

consists of a more ab initio approach that is still accessible to experimentalists, the

tight-binding set of theories. Finally, in Section 3.3, the ubiquitous fitting routines based

upon least-squares type methods are discussed.









3.1 Quantum Mechanics of Transition Metal Ions

The discussion will take place in a building block mode, with examples and asides

inserted where convenient. Of particular interest is the calculation of energy levels, and

how these energies change with the application of an external magnetic field, as these

two pieces of information can be probed directly by experiment. Roughly, one can

begin with a single, free ion and work from the simple hydrogen-like picture, studying

the electron-electron interactions as additional electrons are added to build up a

multi-electronic wavefunction. Next, it is necessary to invoke the so-called ligand field

theory, as the electrostatic interaction and covalency of the ligands with the magnetic

ion will add additional structure to the energy levels. The familiar spin-orbit coupling

and Zeeman splitting terms from quantum mechanics are then discussed in the context

of transition metal ion energy levels. From here, interacting ions are considered, via the

superexchange interaction, and the many-body ground state is approximated, via

mean-field theory. Finally, the motivating interpretation of experimental electron

magnetic resonance, inelastic neutron scattering, UV-Vis spectroscopy, and

magnetization measurements are put forward. Of high interest is the magnetization

example, in which the quantum mechanical treatment discussed is applied to a novel

piece of experimental data, namely the field dependence of the magnetic moment in

potassium ferricyanide. While these methods are old, the advent of modern computers

allows for simultaneous diagonalization of many interactions without the need to invoke

perturbation theory, allowing old problems to be re-Visited with more power and to

provide more insight.









3.1.1 Coulomb Interaction and the Multi-Electron Ion

The hydrogen atom is a standard model, even for more complicated atomic

systems, such as the transition metals [32]. However, even within this framework,

building multi-electron wavefunctions is non-trivial. Therefore, the standard practice is

to learn the empirical set of Hund's rules to formulate the terms that constitute a

multi-electron ion [4]. These rules can be summarized as follows.

(1) Within an electron configuration, the ground state is the term with the maximum

multiplicity, and, strictly speaking, the maximum value of the spin quantum

number.

(2) Within a given spin configuration, the ground state is the term with the largest

value of the angular momentum quantum number.

(3) Finally, for less than half-filled valence shells, the ground state is the term with

the least total angular momentum, and for more than half-filled shells, the

ground state is the term with the most total angular momentum.

However, these empirical rules represent the obfuscated surface of the underlying

Coulombic interactions and wavefunction antisymmetrization that govern the energy

levels of multi-electron ions on a more fundamental level [33]. To treat the problem

using the machinery of quantum mechanics, one must start with the Coulomb repulsion

term,


Ve-e(i,j) = 3.1
i>j= 12









where e is the charge of the electron, r12 is the distance between two electrons, and the

sum is over all interactions within the ion. As this potential represents a pair-wise

electron-electron interaction, the interaction integral can be expressed as

I e2 e2
AB CD = ffDlA (r1) B *(r2) 12 pc(1)) D(r2)dVldV2 3.2
Sr12 / JJ12


where ip is a single-electron wavefunction of a hydrogen-like atom, the subscripts

denote distinct orbitals, and the integral is over all space. The treatment discussed here

only considers the valence electrons of the ion, and ignores higher order effects, such

as potential 4s-3d electron interactions. Practically, the integral in Equation 3.2 is

calculated by expanding Ve-e(i,j) in terms of the natural basis of the wavefunctions,

which are spherical harmonics. After a few lines of calculus and the application of

angular momentum selection rules,


AB --2 CD = 5(msA, msC) 6(msB, msD) 5(mlA+mlB, mlC+mlD) x
r12
3.3

ck(IAmlACmic) ck(IDmlDIBmiB).Rk(ABCD)
k=0


where the ck terms are the angular integrals and the Rk terms are the radial integrals,

and details of these calculations can be found in Condon and Shortley's book [34]. It is

worth noting that the diagonal elements are defined as

J(A,B) = AB 12 AB 3.4
S12


the Coulomb integral, and









e Ie2 \
K(A,B) = AB-- BA ,
rl12

the exchange integral. For equivalent electrons, Rk(ABCD) = Fk, where the Fk terms are

referred to as the Condon and Shortly electron repulsion parameters. The ck integrals

are standard spherical harmonic overlaps, and these can be calculated. Practically, at

this point, one has arrived at a single-ion Hamiltonian where these Fk terms are

parameters that can be used to fit to experimental data to learn about the physics and

chemistry of a given ion. Precedents show that the inclusion of only k = 2 and k = 4

terms is sufficient to parameterize the interaction [34].

A different treatment by Racah is more general and provides a slightly different

parameterization of the electron-electron interactions [35]. While the derivation itself is

more complicated, the Racah parameters themselves are slightly more pleasing

because energy differences between terms of the same multiplicity within a given

configuration only depend upon the Racah parameter B, while in the Condon-Shortley

scheme, two parameters are used. However, separations between terms of different

multiplicities involve both Racah parameters B and C. The electron-electron repulsion

parameters in the two frameworks have a simple linear relationship,

B = F-5F4 and 3.6

C = 35F4 .3.7

In practice, the energies and wavefunctions corresponding to the different states of

the multi-electron single-ion can be solved within the matrix formulation of quantum

mechanics on a personal computer. For the free Fe3+ ion, B = 1,029 cm-1 and C/B = 4.1

(giving C ~ 4,220 cm-1) [5]. An interaction diagram showing the splitting of the terms as










electron-electron repulsion is turned on is shown in Figure 3-1. It is worth noting that

when the Fe3+ forms the hexacyanoferrate complex significant to this thesis,

B = 535 cm-1 and C = 4,219 cm-1 [5].

3.1.2 Ligand Field Theory

While spin orbit coupling is a logical next single ion energy to be considered, for

the 3d transition metals, the energy shifts due to interactions with surrounding ions in a

molecule provide a larger perturbation, and therefore they will be considered [5] [36].

While originally, the interactions between the ion in question and the surrounding

ligands were treated in the framework of an electrostatic interaction [37], quantitative

analysis has shown that most (but not all!) energy shifts are in fact due to covalency

between the ions. Since both treatments are not usually performed ab initio, but rather

semi-empirically, it is a matter of taste as to which approach provides the most personal

insight. Herein, the picture where energy shifts are due to wavefunction overlap, the

angular overlap model [5], will be employed. Octahedral coordination is relevant to the

networks considered in this thesis, Figure 3-2, so a brief outline of such an interaction

will be undertaken. Taking atomic wavefunctions on ligand and ion sites, overlap

integrals can be generated for each metal-ligand interaction, giving a generic angular

overlap interaction matrix for an orthoaxial molecule,

3
4[e,(1)+e,(2) -[e,(1)+e,(3) 0 0 0
+e,(3)+eo(4)] -eo(2)-eo(4)
[eo(1)+eo(3) e,(5)+eo(6)+ [eo(1)+eo(2)+eo(3)+eo(4)] 0 0 0
-e,(2)-e,(4)]
ligand= 0 e (1)+e (4) 0
+ey(2)+ey(3) 3.8
0 0 0 ey(1)+e(6) 0
+e_(5)+e,(3)
0 0 0 0 e()+e
+e0y(5)+e+y(4)+









where a basis of dx2-y2, dz2, dxy, dxz, and dyz was used, with e, and e, denoting overlap of

the ion with o and t orbitals of the ith ligand. The x and y subscripts denote directions of

the overlap of the ligand assuming a right handed coordinated system with the z-axis

vector from the ligand to the ion. For 7 bonding that is symmetric with respect to

rotations about the metal to ligand bond axis, these overlap energies are degenerate.

For Oh symmetry, the standard crystal field splitting parameter of the t2g and eg

strong-field-limit orbitals is simply A= 3e, 4e,.

In practice, a Kronecker tensor product of the single-electron ion-ligand

Hamiltonian, Hligand, must be made for each electron to be considered. Finally the

eigen problem can be solved on a standard personal computer. For a d5 Fe3+ ion in a

symmetric octahedral field, the splitting of the free-ion states due to the ligand

interaction can be calculated, Figure 3-3. It is interesting to note that under the

influence of a sufficiently strong metal-ligand interaction, the ground state actually

comes from the 21 term, rather than the free-ion ground state 6S term, for example,

carbon ligated Fe3+

In addition to the splitting shown in Figure 3-3, due to a symmetrical octahedral

field, lower symmetries further lift the degeneracies. For example, a tetragonal

distortion serves to separate states of different total angular momentum within a given

ligand field multiple. For the 2T2g Fe(CN)63- ground state term, such a distortion is

exemplified in Figure 3-4. Lower symmetries of the six-fold ligand field are also possible,

such as a trigonal distortion. However, in practice these distortions may be small, or

they may serve to overparameterize the problem in such a way as to render the

meaning of the high-order distortion parameters unclear. For the molecules under









consideration in this thesis, a tetragonal distortion is sufficient to capture the salient

features of the energy spectrum.

3.1.3 Spin-Orbit Coupling

Spin-orbit coupling arises from the magnetostatic interactions of the inherent spin

angular momentum of an electron with the orbital angular momentum [4]. The

hydrogen-like single electron Hamiltonian

J single, S-O = 's 3.9

where E is the single electron spin-orbit coupling parameter, I is the angular moment,

and s is the spin moment. The single electron spin-orbit coupling parameter scales

linearly with the effective charge of the nucleus, and therefore becomes more important

the heavier the ion is. For multiple electron ions, the spin-orbit interaction energy

becomes

s-o = ,i (i)/i si = L KL S 3.10

where X is the multi-electron spin-orbit coupling parameter, and Stephens' reduction

factor K has been introduced to take into account the quenching of orbital angular

momentum due to the ligands. The angular momentum reduction factor can vary

between zero and one, depending upon the local environment, and it is often

somewhere in between. The multiple-electron spin-orbit parameter is related to the

single-electron parameter as

k = +/2S 3.11

with a plus sign for less than half-full shells, and the minus sign for greater than half-full

shells, as for the latter, the picture is one of positively charged holes [5] [33].









In practice, the solution of this spin-orbit Hamiltonian is analogous to those posed

in the previous sections. For a d5 Fe3+ ion in a symmetric octahedral field of cyanides, it

was shown that the ground state is a well separated 2T2g, Figure 3-3. The effect of the

spin-orbit coupling on this term can be calculated as the interaction is turned on, and for

different values of the reduction parameter, K (Figure 3-5).

3.1.4 Zeeman Splitting

The response of a magnetic ion to an external magnetic field is called the Zeeman

effect [4]. The Hamiltonian for this effect can be written as

JZeeman=,uBH(K(Lz+2Sz) 3.12

where /iB is the Bohr magneton, H is the applied magnetic field, K is the orbital reduction

parameter, Lz is the component of the angular momentum along the magnetic field, and

Sz is the component of the spin angular momentum along the magnetic field.

Experimentally, this term is the smallest yet considered, with maximum

interaction energies of ~10 cm-1 for fields less than 10 T. However, this term is

essential when considering the magnetization of a sample, since changes in energies

with applied magnetic field are detected with a magnetometer. The effect of this term

on spin-orbit split 2T2g-like ground state of the hexacyanoferrate ion can be calculated

for different values of the orbital reduction factor, Figure 3-6.

3.1.5 Superexchange Interaction

Superexchange is a result of second-order perturbation theory. It is the most

relevant ion-ion interaction for coordination compounds, like those studied in this thesis.

The superexchange interaction arises from a mutual wavefunction overlap with a shared

ligand and two ionic centers. While the connectivity can be greater than two-fold and









span an entire lattice, superexchange is still a two body exchange interaction. This

interaction can give rise to a splitting between the singlet and triplet states of a dimer,

effectively aligning the spins parallel or anti-parallel. Using the molecular orbital theory

model put forward by Hoffman [38], the leading terms in the superexchange energy can

be written in terms of the integrals between dimer A and dimer B, such that


Etriplet Esinglet = JAB perexcha = -2KAB + (2hAB)23.13
JAA-JAB

where K and J are the familiar exchange and Coulomb integrals previously introduced

(Equations 3.4 and 3.5), and hAB is difference of ionization potentials of the dimer A and

dimer B magnetic orbitals. Since the exchange integral is always positive, the first term

in Equation 3.13 leads to ferromagnetic interaction, and the second term in

Equation 3.13 leads to antiferromagnetic interaction. For multiple magnetic orbitals, the

contributions can be summed, to first order. This model effectively reproduces the

empirical Goodenough-Kanamori rules for the sign of superexchange, where the

interactions are ferromagnetic for orthogonal orbitals and antiferromagnetic for orbitals

with substantial overlap [39].

3.1.6 Mean-Field Theory

If the superexchange interaction is present to an appreciable degree, the problem

becomes a many-body system of all ions in the lattice. Many body problems cannot be

solved exactly, but by invoking a mean-field approximation to the free energy, many

features of the many-body ground state can be reproduced [40] [41] [42] [43]. For

simplicity, considering only the superexchange of spins associated with nearest

transition metal neighbors, designated as n.n., under the influence of an applied

magnetic field, the Hamiltonian has the form









-K = -2 JijSi-Sj + gBH YS 3.14
i,j=n.n
where J is an exchange constant (not to be confused with the Coulomb integral, please

note context), g is the Lande factor, /B is the Bohr magneton, S is the electronic spin,

and H is the applied field. The mean-field expansion of the spin operator gives

K = f-MF = -2 J(S)S 2 JijS(S) +2 Y Jij(Si)(S) +gIBH Si 3.1
i,j=n.n. i,j=n.n. i,j=n. i 5

where (S) denotes an average spin polarization value. For the case of a spatially

independent average spin polarization, one can further simplify the problem to a

diagonal Hamiltonian,

1JMF = -2 2ZJo(S).Si + 2ZJoN(S)2+ gBH Si 3.16

where Z is the number of nearest neighbors, Jo is the scalar exchange constant, and N

is the total number of spins.

Expressions for the average spin polarization can be derived by minimizing the

free energy with respect to variation of the spin polarization, yielding,

(g/(B9SHext 2ZJ
(S) = S-Bs + BTS (BT 3.17
kBT kBT

where Bs is the Brillouin function [44], kB is the Boltzmann constant, and (...) denotes

an average. Equation 3.17 is transcendental and therefore cannot be solved exactly,

but must done numerically. For the specific example of a lattice of Fe3+ ions with

spin-orbit split 2T2g-like ground states and fully quenched angular momentum, the

resulting solutions for values of g/,BHext/kB = 0.01 and 2Z/kB = 1 are shown in Figure 3-7.









It should be mentioned that disorder and random effects can give rise to

complicated and highly degenerate microstates, such as the spin-glass (relevant to

Prussian blue analogues), but a detailed discussion of these effects cannot be simply

understood by any low-energy theories, and are therefore beyond the scope of this

thesis [45] [46]. It is worth noting that external stimuli, such as sufficiently strong

magnetic fields, can tune a disordered system away from a spin-glass-like state towards

more standard magnetic states.

3.2 Tight-Binding Approximations

While the previous section outlining semi-empirical methods of quantum

mechanics is useful, an additional level of understanding can be gained by a more

fundamental approach to calculation of energy levels. Because the coordination

networks studied can be understood in terms of a perturbed molecular orbital picture, as

opposed to an itinerant electron picture, tight-binding approximations are appropriate to

approximate the energy levels of the systems. The most common tight-binding

approximation is using a linear combination of atomic orbitals (LCAO), and a further

approximation is the extended HCckel theory [47]. While quantitative results are not

expected with these methods, oftentimes qualitative features of the systems can be

reliably reproduced. Specifically, tight-binding methods are useful for understanding the

nature of the chemical bonding in a system. In the following, a simple example of the

carbon and nitrogen interaction leading to a CN molecule will be outline first. Next,

specific examples of tight-binding calculations to approximate ligand fields,

superexchange interactions, and force-fields between atoms will be discussed. This

final point is relevant when interpreting infrared vibrational spectroscopy data, often









used to delineate between different types of heterobinuclear moieties present in

Prussian blue analogues.

3.2.1 Extended HOckel Theory

The Schrodinger equation can often be written down for a complex system, but

rarely solved exactly. Three standard approximations are employed in solving the

Schr6dinger equation within the HCckel formalism. First, the Born-Oppenheimer

approximation assumes that electrons move in a field of fixed nuclei, due to the

disparate masses of the two types of particles. Second, the independent particle

approximation makes the assumption that the total many-electron wave function can be

written down as a product of the single-electron wave functions. Third, only the valence

electrons are considered in the calculation, as the electrons in filled orbitals are mostly

inert.

Aside from specific fitting of experimental data, tight-binding calculations of

molecules act as a sandbox from which valuable chemical intuition may be extracted.

Each molecular orbital is given as a linear combination of atomic orbitals. The basis set

used is spherical harmonics for the angular part of the wavefunction, and Slater-type

orbitals for the radial part of the wave function. Slater-type orbitals are a further

approximation to the hydrogen-like orbitals in which

R(r) = Nrn-l er, 3.18

where N is a normalization factor, n is the principle quantum number, and z is a

semi-empirical parameter that characterizes the diffuseness of the orbital. For more

diffuse orbitals, such as the 3d set, a double-zeta expansion is used consisting of a









linear combination of two Slater-type orbitals. The problem is then reduced to an eigen

problem for the coefficients of the atomic orbitals in the basis set,

JC C=SCE 3.19

where X is a square matrix containing the one electron energy integrals (analogous to

those discussed in Section 3.1.1 on electron repulsion within the Condon and Shortley

regime), and C is the coefficient matrix, S is the matrix of overlap integrals, and E is the

diagonal matrix of orbital energies. In practice, the coefficient matrix is found by the

variational method to minimize the total energy of the system. The core matrix

elements of the single electrons are given by atomic energies, and for the off-diagonal

elements, by the Wolfsberg and Helmholz approximation,

S=L K(Hii+Hy)ij 3.20

where S is the overlap integral, and K is a scaling parameter introduced to account for

the increased overlap in molecules. While the fundamental calculation of K is not done,

experimental studies of ethane by Hoffman showed that K = 1.75 is suitable [47].

A somewhat straightforward example involving the bonding of carbon and nitrogen

to for the cyanide molecule, which is highly relevant to the cyano-bridged networks

studied in this thesis, will now be presented. The calculation involves the four valence

atomic orbitals of carbon C(2s), C(2px), C(2py), and C(2pz), and the four valence atomic

orbitals of nitrogen N(2s), N(2px), N(2py), and N(2pz). Therefore the cyanide molecule

will have a basis set of 8 atomic wave functions, and Equation 3.18 will require the

inversion of 8 x 8 matrices. The solution to Equation 3.18 gives both the molecular

orbitals and their energies, which are shown in Figure 3-8. The lowest anti-bonding

orbital, the x*, is especially important to the magnetization of the Prussian blue









analogues, because it acts as a strong electron acceptor on the carbon-dominated

molecular orbitals and mediates the superexchange between metal ions.

3.2.2 Ligand Field Theory

While the parameters used in ligand field theory are best utilized as fitting

parameters to experimental data, it is also possible to calculate them from fundamental

principles. However, calculating from first principles is a difficult problem. It is not

surprising that qualitative methods such as LCAO methods are unable to provide

quantitative predictions for the desired parameters, and more complicated methods

such as Density Function Theory (DFT) are not, themselves, parameterized in such a

way to make the ligand field theory parameters assignable from these calculations.

However, it is desirable to glean qualitative trends from first principles to help explain

experimental trends, such as the relative ligand field strengths of different atoms and

the effects of distortions on the ligand field. Two specific examples will be briefly

introduced: (1) the difference in the octahedral ligand field splitting parameter for a

Co(NC)64- molecule compared to the Fe(CN)63- molecule, and (2) the effect of tetragonal

distortions on the energy levels of a Ni(NC)64- and Cu(NC)64- molecules.

3.2.3 Superexchange Interaction

The superexchange interaction introduced in Section 3.1.5 was again dealt with

completely as an empirical parameter. Like the ligand field splitting parameters, the

qualitative changes in the superexchange constant may be investigated with extended

Huckel theory.

3.2.4 Infrared Vibrational Spectroscopy

Quantitative theoretical analysis of the cyanide stretching frequencies in Prussian

blue analogues is still lacking. This situation is unfortunate, given the high degree to









which scientists use cyanide stretching frequencies as diagnostic tools to assign

oxidation states and coordination numbers in cyanometallate compounds. Although a

quantitative solution should be possible using modern density functional theories, such

a study is beyond the scope of this thesis, and instead a tight-binding analysis was

attempted in order to understand the trends observed. Unfortunately, the tight-binding

approximations are inadequate for this level of structure determination and no

meaningful information could be extracted from the studies.

3.3 Fitting Algorithms

More often than not, experimental scientists are forced to deal with

overdetermined systems. The classic example of an overdetermined system is a data

set with many points that a researcher would like to fit to a function having fewer

parameters than there are data points. This situation happens all the time, daily for

many people. All is not lost, especially if one has access to a math machine, such as a

standard personal computer. The ubiquitous technique employed is a method of least

squares.

In Section 3.3.1, the general procedure of least squares fitting is overviewed. In

Section 3.3.2, the Levenberg-Marquardt algorithm that was extensively employed for

fitting functions in this thesis is discussed. In Section 3.3.3, the specific example of

least squares fitting used to fit all diffraction data in the thesis, the Rietveld method, is

overviewed.

3.3.1 Least Squares

The process of fitting data comes down to minimizing the difference between the

function that is fit and the data. These differences are called the residuals of the fit.

The name "least squares" refers to the fact that, in this method, a square of the









difference between the data and the fitting function is used [48]. The squared difference

is used instead of the absolute difference for the simple reason that this allows the

residuals of the fit to be treated as a continuous differentiable quantity. An important

thing to remember when using this method is that the squaring of the residuals

effectively weights outliers stronger than other data. Therefore, if data points are known

to be erroneous they should be excluded, or one can additionally weight the residuals

by the known experimental errors involved.

A straightforward derivation can illustrate these points. The least square fit

parameter, S, is defined as the sum of the square of the residuals

n
S= ,2 3.21
i=1

where there are n discrete data points. Explicitly, these residuals are the difference

between the experimental data and the fitting function,

ri = i f(xi,) 3.22

which depends upon the independent experimental variable and the fitting parameters.

This parameter may be minimized by setting the derivative with respect to changes in

the function parameters equal to zero.

na
as ari
a = 2 ri -= 0, j = 1,...,m 3.23
ap i= ap1

where there are m parameters. By substituting Equation 3.22 into Equation 3.23, one

gets









n -2 ri 0, j = 1,...,m 3.24
i= 1 pJ

If the function f(xi,p) depends linearly upon the fit parameter,

m
f(xi, ) = P Fj(xi) 3.25
j= 1

the derivative is straightforward

8f(xi, )
aj F(xi) 3.26


If a tensor is defined such that

Xij Fj(xi) 3.27

The solution to the linear least square problem becomes clear, namely


p = (XTX) XTy 3.28

3.3.2 Levenberg-Marquardt

While the linear least squares case is straightforward, complicating issues arise

when the function in question is nonlinear, and a more complicated approach must be

used. Here the Levenberg-Marquardt algorithm can be utilized [49] [50]. In order to find

the minimum of a function F(x) that is a sum of squares of nonlinear functions, fi(x), i.e.

m
F(x) = Z[fi(x)]2 3.29
F(x) =
i=1

Let the Jacobian of fi(x) be denoted Ji(x), then the Levenberg-Marquardt method

searches in the direction given by the solution p to the equations









(JkTJk+Akl)k = -JkTfk


3.30


where Xk are nonnegative scalars and I is the identity matrix.

3.3.3 Rietveld Refinement

Rietveld refinement was used to interpret powder diffraction patterns from neutron

and x-ray scattering experiments [51]. Specifically, the GSAS [52] and EXPGUI [53]

computer programs were used for all refinements. This technique refers to the use of

least squares fitting of experimental data with theoretical models, and it is called

refinement because it can only modify parameters within a given test model, rather than

predict the appropriate model a priori. In essence, the refinement consists of minimizing

the function which depends upon the square of the difference between the fit and the

data, such that

F(R) = W(experiment -cxmodel)2 3.31




where W1 is the statistical weight and c is an overall scale factor. Because of this

short-coming, crystallographers must test many different models to arrive at a solution.

Practically, most of the information that goes into a Rietveld refinement comes

from the Bragg condition for constructive interference of scattered waves


4rrsin(e)
Q = 3.32
nk









where Q is the momentum transfer, 0 is the scattering angle, n is the order of the

reflection, and X is the incident wavelength. Or, conversely, the Bragg condition can be

reformulated as the Laue condition

3.33
kK = 2K


where k is the incidence wave vector, and K is the momentum transfer (K k,

where k' is the final wave vector). For a powder pattern, the Bragg reflections can be

thought of in terms of the Ewald sphere [42]. The Ewald sphere is the surface

generated by rotating the incident wave-vector through the origin and testing to see if a

reciprocal lattice point lies on the surface of the sphere at a distance K from the origin,

thus satisfying the Laue condition. The random distribution in a powder sample

averages over all possible scattering angles, so that each reciprocal lattice vector

generates a sphere of radius K This powder scattering sphere will intercept the Ewald

sphere to create a circle, so long as K is less than 2 k. The vector between a point on

the intersecting circle and the end of the incident wave vector is the final momentum k,

that satisfies the Laue scattering condition. Formally,



K = 2k-sin 3.34


therefore, by measuring the angular dependence of the scattered intensities of a

powder, information about all Bragg reflections corresponding to reciprocal lattice

vectors shorter than 2 k is available.









While the gross features are captured by the positions of the reflections, the

intensities of the reflection contain important information about the structure of the

atoms within a unit cell. The geometric structure factor modulates the intensities by

n
S(K) = eKd 3.35
j=1


where S(K) is the geometric structure factor, and dj are the positions of the atoms within

the unit cell. This structure factor can diminish observed Bragg peaks due to

interference between waves scattered within a given unit cell. For neutron refinements,

an additional magnetic factor due to the spin scattering must be added to the atomic

scattering.

Finally, aside from the position and intensities of the Bragg reflections, information

is also contained in the shape of the lines. Crystallite size can serve to broaden the

lines, as a departure from infinite space symmetry occurs. This final point is particularly

relevant to nanostructured materials.









50,000


-50,000


-100,000


-150,000 4F -
C/B = 4.1 4 F
6S
-200,000 I I I
0 500 1000 1500 2000
B (cm1)


Figure 3-1. The energy differences between different d5 Fe3+ free-ion terms arising from
electron-electron repulsion as a function of the Racah repulsion parameter B.
The vertical line corresponds to the standard value for free Fe3+ [5].


Figure 3-2. The octahedral coordination geometry. An octahedrally coordinated smiley
metal ion is shown, with an overlap between a lobe of the central, red, 3d
orbital with an s-wave, green, ligand orbital shown for site 2. The numbers
are relevant to the Hamiltonian angular overlap parameters discussed in the
text.












90 6A Ag(S)
4T2ig(G)
80 Fe3+ in Ligand field / _" 4T2g(G)

70 B = 859 cml1 / 4A1 g4E(G)
-- 4T1g(P)
60 C/B =4.48 4T_ 4T
T2g(D)
450 4Eg(D)
.2T2g(I)
S40 2A2g(I)
30- 2T2g(I)
2T2g(I)
20 2Eg(I)
2Alg(I)
10 T2g(I)

0 2Eg(I)
0 10 20 30 40 5 2Tig(I)
A/B

Figure 3-3. The energy of a molecular term as a function of the octahedral splitting
parameter, A, for a d5 ion, such as Fe3+. A typical value for Fe(CN)63- is
denoted by the vertical line, showing a 2T2g ground state separated from the
next excited state by roughly 10,000 cm-1.









200

100

0

-100

-200

-300

-400


0 100


200 300 400 500


6 (cm-1)

Figure 3-4. Energy shift plotted versus the tetragonal distortion parameter, 6.
Specifically, for a d5 2T2g Fe(CN)63- ground term.


600










400


200

no splitting -6 states
E -200

-400

-600 2 states 1/2 -

-800 spin-orbit coupling of Fe3+ state = 12

-1000 I
0 100 200 300 400 500 600 700

X(cm 1)

Figure 3-5. Energy splitting of the octahedral hexacyanoferrate 2T2g ground state.
Energies are shown for K = 1 (solid lines), the K = 0 totally quenched (green
line), and a partially quenched K = 0.5 state (dashed lines). In the presence
of spin-orbit coupling, the six-fold degeneracy of the 2T2g state is lifted for
different values of the total angular momentum, j. The vertical line at 460 cm1
is the free ion value of spin orbit coupling [5]. Clearly if the orbital moment is
completely quenched, spin-orbit coupling has no effect.










(a)4
3
2
d" 1
21


-2
-3
-4


01234567
H (Tesla)


-



;1 :: 111


-3/2

+-3/2


176
174
172
170
168
-342
-344
-346
-348
-350


350
(c) 348
348
346
344
" 342
I 340,
S-686
-688
-690
-692
-694


01234567
H (Tesla)


TllmlllJr


01234567
H (Tesla)


Figure 3-6. The Zeeman splitting versus applied magnetic field for the spin-orbit split
2T2g-like ground state of hexacyanoferrate. Energies are shown for (a) K = 0,
with no angular momentum, (b) K = 0.5, the angular momentum is partially
quenched, and the magnetic field splits the j = 3/2 into a quartet and the
j = 1/2 into a doublet, and (c) K = 1, with no quenching of the angular moment,
the j = 3/2 term is robust and the j = 12 term is split into a doublet.


0.00256

0.00254

0.00252


I-
A 0.00250
v 0.00248


0.00246

0.00244


I I I '
J=10 (a) .

J=5

J=0


J = -51

I ^JI --^"


100 150 200 250 300
T(K)


0.5

0.4-

0.3 J = 20
A
M J-10
v 0.2

0.1

0.0
0 2 4 6
T (K)


Figure 3-7. The effect of superexchange on magnetization. (a) The effect of
superexchange on the average spin value along the magnetic field above the
magnetically ordered state is shown for an S = 1/2, Fe3+ ion for both
ferromagnetic (J > 0) and antiferromagnetic interactions (J < 0), Equation 3.17.
(b) The average spin value along the magnetic field shows sharp increases
corresponding to long-range magnetic order, with increasing ordering
temperatures for increasing values of the superexchange parameter, J. Here
only ferromagnetic examples are shown, as antiferromagnetic samples have
no net magnetic moment in their ordered state.


-1/2

*1/2


-1/2


+1/2


8 10












160
159
158 (a) = C (b)
157 N
CN fs+p | I

-10 -2P" -- ,- +. 2po- '-
_-15 *--s .
-25 ,
2 p "


-0 2S2-
-35 r.p*- -; I -

-20 I I I I
-30 _




Figure 3-8. The cyanide molecule. (a) An extended HCickel interaction diagram shows
how carbon and nitrogen are able to lower their energy by forming a cyanide
molecule. (b) An illustration of the Kp* and Os+op* orbitals of the CN molecule
relevant for the formation of extended cyano-bridged coordination networks.


100









CHAPTER 4
QUANTITATIVE ANALYSIS OF MAGNETIZATION IN COMPLEX CYANIDES

Using the experimental and theoretical machinery described in the previous two

sections, a quantitative analysis of the magnetization in select Prussian blue analogues

and their paramagnetic precursors can be made. The two magnets of particular interest

to this thesis are Cs2.8Ni4[Cr(CN)6]4 nH20, which has a high ordering temperature of

60 to 90 K [54], and Co4[Fe(CN)6]3-nH20, which can display photoinduced magnetism

when proper proportions of interstitial ions are included in the lattice [1]. In addition,

paramagnetic precursors of these materials, K3Cr(CN)6 and K3Fe(CN)6, will be

presented for comparison. A well parameterized Hamiltonian, whose components may

be determined by experimental measurements, is sought. Specifically, electron

spectroscopy, magnetic neutron diffraction, and magnetization measurements are

sufficient to fully determine the relevant Hamiltonian for magnetization in these systems.

As for most problems, many paths to a possible solution may exist, and the best

approach may depend upon personal taste and the availability of resources. One

recipe of the many possible is outlined in this chapter.

The author would like to stress the dangers of using improper, incomplete, or

misunderstood recipes when modeling the magnetization of the systems in question.

For example, the change in magnetic susceptibility as a function of temperature may be

due to multiple effects, including spin-orbit coupling, structural distortions, and

superexchange. If the model is underdetermined, it may be impossible to separate the

different contributions. In the same vein, the use of simple equations without a full

understanding of their derivation should be avoided at all costs. A common culprit is the

Curie-Weiss formula, X = C/(T-0), where the assumption that all deviations from high


101









temperature Curie-like behavior come only from superexchange is often assumed by

the unaware researcher, even in systems with first order orbital angular momentum.

This issue, and others, may be avoided if proper companion measurements to

magnetization are performed, where the Hamiltonian to be considered contains only

well understood parameters.

In Section 4.1, spectroscopy experiments, designed to determine the single-ion

parameters are discussed. These measurements help in determining possible spin and

orbital states for ions. In Section 4.2, magnetization measurements performed to detect

the presence of magnetic order are presented. In Section 4.3, microscopic probes of

the magnetic structure are presented. Finally, in Section 4.4, known parameters are

summarized and a fit of the magnetization data is presented.

4.1 Synthesis and Chemical Composition

The precursor materials were used without modification after purchase from

Acros. The powders of Cs2.8Ni4[Cr(CN)6]4nH20 and Co4[Fe(CN)6]3-nH20 were

synthesized by Matthieu F. Dumont. Chemical formulae were determined using

ICP-MS for the relative ratios of of all elements excepting hydrogen, which was

determined by multiplying the oxygen content by two. The ICP-MS was performed by

Complete Analysis Laboratories, Inc. (www.calilabs.com).

4.2 Spectroscopy

Even before beginning a spectroscopy experiment, it is useful to have knowledge

of the general atomic environment and structure of the material. For Prussian blue

analogues, a good starting point is always the simple cubic structure that was assigned

after single-crystal diffraction studies of Prussian blue [55]. The structure consists of

metal ions linked by cyanides in a unit cell with two metals, Figure 4-1. From this space


102









model, the point symmetry and local environments of the magnetic atoms can then be

determined.

Two different types of spectroscopy may be applied to the materials in question,

INS for excitations from ~25 meV to ~1 eV (spanning the energies of spin-orbit coupling

and structural distortions) and UV-Vis for excitations from ~1 eV to ~6 eV (for which

ligand field interactions and electron repulsion are relevant energies). Inelastic neutron

scattering has not yet been exploited for complex cyanides, but at the end of May 2010,

a collaboration between UF and ORNL should remedy this lack of data. Ultraviolet and

Visible spectroscopy, on the other hand, has been performed on

Cs2.8Ni4[Cr(CN)6]4-nH20, C04[Fe(CN)6]3.nH20, K3Cr(CN)6,and K3Fe(CN)6. Results are

conclusive for determining the ligand field splitting of nickel and chromium ions, Figure

4-2. However, iron has transitions from the 7 of the CN- to the d-levels of the metal in

the same energy range. The ligand to metal transitions have much larger probabilities

than d-d transitions, rendering the standard UV-Vis spectroscopic determination of iron

ligand field states with standard UV-Vis spectrometers difficult.

4.3 Magnetic Susceptibility

The next experiment consists of a temperature dependent magnetic susceptibility

measurement down to the lowest temperatures available. The point of this

measurement is to look for anomalies in the shape that may be characteristic of

magnetic phase transitions. The magnetization data for K3Cr(CN)6 and K3Fe(CN)6 do

not show any anomalies, whereas the Prussian blue analogues,

Cs2.8Ni4[Cr(CN)6]4 nH20 and Co4[Fe(CN)6]3 nH20, show divergence of the magnetic

susceptibility at ~90 K and ~13 K, respectively, Figure 4-3. These transition


103









temperatures may be used to estimate the magnitude of the superexchange parameter

using mean-field theory,


Tc JBSA(SA+1)SB(SB+l) 4.1

where Tc is the ordering temperature, Z is the number of nearest neighbors, J is the

superexchange constant, S is the spin quantum number, and kB is the Boltzmann

constant. It is important to note that Equation 4.1 can only give the magnitude of the

superexchange parameter, but not the sign.

4.4 Microscopic Probe of Magnetization

While magnetization or specific heat measurements can determine the

magnitude of the superexchange constant, true determination of the magnetic structure

requires a microscopic probe. One possible probe is magnetic neutron diffraction.

Neutron diffraction of Cs2.8Ni4[Cr(CN)6]4 nH20 has been performed, and a ferromagnetic

structure was found [56]. For Co4[Fe(CN)6]3 nH20, an x-ray magnetic circular dichroism

measurement has been reported [57], however the electronic transitions present in the

spectra are not well enough understood to be quantitatively modeled, and macroscopic

magnetic susceptibility data was relied upon for analysis of the microscopic data. For

this reason, it is crucial to perform a study of the magnetic structure of

Co4[Fe(CN)6]3 nH20 using a more easily modeled probe, such as neutron diffraction,

where the relevant interactions are understood. The proposal to perform such an

experiment has been accepted and is scheduled to be performed at ORNL on the HB2A

beamline of HFIR in May 2010.


104









4.5 Magnetization Fitting

At this point, a parameter set is available for the different compounds, Table 4-1.

Here, certain parameters could not be determined from the available measurements,

but plausible estimates can be made based upon spectroscopic and nephelauxetic

series [5]. Once the ground term has been assigned, the first order relevant energy

scales are spin-orbit coupling and superexchange.

Magnetization [3] [58] is one of the more complex quantities to critically

understand because it does not just depend upon the energy levels of a system, but

rather how those energy levels respond to an applied magnetic field

dE
M -4.2
dH

One can use Boltzmann statistics to calculate the average magnetization resulting from

the thermal population of a set of calculated energy levels, Ei,

SdEi -E /k T
= --I) 7 4.3
Zi e-Ei/kBT

For the magnetization calculations in this section, the well separated, ligand-field ground

term will be used as a starting point.

4.5.1 K3Cr(CN)6

First, consider the magnetization of the K3Cr(CN)6 material. The parameters in

Table 4-1 suggest that a good Hamiltonian to calculate magnetization between 2 K and

300 K may be

x = uBH(2Sz) 4.4


105









where /s is the Bohr magneton, H is the external magnetic field, and Sz is the spin

moment along the field direction. The agreement between model calculations, using

Equation 4.4, and experiments is astonishing, as no fitting parameters are used,

Figure 4-4. The slight deviations between experiment and model may be due to errors

in background subtraction, or second order spin-orbit coupling effects coming from

mixing with excited states of similar symmetry. The decrease in the XT values at low

temperatures is real, present in both the model and the experimental, and is due to a

violation of the HIT << 1 limit for Curie behavior.

4.5.2 K3Fe(CN)6

The next material, K3Fe(CN)6, has unquenched orbital angular momentum that

gives an additional level of complexity. In light of this orbital contribution, a plausible

Hamiltonian is then

i = /BH(KLz + 2Sz) + A(KL S) 4.5

where /s is the Bohr magneton, H is the external magnetic field, A is the spin-orbit

coupling constant, Lz is the orbital moment along the field direction, Sz is the spin

moment along the field direction, and K is the orbital reduction parameter. While

structural distortions might exist (and in the most rigorous case, should be included), for

simplicity, these effects may be approximated by the K parameter. It is also noteworthy

that the use of powder data makes additional parameterization suspicious. The

experimental data of K3Fe(CN)6 may be compared to a model without orbital

contributions (Equation 4.4) and one with orbital contributions (Equation 4.5), Figure 4-5.

The failure of the spin-only model to reproduce the magnitude of the magnetization as a

function of field and temperature is striking for this material, in contrast with K3Cr(CN)6.


106









The fact that the moment is anomalously large at high temperatures and small at low

temperatures is a suggestive signature of the presence of orbital moments and

spin-orbit coupling. With spin-orbit coupling parameters of ~100 cm-1 for the iron series

ions, the width of the energy multiple is nearly equal to the thermal difference between

300 K and 2 K. Discrepancies of the fit can be attributed to the known structural

transition at 70 K and the lack of a structural distortion parameter in the fitting

Hamiltonian. These higher-order effects go beyond the scope of the current discussion

and analysis.

These data clearly illustrate two important lessons for fitting magnetic data of

organometallic compounds. First, a simple application of X = C/(T 0) would yield

0 ~ 40 K, although spin correlations of that magnitude can be completely ruled out by

the absence of a divergence in the susceptibility in the relevant temperature range.

Second, a simple application of a spin-only Hamiltonian with a scaled g-factor cannot

reproduce the data, unless an appreciable temperature dependence of the effective

g-factor was introduced.

4.5.3 Cs2.8Ni4[Cr(CN)6]4-nH20

The next material to be considered is Cs2.8Ni4[Cr(CN)6]4 nH20. This material

possesses two magnetic ions, Ni2+ and Cr3+, that both have orbital singlet ground states.

Therefore, to first order, orbital angular momentum should not affect the magnetization.

However, the observed divergence of the magnetic susceptibility and subsequent

neutron diffraction studies showed strong spin-spin correlations associated with

superexchange. Therefore, a reasonable Hamiltonian should be

x = p/BH(2Sz)-2 i,j=n.n.JijSiSj 4.6


107









where /s is the Bohr magneton, H is the external magnetic field, A is the spin-orbit

coupling constant, Sz is the spin moment along the field direction, and J is the

superexchange parameter. The many body portion of Equation 4.6 can be

approximated with a mean-field solution, as described in Section 3.1.6. In addition,

demagnetizing effects will be present in the low field magnetically ordered data and the

phenomenological approach described in Section 6.4.1, that scales the magnetization

by a domain factor will be used. Good global agreement of the magnitude and shape of

the model calculation compared to the experimental data is found, Figure 4-6.

Deviations near the onset of magnetic order are expected due to the approximate

nature of the mean-field approach. Additional differences may be due to 2nd order

orbital momentum effects, notorious for these ions [36].

4.5.4 Co4[Fe(CN)6]3-nH20

Co4[Fe(CN)6]3 nH20 is the final, most complicated material to be considered. As

seen by the parameters in Table 4-1, Co4[Fe(CN)6]3 nH20 has both spin-spin

correlations and first order orbital angular momentum. A reasonable Hamiltonian to try

modeling the system is

if = /BH( Lz+ 2Sz) + (K(L S) 2 i,j=n.n. JijSi Sj 4.7

where /iB is the Bohr magneton, H is the external magnetic field, A is the spin-orbit

coupling constant, Lz is the orbital moment along the field direction, Sz is the orbital

moment along the field direction, J is the superexchange parameter and K is the orbital

reduction parameter. Like the Cs2.8Ni4[Cr(CN)6]4 nH20, the many body interaction is

solved using mean-field theory, and the low field, ordered state is expected to be

multi-domain. The author is currently in the process of debugging his full-fledged code


108









for this Hamiltonian, so at this point, only high temperature expansion fits are made.

Similar to the paramagnetic K3Fe(CN)6 precursor that has one of the chromophores

present in Co4[Fe(CN)6]3-nH20, first an illustrative attempt to fit the data using a

spin-only formula is made, Figure 4-7. For all fits, both signs of J will be used in order to

see which one produces smaller residuals.

While the antiferromagnetic J can reproduce the shape of the susceptibility as a

function of temperature quite well without needing to include orbital moments, the

magnitude of the magnetization is terribly wrong, Figure 4-7. This disagreement is good

reason to include orbital moments, as in the originally hypothesized formula,

Equation 4.7. The magnitude of the magnetization after including first order angular

momentum is in much better agreement with experimental data. Perhaps surprisingly,

the shape can now also be reproduced with a ferromagnetic J, as a decrease in the

susceptibility with temperature now also takes place because of the depopulation of

high orbital momentum states, just as in K3Fe(CN)6. Preliminary fits of the field

dependence yield similar conclusions. One tell-tale sign of orbital contributions in these

compounds is the finite slope of magnetization at high magnetic fields, which is absent

in systems with first order orbital singlets.

In the end, the Co4[Fe(CN)6]3-nH20 fitting is the most complicated, but most

intriguing as well. Although XT decreases with temperature, an observation that has

lead scientists to conclude antiferromagnetic interactions based upon application of the

Curie-Weiss law, a full fit that properly concludes orbital contributions actually suggests

ferromagnetic interactions may be present. Whichever the case, these questions


109










should be answered with the inelastic neutron scattering and magnetic diffraction


experiments scheduled at ORNL at the end of May 2010.


6% H20


Figure 4-1. The cubic complex cyanide Prussian blue analogue structure. (a) A
representation of the crystal structure of a generic Prussian blue analogue,
AjM1k[M2(CN)6]r nH20. (b) The M1(NC)6 molecular sub-unit to be considered
for single-ion excitations. (c) The M2(CN)6 molecular sub-unit to be
considered for single-ion excitations.


Energy (eV)
1.5 2.0 2.5 3.0 3.5
3) -Cr(N)6
-Ni[Cr(CN)6]nH20 powder


10,000 15,000 20,000 25,00C
Energy (cm'1)


30,000 35,000


100,000- () (n
90,000 (b)
80,000 2T
70,000 T (i
60,000 ,
E
S50,000
40,000 2T 'E 'T2
a 30,000 '- ,
LU 20,000 E
10,000
0 A A
Cr "(CN), Ni"(NC)6


Figure 4-2. UV-Vis of Rbo.7Ni4.o[Cr(CN)6]2.9-nH20. (a) Room temperature UV-Vis
spectroscopy measurements of the d-d transitions present in 10 mM Cr(CN)6
precursor (---), and a 10 mM Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 powder with
background subtracted by using the functional form of a diamagnetic
Rbo.5Zn4.o[Fe(CN)6]2.8 nH20 (-). (b) Using the transitions shown in
Figure 4-2 (a), a multiple calculation can be performed to show the electronic
energy levels for the Ni2+ and Cr3+ ions in the Prussian blue network.
Chromium energy levels are shown for (i) no spin-orbit coupling and (ii) using
reported values of spin-orbit coupling for the free ion [5]. Nickel energy levels
are shown for (iii) no spin-orbit coupling and (iv) using reported values of spin-
orbit coupling for the free ion [5].


110


1.2
1.0
S0.8
- 0.6
a0.4
0.2


12
10
85
6
4 (
LU
2
0










0

3Fe(CN)6
E -200 K3Cr(CN)6

E Cs7Ni4[Cr(CN)4nH20
[- Co4Fe(CN)63nH20

-400

0 50 100 150 200 250 300
Temperature (K)

Figure 4-3. Measurement of magnetic ordering temperature in Cs2.8Ni4[Cr(CN)6]4-nH20,
C04[Fe(CN)6]3. nH20, K3Cr(CN)6,and K3Fe(CN)6. A derivative of the DC
magnetic susceptibility, in an applied field of 100 G, as a function of
temperature is shown to display magnetic transitions, from paramagnetic to
either ferromagnetic or ferrimagnetic states. The K3Fe(CN)6 and K3Cr(CN)6
materials are seen to be paramagnetic from 2 K to 300 K. The
Cs2.8Ni4[Cr(CN)6]4. nH20 and C04[Fe(CN)6]3. nH20 materials show divergence
of the magnetic susceptibility at ~13 K and ~90 K, respectively.


Table 4-1. Parameters to be used in modeling magnetization data for
Cs2.8Ni4[Cr(CN)6]4.nH20, C04[Fe(CN)6]3.nH20, K3Cr(CN)6,and K3Fe(CN)6.
A is the octahedral splitting parameter, B and C are electron repulsion
parameters, C is the spin-orbit coupling parameter, K is the orbital reduction
factor, J is the superexchange parameter, and the ground states have been
calculated based upon the previous parameters. Units are all in cm-1, unless
specified.
complex A e, e, B C C K J ground
state
Co2+(NC)6 11,000 0 3,667 885 4,253 515 0-1.5 13 4Tq
Fe+(CN)6 35,000 -1,000 10,333 535 4,219 460 0-1 -T2q
Ni (NC)6 11,000 0 3,667 1,044 4,594 630 3+90 A1
Cr3+(CN)6 26,000 -1,000 7,333 933 3,732 275 4A,


111









18000
16000
14000
12000
10000
8000
6000
4000
2000
0


0 50 100 150 200 250 300
T(K)


0 10 20 30 40 50 60 70
H (kG)


Figure 4-4. Magnetic properties of K3Cr(CN)6. (a) XT versus T and (b) magnetization
versus field for K3Cr(CN)6, from a SQUID magnetometer experiment (E), and
model calculations ( ).


6000

5000

0 4000

; 3000
E
2000

1000

0


0 50 100 150 200 250 300
T(K)


0 10 20 30 40 50 60 70
H (kG)


Figure 4-5. Magnetic properties of K3Fe(CN)6. (a) X T versus T and (b) magnetization
versus field for potassium ferricyanide, from a SQUID magnetometer
experiment (E), a simple model with only Zeeman splitting of the 2T2g ground
state (-), and a model including both spin-orbit interaction and Zeeman
splitting of the ground state (-). A reduction parameter of 3/4 was found to fit
best. It is clear that even qualitative features of the data cannot be fit without
spin-orbit coupling.


112


0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20


















0 20 40 60 80 100120
T (K)


18
16
14
12
10 Li
10
8
6
4
2
0
0 =----------
100 150 200 250 300
T (K)


'30

20

10
75


0 10 20 30 40 50 60 70
H (kG)


Figure 4-6. Magnetic properties of Cs2.8Ni4[Cr(CN)6]4-nH20. (a) Magnetization versus
temperature, (b) X T versus temperature, and (c) magnetization versus field
for Cs2.8Ni4[Cr(CN)6]4 nH20, from a SQUID magnetometer experiment (o), a
mean-field model taking into account interacting spins ( ), and a model
neglecting spin-spin interactions (-).


7

- 6
E 5

I4
I-
53


-D-





0 5 10 15 20 25 30
T (K)


200 300
T (K)


0 10 20 30 40 50 60 70
H (kG)


Figure 4-7. Magnetic properties of Co4[Fe(CN)6]3nH20. (a) Magnetization versus
temperature, (b) X T versus temperature, and (c) magnetization versus field
for C04[Fe(CN)6]3.nH20 (actually normalized to Coi.5[Fe(CN)6] nH20 to
compare directly to literature), from a SQUID magnetometer experiment (o), a
model without orbital moments(-), and different models having degrees of
quenching of the orbital moments, with full moments on Co and Fe (-),
reduced orbital momentum on the Fe ( ), reduced orbital momentum on the
Co ( ), and reduced orbital moments on both ions(-). Here, bold lines
correspond to ferromagnetic models and thin lines correspond to
ferrimagnetic models.


113









CHAPTER 5
COBALT HEXACYANOFERRATE NANOPARTICLES

Nanotechnology represents an exciting new frontier of science, and the study of

nanoparticles is bound to uncover additional scientifically and technologically relevant

phenomenon in the coming years. Nanoparticles are materials between a few

nanometers and a few hundred nanometers, and often behave differently from

microparticles and bulk crystals of the same material. For this reason, nanoparticles of

the photoactive Prussian blue analogue cobalt hexacyanoferrate were studied and

presented in this thesis. Different interstitial cations were used to investigate different

material properties, as the choice of rubidium, potassium, or sodium has subtle effects

on the magnetization of the host material. These results for the nanoparticles are best

understood within the greater context of past studies on similar bulk materials.

The investigation of the magnetism of Prussian blue, Fe4[Fe(CN)6]3. nH20, and

related analogues has a rich history [59] [60], dating back to 1928 [61]. Measurements

down to liquid helium temperatures identified the transition in Prussian blue to be long-

range ferromagnetic order [62-64], but an understanding of the magnetic interactions

remained elusive until the 1970s, when x-ray [65] and neutron [55] diffraction data

identified the crystal structure and the spin transfer from high-spin Fe3+ to low-spin Fe2+.

Subsequent to this identification, the class of isostructural, face centered cubic

cyanometallates were dubbed Prussian blue analogues, Figure 5-1.

Interest in Prussian blue analogues was renewed by the 1996 discovery of

long-lived, photoinduced magnetism in Ko.2Co1.4[Fe(CN)6]-6.9H20 [1]. A flurry of

experimental and theoretical research has elucidated the fundamental nature of the

phenomena in three-dimensional bulk materials [66] [67]. In the following a brief


114









summary of the photoinduced magnetization mechanism elucidated by these studies

will be presented.

Magnetic transitions in cobalt hexacyanoferrate systems depend strongly on the

single-ion parameters introduced in Section 3.1. Iron is in a strong ligand field of carbon

and is therefore found to be either Fe3+ (LS, S = 1/2) or Fe2+ (LS, S = 0). Cobalt is in an

intermediate ligand field of nitrogen and is found to be either Co2+(HS, S = 3/2) or

Co3+(LS, S = 0). The nomenclature of the following is such that pairs displaying defined

local minima in configuration space with respect to energy have a "0" appending their

label and short-lived metastable states are appended with a prime.

Co3+Fe2+(Sco = 0, SFe = 0, Stot = 0) is the diamagnetic LSO pair and

Co2+Fe3+(Sco = 3/2, SFe = 1/2, Stot = 1) is the ferrimagnetic HSO pair with a

superexchange through the bridging cyanides. Co3+Fe2+(Sco = 1, SFe = 0, Stot = 1) is the

intermediate HS' pair and Co2+Fe3+(Sco = 1/2, SFe = 1/2, Stot = 0) is intermediate the LS'

pair. Other pairings, appearing to be oxidized and reduced species when compared to

HSO and LSO, exist mainly at the boundary of HS and LS pairs and at surfaces where

metal coordination numbers change. There are also structural differences between the

different pairs, with LSO being the shortest (~10 A Co-Co) and HSO being the longest

(~10.2 A Co-Co) [68].

The ability to increase magnetization with blue light and decrease it with red light

can be understood as changing the relative populations of LSO and HSO in the sample,

Figure 5-2. To increase the magnetization in Ko.2Co1.4[Fe(CN)6]-6.9H20, an LSO state is

irradiated with blue light, exciting an electron from Fe to Co, giving LS'. The LS' state

then transitions to the metastable HSO state, separated from LSO by an energy barrier


115









of the order of 1 eV [1] [69]. To decrease the magnetization in

Ko.2Coi.4[Fe(CN)6]-6.9H20, HSO regions are irradiated with red light, exciting an electron

from Co to Fe, giving HS'. The HS' state then transitions to the metastable LSO state,

separated from HSO by an energy barrier of the order of 1 eV [1] [69].

In this chapter, starting with Section 5.1, nanoparticles of rubidium cobalt

hexacyanoferrate will be discussed, with specific attention to finite size effects on the

photoinduced magnetism and magnetically ordered states. In the subsections of 5.1,

synthesis and chemical composition (5.1.2), structure (5.1.3), and magnetization (5.1.4)

will be presented and discussed (5.1.5). Second, Section 5.2 compares nanoparticles

and bulk powder of cobalt hexacyanoferrate. This potassium cation system is

interesting because it allows for clear thermal transitions and thermal quenching of

magnetic states in addition to photoinduced magnetization. Within Section 5.2,

synthesis and chemical composition (5.2.2), structure (5.2.3), and magnetization (5.2.4)

are to be presented and discussed (5.2.5). Finally, in Section 5.3, sodium cobalt

hexacyanoferrate nanoparticles showing large thermal hysteresis are discussed to show

the size dependence of this effect.

5.1 Nanoparticles of Rubidium Cobalt Hexacyanoferrate

Nanoparticles of rubidium cobalt hexacyanoferrate (RbjCok[Fe(CN)6]r nH20) were

synthesized using different concentrations of the organic ligand

polyvinylpyrrolidone (PVP) to produce four different batches of particles with

characteristic diameters ranging from nominally 3 to 13 nm. Upon illumination with

white light at 5 K, the magnetization of these particles increases. In addition, the

magnetic properties, namely the long-range ferrimagnetic ordering temperature (Tc) and


116









the coercive field (Hc), of the nanoparticles evolve with preparation protocol. At 2 K,

particles with diameters less than 10 nm are in the superparamagnetic limit. This work

was published, in part, in the New Journal of Physics

(http://www.iop.org/EJ/journal/NJP). Those sections contained within the NJP article

are copyright of the IOP (copyright release form in Appendix C) and the online abstract

can be found at http://www.iop.org/EJ/abstract/1367-2630/9/7/222/ [8].

5.1.1 Introduction

Numerous efforts to synthesize nanoparticles of Prussian blue analogues have

been made, but only a few examples of photoinduced magnetism have been reported,

including work that isolated K.Cok[Fe(CN)6],nH20 particles, with typical diameters of

8-10 nm, within a silica xerogel [6], and other research producing 11 nm x 70 nm

nanorods of Mo(CN) Cu2 protected by polyvinylpyrrolidone (PVP) [7]. In each case,

although photoinduced magnetism was observed, the particles did not exhibit long-

range order.

This section reports RbjCok[Fe(CN)6]r nH20 nanoparticles, protected by PVP, that

exhibit photoinduced magnetism for all sizes and that possess long-range ordering, with

coercive fields ranging between 0.25-1.5 kG, in the larger particles. From the data, the

superparamagnetic limit at 2 K is identified, and the magnetic signal generated from

particles in this limit can be estimated for the different batches of particles.

5.1.2 Synthesis and Chemical Composition

The nanoparticles were synthesized by Dr. Franz Frye, modifying the procedure

described by Uemura and coworkers [70] [71]. The Prussian blue analogue powder is

encapsulated in a polyvinylpyrrolidone polymer (PVP) during synthesis. By varying the


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amount of PVP (Table 5-1), the protocol produced specimens with different sizes and

distributions. After 30 minutes of stirring, the solution was allowed to sit for one week.

To isolate the particles, three volumes of acetone were added to the synthesis solution,

which was centrifuged, and then further washed with acetone and dried under vacuum.

Chemical analysis was obtained from a combination of CHN combustion analysis and

inductively coupled plasma mass spectroscopy (ICP-MS), and the resulting formula are

listed in Table 5-1, along with the ratio of the PVP repeat unit per cobalt. The

concentration of H20 was estimated by considering the measured Fe vacancies and

charge balance.

5.1.3 Structure

In order to characterize the nanoscopic nature of the samples, transmission

electron microscopy (TEM) and fourier transform infrared (FT-IR) spectroscopy was

performed. Other techniques that would provide analysis of large sample sizes, such as

dynamic light scattering of samples in suspension, were tried but lacked the requisite

resolution for the size regimes and distributions of interest.

5.1.3.1 Transmission electron microscopy

For the transmission electron microscopy (TEM) studies, a 50 pL aliquot of the

suspension was diluted 2000 times, and 8 pL of the diluted suspension was placed on a

holey carbon grid. Selected area electron diffraction was compared to powder x-ray

diffraction patterns to confirm the structure [72]. Using Image J imaging software [73],

the TEM images were analyzed to obtain the particle size distributions shown in Figure

5-3, similar size distributions have been obtained for Prussian blue nanoparticles


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protected by PVP [74]. These data were fit to a log-normal function that yielded the

characteristic diameters shown in Table 5-1.

5.1.3.2 Fourier transform infrared spectroscopy

FT-IR was performed on samples analogous to those studied in more detail in the

following subsections, Figure 5-4. Similar synthesis protocols yielded batches with

characteristic diameters, d, of d ~ 240 nm (a 'bulk' powder), d ~ 13 nm (nanoparticles

analogous to Batch D), and d ~ 3 nm nanoparticles (analogous to Batch A). It was

necessary to synthesize new samples as these studies were performed after the

original samples had degraded over time. The FT-IR spectrum of the pure cobalt

hexacyanoferrate displays peaks near 2163, 2120, 2090, and 2040 cm-1, corresponding

to the cyanide stretches of the Co2+Fe3+ (HS), Co3+Fe2+ (LS), Co2+Fe2+ (reduced) and

linkage isomerized Co2+Fe2+ phases, respectively [75]. As the size of the particle is

reduced, the intensity of the HS peak near 2163 cm- decreases, while that of the LS

and reduced peaks near 2120 and 2090 cm- emerge and grow.

5.1.4 Magnetization

The quantity of central interest is magnetization. To this end, a standard

commercial SQUID magnetometer was employed. The samples were mounted to

commercial transparent tape and could be irradiated with light from a room temperature,

halogen source by using a homemade probe equipped with a bundle of optical

fibers [76]. Background contributions from the holder and tape are independently

measured and subtracted from the data.

5.1.5.1 DC susceptibility

The temperature dependence of the DC magnetic susceptibilities, x(T), of the

four batches of particles are shown in Figure 5-5. The magnetic signals are expressed


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per mole of Prussian blue analogue (PBA), Table 5-1. The dark state ZFC data were

obtained after cooling in zero applied field from 300 K, while the dark state FC data

were taken after cooling in 100 G from 300 K. The light state was established after field

cooling the samples from 300 K to 5 K in 100 G and subsequently irradiating with light

for 5 hours. The light state FC data were obtained after cooling from 30 K in 100 G.

All samples reported show a photoinduced increase in their magnetic signals and

ordering temperatures, and the strength of the change is correlated with the size of the

particles. The differences between the FC susceptibilities of the light and dark states,

light dark
X=XFC -XFC are plotted in the insets of Figure 5-5; finite values can only arise from the

photoinduced magnetism.

5.1.5.2 DC magnetization

The field dependence of the DC magnetization, M(H), of the four batches of

particles are shown in Figure 5-6. The magnetic signals are expressed per mole of

Prussian blue analogue (PBA), Table 5-1. The dark and light states are the same as for

the DC susceptibility measurements. All samples reported show a photoinduced

increase in their coercive fields and saturation magnetizations, and the strength of the

change is correlated with the size of the particles.

5.1.5.3 AC susceptibility

The temperature dependence of the real (X') and imaginary (X") AC

susceptibilities of the ZFC dark states, i.e. dark states, of all four batches are shown in

Figure 5-7. The phenomenological parameter for frequency dependence of the peak is

ATf
STfA(Iog) 5.1
TfA(logw)


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where Tfis the freezing temperature given by the cusp in x'(T) and co is the angular

frequency, is 0.024 + 0.004 for batches C and D, and this observation is consistent with

spin glass or cluster glass behavior [77] [78].

5.1.5 Discussion

It is important to recall that the photoinduced magnetic properties of the Prussian

blue analogues depends upon a spin crossover effect and the presence of vacancies

that allow crystalline flexibility [57][79-85]. More specifically for the

RbjCok[Fe(CN)6]1 nH20 nanoparticles, the spins can exist in either of two arrangements.

The low-spin state consists of Fe 2+(S = 0) and Co3+ (S = 0), while the high-spin state

possesses Fe3+ (S = 1/2) and Co2+ (S = 3/2). Depending on the local chemical

environment due to the values ofj,k,I, and n in the chemical formula, the Fe and Co

spins can be locked into either their high-spin or low-spin states for all accessible

temperatures. Alternatively, with the proper tuning of constituent elements, the Fe and

Co ions can exist in high-spin states at room temperature and then experience a

crossover to their low-spin states at approximately 150 K. This spin crossover

phenomenon prepares the system for the possibility of experiencing photoinduced

magnetism. However, since the spin crossover effect may not be 100% efficient, some

regions remain locked in their high-spin states, giving rise to the magnetic signals

observed for the dark state of the particles. When irradiated, the low-spin regions are

photoinduced to the high-spin magnetic state, resulting in a growth of the magnetic

domain. This scenario is supported by the frequency dependent AC susceptibility

studies, and by the local probe investigations of others [68] [86].


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Two main features, onset of long-range magnetic order and increasing net

magnetization, are important when considering the evolution of material properties due

to the increasing average size of the separate batches. The ordering temperatures of

the particles and the magnetic susceptibility are both seen to decrease as particles

become smaller, Figure 5-5. This scaling of magnetization is linked to an increased

diamagnetic-surface to magnetically-active-volume ratio at smaller particle sizes. This

contention is supported by the FT-IR spectra, which show an increase in the LS and

reduced content as particle size decreases, although the precise location of these

moieties cannot be gleaned from such methods. At low temperatures, the high-spin Fe

and Co ions interact antiferromagnetically, giving rise to a ferrimagnetic transition at

Tc [87]. For the magnetic data shown in Figure 5-5 and Figure 5-8, the onset of this

transition can be estimated, and these macroscopic temperatures are listed in Table 5-2.

Therefore, it is plausible that particles larger than a critical size will allow domains large

enough to approach bulk-like magnetic properties. Conversely, smaller particles may

put limits on allowed domain size, suppressing the ordering temperature.

Microscopically, if the size of the magnetic domains is less than or of the order of the

magnetic coherence length, then a spectrum of Tc values can be expected until the

superparamagnetic limit is achieved.

Consider the magnetic properties of the samples presented in conjunction with the

TEM analysis. Batch A has no observed coercivity and follows Curie-like behavior.

Batches B and C show a combination of Curie-tail and partial ordering with a reduced T
C

(Figure 5-5 and 5-8), as well as finite coercive fields. Finally, the active sites in Batch D

are almost entirely ferrimagnetically ordered with the largest coercive field of all batches


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presented. In addition, the differences between the FC and ZFC data for the dark state

in batch D is consistent with spin glass or cluster glass behavior [77] [78], in accord with

the presence of large magnetic domains.

Based upon fitting the macroscopic magnetization data available in the Curie-like

contributions at low temperature, the superparamagnetic contribution for each of the

four batches of nanoparticles (smallest to largest) is 100%, 90%, 50%, and 10%.

Consequently, at least down to 2 K, nanoparticles with sizes below -10 nm are in the

superparamagnetic limit. These interpretations are consistent with the M versus H

measurements performed at 2 K, where significant coercive fields, Hc, and remnant

magnetization values are observed for the two largest sets of particles but not for the

two smallest sets of particles, Figure 5-6 and Table 5-2. It is interesting to note that

even at 70 kG, there is still a finite slope to the magnetization, and this slope actually

gets larger with particle size. Two plausible explanations for this behavior exist:

(1) there is local flipping of minor Fe spins to be aligned with the increasing field, without

a clear spin-flip field due to the disordered nature of the magnetism, and (2) there is a

field dependent magnetic moment on either the cobalt or iron sites due to the first order

angular momentum not being completely quenched by the crystal environment (as

detailed in Chapter 4).

5.1.6 Conclusions

In conclusion, four different sizes of RbjCok[Fe(CN)6]r nH20 nanoparticles

protected by PVP were synthesized and characterized. Each batch of particles is

photoinducible, but the strength of this effect, as well as other global properties, e.g. Tc

and Hc, are correlated with the intrinsic particle size distributions of each batch. The


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combination of photoinduced magnetism and nanosized Prussian blue analogue

particles with finite coercive fields is unique and establishes a length scale limit of

-10 nm diameter for these properties. Since publication, these findings have been used

to understand the results of additional nanoparticle studies [88] [89].

5.2 Nanoparticles of Potassium Cobalt Hexacyanoferrate

Nanoparticles and bulk powder of potassium cobalt hexacyanoferrate

(KjCok[Fe(CN)6]1 nH20) were synthesized by changing the density of reactants in

solution, without using surfactant, excluding additional experimental uncertainties from

estimating the amount of surfactant present. Bulk powder with characteristic diameters,

d, of d ~ 200 nm and nanoparticles of d ~ 27 nm were studied. Both sizes show

photoinduced magnetization and the ability to trap magnetic states by thermal

quenching. However, the magnetic properties are modified in the nanoparticles,

showing less total magnetization, greater magnetic coercivities, and longer isothermal

relaxation constants. In addition, macroscopic differences between photoinduced and

thermally quenched low temperature magnetic states of KjCok[Fe(CN)6]r nH20 are

presented for the first time. Magnetic data is complemented by infrared spectroscopy,

transmission electron microscopy, x-ray powder diffraction, and temperature dependent

neutron diffraction. This work is expected to be submitted for publication at a later date.

5.2.1 Introduction

While there are reports of photoswitchable nanoparticles by other groups [6] [7], as

well as a study of rubidium cobalt hexacyanoferrate nanoparticles that the author

co-authored [8], these studies focused mainly on the low temperature magnetically

ordered state. However, charge transfer induced spin transitions (CTIST) between the

high-spin (HS) and low-spin (LS) states can also be incited by changing the


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temperature [1]. The relevant states to be considered for these transitions are

summarized in Table 5-3 and Table 5-4.

In the high temperature limit (~300 K), most Co-NC-Fe pairs will be in a

HS (S = 1) state. Upon reaching a sufficiently low temperature (~200 K), Co-NC-Fe

pairs begin transitioning to the LS state, and the process can be summarized as



Co3+ (S = 0)-Fe2+ (S = 0) <> Co2+ (S = 3/2)-Fe3+ (S = 1/2)

(low temperature) (high temperature).



The K cation samples are of particular interest because it was shown that in

addition to photoinducing, thermal quenching can trap variable amounts of HS

pairs [9] [76]. In the extreme limit of cyanide bridged molecular cobalt iron, it was found

that quenched and photoinduced states are identical [90]. While the quenched and

photoinduced states in Na cation, bulk powders have been studied by x-ray

diffraction [68], and quenching has been studied with magnetic susceptibility,

Mossbauer spectroscopy [91] and specific heat [92], no systematic study has been

presented on the macroscopic measurements of such a material or of how finite size

effects might play a role and further elucidate the fundamental mechanisms involved.

This thesis chapter reports on KjCok[Fe(CN)6]r nH20 (henceforth K-Co-Fe)

nanoparticles, synthesized without surfactant, that exhibit photoinduced magnetism and

thermal quenching of magnetic states for characteristic diameters, d, of d ~ 27 nm and

d ~ 200 nm batches of nanoparticles. Clear modifications of the magnetic properties


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with size are observed and macroscopic differences between the photoinduced and

thermally quenched magnetic states are detected.

5.2.2 Synthesis and Chemical Composition

Bulk K-Co-Fe Prussian blue analogue powders were prepared by Dr. Justin E.

Gardner and Matthew J. Andrus, using procedures previously described by Shimamoto

and coworkers [84]. Nanoparticles of K-Co-Fe Prussian blue analogues were

synthesized Dr. Justin E. Gardner and Matthew J. Andrus, by modifying the procedures

previously reported by Yamada and coworkers [93]. In order to estimate the chemical

formulae, EDS and CHN were performed on the samples, and the results are part of

Table 5-5.

5.2.2.1 Fourier transform infrared spectroscopy

Infrared spectroscopy of cobalt hexacyanoferrates are typically performed in the

energy region of the cyanide stretch of the compound, where the structure evolves due

to the changing oxidation states of the coordinating metals of the cyanide, Figure 5-9.

The first peak at 2160 cm-1 represents the Fe3+(LS)-CN-Co2+(HS) stretch, while the

second broad peak is a combination of the Fe2+(LS)-CN-Co3+(LS) and Fe2+(LS)-CN-

Co2+(HS) stretches which appear at 2115 cm-1 and 2090 cm-1, respectively. Nanosized

powders are known to exhibit spectra with a smaller Fe3+/Fe2+ ratio [8] [94]. Results of

fitting the FT-IR spectra are detailed in Table 5-5.

5.2.3 Structure

The macrostructure of the samples were characterized with HR-TEM, in order to

estimate the average particle size. XRD at room temperature was performed to show

the structure of the heavier elements. Temperature dependent neutron diffraction from

5 K to 300 K was performed on the d ~ 27 nm nanoparticles to probe the structural


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transition and to look for magnetic scattering. While structural transitions correlated to

changes in the magnetic states have been observed in the bulk, they have yet to be

reported for samples showing finite size effects.

5.2.3.1 Transmission electron microscopy

For the transmission electron microscopy (TEM) studies, a 50 pL aliquot of the

suspension was diluted 2000 times, and 8 pL of the diluted suspension was placed on a

holey carbon grid. The TEM images were analyzed by printing on standard 8.5 in x

11 in paper and measuring particles by hand with a digital caliper to obtain the particle

size distributions shown in Figure 5-10, and similar size distributions have been

obtained for Prussian blue nanoparticles protected by PVP [8] [74]. This size analysis

method was found to yield the same results as the previous method in which a

computer program was used, however the paper method involved less eye-strain and

the ability to perform the measurements while enjoying Florida sunshine. These data

were fit to a log-normal function that yielded the characteristic diameters shown in

Table 5-6.

5.2.3.2 X-ray diffraction

To investigate the lattice constants and crystal structure, a Philips APD 3720

powder diffractometer, housed in the Major Analytical Instrument Center at the Univesity

of Florida, was used to perform room temperature x-ray diffraction (XRD) using a Cu Ka

source. It is worth noting that two lines, 1.54056 A and 1.54441 A, are present for the

K-edge of Cu, and stripping of the weaker K2 line was performed, however the

line-widths are so large for these samples that such an analysis is not necessary.

Diffraction studies on similar compounds have assigned the HS unit cell to ~10.3 A and


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the LS unit cell to -10.0 A [68]. As extended x-ray fine structure (EXAFS)

measurements have shown that the Co-N distance is most sensitive to the changing

oxidation states of a Co-NC-Fe pair [95] [96], the Fe3+(LS)-CN-Co2+(HS) and

Fe2+(LS)-CN-Co2+(HS) moieties should have similar lengths near ~10.3 A. Between

10-20 mg of the same samples used for all other characterizations were mounted on

glass slides and pressed onto squares of double-sided cellophane tape of ~ 2.3 cm2

The room temperature x-ray powder diffractograms, shown in Figure 5-11, were used to

model the structure by a Rietveld refinement using the EXPGUI [53] interface for

GSAS [52]. In order to approximate the complicated Prussian blue analogue structure,

a single-phase model with Fm3'm (No. 225) space group symmetry was used.

Specifically, the cobalt and nickel atoms were forced to occupy the same site. Atomic

occupancies were set by the experimentally determined chemical formulas, excepting

the oxygen atoms of the interstitial waters that were allowed to vary as the samples may

have dehydrated or hydrated between synthesis and diffraction. The same site

symmetries as in Prussian blue were used, where the iron vacancies were replaced by

the six coordinated oxygen atoms of the ligand water molecules [65]. Placement of the

oxygen atoms of the interstitial water molecules at the 32fWyckoff position [68] and a

relatively small percentage at the 192/position was found to yield a robust local minima

during refinement procedure.

The bulk powder shows one phase, with a lattice constant of 10.35 A and a FWHM

of 0.120 for the (2, 2, 0) reflection. The nanoparticles show two phases, with lattice

constants of 10.31 A and 10.07 A and a FWHM of 0.50 for the (2, 2, 0) reflection.


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5.2.3.3 Neutron diffraction

As compared to x-ray diffraction, neutron diffraction is more sensitive to light

elements and may also scatter off local magnetic moments in a lattice. In order to

probe the structure of the quenched phase in nanoparticles, as compared to the bulk

materials that have already been studied [68], 5 grams of deuterated K-Co-Fe were

used. A full neutron powder diffraction pattern of the nanoparticles gives the same unit

cell parameters as x-rays, Figure 5-12. Quenching the sample by quickly inserting into

a liquid helium filled cryostat (AT/Atime ~ 100 K/min) shows clear changes in the

diffraction pattern, corresponding to the thermally quenched state (Q). Upon warming to

200 K, a transition of the unit cell to the low temperature ground state (G) can be seen,

Figure 5-13 (a). Conversely, by slowly cooling the sample from room temperature at

~1 K/min, a transition from the high temperature equilibrium phase (RT) to the G state is

observed, Figure 5-13 (b).

These peaks can be fit with a three phase model analogous to that used for the

infrared measurements, with the HS, LS, and 'reduced' components taken into account.

Unfortunately, the neutron data is more convoluted due to the HS and 'reduced' phases

having the same lattice constant and the overlap of the lines due to the structural

disorder in the samples. To mitigate this issue, the LS component was fit with a

temperature independent lattice constant, and this assumption is corroborated by a set

of fits done with free parameters showing no clear trend in the LS unit cell parameter.

On the other hand, the HS unit cell parameter shows a clear dependence on the

high-spin fraction and must be allowed to vary. Although both a Lorentzian and

Gaussian character is present in the peaks, and in fact the Gaussian nature is


129









somewhat stronger, a Lorentzian shape was used to fit the temperature dependence to

avoid insurmountable variable co-variance as the HS/'reduced' and LS lines get close at

lower temperatures. The results of fitting the lines yield the temperature dependence

of the lattice constants, Figure 5-13 (c) and the amount of scattering associated with

each unit cell, Figure 5-13 (d). The full-width-half-maximum values did not shown any

clear trends above the noise.

Finally, an attempt was made to observe magnetic scattering of the nanoparticles,

in order to elucidate the magnetic structure. The expected nature of the scattering was

modeled by positioning point spins on the nuclear positions of the metals and

comparing the magnetic signal for parallel and antiparallel alignment, Figure 5-14 (a). A

main result of the model calculations is that the relative intensity of the first two peaks is

different for the ferrimagnetic versus ferromagnetic states. However, any magnetic

scattering present, as potentially evidenced by the difference between the spectra at

5 K in the magnetically ordered state and 30 K in the paramagnetic state, was smaller

than the experimental resolution of the setup used, Figure 5-14 (b). In addition, no

difference was observed between 0.5 T and 5 T at 5 K.

5.2.4 Magnetization

Magnetization is a key parameter to further probe the quenched state (Q), low

temperature ground state (G), and photoinduced states (P), which are defined explicitly

in Table 5-4. To this end, a standard commercial SQUID magnetometer was employed.

For photoinduced measurements, the samples were mounted to commercial

transparent tape and could be irradiated with light from a room temperature, halogen

source by using a homemade probe equipped with a bundle of optical fibers [76].

Background contributions from the holder and tape are independently measured and


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subtracted from the data. For the quenching measurements, in order to achieve better

signal to noise in the paramagnetic states, gelcaps were packed with powder and

mounted on a standard sample holder. Backgrounds from the gelcaps were subtracted

based upon the mass susceptibility of an analogous gelcap.

5.2.4.1 Quenched high temperature DC susceptibility

The temperature dependence of the DC magnetic susceptibility temperature

product, x(T) T, in the paramagnetic limit are shown in Figure 5-15. The magnetic

signals are expressed per mole, Table 5-6. Quenching was achieved by stabilizing the

cryostat temperature to 100 K, and quickly inserting the sample. While infinitely

slow-cooling will reach the true G state, this was realized experimentally by quenching

to 100 K, warming to 200 K at less than 1 K/min, and subsequently cooling again.

Both samples show a clear trapping of magnetic states with thermal quenching,

and the ability to trap the magnetic states is correlated with the size of the particles. In

fact, the room temperature HS content of the samples, nHS, is strongly affected by the

particle size.

5.2.4.2 Quenched low temperature DC susceptibility

The temperature dependence of the DC magnetic susceptibilities, x(T), in the

ordered state before and after thermal quenching are shown in Figure 5-16. The

magnetic signals are expressed per mole, Table 5-6. Quenched states were achieved

by stabilizing the cryostat temperature to 100 K, and inserting the sample. The G state

was reached by quenching to 100 K, warming to 200 K at less than 1 K/min, and

subsequently re-cooling. The ZFC data were obtained after cooling in zero applied field

from 100 K, while the dark state FC data were taken after cooling in 100 G from 100 K.


131









Both samples show an increase in their magnetic signals as more spins are

thermally trapped by quenching, and the strength of the change is correlated with the

size of the particles. Insets of Figure 5-16 show a weak increase in the magnetic

ordering temperature for the Q states compared to the G states.

5.2.4.3 Quenched low temperature magnetization

The temperature dependence of the DC magnetization, M(H), before and after

thermal quenching are shown in Figure 5-17. The magnetic signals are expressed per

mole, Table 5-6. The history of the cooling is the same as for the DC susceptibility

measurements. Both samples show clear increases in their high field magnetization

with quenching, and a weak increase in the coercive fields. The ~27 nm nanoparticles

have a much larger coercive field than the ~200 nm nanoparticles.

5.2.4.4 Isothermal relaxation

The time dependence of the DC magnetic susceptibility temperature product,

x(T) T, of the quenched states in the paramagnetic limit are shown in Figure 5-18. The

magnetic signals are expressed per mole, Table 5-6. Thermal quenching was achieved

by stabilizing the cryostat temperature to 100 K, and quickly inserting the sample.

Three different isothermal relaxation temperatures were chosen for each sample. Both

samples show non-exponential relaxation of the metastable Q state at elevated

temperatures, and the nanoparticles show elongated relaxation times.

5.2.4.5 Photoinduced low temperature DC susceptibility

The temperature dependence of the DC magnetic susceptibilities, x(T), in the

ordered state before and after photoirradiation are shown in Figure 5-19. The magnetic

signals are expressed per mole, Table 5-6. The light state was established after field

cooling the samples from 300 K to 5 K in 100 G and subsequently irradiating with light


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for 5 hours. The ZFC data were obtained after cooling in zero applied field from 100 K,

while the dark state FC data were taken after cooling in 100 G from 100 K. The light

state FC data were obtained after cooling from 30 K in 100 G.

Both bulk and nanoparticle samples showed a dramatic increase in the magnetic

ordering temperature. The nanoparticles show a reduced ordering temperature

compared to the bulk powder due to finite size effects. The differences between the FC

susceptibilities of the P and G states are plotted in the insets of Figure 5-19.

5.2.4.6 Photoinduced low temperature magnetization

The temperature dependence of the DC magnetization, M(H), are shown in

Figure 5-20. The magnetic signals are expressed per mole of Prussian blue analogue

(PBA), Table 5-6. The history of the cooling is the same as for the DC susceptibility

measurements. Both samples show clear increases in their high field magnetization

with quenching, and a weak increase in the coercive fields. The ~27 nm nanoparticles

have a much larger coercive field than the ~200 nm nanoparticles.

5.2.5 Discussion

First, details of the models used to interpret the data will be introduced. Second,

the size dependence of the quenching effect will be discussed. Third, the difference

between the photoinduced and quenched states, and particularly how the nanoparticles

add to this understanding, will be given. Finally, a schema of the particles that can

qualitatively reproduce the previous points will be presented.

5.2.5.1 Details of modeling

While it is possible to model the complicated system of temperature dependent

moments and superexchange interactions using a combination of ligand field theory and

mean-field theory, a more transparent and simple approach will be applied for the


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analysis of the magnetization data presented. A few assumptions are necessary for the

simple model and are



(1) XT versus T is assumed to be linear in the range from 100 K to 300 K,

(2) the oxidation states at room temperature are taken from the infrared

spectroscopy measurements,

(3) the ground state in the bulk, reached by quenching and subsequently slow

cooling, is assumed to have transitioned all switchable Fe ions into the low-spin

state, and

(4) a linear dependence of the susceptibility on the high-spin fraction is assumed.



The validity of assumption (1) is clear, to a high degree, by inspection of the data.

Assumption (2) is only an assumption in the sense that the extinction coefficients of the

different moieties are not exactly known. Assumption (3) is the biggest conjecture,

however, microscopic probes on analogous bulk samples show that complete transition

is plausible [68]. The final assumption, the linearity of the effective moment with respect

to the high-spin fraction, may not be obvious. However, this temperature dependence

can be justified from mean-field theory predictions by using a plausible set of

parameters and plotting the effective moment as a function of the high-spin fraction,

Figure 5-21, where the approximate linearity becomes clear.

The results of the aforementioned assumptions, is a semi-empirical expression for

XT as a function of the high-spin fraction


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(1.996 + 2.591 nHs)
XT(nHs) = T1000 + (0.407 + 5.002nHS) 5.2
1000

In order to study the bi-stability in the paramagnetic region, an Ising-like model for the

charge transfer induced spin transition is used. For this approach, the Hamiltonian is


K = -J sis kBTIn -A i 5.3
i

where is the sum over nearest neighbors, kB is the Boltzmann constant, Tis the

temperature, s is the pseudo-spin that keeps track of the CTIST state of a pair, A takes

into account the configuration interaction due to electron-electron interaction and the

ligand field, g+ is the degeneracy of the HS state and g_ is the degeneracy of the LS

state. Later, for convenience, g will be defined as the ratio -+. Using a
g_

Bethe-Peierls-Weiss approximation, this equation can be solved to get the high-spin

fraction, nHs, and the relative population of the mixing of the states, nHL [97].

In order to fit the magnetization curves, various constraints can be applied. To

begin, the equilibrium temperature of the transition is well defined,

2A
Teq kn(g) 5.4


Equation 5.4 can be used to constrain the parameters of Equation 5.3. First, Teq can be

fixed according to the XT versus T data (~225 K), and the relative degeneracy of the

states can be taken from specific heat measurements on similar materials

(In(g) ~ 12) [92]. These constraints give a value for A of 1315 K. From here all

parameters can be fit simultaneously to give an activation energy of 5000 K, and an


135









interaction energy of 90 K, with a fundamental relaxation rate of

1/TO = 1/(0.12 x 1012 minutes).

Aside from scale, the temperature dependence of the magnetization is identical in

the nanoparticles and the bulk powder. Based upon this similarity, it is logical to

assume that the phenomenological equilibrium parameters are the same, rather than

coincidentally modified in such a way as to give nearly identical macroscopic equilibrium

states. Two models were tested by studying the relative homogeneity of the state, nHL.

In the first model, the amount of high-spin material was distributed through a

nanoparticle volume, and in the second, the nanoparticle acted essentially as regions of

a bulk powder and regions of locked pairs in 'reduced' or LS states at all temperatures.

It was found that the second model, having discrete regions of trapped and bistable

material, had better fits to experimental data. However, in order to more properly fit the

nanoparticle relaxation data, an additional temperature dependent interaction term had

to be included in the fits to account for the extended relaxation times. This interaction is

proposed to be between the bistable pairs and the locked 'reduced' and LS regions in

the sample. This association is consistent with the clear temperature dependence of

the strain that was detected in lattice constants from the neutron scattering data.

5.2.5.2 Size dependence of thermal quenching

To begin, the TEM images show clear differences in the sizes of the bulk and

nanoparticle samples. Although the chemical formulae are similar, the FT-IR displays

the decrease in HS material in the nanoparticles at the expense of 'reduced' and LS

fractions. Neutron powder diffraction is a microscopic probe that delineated between

the HS/'reduced' phases and LS phase in the nanoparticle sample as a function of


136









temperature and provided clear evidence of the strain induced on the lattice for different

degrees of thermal quenching. From the paramagnetic susceptibility, nHs could be

approximated, and the similarity of the equilibrium temperatures for the bulk and

nanoparticles is evidence that the local chemical formula is similar for both samples.

Although dramatic increases in the magnetic moment are seen with quenching, only

small modifications of the coercive fields and ordering temperatures are observed,

Table 5-7. It is interesting to note that the nanoparticles actually have a significantly

larger coercive field, roughly 10 times, although their magnetic ordering temperature is

reduced. Isothermal relaxation experiments of the dynamics in the quenched state

provide additional evidence for the similarity between the bulk and nanoparticle samples,

while also displaying subtle differences due to the strain between the bistable and

locked pairs in the nanoparticles.

5.2.5.3 Photoinduced versus quenched states

Although the photoinduced and quenched states are identical on the molecular

level [90], the resulting many-molecule state in a lattice turns out to be different for

quenched or photoinduced pairs due to the interactions between molecular units. This

difference can clearly be seen in the macroscopic magnetization, and the manner in

which it changes under photoexcitation as compared to thermal quenching, Table 5-8.

While the quenched states show little dependence of the ordering temperature and

coercive field on the degree of quenching, photoexcitation brings dramatic changes to

these parameters. Within the context of a mean-field picture, these differences may be

due to better interconnectivity of the lattice in the photoinduced as opposed to quenched

states. Also, in the photoinduced state, the difference in the ordering temperature of the

bulk and nanoparticles becomes more pronounced, suggesting that the magnetic


137









domains in the nanoparticles are being limited by structural constraints for the

photoinduced states, but not in the quenched states. This observation is consistent with

the current microscopic picture of smaller domains in the quenched as compared to

photoinduced states for bulk materials [68]. Furthermore, the nanoparticle data implies

that the photoinduced domains in the bulk are larger than ~27 nm. In a similar way, the

increased ordering temperature of the quenched nanoparticles implies the finite size is

constraining trapped spins to be more interconnected in the nanoparticles.

5.2.5.4 Resulting schema of bulk and nanoparticles

Based upon the microscopic data already discussed, as well as the macroscopic

magnetization data taken as a whole, Figure 5-22, a qualitative model for the bulk and

nanoparticles in the quenched and photoinduced states can be formulated, Figure 5-23.

This model is consistent with the quenched and photoinduced structural domains

proposed based upon microscopic x-ray diffraction experiments [68], where the

magnetism in quenched states is less connected than in the photoinduced states. A

consistent extension of the previous bulk model is made for the nanoparticle samples, in

which size effects and surface effects now play a role in how the macroscopic

magnetization manifests itself based upon the microscopic domains.

5.2.6 Conclusions

In conclusion, bulk powder and nanoparticles of KjCok[Fe(CN)6]-nH20 were

synthesized and characterized. The experimental data was analyzed using

semi-empirical methods to show clear trends suggesting that nanoparticles consist of a

core-shell type distribution of states. While the particles are large enough to display

long-range magnetic order, the amount of photoswitchable material is dramatically


138







reduced. The coercive fields of the nanoparticles are enhanced more than ten times


compared to the bulk.


O M1


. J


Seb
n40


0

0


0
O


H20


NC S CN 0


Figure 5-1. Prussian blue analogue structure. Prussian blue analogues have a
chemical formula of AyM1k[M2(CN)6] -nH20, j,k,I and n are constrained by
charge balance. Cations (A = Cs+, Rb+, K+, Na+) are incorporated based
upon the number of M2 vacancies. M2 vacancies are coordinated by water
as shown. The distance between M1 and M2 is generally ~5 A.


hVa

eg
2 g
Co3+(S = 0) Fe2+(S = 0)

Intersystem
crossing


.............. LS' .******* ". Intersystem
**' Intersystem
-- L eg ,rossiqn

Co2+(S = 1/2) Fe3+(S = 1/2) e

........... HS' ......... Co2+(S=3/2) Fe3+(S=1/2)
eg
Se dz- dx _y
t24 g 2 gtj 4 44yz hb eg
Co3+(S= 1) Fe2+(S= 0) dxyoct
'""""""""'""""dxz


Figure 5-2. A detail of the photoexcitation processes in Ko.2Co1.4[Fe(CN)6]-6.9H20.
Strong-field eg and t2g orbitals are used for simplicity.


139











Table 5-1. Synthesis and chemical composition of rubidium cobalt hexacyanoferrate
nanoparticles.
Starting PVP:Co
Batch paig Resulting chemical formula Diameter (nm)
PVP (g) ratio
A 1.0 Rb1.9Co4[Fe(CN)6]3.2 5.8 H20 360 3.3 0.8

B 0.5 Rb1.8Co4[Fe(CN)6]3.2- 5.8 H20 200 6.9 2.5

C 0.2 Rb1.7Co4[Fe(CN)6]3.2 5.8 H20 60 9.7 2.1

D 0.1 Rbo.9Co4[Fe(CN)6]2.9 6.6 H20 20 13.0 3.2


Batch A


Batch D


Batch A



Batch B



Batch C



Batch D


0 5 10 15 20
Diameter (nm)


Figure 5-3. TEM of RbjCok[Fe(CN)6]r nH20 nanoparticles. (left) Typical TEM images.
(right) The particle distributions, normalized to the largest bin, versus
diameter for the four batches of particles, see Table 5-1. The total number of
particles for each distribution, smallest to largest, is 44, 27, 53, and 62,
respectively. The solid lines are the results of log-normal fits that provide the
characteristic diameters shown for each distribution.


140










A e -2w (a) d ~ 240 nm 'bulk' (b) d ~ 13 nm 'Batch D' (c) d ~ 3 nm 'Batch A'

xc(cm-1) 2157 2122 2088 2156 2116 2088 2088 2120 2160
w (cm1) 17.6 18.41 22.3 15.7 35.9 45.2 42.8 38.6 15.4
A (arb. Units) 0.72 0.04 0.24 0.22 0.23 0.52 0.28 0.60 0.12


(a)"
4



0

22002150210020502000
wavenumber (cm'1)


S(b)

CU


0

22002150210020502000
wavenumber (cm'1)


- I I I

CU





22002150210020502000
wavenumber (cm' )


Figure 5-4. FT-IR absorption spectra of RbjCok[Fe(CN)6]1-nH20 nanoparticles.
(a) d ~ 240 nm 'bulk' powder, (b) d ~ 13 nm nanoparticles analogous to Batch
D, and (c) d ~ 3 nm nanoparticles analogous to Batch A. Fits to Gaussian
lines are shown for HS (-), LS ( ), reduced (-), and total intensity (-), and
the values of the fitting parameters are tabulated.


141





















0.4 -


20 -


Batch A --.
* ] o' 0.032

S 0.01

0 5 10 15 20 25

Q49QQQQQ -


-0
-0


E E -

0.001 2
0 5 10 15 20 25


Batch C 1.5
1.0


o 0 5 10 15 20 25
T(K)

Batch D -8 L-~".
6- *
E=4

o. ..
So 0 5 10 15 20 25
S T (K).
^-A^^f T I ft TTI m r "T r i r ri r-i r- -.-- -- r


Batch B 0,15
0.10


0 5 10 15 20 25 30
Temperature (K)



Figure 5-5. The temperature dependence of the low field, 100 G, susceptibilities of
RbjCok[Fe(CN)6]1-nH20 nanoparticles. The zero-field cooled (ZFC) dark (*),
field-cooled (FC) dark (m), and FC light (r) states of each batch produced are
shown. The insets display the differences between the FC light and dark
states, as described in the text. Finite values for this difference can only arise
from photoinduced magnetism.


142









0 40
20
Z 0
E
E-20
"0o
-40


40
E
S20
(D
E
-20
"'-40


-60-40-20 0 20 40 60
H (kG)


: p 10-
E 5-


"o -5 I-"
Z3 0 "--
E

-10
2 -1.0
U,
cn
C5F,.


20


O
E

-10
-20
2 -1


10


-5 0
H (kG)


Figure 5-6. The T = 2 K magnetization versus magnetic field sweeps of
RbjCok[Fe(CN)6]1-nH20 nanoparticles. Here Batch C for (a) high fields and
(b) the field region relevant to the hysteresis in both light (o) and dark (*)
states are shown. In addition, Batch D for (c) high fields and (d) the field
region relevant to the hysteresis in both light (E) and dark (m) states. The
coercive fields, Hc, for the light and dark states for each batch are listed in
Table 5-2, and the lines are guides for the eyes.


143


W- (d) ''


- -
*. *


5 10


-0.5 0.0 0.5
H (kG)






























0 5 10 15 20
Temperature (K)


0.1
SBatch A
0.0 P ..
4-

Batch B
0.0


0.l Batch C




1.0 -
0.5- -
1.5 'l .... i .. i D. "
0.0

0 5 10 15 20
Temperature (K)


Figure 5-7. The temperature dependence of the real (X') and imaginary (X") AC
susceptibilities of RbjCok[Fe(CN)6]1-nH20 nanoparticles. Zero field cooled
dark states were measured with no applied static field and an alternating field
of 4 G, except for batch D, which was measured in 1G. The frequency
dependence was studied at 1 Hz (m), 10 Hz (m), 100 Hz (n), and 1 kHz (m) for
all batches, except for batch D, which has an additional measurement at
333 Hz ( ). Arrows are guides for the eyes and pass through the peaks.


Table 5-2. Magnetic properties of rubidium cobalt hexacyanoferrate nanoparticles.
Batch Diameter (nm) TCdark (K) Tclight (K) HCdark (G) Hclight (G)


A 3.30.8 <2 <2 <10 < 10


B 6.9 2.5 -10 -13 -15 -30


C 9.7 2.1 13 17 250 330


D 13.0 3.2 19 22 1000 1500


144












0.04

0.02

0.00
2

1


10
Temperature (K)


15 20


Figure 5-8. Ordered magnetic components of the smaller batches. The ZFC (*) and FC
(m) dark states show a signal associated with long-range order for the two
batches (Batch B and Batch C) displaying a mix of magnetic behavior after
subtracting the Curie-like contribution.


Table 5-3. The microscopic states relevant to KiCok[Fe(CN)6]r nH20.
microstate
mcrosae oxidation states bond length (A) Co spin Fe spin
shorthand
HS Co2-NC-Fe3+ 10.3 3/2 1/2
LS Co3+-NC-Fe2+ 10.0 0 0
'reduced' Co2+-NC-Fe2+ 10.3 3/2 0


Table 5-4. The macroscopic states relevant to KiCok[Fe(CN)6]r nH2O.
macrostate expected
Meaning realization
shorthand microstates
rapid cooling to
Q Quenched 100 K at mostly HS, some LS
-100 K/min
photoirradiation at
P Photoinduced temperatures below mostly HS, some LS
100 K
G low temperature slow cooling at LS
Ground state -1 K/min to 100 K


145


Batch B



. C

SBatch C

-i











8 1.0
C
- 0.5
0

1.0
N
N0.5
0
z0.0 -
2200


2150 2100 2050
wavenumber (cm-1)


2000


Figure 5-9. FT-IR spectra of bulk and nanoparticles of K-Co-Fe. The nanoparticles
show a reduction in the amount of switchable material, the leftmost peak.

Table 5-5. Metal oxidation states of bulk powder and nanoparticles, in addition to fitting
parameters used for Figure 5-9. Fits are to a Gaussian function for the lines,
2
i.e. wA e-2(e )

Sample Oxidation States wo (cm1) W (cm1) A (I cm-1)
2159.4 0.2 19.78 0.3 21.6 0.3

bulk Co3+0.25002+0.75 [Fe2+(CN)610.18[Fe3+(CN)6]0.56 2125.0 1.1 18.7 2.2 5.9 1.0

2095.9 0.8 32.1 1.3 25.0 1.0
2162.4 3.7 32.7 8.8 13.0 9.3
nano Co3+.o07Co2+o.93[Fe2+(CN)6]o.32[Fe3+(CN)6].36 2123.2 1.3 36.5 9.8 37.5 18.8

2096.7 2.9 45.7 5.5 33.0 10.3


Table 5-6. Chemical composition and characteristic sizes of potassium cobalt
hexacyanoferrate nanoparticles and bulk powder.
Sample Chemical Formula Diameter (n
bulk KO.39 Co3+o.25Co2+0.75 [Fe2+(CN)60.18[Fe3+(CN)60.56 3.30 H20 200 38
nano Ko.32Co3+o.o7Co2+o.93[Fe2+(CN)6]0.32[Fe3+(CN)6]0.36 3.30 D20 27.4 5.7


146













150


10 -200 nm'bulk'


E 5-



I. 0 50 100 150 200 250 300 350
edge length (nm)


Figure 5-10. TEM of K-Co-Fe. (left) Typical TEM images are shown. (right) The
particle distributions, versus diameter for the bulk and small particles, see
Table 5-6. The solid lines are the results of log-normal fits that provide the
characteristic diameters shown for each distribution.


147










1000


800 () 400 (D)

--600 -
300 -
400
8
200 200-
0 0
0 i ", I I 0o1 A
OL r_ 100 jW M I1 ,
-200 100
0
-400 -
S I I I -100
10 20 30 40 50 60 20 30 40 50 60
26 (degrees) 20 (degrees)

Figure 5-11. XRD of K-Co-Fe. Room temperature x-ray powder diffractograms of
(a) bulk and (b) nanoparticles of K-Co-Fe. The wavelength is 1.54 A. The
nanoparticles clearly show two peaks, a larger peak accounting for 65% of
the scattering near 10.313 A corresponding to the HS and 'reduced' phases,
and a smaller peak accounting for the remaining 35% of the structured peak
near 10.068 A corresponding to the LS phase. Experimental counts are
shown in black, with Rietveld refinements in red, and residuals of the fit below
the data in blue. Peak nomenclature is described in Table 5-3.


148









3000


2000 1000

34 35 36
U)



0
1000 -






20 40 60 80 100 120
20 (degrees)

Figure 5-12. Neutron scattering of K-Co-Fe. Room temperature neutron powder
diffractograms of nanoparticles of K-Co-Fe are shown, for \ = 1.54 A. The
nanoparticles clearly show two peaks, a larger peak accounting for 73% of
the scattering near 10.312 A corresponding to the HS and 'reduced' phases,
and a smaller peak accounting for the remaining 27% of the structured peak
near 10.061 A corresponding to the LS phase. (inset) The (4, 0, 0) reflection
showing the structure of the reflections, experimental counts are shown as
white circles, with a HS/'reduced' peak in blue and the LS peak in red. Peak
nomenclature is described in Table 5-3.


149








(a)

16 100 K 200K
S142 10.3 (c)
8 1 2HS and 'reduced' peak
8 12 -
10- 10.2- -,
c: 8- <

6 10.1
Z LS peak
34 35 36 37
(b) 20(degrees) 0.9
26 (d)
24 140K 300 (d)
S 0.8
S22 -
S 20 -
8 18 + 0.7I +
Co 16






20 (degrees) T(K)
14 0.6 T
cn 12
e o 10 0.5

6 4 50 100 150 200 250 300
-58 -56 -54
20 (degrees) T (K)

Figure 5-13. Neutron diffraction as a function of temperature for K-Co-Fe. The
evolution of the structure is seen by tracking the (4,0,0) reflection for
(a) warming after quenching to 100 K and (b) warming after slowly cooling at
~1 K/min. Results of fitting these reflections can be seen in the temperature
dependence of (c) the unit cell size and (d) the fraction of high spin and
'reduced' material, nH + reduced, having the larger lattice constant.
Quenching is shown in green, slow-cooling in blue, and room temperature as
a red star for data taken at HFIR on HB2A (X ~ 1.54 A), data taken slow
cooling on HB1A (X ~ 2.36 A) is black. The different horizontal axes on the
raw scattering data is due to slightly different ways of counting the angle on
the experimental detector bank, and these angles have been kept as they
were recorded by the machines.


150























12 14 16 18 20 22 24 26 28 30
20 (degrees)


- I I I I I I '
c 20 (b)

S10

*r 0

8-10 -
o
-20

.-30 I
E 12 14 16 18 20 22 24 26
20 (degrees)


Figure 5-14. Magnetic neutron scattering in K-Co-Fe. (a) Calculated magnetic
scattering for K-Co-Fe predicting different intensities for ferromagnetic (-)
and ferrimagnetic (---) structures are shown for an incident wavelength of
1.54 A. (b) Experimental observed magnetic scattering contribution taken as
the difference between the magnetically ordered state at 5 K and the
magnetically disordered state at 30 K.


4


S3

E
U2


150 200 250 300
T(K)


150 200 250 300
T(K)


Figure 5-15. Temperature dependent magnetic moment of quenched states for
K-Co-Fe. The temperature dependence of XT are shown in the
paramagnetic state at 5 kG for Q ( ), intermediately quenched (i), and
G states (A) of (a) ~200 nm particles and (b) ~27 nm particles. A clear
reduction in the magnetism in the quenched state can be seen for the smaller,
~27 nm nanoparticles. Solid lines are fits to extract the HS fraction (nHs), and
the details of the fits are described in the text.


151


4

E
S3
E

-2
















1.5 1.5 -
05- E 05 A
005 A
1.0 oo 1.0 oo
5 10 15 20 5 10 15 2




5 10 15(K 20 25 30 0 5 10 15 20 25 30 35 40
S(K) 'T (K)

Figure 5-16. Magnetic ordering of quenched states in K-Co-Fe. The temperature
dependence of the FC and ZFC DC magnetization, M, are shown in the
ordered state at 100 G for Q ( ), intermediately quenched (m), and G states
(A) of (a) -200 nm particles and (b) -27 nm particles. A clear reduction in
the magnetism in the quenched state can be seen for the smaller, -27 nm
nanoparticles. The ordering temperatures of both samples are similar.
(inset) The FC susceptibility normalized to the low temperature limit is shown
to more clearly display the changing ordering temperatures.


152










12
(a)
7-10
E
- 8

2: 6

4 g=2, S
2

0 1 2 3H(T)4


12 (b)

ml0 -
E

......
6


L-----...... g=2,S=1<:::::::
S0 I
5 6 7 0 1 2 3 4 5 6 7
H (T)


-0.1 0.0
H (T)


0.1 0.2


3
-2
0
-1

Sa-1
--2

-3


-0.4 -0.2 0.0 0.2 0.4
H (T)


Figure 5-17. Magnetization versus field of quenched states for K-Co-Fe. The T= 2 K
magnetization versus magnetic field sweeps are shown in the ordered state at
100 G for Q ( ), intermediately quenched (m), and G states (A) of
(a) ~200 nm particles and (b) ~27 nm particles. The coercive fields, Hc,
strongly depend upon the size of the particle, as shown in the expanded
views for (c) ~200 nm particles and (d) ~27 nm particles.


153


3
2


0
E




-31
-3


(c)


).2























S0.04
~ 0.1 -
S0.02 -
00[ (c). (d) n
0.0 I I 0.00
0.0 0.5 1.0 1.5 2.0 0.0 0.2 0.4 0.6 0.8 1.0
Time (Days) Time (Days)

Figure 5-18. Relaxation of magnetization in quenched states of K-Co-Fe. (a) The time
dependence of XT in 5 kG is shown for the Q state of the ~200 nm particles
for 120 K (*), 130 K (m), and 140 K (A). (b) The time dependence of XT in
5 kG is shown for the Q state of the -27 nm particles for 120 K (*), 100 K (m),
and 160 K (A). Using the XT data, the active HS fraction, nHS nHso, can be
extracted and fit (-) to extract the HS-LS mixing parameter, nHL ( ), and the
semi-empirical spin-crossover parameters, which are shown for (c) ~200 and
(d) ~27 nm particles. In order to fit the ~27 nm particle data, an additional
parameter for the strain due to 'reduced' states must be added to reproduce
the relaxation. Details of models used for the fits are explained in the text.


154














S 0 .0 0 AA 2 0 2 5
(?o 5? 10 15 20 25 E E
0.2 b TAA
0 1.5

0.0a
00.0
5 10 15) 20 25 30 0 5 10 15 20 25 30 35 40
/(K) T (K)

Figure 5-19. Magnetic ordering of photoinduced states in K-Co-Fe. The temperature
dependence of the FC and ZFC DC susceptibility, X, are shown in the
ordered state at 100 G for ground states (m) and photoinduced (E) of
(a) -200 nm particles and (b) -27 nm particles. Clear increases in the
magnetic signals after photoirradiation are present. The arrows point out
photoinduced and ground state magnetic signals, most likely due to
inhomogeneity of photoirradiation. (inset) The FC susceptibility normalized to
the low temperature limit is shown to more clearly display the changing
ordering temperatures.


155










12
(a)
7l0
E
8-

-U6

)- 4-


0123
0 1 2 3
H (T)


2 (c)
.10


0
-2 -
-3*


12


E
8


326


*0 I ***
4 5 6 7 0 1 2 3 4 5 6 7
H (T)

3
2 (d)



-2.
S-1



0.2 0.4 -0.4 -0.2 0.0 0.2 0.4
H (T)


Figure 5-20. Magnetization versus field of photoinduced states for K-Co-Fe. The
T = 2 K magnetization versus magnetic field sweeps are shown in the ordered
state at 100 G ground states (m) and photoinduced (E) of (a) ~200 nm
particles and (b) ~27 nm particles. The coercive fields, Hc, strongly depend
upon the size of the particle, and to illustrate this clearly zoomed plots are
shown for (c) ~200 nm particles and (d) ~27 nm particles. Lines are guides
for the eyes.


156




















0.0 0.2 0.4 0.6 0.8 1.0
nHS

Figure 5-21. Linearization of modeling. The value of XT as a function of the high-spin
fraction (nHs), calculated using mean-field theory and ligand field theory and a
plausible set of parameters for 100 K and 300 K shows approximately linear
behavior.


Table 5-7. Magnetic properties of quenched potassium cobalt hexacyanoferrate
nanoparticles and bulk powder.
Batch Diameter (nm) Tc" (K) Tc' (K) Tc" (K) Hc" (G) Hc (G) Hc" (G)

bulk 200 38 12.3 12.4 13.8 130 165 200

nano 27.4 5.7 12.4 15.0 1500 2200



Table 5-8. Magnetic properties of photoinduced potassium cobalt hexacyanoferrate
nanoparticles and bulk powder.
Batch Diameter (nm) Tc" (K) Tc (K) Hc" (G) Hc (G)

bulk 200 38 12.3 19.2 130 690

nano 27.4 5.7 12.4 15.7 1500 2440


157









19 -I I' I I I I 3.0 I I I I I I I I I
2.5 nanoparticles
18 o
18 (a) 2 (b)
17 2.0
Bulk
16 nanoparticles 1.5
15 11.0
14 -
13 / 0.5 Bulk
12 I 0.0
15 20 25 30 35 40 45 50 55 15 20 25 30 35 40 45 50 55
HS (%) HS (%)

Figure 5-22. Ordering temperatures and coercive fields of batches in different
macroscopic states. (a) The dependence of the coercive field upon HS,
showing clear increase in coercivities for the nanoparticles, as well as an
increase in the coercivity of the P state as compared to the Q state. (b) The
dependence of the magnetic ordering temperature upon HS, showing an
increase in the ordering temperature of the P state as compared to the Q
state, as well as a suppression of the magnetic ordering temperature in the
nanoparticles.

(a) G (b)




OG


Q 4 nQ 40 n





Figure 5-23. Microscopic schema based upon all data. A plausible microscopic picture
is shown of the different low temperature ground state (G), quenched
state (Q) and photoinduced state (P) in (a) the bulk powder and (b) the
nanoparticles, where in the latter, the states are prefixed with "n." In the bulk,
the similar ordering temperatures of the G and Q states imply similar domain
structure, as opposed to the increased ordering temperature of the P state,
which implies a larger domain structure. In the nanoparticles, the nG versus
nQ states are analogous to the bulk G and Q states, however in the nP state,
the domains are larger giving an increased ordering temperature, but one that
is reduced compared to the bulk P state. These schematics can be
compared to those by other workers [68].


158









CHAPTER 6
THIN FILMS OF PRUSSIAN BLUE ANALOGUES

6.1 Introduction

From a technological standpoint, magnetic thin films are of a high interest due to

their applications in memory storage. While modern devices utilize mainly metals and

alloys, the study of magnetization in thin films of coordination compounds is continuing

to provide new twists on magnetic thin films [10] [29] [76] [93] [94] [98-112]. Aside

from the benign conditions necessary for synthesis, compared to metallurgy, the ability

for magnetic properties to be tuned with external stimuli makes these systems

especially attractive. For example, Prussian blue analogues (PBAs),

AjM1k[M2(CN)6]r/nH20 (Figure 6-1), have been shown to display interesting behavior

when in thin film geometries [10] [76] [93] [94] [100] [102].

In this chapter, a close look is taken at the magnetic properties of PBA thin films

generated by a sequential adsorption technique, Figure 6-2. This synthesis technique is

attractive due to fine thickness control and the ability to change transition metal centers

in a straightforward fashion. Previously, Park et al. had seen anisotropy in

photoinduced magnetization for thin films of a RbjCok[Fe(CN)6]r nH20 Prussian blue

analogue and arrived at a plausible schematic explanation for the effect [100]. However,

the RbjCok[Fe(CN)6]r nH20 is complicated due to the ability for iron and cobalt ions to

have multiple stable oxidation states, as well as the orbital angular moment contribution

to the net magnetization. Therefore, additional systems should be studied with

microscopic probes to understand these films on a more fundamental level.

The hypothesis of this work is that when films are oriented at different directions

with respect to an applied magnetic field, Figure 6-3, different magnetic susceptibilities


159









will be observed due to the geometry of the samples. To this end, a series of nickel

hexacyanochromate films were studied in the greatest detail, but other transition metal

analogues were also studied. The main probe utilized was a SQUID magnetometer,

however to get a more detailed microscopic understanding of the effects observed in

the magnetization, UV-Vis, FT-IR, EMR, and XRD, among other probes, were employed.

The results of these studies are to be presented in the following chapter. Portions of

this work have already been published in Dr. Justin E. Gardner's thesis [93], and the

main results are to be submitted for future publication.

In Section 6.2, experimental work on thin films of Rbo.7Ni4.o[Cr(CN)6]2.9- nH20 thin

films is presented, including magnetization and magnetic resonance experiments. In

Section 6.3, experimental data resulting from studying additional Prussian blue

analogue films are given. The source of anisotropy in the different systems, as well as

ion dependence, is discussed (Section 6.4) and possible future experiments are

mentioned.

6.2 Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 thin films

The material chosen for an in-depth study of the thin film geometry, having a

Melinex solid support, was the rubidium nickel hexacyanochromate Prussian blue

analogue, RbjNik[Cr(CN)6]r nH20. The reasons for choosing this material are three-fold.

First, the samples are robust for months after synthesis, with little detectable changes in

their materials properties. Second, RbiNik[Cr(CN)6]r nH20 has a convenient magnetic

ordering temperature that is quite high for coordination networks, varying between 60

and 90 K, depending upon the stoichiometry of the sample [54]. Finally, the single-ion


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ground states are well defined, having stable oxidation states and no first-order orbital

angular momentum.

6.2.1 Sample Characterization

To begin, thin films of nickel hexacyanochromate were characterized with

scanning electron microscopy to determine the chemical formula, atomic force

microscopy to determine the thickness and surface morphology, infrared spectroscopy

to determine the cyanide stretching frequencies, and ultraviolet and Visible electron

spectroscopy to determine the electronic structure of the ions. Results of these studies

will be presented in the following sections, to set a frame of reference for magnetization,

resonance, and x-ray scattering experiments.

6.2.1.1 Chemical composition

Chemical composition was determined using scanning electron microscopy (SEM).

Films were cut into squares of 1 cm2, mounted onto a metal puck, and coated with a

thin film of carbon to enhance conductivity. The results gave chemical formulas of

Rbo.7Ni4.o[Cr(CN)6]2.9.nH20 for the films, and additional details can be found in

Dr. Justin E. Gardner's thesis [93].

6.2.1.2 AFM

Detailed atomic force microscopy (AFM) studies of thin films of different Prussian

blue analogues were performed by Dr. Justin E. Gardner, as detailed in his thesis [93].

Briefly, the nickel hexacyanochromate analogue shows a linear dependence of

thickness upon the number of deposition cycles, and an approximately linear

dependence of the root-mean-square (RMS) roughness upon the number of deposition

cycles, Figure 6-4. The curious dip in roughness at 20 cycles is reproducible, and likely


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dependent upon the inherent structural coherence of the material under the given

growth conditions.

6.2.1.3 FT-IR

Fourier transform infrared (FT-IR) spectroscopy measurements provide

information about the structure of cyanometallate networks, based upon the energies of

the cyanide stretches. Room temperature FT-IR was performed on a K3Cr(CN)6

precursor, an 80 cycle Rbo.7Ni4.o[Cr(CN)6]2.9.nH20 thin film, and a

Rbo.7Ni4.o[Cr(CN)6]2.9.nH20 powder. While the films could be mounted directly, the

powders were measured in diluted KBr solid solutions The cyanide stretches associate

with the Ni2+-NC-Cr3+ unit of the bulk powder and thin films were observed to be similar,

Figure 6-5. The sharp peak in the precursor also appears to be present in both the thin

films and the powder samples.

6.2.1.4 UV-Vis

Electronic spectroscopy in the Visible and ultraviolet (UV-Vis) provides important

information about the electronic structure of a K3Cr(CN)6 precursor, an 80 cycle

Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 thin film and a Rbo.7Ni4.o[Cr(CN)6]2.9.nH20 powder. Room

temperature UV-Vis spectroscopy shows d-d transitions associated with the molecular

building blocks of the materials, Figure 6-6, namely Cr(CN)6 and Ni(NC)6. Ligand field

multiple calculations were performed using the methods described in Chapter 3. Two

clear transitions are present in all samples, one for Ni2+ at 17,100 cm-1 corresponding to

a 3A2g(F) 3Tlg(F) type transition and one for Cr3+ at 26,600 cm-1 corresponding to a

4A2g(F) 4T2g(F) type transition. In the precursor, an additional transition at

32,600 cm- can be resolved. In the PBA film and powder, a less resolvable 3A2g(F)

3T2g(F) type transition for Ni2+ can be seen near 10,000 cm-1. There is no clear


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evolution from precursors, to bulk, to film, possibly due to the large line widths. All

samples show ground states consistent with quenched orbital angular momentum.

6.2.2 Magnetization

A commercial magnetometer from Quantum Design was used for the DC-SQUID

measurements. Powder samples were mounted in diamagnetic gelcaps, thin film

samples were either stacked in boxes or measured individually in a straw without

additional mounting. Commercial straws were used as a diamagnetic sample rod to

allow translation of the sample through the SQUID magnetometer detector coils.

6.2.2.1 DC susceptibility in 100 G

The low field magnetization as a function of temperature for the bulk

Rbo.7Ni4.o[Cr(CN)6]2.9 nH20 and thin films of Rbo.7Ni4.o[Cr(CN)6]2.9 nH20 are shown in

Figure 6-7, with both parallel and perpendicular orientations with respect to the applied

magnetic field shown for the film samples. Bulk material is expressed per mole and film

samples are expressed per unit area. The ZFC data were obtained after cooling in zero

applied field from 300 K, while the FC data were taken after cooling in 100 G from 300 K.

All samples show a change in the inflection of the magnetization versus temperature,

indicative of the three dimensional magnetic order known to exist between 60-90 K,

depending upon stoichiometry [54]. The thin film samples show clear anisotropy

between the susceptibility in the parallel and perpendicular orientations.

6.2.2.2 DC magnetization in 40 kG

The temperature dependence of the high field magnetization for the bulk

Rbo.7Ni4.o[Cr(CN)6]2.9 nH20 and thin films of Rbo.7Ni4.o[Cr(CN)6]2.9 nH20 are shown in

Figure 6-8, with both parallel and perpendicular orientations with respect to the applied

magnetic field shown for the film samples. Bulk material is expressed per mole and film


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samples are expressed per unit area. For these measurements in H = 40 kG, the ZFC

and FC data were found to lie on top of each other. This measuring field is particularly

relevant for comparison with f ~ 116 GHz microwave magnetic resonance experiments.

All samples show the usual increased ordering temperature and a more gradual slope

of the onset compared to the low field measurements. Even at these high fields, the

thin film samples show clear anisotropy between the susceptibility in the parallel and

perpendicular orientations.

6.2.2.3 DC magnetization field dependence

The field dependence of the DC magnetization at T = 2 K for bulk

Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 and thin films of Rbo.7Ni4.o[C(CN)6]2.9 nH20 are shown in

Figure 6-9, with both parallel and perpendicular orientations with respect to the applied

magnetic field shown for the film samples. Bulk material is expressed per mole and film

samples are expressed per unit area. All samples show clear saturation indicative of

ferromagnetic interactions in the complex. Thin films show anisotropy persisting up to

the largest available field of 70 kG.

6.2.2.4 Magnetization Process

The field dependence during the magnetization process, H = 0 to H = 2.5 kG, was

mapped at many temperatures, from 10 K to 70 K in 10 K increments. This series of

scans was performed on the bulk powder of Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 (Figure 6-10),

40 cycle Rbo.7Ni4.o[Cr(CN)6]2.9 nH20 thin films (Figure 6-11), and 400 cycle

Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 thin films (Figure 6-12), with both parallel and perpendicular

orientations with respect to the applied field measured for the films.


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6.2.2.5 DC magnetization angular dependence

In order to determine the functional form of the angular dependent magnetic

anisotropy, in-situ rotation measurements were performed in a SQUID magnetometer

using a custom made rotation probe [28]. Measurements were performed on 40 cycle

and 400 cycle thin films of Rbo.7Ni4.o[Cr(CN)6]2.9- nH20 at T = 10 K and H = 40 kG, Figure

6-13. These field and temperature values were chosen to be compared to f ~ 116 GHz

microwave electron magnetic resonance experiments. Both films show clear anisotropy,

with an in-plane easy axis.

6.2.3 Electron Magnetic Resonance

To further probe the magnetic transitions in the Rbo.7Ni4.o[Cr(CN)6]2.9-nH20

magnets, and particularly the anisotropic thin films, electron magnetic resonance

experiments were performed on powders and films. Continuous wave absorption

measurements were performed using a resonant cavity technique at the NHMFL in

Tallahassee. Temperature dependence, angular dependence and frequency

dependence of the microwave absorption will be presented in the following sections.

Previously, EMR researchers at the NHMFL studied powders of the charge transfer

Mn-Fe Prussian blue analogue [112bis].

6.2.3.1 EMR temperature dependence

The temperature dependence of the microwave absorption for a

Rbo.7Ni4.o[Cr(CN)6]2.9nH20 powder was measured at a continuous frequency of

~116 GHz, Figure 6-14. One clear absorption peak can be seen, with an effective

g-factor of 2.05. The full-width half-maximum of the line-width, the position, and the

area of the peak all show dependence on the magnetic order parameter, Figure 6-15.

More specifically, the change in the shape of the curves, near 90 K, is the temperature


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at which the magnetization curves also have a change in shape, which has been

assigned to the development of long-range magnetic order in the samples.

The temperature dependence of the microwave absorption for a

Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 40 cycle film was measured perpendicular to the applied

magnetic field at f ~ 116 GHz, Figure 6-16. In contrast with the powder, two clear

absorption peaks can be seen, with effective g-factors of 1.97 and 2.05 at 10 K. These

shifts in effective g-factors (and those in the following paragraphs) may not actually be

associated with changing g-factors, but are merely stated to give an additional frame of

reference for the magnitude of the shifts of the lines. The full-width half-maximum of the

line-widths, the position of the peaks, and the area of the peaks all show dependence

on the magnetic order parameter, Figure 6-17. Just as in the powder resonance

experiment, the lines are showing changes in behavior near 90 K, which is the magnetic

ordering temperature of the sample for external fields of 40 kG.

The temperature dependence of the microwave absorption for a

Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 40 cycle film was measured parallel to the applied magnetic

field at f ~ 116 GHz, Figure 6-18. Similar to the perpendicular orientation, two clear

absorption peaks can be seen. However, effective g-factors of 2.11 and 2.05 are

observed at 10 K. The full-width half-maximum of the line-widths, the position of the

peaks, and the area of the peaks all show dependence on the magnetic order

parameter, Figure 6-19, with a change in shape of the temperature dependence

happening near the magnetic ordering temperature of 90 K.

The temperature dependence of the microwave absorption for a

Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 400 cycle film was measured perpendicular to the applied


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magnetic field at f ~ 116 GHz, Figure 6-20. Similar to the 40 cycle film measured

perpendicularly, two peaks are seen, with effective g-factors of 1.97 and 2.05 at 10 K.

The full-width half-maximum of the line-widths, the position of the peaks, and the area of

the peaks all show dependence on the magnetic order parameter, Figure 6-21, in the

same manner as the thinner films.

The temperature dependence of the microwave absorption for a

Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 400 cycle film was measured parallel to the applied

magnetic field at f ~ 116 GHz, Figure 6-22. Similar to the 40 cycle film measured

parallel to the field, two peaks are seen, with effective g-factors of 2.11 and 2.05 at 10 K.

The full-width half-maximum of the line-widths, the position of the peaks and the area of

the peaks all show dependence on the magnetic order parameter, Figure 6-23, in the

same manner as the thinner films.

6.2.3.2 EMR angular dependence

The angular dependence of the microwave absorption was measured for 40 cycle

thin films of Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 in ~116 GHz radiation at a temperature of 10 K,

Figure 6-24, which is well within the magnetically ordered state for the material. Angle

dependence of the peak position and line-width show a planar anisotropy, Figure 6-25

(a) and (b), which cannot be resolved above the ordering temperature, Figure 6-25 (c).

The angular dependence of the microwave absorption was measured for

400 cycle thin films of Rbo.7Ni4.o[Cr(CN)6]2.9nH20 in ~116 GHz radiation at a

temperature of 10 K, Figure 6-26, which is well within the magnetically ordered state for

the material. Analogous to the angle dependence of the 40 cycle films, the 400 cycle

films show planar anisotropy in the peak position and line-width, Figure 6-27 (a) and (b),

which cannot be resolved above the ordering temperature, Figure 6-27 (c).


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6.2.3.3 EMR frequency dependence

The angular dependence of the microwave absorption was measured for 400

cycle thin films of Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 in 50 GHz radiation at a temperature of

10 K, Figure 6-28, which is well within the magnetically ordered state for the material.

The observed behavior is reminiscent of the f ~ 116 GHz data, again showing planar

anisotropy in the peak position and line-width, Figure 6-29 (a) and (b), with similar

splitting of the lines at both frequencies, Figure 6-29 (c).

6.2.4 X-ray Diffraction

X-ray diffraction experiments are currently underway. After room temperature

experiments were performed at MAIC with the standard setup, samples were sent to

Professor Stefan Kycia at the University of Guelph, and his research group is in the

process of looking for structural distortions as a function of temperature.

6.3 Additional Prussian Blue Analogue Thin Films

In addition to the detailed study of Rbo.7Ni4.o[Cr(CN)6]2.9 nH20 thin films, an array

of 3-d block transition metal Prussian blue analogue thin films were studied. Films were

synthesized with the sequential adsorption technique, using Co2+, Ni2+, Cu2+ and Zn2+

for the divalent nitrogen coordinated metal site and hexacyanometallic Cr3+ and Fe3+ for

the trivalent, carbon coordinated site of the network, Table 6-1. All thin films

synthesized display magnetic anisotropy, which can be tuned depending upon the

constituent metals. Details of magnetization studies on each material, as well as

additional modeling and experimental probes will be discussed in the following sections.

6.3.1 Rbo.6Co4.0[Cr(CN)6]2.9-nH20 Thin Films

Thin films of Rbo.6Co4.o[Cr(CN)6]2.9-nH20 generated with 200 sequential adsorption

cycles on a Melinex solid support were characterized by SEM to obtain their chemical


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formula [93]. Similar to the nickel based films, these materials can be thought of as

consisting of Co(NC)6 and Cr(CN)6 molecular building blocks. The single-ion ground

states of these building blocks are important to understand the magnetization of the

sample, and be calculated using ligand field multiple calculations with known

spectroscopic and nephelauxetic parameters, Figure 6-30 [5]. The results show a Co2+

ground state with S = 3/2 and unquenched orbital angular momentum and a Cr3+ ground

state with S = 3/2 and no orbital angular momentum contributions.

The low field magnetization as a function of temperature for thin films of

Rbo.6Co4.o[Cr(CN)6]2.9 nH20 are shown in Figure 6-31, in both parallel and perpendicular

orientations with respect to the applied magnetic field. The ZFC data were obtained

after cooling in zero applied field from 300 K, while the FC data were taken after cooling

in 100 G from 300 K. The films show a change in the inflection of the magnetization

versus temperature, indicative of the three dimensional magnetic order known to exist

near 23 K, depending upon stoichiometry [60]. In addition, a clear anisotropy in the low

field susceptibility is present below the magnetic ordering temperature.

6.3.2 Rbo.7Cu4.o[Cr(CN)6]2.9-nH20 Thin Films

Thin films of Rbo.7Cu4.o[Cr(CN)6]2.9-nH20 generated with 200 sequential adsorption

cycles on a Melinex solid support were characterized by SEM to obtain their chemical

formula [93]. The single-ion ground states have calculated using ligand field multiple

calculations with known spectroscopic and nephelauxetic parameters, Figure 6-32 [5].

The results show a Cu2+ ground state with S = 1/2 and unquenched orbital angular

momentum and a Cr3+ ground state with S = 3/2 and no orbital angular momentum

contributions.


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The low field magnetization as a function of temperature for thin films of

Rbo.7Cu4.o[Cr(CN)6]2.9-nH20 are shown in Figure 6-33, in both parallel and perpendicular

orientations with respect to the applied magnetic field. The ZFC data were obtained

after cooling in zero applied field from 300 K, while the FC data were taken after cooling

in 100 G from 300 K. The films show a change in the inflection of the magnetization

versus temperature, indicative of the three dimensional magnetic order. In addition, a

clear anisotropy in the low field susceptibility is present below the magnetic ordering

temperature.

Temperature dependent ultraviolet and Visible spectroscopy of an 80 cycle

Rbo.7Cu4.o[Cr(CN)6]2.9-nH20 thin film on a quartz solid support was performed to study

the electronic structure of the Cr3+ ion. The highly transmitting quartz was used instead

of Melinex, which has many UV-Vis transitions that make transmission experiments

impossible with the available setups. An absorption peak at ~24,000 cm-1 may be

assigned to a 4A2g(F) 4T2g(F) type transition on the chromium ion. A shift and

sharpening of the absorption line can be seen in the background subtracted spectra of

the film, Figure 6-34 (a), and even more clearly when taking differences between room

temperature and low temperature scans, Figure 6-34 (b).

6.3.3 Rbo.3Zn4.0[Cr(CN)6]2.8-nH20 Thin Films

Thin films of Rbo.3Zn4.o[Cr(CN)6]2.8 nH20 generated with 200 sequential adsorption

cycles on a Melinex solid support were characterized by SEM to obtain their chemical

formula [93]. The single-ion ground states have calculated using ligand field multiple

calculations with known spectroscopic and nephelauxetic parameters, Figure 6-35 [5].

The results show a Zn2+ ground state with S = 0 and no orbital angular momentum and

a Cr3+ ground state with S = 3/2 and no orbital angular momentum contributions.


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The field dependence of the DC magnetization at T = 2 K for thin films of

Rbo.3Zn4.o[Cr(CN)6]2.8- nH20 are shown in Figure 6-36, with both parallel and

perpendicular orientations with respect to the applied magnetic field shown for the film

samples. These films show anisotropy persisting up to the largest available field of

70 kG. As these materials are paramagnetic, the standard low field susceptibility plots

were not made, because the background of the Melinex solid support is the same order

of magnitude as the paramagnetic Prussian blue analogue signal.

6.3.4 Rbo.9Ni4.o[Fe(CN)6]2.8-nH20 Thin Films

Thin films of Rbo.9Ni4.o[Fe(CN)6]2.8-nH20 generated with 200 sequential adsorption

cycles on a Melinex solid support were characterized by SEM to obtain their chemical

formula [93]. Similar to the hexacyanochromate based films, these materials can be

thought of as being made up of Ni(NC)6 and Fe(CN)6 molecular building blocks. The

single-ion ground states of these building blocks are important to understand the

magnetization of the sample, and be calculated using ligand field multiple calculations

with known spectroscopic and nephelauxetic parameters, Figure 6-37 [5]. The results

show a Ni2+ ground state with S = 1 and no orbital angular momentum and a Fe3+

ground state with S = 1/2 and unquenched orbital momentum contributions.

The low field magnetization as a function of temperature for thin films of

Rbo.7Ni4.o[Fe(CN)6]2.9.nH20 are shown in Figure 6-38, in both parallel and perpendicular

orientations with respect to the applied magnetic field. The ZFC data were obtained

after cooling in zero applied field from 300 K, while the FC data were taken after cooling

in 100 G from 300 K. The films show a change in the inflection of the magnetization

versus temperature, indicative of the three dimensional magnetic order. In addition, a


171









clear anisotropy in the low field susceptibility is present below the magnetic ordering

temperature.

6.3.5 Rbo.7Co4.o[Fe(CN)6]2.8-nH20 Thin Films

Thin films of Rbo.7Co4.o[Fe(CN)6]2.8 nH20 have already been studied in detail, both

for their photoinduced anisotropy [10] and for the thickness dependence of that

photoinduced anisotropy [103]. The photoinduced magnetism in the cobalt-iron

analogue is due to bistabilities of oxidation states in the material, which may be

changed with external stimuli. Therefore, two oxidation states for each ion must be

considered when interpreting experimental data, and when performing ligand field

multiple calculations, Figure 6-39 (a). Known values of spectroscopic and

nephelauxetic parameters give energy level schemes for the Co3+/Fe2+ dark state and

the Co2+/Fe3+ light state. Results give well separated diamagnetic ground states in the

Co3+/Fe2+ dark state, and Co2+ S = 3/2, Fe3+ S = 1/2 ground states with unquenched

orbital angular momentum for the Co2+/Fe3+ light state.

Although parallel and perpendicular magnetization and photoinduced

magnetization has already been reported, the development of a SQUID magnetometer

probe that is capable of performing in situ photoexcitation and sample rotation allowed

for a new twist on the previous results [28]. Rotation of films modifies the magnetization

of the photomagnetic state, Figure 6-39 (b) and (c).

6.3.6 Rbo.5Cu4.o[Fe(CN)6]2.7-nH20 Thin Films

Thin films of Rbo.5Cu4.o[Fe(CN)6]2.7-nH20 generated with 200 sequential adsorption

cycles on a Melinex solid support were characterized by SEM to obtain their chemical

formula [93]. The single-ion ground states have calculated using ligand field multiple

calculations with known spectroscopic and nephelauxetic parameters, Figure 6-40 [5].


172









The results show a Cu2+ ground state with S = 1/2 and unquenched orbital angular

momentum and a Fe3+ ground state with S = 1/2 and unquenched orbital angular

momentum contributions.

The low field magnetization as a function of temperature for thin films of

Rbo.5Cu4.o[Fe(CN)6]2.7 nH20 are shown in Figure 6-41, in both parallel and perpendicular

orientations with respect to the applied magnetic field. The ZFC data were obtained

after cooling in zero applied field from 300 K, while the FC data were taken after cooling

in 100 G from 300 K. The films show a, upturn in the magnetization versus temperature,

indicative of the three dimensional magnetic order. In addition, a clear anisotropy in the

low field susceptibility is present in the magnetically ordered state.

6.3.7 Rbo.5Zn4.o[Fe(CN)6]2.8-nH20 Thin Films

This film is expected to be diamagnetic, and various measurements have shown

that to be true. It is useful to have a diamagnetic sample available if magnetically quiet

capping layers are to be used for heterostructured films.

6.4 Discussion

The principle result of the thin film studies is that films of Prussian blue analogues

show magnetic anisotropy not present in the bulk solid state. This anisotropy manifests

itself in magnetization and microwave spectroscopy experiments. However, no clear

trends are observed in vibrational infrared and UV-Vis electron spectroscopy

measurements. The functional form of the fundamental source of the anisotropy has

been determined, and good candidates for the fundamental source of the anisotropy are

presented. The state of what is known and unknown about the newfound anisotropies

will be discussed in the following.


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6.4.1 Rbo.7Ni4.0[Cr(CN)6]2.9-nH20 Thin Films

Since the Rbo.7Ni4.o[Cr(CN)6]2.9 nH20 Prussian blue analogues are of primary

interest due to their high ordering temperature and their large magnetic anisotropy, they

were studied in the most detail. Four types of anisotropy are possible in the Prussian

blue analogue films, namely magnetostatic, superexchange, magnetocrystalline, and

g-factor [113]. Frequency dependent magnetic resonance experiments have ruled out

g-factor anisotropy, Figure 6-29 (c). Temperature dependent x-ray diffraction has

shown no departure from cubic structure, even in the ordered stated, making

magnetocrystalline effects highly unlikely [113bis]. Superexchange anisotropy cannot

be present as no change in the ordering temperature is observed for different film

orientations, a fact that will be reiterated in the following discussion section.

Consequently, anisotropy magnetostatic effects, especially those associated with

demagnetization within magnetic domains, will be the focus of the present discussion of

the anisotropy in the Rbo.7Ni4.o[C(CN)6]2.9. nH20 films.

The low field temperature dependence of the magnetization is where the

magnetic anisotropy of the films was first observed as a difference between the

magnetic susceptibility when the surface of the films was oriented parallel or

perpendicular to the applied magnetic fields. The simplest Hamiltonian for a magnetic

system with superexchange coupling is

K = -2 JijSiSj + gBH Si 6.1
i,j=n.n

where J is the superexchange parameter, S is an electron spin, g is the parameter that

scales the magnetic field dependence of the energy, uB is the Bohr magneton, and H is


174









the applied field. Anisotropy may be introduced into Equation 6.1 through either the first

or second term, however, changing the superexchange parameter, J, changes the

ordering temperature, which is not experimentally observed. On the other hand, a

modification of the internal field due to demagnetizing effects may scale the

magnetization without modifying the ordering temperature. A demagnetizing field

consists of modifying the applied field to be

Heff = Hiab-NM 6.2

where Heff is the effective field, Hlab is the applied field, M is the magnetization, and N is

the demagnetizing factor. Immediately, this functional form is attractive because it is

observed that once magnetic order begins to take place in the films while cooling, an

anisotropy develops that depends upon the macroscopic magnetization. To begin, the

magnetic fields produced by a 1 cm x 1 cm x 2 pm film, analogous to the 400 cycle

nickel hexacyanochromate film, may be considered from a theoretical perspective.

Using the magnetic charge formalism [3], calculations of internal fields are

straightforward for both parallel and perpendicular orientations of films. In the absence

of electric current, Maxwell's equations for magnetostatics have a reduced number of

degrees of freedom that allow all of the information for magnetic fields to be tabulated in

the same way as electric fields so that vector quantities are simply gradients of scalars.

Thus,

d
Hz = M 6.3



where pM is the magnetic potential, which is determined from an analogue of Poisson's

equation, and magnetic dipoles may be modeled as dumb-bells with a positive magnetic


175








charge on one end and a negative magnetic charge on the other. First, the potential of

the oppositely, magnetically charged upper and lower surfaces of the film is calculated,

and then a derivative is taken to find the field along the z-axis, so


b/2 a/2 b/2 a/2
d f f poModxdy f f poModxdy
H dz 6.4
-b/2 -a/2 4- ox2 + y+ + )2 -b/2 -a/2 4-n X2 +y2+ (Z-


where for parallel orientations, a = 1 cm, b = 1 pm, and c = 1 cm, and for perpendicular

orientations, a = 1 cm, b = 1 cm, and c = 1 pm, with the permanent magnetization along

the z-axis for both cases. These calculations are performed using a permanent

magnetization with no applied field, although applied fields are easily considered using

the superposition principle. (Note: for this subsection calculations will use SI units by

which [B] = T, [H] = A/m, and [to] = 4n x 10-7, and volume susceptibility for clarity, and

the units are explained in Appendix A.) In the perpendicular orientation, the field

produced is small, Figure 6-42 (a) and (b) and the H field is actually negative within the

sample and equal to Mo, Figure 6-42 (c). Therefore, the perpendicular orientation is

equivalent to N = 1 in Equation 6.2. The integral and subsequent differentiation for the

parallel orientations is less well behaved and was computationally more expensive, so

less data were calculated. However, it is clear that the B fields produced are much

larger in the parallel orientation compared to the perpendicular orientation,

Figure 6-43 (a), and that there is no demagnetizing H field in the parallel orientation,

Figure 6-43 (b). Therefore, in the parallel orientation, N = 0 in Equation 6.2 and there is

no modification of the external field.


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The effect of the demagnetizing fields on the magnetization can be seen by

assuming a functional form for the magnetization in 100 G, namely,

M = owHeff 6.5

where iow is the low field volume susceptibility. This assumption of linearity is

customary for fields that are small compared to temperature, and is clearly justified by

the magnetization data, Figures 6-10, 6-11, and 6-12. Plugging Equation 6.5 into

Equation 6.2 yields

M = Xiow (Hab NM) 6.6

Equation 6.6 can be solved self-consistently to give an expression for the magnetization

in the presence of a demagnetizing field,

M iowHlab 6.7
M-
1 + iowN
Therefore, to calculate the magnetization in the presence of a demagnetizing field, one

needs to estimate liow and N. The low field susceptibility may be found by using

M
low = Hef 6.8


to fit the magnetization data between 20 G (2 mT) and 100 G (10 mT) and N may be

kept as a fitting parameter. Practically, to estimate the low field susceptibility, the

powder data may be used, under the assumption that the magnetization at 70 kG (7 T)

and 2 K reaches the theoretical saturation value of

Msat gBS 2.05(9.27 x10-24 J/T)(4 x1+3 x1.5)
V V =( 1.47 x105 A/m2 6.9
V V (10.33 x01- m)3


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First attempts to fit the low field magnetization data to N = 0 for the parallel orientation

and N = 1 for the perpendicular orientation failed. In retrospect, this is not surprising as

domains are present in low fields. Therefore, a more general spheroid may be

considered [113], where the demagnetization of the principle axes are related by

N +N +N = 1 6.10

or for Nx= N,, as in the case of the film geometry,


2N + N = 1 6.11

With these conditions, impressive agreement of calculated and measured magnetization

of the films at low field is seen for N, = 0.07 and N, = 0.86, Figure 6-44.

In a similar way as for the H = 100 G temperature dependence, the H = 40 kG

temperature dependent magnetization may be modeled. First, it is clear that the

increase in the ordering temperature is reproduced in the mean-field model, due to

increased spin-spin correlations in a high magnetic field, Figure 6-45. An important

difference between the two regimes is the functional form of the field dependence of the

magnetization. For the high field limit,

M = Mo + XhighHeff 6.12

where Mo and Xhigh are empirical parameters, and the introduction of a demagnetizing

factor, Equation 6.2, gives

M = Mo+Xhigh (Hlab NM) 6.13

and solving self-consistently as before yields

M + Xhigh Hlab 6.14
+Mhig
1 + Xhigh N


178









To find Mo and Zhigh, the magnetization of the powder data scaled to the theoretical

maximum (Equation 6.9) was fit between 20 kG and 40 kG, Figure 6-45 (a) and (b).

The magnetization data for the thin films in the high field limit can then be fit using

NI = 0 and N_ = 1, Figure 6-45 (c), however an additional scaling of the %high by a factor

of 5 is necessary to achieve the magnitude of magnetic anisotropy observed, which may

simply be experimental uncertainty in the determination of Zhigh.

It is also worth noting the effect of film thickness on the anisotropy of the low field

susceptibilities, Figure 6-46 (a), where the anisotropy persists, even in the thickest films,

albeit at slightly reduced levels. Perhaps even more startling is that anisotropy is

present even in the caustically manufactured spin cast films [93]. The reduction in

anisotropy may be attributed to a departure from an ideal thin film geometry, since films

of increasing thickness also have an increasing presence of powder-like features on the

surface, Figure 6-46 (b) and (c) [93].

The complex cyanides are ideal candidates for resonance experiments because

their high resistivity eliminates complications coming from potential skin effects that

plague metallic magnets. For the resonance data, the powdered sample is a logical

place to begin as it has the simplest spectrum. The presence of only one line in the

powder spectrum of Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 indicates that only one species is in

resonance, which might be a well-coupled Ni2+ and Cr3+ line, a single Cr3+ line, or a Ni2+

line. The possibility that the signal is due to an impurity can be ruled out because the

area of the line as a function of temperature, which is proportional to the magnetization,

tracks the measured magnetization in the SQUID magnetometer at 40 kG. In addition,

the full-width-half-maximum of the absorption peak decreases as the temperature is


179









lowered, until the magnet becomes ordered, where the line width increases. These

changes can be understood quite well in terms of the simple expression

AH = 2a Heff 6.15

where a is the damping parameter and Heff is the effective field seen by the spin [113].

The general decrease above the ordering temperature may be attributed to reduction in

spin-lattice relaxation as the number of phonons decreases with temperature. The

increase in width of the line from the onset of magnetic order and down to the coldest

temperatures may be associated with an increased spin-spin relaxation as the sample

enters more deeply into the magnetically ordered state. The position of the peak as a

function of temperature may be analyzed in a similar demagnetization formalism as the

magnetization data was analyzed. To begin, the same functional form of the

demagnetization effects is assumed

Hx = Hlab,x + NxMx Hy = Hab,y + NyMy Hz = Hlab,z + NzMz 6.16

where for resonance experiments all principle axes of the magnet and magnetic field

are important. Using the equations of motion for a spin in an applied magnetic field

along the z-axis, the resonance condition may be written [113],

W2 =g22[Hab,z +(Ny- Nz)Mz][Hab, z +(Nx- N)z]M 6.17

For a perfect sphere, Nx = Ny = Nz = 1/3, so the resonance condition should be isotropic

and have no magnetization dependence,


WO,sphere = gloHlab,z 6.18

In practice, there may be small deviations from spherical symmetry for the powder, and

the resonance condition may be written as


180










W0,powder= g 0o[HIab,z 6.19

where 6 takes care of deviations from spherical symmetry. For the powder data of

Rbo.7Ni4.o[Cr(CN)6]2.9.nH20, a shift of ~100 G is observed in the fully magnetized state

compared to the paramagnetic state, and this is consistent with a value of 6 ~ 0.05.

Compared to the powder, things begin to get more complicated in the resonance

experiments of the thin films. The variation of the line thickness with temperature is

reminiscent of the bulk powder, however, the most striking feature is the evolution of

two resonance conditions in the films when cooled below the magnetic ordering

temperature. The two lines, most clearly seen in the parallel and perpendicular

orientations, cannot reproduce the powder line when integrated over the angular degree

of freedom, as can be clearly demonstrated by observing that the lines in the powder

are actually sharper than the lines in the films, Figure 6-47 (a). It is also worth noting

that the magnitudes of the absorption lines for different orientations may be different

due to the complicated coupling term between the modes of the cavity and the sample.

More clearly than the magnetic susceptibility measurements, this experimental fact

confirms that there is magnetic anisotropy in the films that is not present in the bulk.

Restated, the thin film samples have a magnetic anisotropy that is induced by their thin

film character.

A quantitative treatment of the resonance in the thin films can be carried out using

the demagnetization formalism used to analyze the magnetization data of the thin films

and the general resonance condition in Equation 6.17. For a film oriented perpendicular

to an applied field, Nx = Ny = 0 and Nz = 1 in the limit that the film is uniformly

magnetized, and the resonance condition for the perpendicular orientation is


181









WO,perp = glo [HIab,z M 6.20

For the parallel orientation of the film, Nx = Nz = 0 and Ny = 1 in the limit that the film is

uniformly magnetized, and the resonance condition for the parallel orientation is


WO,par = gJo[Hlabz(Habz + Mz)2 6.21

Just as in the modeling of the magnetization data, Mz will be taken from the powder

magnetization scaled to reach the theoretical value at 2 K and 70 kG (7 T). The results

of these calculations can be seen in Figure 6-47 (b). The quantitative agreement

between model and the position of the largest absorption line is striking. One may

argue that qualitative agreement breaks down above approximately 100 K, and this may

be explained as a breakdown of the applicability of the model. One potential problem

as temperature increases is the loss of the single-domain state, which gave Nx = Ny = 0

and Nz = 1 in the perpendicular orientation and Nx = Nz = 0 and Ny = 1 in the parallel

orientation.

Another potential discrepancy between the model and experimental data is the

existence of two absorption lines in the resonance experiments of the films. While

demagnetization may produce inhomogeneous internal fields for some samples, giving

rise to such a behavior, calculations of the internal field showed a high degree of

homogeneity for the chosen geometries, Figure 6-42 and Figure 6-43. However, the

same powder-like component that explains the observed thickness dependence of the

magnetization can also explain the smaller absorption line that does not shift. A portion

of the Prussian blue analogue material in the film actually exists as a powder-like phase


182









on the surface, and therefore has a weak temperature dependence in the resonance

experiments that is different than the temperature dependence of the film-like material.

The angular dependence of the resonance line provides further confirmation of the

uniaxial, ~ sin2(0), nature of the anisotropy that was deduced from the angular

dependent magnetization measurements. Just as in the temperature sweeps, two lines

are seen, one that moves with angle and another that remains still. The angular

dependence of the line width shows wider lines in the perpendicular compared to the

parallel orientations, consistent with previous resonance experiments on thin films [105].

Finally, the resonance data for the thin films looks similar whether 40 cycle films or

400 cycle films were used. One key difference, however, is that in the thin films, a

Lorentzian fits the line-shape better, while in the thicker films, a Gaussian fits the

line-shape better. This observation implies that the disorder increases as the films

become thicker and is consistent with the magnetization data.

6.4.2 Additional Prussian Blue Analogue Thin Films

Taking what has been learned from the Rbo.7Ni4.o[Cr(CN)6]2.9- nH20 films as a fixed

point for understanding magnetostatic effects in these materials, the other films may be

analyzed to see if additional anisotropy may be present.

The Rbo.6Co4.o[Cr(CN)6]2.9 nH20 film shows a large anisotropy in the low field

magnetization, reminiscent of the Rbo.7Ni4.o[Cr(CN)6]2.9- nH20 film. While both films are

ferromagnets, one difference between them is that Ni2+ is a L = 0, S = 1 ion, while Co2+

is a L = 1, S = 3/2 ion (in the unquenched limit). If the anisotropy was magnetostatic,

one would expect a larger anisotropy in the Rbo.6Co4.o[Cr(CN)6]2.9 nH20 compound

compared to the Rbo.7Ni4.o[Cr(CN)6]2.9-nH20, as the moments in


183









Rbo.6Co4.o[Cr(CN)6]2.9-nH20 are larger. In fact, the nickel based film is found to have a

larger angular dependence of susceptibility. In addition, Co2+ is known to be a highly

anisotropy ion, due to the large amount of orbital momentum. Therefore additional

measurements may be required to fully understand the cobalt film.

The Rbo.7Cu4.o[Cr(CN)6]2.9.nH20 film is in a different class than the Ni or Co films,

as a tetragonal structure has been resolved in x-ray diffraction measurements [106].

This distortion from simple cubicity is understood within the confines of the classic

Jahn-Teller distortion that lowers the electronic energy by elongating or compressing

the coordinating octahedral, also relieving the orbital degeneracy of the upper crystal

field split doublet [5]. In light of this distortion, it is not surprising that thin films would be

anisotropic, as both single ion anisotropy of the Cr3+ ions, and g-factor anisotropy of the

Cu2+ are expected to be first order effects in such a compound, in addition to

magnetostatic effects. Further evidence changes in the electron energy levels in the

ordered state are shown in the temperature dependent UV-Vis spectroscopy studies.

The final hexacyanochromate material studied was the zinc analogue. As Zn2+

has a full d-shell, the Rbo.3Zn4.o[Cr(CN)6]2.8- nH20 material remains paramagnetic down

to the lowest temperature measured, which was 2 K. Anisotropy is observed, which

becomes enhanced at higher fields when the magnetization is greater, showing the

familiar dependence upon the sample magnetization. Therefore, the

Rbo.3Zn4.o[Cr(CN)6]2.8 nH20 film anisotropy is probably magnetostatic.

In addition to chromate cyanides, iron cyanides were also studied. The

magnetization of Rbo.9Ni4.o[Fe(CN)6]2.8- nH20 thin films is actually quite similar to

Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 thin films. The only difference, besides the ordering


184









temperature, is the magnitude of the difference between the parallel and perpendicular

low field susceptibilities. The Rbo.7Co4.o[Fe(CN)6]2.8-nH20 thin films actually add

confusion instead of enlightenment. It was found with the new probe setup that rotation

in field causes a change in the magnetization, the mechanism of which is still unclear.

The Rbo.5Cu4.o[Fe(CN)6]2.7 nH20 films are also different than the other films

studied, and of acute interest for this reason. Unlike all other analogues, the

Rbo.5Cu4.o[Fe(CN)6]2.7-nH20 has the less common out-of-plane anisotropy. Although it

has yet to be observed, the Rbo.5Cu4.o[Fe(CN)6]2.7-nH20 analogue is expected to have a

tetragonal structure analogous to the Rbo.7Cu4.o[Cr(CN)6]2.9- nH20. However, since

both copper and iron ions are S = 1/2, no zero-field splitting are possible. Therefore,

the only first order effects should be g-factor anisotropy, which may explain the out-of-

plane anisotropy. It is much more difficult to explain out-of-plane anisotropy with

demagnetization as the source. One possibility would be if the

Rbo.5Cu4.o[Fe(CN)6]2.7-nH20 film had a spire-like structure on the surface. However,

atomic force microscopy measurements actually show Rbo.5Cu4.o[Fe(CN)6]2.7. nH20 films

to be the smoothest of all studied [93].

6.5 Conclusions

In conclusion, all Prussian blue analogue thin films studied show magnetic

anisotropy. The Rbo.7Ni4.o[Cr(CN)6]2.9 nH20 films have been studied in the most detail

and show a uniaxial, sin2(0) dependence to the anisotropy that can be well understood

within the context of a demagnetization model. Other noteworthy results are an

out-of-plane anisotropy in Rbo.5Cu4.o[Fe(CN)6]2.7-nH20, and a field rotation induced

change in susceptibility in Rbo.7Co4.o[Fe(CN)6]2.8. nH20. It is important that each


185








transition metal film be examined in detail, as multiple sources or magnetic anisotropy
are likely to be present. Additional probes of the films might include Lorentz microscopy
to analyze the domain structure of the films, or dispersive x-ray spectroscopy to look for
anisotropy of the fundamental parameters, specifically in spin-orbit coupling.



U* u D B M1
0 0 i Q ,

0 0 M2

"C 0c

0 0 0 0 A


0 o C o
S0 Nc cN 0 H20

Figure 6-1. Prussian blue analogue structure. Prussian blue analogues have a
chemical formula of AyM1k[M2(CN)6]. nH20, j,k,l and n are constrained by
charge balance. Cations (A = Cs+, Rb+, K+, Na+) are incorporated based
upon the number of M2 vacancies, which are coordinated by water as shown.


186











e


e I
o o +/i


T


I I

Cr
^r- (r


j2+ Cr3+rrN
iZaq) Cr3N) 6(aq)
& Rb+

"x" cycles


Figure 6-2. The multiple sequential adsorption method that can be used for generating
thin films of Prussian blue analogues. For example, synthesis of a Ni-Cr
analogue would consist of using a solid support (typically Melinex) and
immersing it in an aqueous solution of hexacyanochromate, and into separate
solution of Rb1 and Ni2+ ions, thus depositing approximately one layer of Ni-
CN-Cr. After each cycle, a simple washing with water is essential to remove
excess ions. This process can be iterated to yield films of varying
thicknesses and morphologies.


HPAR


HPERP


0=0 = 45


0=90


Figure 6-3. Different orientations of the magnetic thin films with respect to the applied
magnetic field are expected to have different behavior.


187


A












1200 -
(a) 1 0 (b)
I 75 -
U)U 800
I 50
0 400
of 25- / -
SI I 0 I I

0 20 40 60 0 50 100 150 200
Deposition Cycles Deposition Cycles

Figure 6-4. AFM of thin films. (a) Multiple sequential adsorption film roughness
generally increases with number of cycles, but the 20 cycle film is smoothest.
This minimum in the roughness is due to the inherent structural coherence of
the material and is reproducible. (b) The thickness of the multiple sequential
adsorption films is directly proportional to the number of cycles. The red line
is a fit yielding 5.7 nm/cycle.





1.0

0
c 0.8
S0.6
S0.4
S0.2
0 ------------- -
z 0.0
2300 2250 2200 2150 2100 2050
Energy (cm1)
Figure 6-5. Room temperature FT-IR spectroscopy measurements of the cyanide
stretches present in Ni-Cr materials. Cyanide stretches are seen in the
K3Cr(CN)6 precursor (---), Rbo.7Ni4.o[Cr(CN)6]2.9.nH20 powder (-), and a
80 cycle thin film of Rbo.7Ni4.o[Cr(CN)6]2.9nH20 (-). The K3Cr(CN)6 sample
shows one clear stretch at 2131 cm1. The thin film and powder samples
show peaks at 2173 cm-1 and 2131 cm-1 with the latter being associated with
free cyanides at the surface and coordinating vacancies.


188


. I I I









Energy (eV)
1.5 2.0 2.5 3.0 3.5 4.0 100,o 000 b E ( 12
90,000 (b)
1.2 (a) r(eN )6 80,000 2 .-. 10
Ni[Cr(CN)6]nH20 film on quartz 5 80,000 10
1.0 -- Ni[Cr(CN)6]nH20 powder i 70,000 T (lv)
0.8 -
CO 0 50,000 6
- 0.6 40,000 1- E E
S0.4 30,000 I
< L 20,000 2
0.2 10,000- E 3
0.0 ------ -- ... 0 0
10,000 15,000 20,000 25,000 30,000 35,000 CrJ(CN)6 Nil(NC)6
Energy (cm1)

Figure 6-6. UV-Vis spectroscopy of Ni-Cr materials. (a) Room temperature UV-Vis
spectroscopy measurements of the d-d transitions present in 10 mM Cr(CN)6
precursor (---), 10 mM Rbo.7Ni4.o[Cr(CN)6]2.9nH20 powder with background
subtracted by using the functional form of a diamagnetic
Rbo.5Zn4.0[Fe(CN)6]2.8'nH20 (-), and an 80 cycle thin film of
Rbo.7Ni4.o[Cr(CN)6]2.9.nH20 with background subtracted by using the
functional form of a diamagnetic Rbo.5Zn4.0[Fe(CN)6]2.8. nH20 film (-).
(b) Using the transitions shown in Figure 6-6 (a), a multiple calculation can
be performed to show the electronic energy levels for the Ni2+ and Cr3+ ions in
the Prussian blue network. Chromium energy levels are shown for (i) no spin-
orbit coupling and (ii) using reported values of spin-orbit coupling for the free
ion [5]. Nickel energy levels are shown for (iii) no spin-orbit coupling and (iv)
using reported values of spin-orbit coupling for the free ion [5].




3.0 4.0
6 2(a) 2.5/"M... (b) 3.5 ( (c)
4 o 2.5
S1.5 2.0
S1.0 1.5

0.00 *.. .-.-m-m 0.0 ....-.-. .
0 204 6 8 1 120140 20 40 60 80100120140 0 20 40 60 80 100120140
0 20 40 60 80 100 120 140
(K) T(K) T(K)
Figure 6-7. Temperature dependent magnetization of Ni-Cr materials. The temperature
dependence of the low field, 100 G, magnetizations are shown for
Rbo.7Ni4.o[Cr(CN)6]2.9nH20 (a) powder, zero-field cooled (ZFC) (E) and
field-cooled (FC) (m), (b) 400 cycle thin film, parallel ZFC (E), parallel FC (i),
perpendicular ZFC (E) and perpendicular FC (m), (c) 40 cycle thin film,
parallel ZFC (o), parallel FC (m), perpendicular ZFC (E) and
perpendicular FC (m). Connecting lines are guides to the eye.


189



















0 20 40 60 80 100120140
T(K)


4 :: (b)





0 20 40 60 80 100120 140


1.0
0.8 (c)
1 0.6 %
o0.4
0 0.2
0.0 '**-..-
0 20 40 60 80 100 120 140
T(9K


Figure 6-8. Temperature dependent magnetization of Ni-Cr materials at high fields.
The temperature dependence of the high field, H = 40 kG, magnetizations
are shown for Rbo.7Ni4.o[Cr(CN)6]2.9nH20 (a) powder (m), (b) 400 cycle thin
film, parallel (m) and perpendicular FC (m), (c) 40 cycle thin film, parallel (m)
and perpendicular FC (m). Lines connecting discrete data points are guides
to the eye.


-]
4



2

1 (b)

0 20 40 60
H (kG)


1.00 ::.- ----........
.


0.50
0.25
00(c)
0.00
0 2 20 40 60
H (kG)


Figure 6-9. Field dependent magnetization of Ni-Cr materials. The field dependence
of the low temperature, 2 K, magnetizations are shown for
Rbo.7Ni4.o[Cr(CN)6]2.9nH20 (a) powder (m), (b) 400 cycle thin film, parallel (m)
and perpendicular FC (m), (c) 40 cycle thin film, parallel (m) and
perpendicular FC (m). The negative high field slope of the magnetization in
the 40 cycle film can be attributed to the diamagnetic substrate. Lines
connecting discrete data points are guides to the eye.


190


H (kG)













(a) 120 "10 K
0.8 I- I 'o n 30K
.H o .. 40K
o DO. .s
0.6-

E 0.4 ..-
b 060 K
0.2 .
o ., .O70 K
0.0 0g -.0 no o- o?0 D. _
0.0 0.5 1.0 1.5 2.0 2.5
H (kG)


(b) 1.0 K
M/H
0.8 30K
"E 40K
0.6 .-
.5 5.. .0K
0.4
60 K
0.2

0.0DDD
0.0 0.5 1.0 1.5 2.0 2.5
H (kG)


Figure 6-10. Magnetizing process of thin Ni-Cr film. Magnetization as a function of
external field for 40 cycle film of Rbo.7Ni4.o[Cr(CN)6]2.9 nH20 in (a) parallel and
(b) perpendicular orientations with respect to the applied magnetic field. Here
the field dependence of the low field data is fit to a linear model, M = y H,
where r = M/H. In a similar manner, the high field data can be fit to M = Mo +
XE H. These fits are relevant to understanding the magnetization process and
more specifically to understand the potential roles of demagnetizing fields in
the materials.


0.5 1.0 1.5 2.0 2.5
H(kG)


M/H g 30 K
40K
50 K

2 D o60K-


70K

0
0.0 0.5 1.0 1.5 2.0 2.5
H(kG)


Figure 6-11. Magnetizing process of thick Ni-Cr film. Magnetization as a function of
external field for 400 cycle film of Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 in (a) parallel
and (b) perpendicular orientations with respect to the applied magnetic field.
The meaning of the fitting lines are described in the figure caption of
Figure 6-10.


191


"Mo
c- 3
3
E


(,


S


20 K
M/H ..... .30 K
S D 040K
50 K

h? o o O O760K


7 o70K


0.0












M/H 10K
1.2 0 0K

0.9 50""

0.6 ..
0370K

0.0
0.0 0.5 1.0 1.5 2.0 2.5
H (kG)


Figure 6-12. Magnetizing process of Ni-Cr powder. Magnetization as a function of
external field for Rbo.7Ni4.0[Cr(CN)6]2.9-nH20 bulk powder. The meaning of the
fitting lines are described in the figure caption of Figure 6-10.


E 1.00

E 0.95

5 0.90


90 180 270 360
0 (degrees)


0 90 180 270 360
0 (degrees)


140 141 142 143 144 145 146 147 148
6 (degrees)


Figure 6-13. Angle dependence of magnetization in Ni-Cr materials. (a) Magnetization
as a function of angle, at T = 10 K and H = 40 kG, for (a) 400 cycle
Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 thin films and (b) 40 cycle
Rbo.7Ni4.o[Cr(CN)6]2.9nH20 thin films. Both data sets are raw magnetization
from the SQUID, without any additional processing. A detail of the 400 cycle
film data shows the discrete 1.50 steps used during rotation, as data was
taken continuously. The difference in the signal to noise for the two
measurements may due to the different amount of sample present in a 40
versus 400 cycle film.


192


E 4.10

S4.05

" 4.00


















C
30







E
c
1 -



. 30

(3


10K
20 K
30 K
40 K
50 K
60 K
70 K
80 K
90 K
100 K
110K
120 K


15K
25 K
35 K
45 K
55 K
65 K
75 K
85 K
95 K
105 K
115K
125 K


f 116 GHz


50


Figure 6-14. EMR lines of Ni-Cr powder. The field dependence of the microwave
transmission through the resonance cavity as a function of temperature,
measured at a constant frequency of ~116 GHz, for
Rbo.7Ni4.o[Cr(CN)6]2.9.nH20 powder. Here the data have been offset to more
clearly display the evolution of the line, and the data have not been adjusted
to reflect the temperature dependent resonance in the cavity.


1300 .
1200- (a)
"11o 00
g 1000
8 900.-

800) 2 6 8
500.

0 20 40 60 80 100 120 140


40420
40400
40380
S40360
140340
o 40320
' 40300
40280
40260
0


*
E*

U.
U
**
*


* m
*


20 40 60 80 100 120 140
T(K)


EU.




** ..
0 20 40 60 80 100 120 140
T(K)


Figure 6-15. Results of fitting EMR lines of Ni-Cr powder. Fitting the temperature
dependence in Figure 6-14 to a Lorentzian line yields (a) the full-width
half-maximum (FWHM) of the line, (b) the center position of the absorption
line and (c) the relative area of the absorption peak as a function of
temperature.


193


40

H (kG)













150 K
-130 K
*C ._ _________ ------------ 115 K
115 K
100 K
85 K
70 K
-- 55 K
40 K
0 -25 K
\-10K

U 7f~116 GHz
H
St 'PERP

S30.0 35.0 40.0 45.0 50.0

SH (kG)

Figure 6-16. EMR lines of Ni-Cr thin film perpendicular. The field dependence of the
microwave transmission through the resonance cavity as a function of
temperature, measured at f ~ 116 GHz, for Rbo.7Ni4.o[Cr(CN)6]2.9 nH20
40 cycle films perpendicular to the applied field. Here the data have been
offset to more clearly display the evolution of the line, and the data have not
been adjusted to reflect the temperature dependent resonance in the cavity.




3000
(a) bigline (b) 42000 (c)
2500 o little line big line
5,3 41500 o little line
S 2000 ,E

I0| o 40500 .
1000 D on o o o o E n E
E i& 40000 o .
0 20 40 60 80 100120140160 202 40 60 80 100 120140160 0 20 40 60 80 100120140160
T (K T(K) T (K
Figure 6-17. Results of fitting EMR lines of Ni-Cr thin film perpendicular. Fitting the
temperature dependence in Figure 6-16 to Lorentzian lines yield (a) the
full-width half-maximum (FWHM) of the lines, (b) the center positions and
(c) the relative area of both peaks as a function of temperature.











194





















0





E
C)

I30

^3


.0


150 K
130 K
115K
100 K
85 K
70 K
55 K
40 K
25 K
10K


35.0 40.0 45.0 50.0


U H (kG)
Figure 6-18. EMR lines of Ni-Cr thin film parallel. The field dependence of the
microwave transmission through the resonance cavity as a function of
temperature, measured at f ~ 116 GHz, for Rbo.7Ni4.o[Cr(CN)6]2.9 nH20
40 cycle films parallel to the applied field. Here the data have been offset to
more clearly display the evolution of the line, and the data have not been
adjusted to reflect the temperature dependent resonance in the cavity.


(a) 1600 i big line
1400 o o little line
-" 1200 o
| 1000
800 0
600 E o [ [
400 E
200
0 20 40 60 80 100 120 140160
T (K)


(b) 40400
40200
r 40000
39800
39600
39400
39200


n big line
o little line






20 40 60 80 100 120 140 160
T(K)


(c)


0 20 40 60 80 100120140160
T (K)


Figure 6-19. Results of fitting EMR lines of Ni-Cr thin film parallel. Fitting the
temperature dependence in Figure 6-18 to Lorentzian lines yield (a) the
full-width half-maximum (FWHM) of the lines, (b) the center positions and
(c) the relative area of both peaks as a function of temperature.


195


f- 116 GHz
SH AR















--15K
--25 K
--35K
--45K
--55K
--65K
-75 K
--85K
95K
110K
--130K
--150K


GHz


35.0 40.0 45.0
H (kG)


Figure 6-20. EMR lines of Ni-Cr thick film perpendicular. The field dependence of the
microwave transmission through the resonance cavity as a function of
temperature, measured at f ~ 116 GHz, for Rbo.7Ni4.o[Cr(CN)6]2.9 nH20
400 cycle films perpendicular to the applied field. Here the data have been
offset to more clearly display the evolution of the line, and the data have not
been adjusted to reflect the temperature dependent resonance in the cavity.


o
1 11 EP


20 40 60 80 10
T(K)


big line (b) 42000 bl
little line K 4 big line
41500 little line
S41000 E
S40500
40000 D ED E
& cccoxxoxxmcox o
39500
0120140160 0 20 40 60 80100 120140160
T(K)


(c)(K)







T(K)
T (K)


Figure 6-21. Results of fitting EMR lines of Ni-Cr thick film perpendicular. Fitting the
temperature dependence in Figure 6-20 to Gaussian lines yield (a) the
full-width half-maximum (FWHM) of the lines, (b) the center positions and
(c) the relative area of both peaks as a function of temperature.


196


(a) 2000

S1500

00

500
0























0






E
C-

I-

<35

(9


0.o


40.0

H (kG)


10K
20K
30K
40K
50K
60K
70 K
80K
90K
100K
110K
120K
140K


15K
25K
35K
45K
55K
65K
75 K
85K
95K
105K
115K
130K
150K


H PAR

f- 116 GHz

45.0


Figure 6-22. EMR lines of Ni-Cr thick film parallel. The field dependence of the
microwave transmission through the resonance cavity as a function of
temperature, measured at f ~ 116 GHz, for Rbo.7Ni4.o[Cr(CN)6]2.9 nH20
400 cycle films parallel to the applied field. Here the data have been offset to
more clearly display the evolution of the line, and the data have not been
adjusted to reflect the temperature dependent resonance in the cavity.


1400 40500
(a) Eo big line (b) (c)
1200 o o little line oco "
0 0 9 40000 Ennn l *
CU 00U
S1000 N0
l800 E c' 39500 DD
Sbig line U m.
600 Oc, 3 000 o little line ..

S20 40 60 80 100120140160 0 20 40 60 80 100 120 140160 0 20 40 60 80 0 120 140 160
T(K) T(K) T(K
Figure 6-23. Results of fitting EMR lines of Ni-Cr thick film parallel. Fitting the
temperature dependence in Figure 6-22 to Gaussian lines yield (a) the
full-width half-maximum (FWHM) of the lines, (b) the center positions and
(c) the relative area of both peaks as a function of temperature.


197


//

J
















t '-
03









O
E
u)




0
85


35 40 45 50

H (kG)


Figure 6-24. EMR lines of Ni-Cr thin film as a function of angle. The field dependence
of the microwave transmission through the resonance cavity as a function of
angle, measured at f ~ 116 GHz and T = 10 K, for Rbo.7Ni4.o[Cr(CN)6]2.9-nH20
40 cycle films. Here the data have been offset to more clearly display the
evolution of the line. The vertical lines delineate the extremum values of the
big line, labeled by the numbers near each line in units of kG.


42000 E big line
41500 little line E
41500
S41000 0
.40500 oo Ooo
S40000

U 39500 0
39000
0 30 60 90 120
Angle (0)


2600
big line
2400 o little line,
2200
S2000
1800
1600 IIr
L 1
1400
1200 -
0 50 100 150
Angle (B)


2 --




(c T= 15
2-,30 35 40 45 50
0 H (kG)


Figure 6-25. Results of fitting EMR lines of Ni-Cr thin film as a function of angle. Fitting
the temperature dependence in Figure 6-24 to Lorentzian lines yield (a) the
center positions and (b) the full-width half-maximum (FWHM) of the lines.
(c) The angular dependence above the ordering temperature. Here the data
have been offset to more clearly display the evolution of the line. The vertical
line delineates the peak position, labeled by the number at the top in units of
kG.


198






















0
O


C -

I 7

. 30.0
85


35.0 40.0 45.0

H (kG)


9(P
80
70
50
40
30
20
10
00
-10Y
-20
-30
-40
-50o

-70

-90
-100
-111P


Figure 6-26. EMR lines of Ni-Cr thick film as a function of angle. The field dependence
of the microwave transmission through the resonance cavity as a function of
angle, measured at f ~ 116 GHz and T = 10 K, for Rbo.7Ni4.o[Cr(CN)6]2.9-nH20
400 cycle films. Here the data have been offset to more clearly display the
evolution of the line. The vertical lines delineate the extremum values of the
big line, labeled by the numbers near each line in units of kG.


(a) 42000 n big line
41500 o little line
| 41000
S40500 E
40000O oOoooo ooo
39500
39000 D-]
38500
-120-90 -60 -30 0 30 60 90
Angle (0)


2200
(b)
2000
1800
1600
1400
-1
1200
-12


o big line
o little line


rjnr

0 -90 -60 -30 0 30 60 90
Angle (0)


(c) 40.1

E
-2



I- T = 150,l K ,I
30.0 35.0 40.0 45.0 50.0
B H(kG)


Figure 6-27. Results of fitting EMR lines of Ni-Cr thick film as a function of angle.
Fitting the temperature dependence in Figure 6-26 to Gaussian lines yield (a)
the center positions and (b) the full-width half-maximum (FWHM) of the lines.
(c) The angular dependence above the ordering temperature. Here the data
have been offset to more clearly display the evolution of the line. The vertical
line delineates the peak position, labeled by the number at the top in units of
kG.


199






























H (kG)


Figure 6-28. EMR lines of Ni-Cr thick film as a function of angle in lower field. The field
dependence of the microwave transmission through the resonance cavity as
a function of angle, measured at f ~ 50 GHz and T = 10 K, for
Rbo.7Ni4.o[Cr(CN)6]2.9.nH20 400 cycle films. Here the data have been offset to
more clearly display the evolution of the line.


19500
(a) 19000 En big line
190 o little line
18500
18000
0
S17500 ooooO Ooo O
S17000
16500 El
16000
-120-90 -60 -30 0 30 60 90
Angle (0)


(b) 2000
1800
1600
S1400
S1200
1000


big line
little line


-120 -90 -60-30 0 30 60 90
Angle (0)


3500
(C) 300----------o-------
S2500
2000
S1500
1000
S500
0
0 20 40
Resonance Field (kG)


Figure 6-29. Results of fitting EMR lines of Ni-Cr thick film as a function of angle in
lower field. Fitting the temperature dependence of the 400 cycle film shown
in Figure 6-28 to Lorentzian lines yield (a) the center positions and (b) the
full-width half-maximum (FWHM) of the lines. (c) The difference between
parallel and perpendicular resonance as a function of frequency shows the
lack of field dependence for the splitting of the line. This lack of field
dependence rules out g-factor anisotropy as the source of the observed
splitting. The dotted line is a guide to the eye.


200









Table 6-1. Molecular formulas of films measured and techniques used.
constituent ions chemical formula measurements

Co2+(S = 3/2) Cr+(S = 3/2) Rbo.6Co4.o[Cr(CN)6]2.9-nH20 magnetization

Cu2+(S = 1/2) Cr+(S = 3/2) Rbo.7Cu4.o[Cr(CN)6]2.9nH20 magnetization, UV-Vis

Zn2+(S = 0) Cr3(S = 3/2) Rbo.3Zn4.o[Cr(CN)6]2.8-nH20 magnetization

Ni2+(S = 1) Fe3+(S = 1/2) Rbo.9Ni4.o[Fe(CN)6]2.8 nH20 magnetization

Cu2+(S = 1/2) Fe3+(S = 1/2) Rbo.5Cu4.o[Fe(CN)6]2.7-nH20 magnetization, thickness
dependence, EMR

Co+(S = 3/2) Fe3+(S = 1/2) Rbo.7Co4.o[Fe(CN)6]2.8-nH20 magnetization,
photoinduced
magnetization, in situ
rotation

Zn 2(S = 0) Fe 2(S = 0) Rbo.5Zn4.o[Fe(CN)6]2.8- nH20 magnetization


100,000
90,000
80,000
70,000
S60,000
S50,000
;40,000
30,000
20,000
10,000
0


-
2T2,
T- (iii) (iv)
2Tg 2T2g 2E 2T
4g 2E
T 2
-2Ag :T 2E ------

SA2g "Tlg 2Tl
2T ---- 2 2T
2E 2Tg 2Eg 4T
4A2g 4Tg
Cr"(CN)6 Co"l(NC)6


Figure 6-30. Ligand field levels of Co-Cr. A multiple calculation can be performed to
show the electronic energy levels for the Co2+ and Cr3+ ions in the Prussian
blue network. On the left, chromium energy levels are shown for (i) no spin-
orbit coupling and (ii) using reported values of spin-orbit coupling for the free
ion [5]. On the right, cobalt energy levels are shown for (iii) no spin-orbit
coupling and (iv) using reported values of spin-orbit coupling for the free
ion [5].


201










12.5

10.0

7.5

E 5.0

- 2.5

0.0


0 10 20 30 40
T (K)

Figure 6-31. Magnetic susceptibility of Co-Cr thin film. The temperature dependence
of the low field, 100 G, magnetizations are shown for 200 cycle
Rbo.6Co4.0[Cr(CN)6]2.9-nH20 thin films, parallel ZFC (o), parallel FC (m),
perpendicular ZFC (w) and perpendicular FC (m). Here connecting lines of
orange and blue are guides to the eye.


100,000
90,000
80,000
70,000
60,000
S50,000
40,000
30,000
20,000
10,000
0


12

10

8.
a,
6

4
LU
2

0


Cull(NC)6


Figure 6-32. Ligand field energies of Cu-Cr. A multiple calculation can be performed to
show the electronic energy levels for the Cu2+ and Cr3+ ions in the Prussian
blue network. On the left, chromium energy levels are shown for (i) no spin-
orbit coupling and (ii) using reported values of spin-orbit coupling for the free
ion [5]. On the right, copper energy levels are shown for (iii) no spin-orbit
coupling and (iv) using reported values of spin-orbit coupling for the free
ion [5].


202


- E (I) II
-T
-~~ \- -
2T2,
2T 2T2 T
- T
2g 2E
2g Tg2
- VT
T9
(iii) (iv)
E -

- 4A2g 2E2g


Cr"(CN)6












1.5


0 1.0


a0.5


S0.0


0 20 40 60 80 100

T (K)


Figure 6-33. Magnetic susceptibility of Cu-Cr thin film. The temperature dependence
of the low field, 100 G, magnetizations are shown for 200 cycle
Rbo.7Cu4.o[Cr(CN)6]2.9-nH20 thin films, parallel ZFC (o), parallel FC (m),
perpendicular ZFC (E) and perpendicular FC (m). Here connecting lines of
orange and blue are guides to the eye.


E (eV)


2.48 2.73


3.22


2.73


E (eV)
2.98


3.22


E (cm1) E (cm1)


Figure 6-34. UV-Vis of Cu-Cr thin film. (a) Temperature dependent UV-Vis
spectroscopy measurements of an 4A2g(F) -* 4T2g(F) type transition on the
Cr3+ ion of an 80 cycle Rbo.7Cu4.o[Cr(CN)6]2.9 nH20 thin film on a quartz solid
support. (b) A difference plot of the UV-Vis absorption, displaying the
temperature dependent shift and sharpening of the line.


203


U
U


---m-m-m -


-
-













100,000
90,000
80,000
70,000
S60,000
50,000
;40,000
30,000
20,000
10,000
0


12

10

8.

6"

4
LU
2

0


Zn"(NC)6


Figure 6-35. Ligand field energies of Zn-Cr. A multiple calculation can be performed to
show the electronic energy levels for the Zn2+ and Cr3+ ions in the Prussian
blue network. On the left, chromium energy levels are shown for (i) no spin-
orbit coupling and (ii) using reported values of spin-orbit coupling for the free
ion [5]. On the right, zinc energy levels are shown for (iii) no spin-orbit
coupling and (iv) using reported values of spin-orbit coupling for the free
ion [5].


0.5


0

E
a
b
75


0.3

0.2

0.1

0.0-
0


*
-

-U
-U
I

I
I


/



20 40 60


H (kG)

Figure 6-36. Magnetization of Zn-Cr versus field. The magnetic field dependence of
the low temperature, 2 K, magnetizations are shown for
Rbo.3Zn4.o[Cr(CN)6]2.8- nH20 thin films in parallel (m) and perpendicular (m)
orientations. Here connecting lines of orange and blue are guides to the eye.


204


2E (I) (II)


T-

-yA
2T, 2T21

Ig 2? g 2E
g T g 4 E
T9
4T2g
2T2g
ET
2EA 2Tg
4 AA (iii) (iv)










100,000
90,000
80,000
70,000
S60,000
S50,000
;40,000
30,000
20,000
10,000
0


Fe "(CN)6


12

10

85



LU
6
4

2


Ni"(NC)6


Figure 6-37. Ligand field energies of Ni-Fe. A multiple calculation can be performed to
show the electronic energy levels for the Ni2+ and Fe3+ ions in the Prussian
blue network. On the left, iron energy levels are shown for (i) no spin-orbit
coupling and (ii) using reported values of spin-orbit coupling for the free ion
[5]. On the right, nickel energy levels are shown for (iii) no spin-orbit coupling
and (iv) using reported values of spin-orbit coupling for the free ion [5].


2.0

1.5

S1.0

S0.5

0.0


0 10 20 30
T (K)

Figure 6-38. Magnetic susceptibility of Ni-Fe thin films. The temperature dependence
of the low field, 100 G, magnetizations are shown for 200 cycle
Rbo.9Ni4.o[Fe(CN)6]2.8-nH20 thin films, parallel ZFC (o), parallel FC (m),
perpendicular ZFC (E) and perpendicular FC (m). Here connecting lines of
orange and blue are guides to the eye.


205


(1) (II)
-T T
-
T T E
2 T jg T2T g 2 E g

S- 2T




- T
^2T 3--
'2a^^^ 1T2a---------

23
T2g A 2a











(i) (ii)


(iii) (iv)


3.0

2.5

2.0

1.5

1.0

0.5

0.0


Dark state
ColFel


20,000


-* -- (a).
T- A -A
IT
T -A
2
ST,




Fe
-
- E -- --
E



AA 4T 2T Co
1~, T1 2, ,
=o0 40 =0 o 0


photoinduced state
Co Fef


II ------light----- II 1iIII.


(b)
.-

-3 0 5 90 95 100105
Time(minutes)
0 30 60 90
Time (minutes)
11 ||-----light---- || || Ill-lights

(c),





-30 0 30 60 90 120150180
Time (minutes)


Figure 6-39. Ligand field energy levels and rotational magnetism of Co-Fe. (a) A
multiple calculation can be performed to show the electronic energy levels for
the Co2+/Co3+ and Fe3+/Fe2+ ions in the Prussian blue network. On the left,
Co3+Fe2+ energy levels are shown for (i) no spin-orbit coupling and (ii) using
reported values of spin-orbit coupling for the free ions [5]. On the right,
Co2+Fe3+ energy levels are shown for (iii) no spin-orbit coupling and (iv) using
reported values of spin-orbit coupling for the free ions [5]. (b) Rotating the
Co-Fe powder after photoirradiation at 5 K and 100 G has little effect on the
magnetism. Inset: Detail showing the robustness of the magnetization with
respect to rotation. (c) Rotating the Co-Fe film after photoirradiation at 5 K
and 100 G reduces the magnetism. For (b) and (c), orientation and
photoirradiation is shown in the timeline above the graph.


206


15,000

E
10,000 o


5,000


0









100,000
90,000
80,000
70,000
' 60,000
, 50,000
S40,000
30,000
20,000
10,000
0


Fe "(CN)6


\~ \
- (i) (ii)
T T
T 2T
4T 2T,
2T 2T
2T 2

'hg 4T2g 2E 4Eg --
2T- 2T\ g 2' g

- T 'T 2-
2A- 2T 2T 2Eg
% (iii) (iv)

2g 2E2g


Cu"(NC)6


Figure 6-40. Ligand field energies of Cu-Fe. A multiple calculation can be performed
to show the electronic energy levels for the Cu2+ and Fe3+ ions in the Prussian
blue network. On the left, iron energy levels are shown for (i) no spin-orbit
coupling and (ii) using reported values of spin-orbit coupling for the free
ion [5]. On the right, copper energy levels are shown for (iii) no spin-orbit
coupling and (iv) using reported values of spin-orbit coupling for the free
ion [5].


-....E.
::::::::::.,







) 10 20 3(
T(K)


0.05
0.04
0.03


-0.01L
0


ME.M
nUi.n..







10 20 30
T(K)


Figure 6-41. Magnetic susceptibility of Cu-Fe thin films. The temperature dependence
of the low field, 100 G, magnetizations are shown for (a) 200 cycle
Rbo.5Cu4.o[Fe(CN)6]2.7-nH20 thin films and (b) 20 cycle
Rbo.5Cu4.o[Fe(CN)6]2.7-nH20 thin films, parallel ZFC (o), parallel FC (m),
perpendicular ZFC (E) and perpendicular FC (m). Here connecting lines of
orange and blue are guides to the eye.


207


12

10

85

6

4
LU
2

0


0.20

" 0.15

S0.10

b 0.05

r 0.00











8 (a)
6 -
' 4
2
0
-10 -5 0 5 10
distance from center of film (cm)


S-2 -1 0 1 2
distance from center of film (cm)


(c)


-0.51


-1.0 -
-2 -1 0 1 2
distance from center of film (pm)


Figure 6-42. Demagnetizing fields in films uniformly magnetized perpendicular to the
surface. (a) The change in the B field in and outside the 2 pm high film with
respect to the value at the center of the film, B[0] = 9.0031631 x 10-5 [oMo.
(b) The B field is small compared to toMo, and continuous across the film
boundary. (c) Inside the film, the H field opposes the magnetization with
maximum demagnetization, which is equivalent to N = 1 in Equation 6.2.


1.0 -
0.8- (a)
- 0.6
_ 0.4
m 0.2
00 0 0 oo u1
-20 -10 0 10 20


1.0
0.8
0.6
S0.4
0.2
0.0


(b)





-4 -2 0 2 4


distance from center of film (cm) distance from center of film (cm)
Figure 6-43. Demagnetizing fields in films uniformly magnetized parallel to the surface.
(a) The B field in and outside the 1 cm long film. (b) Inside the film, there is
no demagnetizing H field. This lack of demagnetization is equivalent to N = 0
in Equation 6.2.


208











(a)
8

6

4-

2-
0


0 50 100 150
T (K)


0 20 40 60 80 100
T (K)


Figure 6-44. Fitting magnetization of Ni-Cr thin films in low field. (a) The low field
volume susceptibility used for modeling magnetization with Equation 6.7.
(b) The magnetization at H = 100 G of a thin film of
Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 in parallel (-i-) and perpendicular (-i-)
orientations is modeled with mean-field theory (k), an anisotropic
superexchange constant ( ), mean-field theory with a demagnetizing
factor (-), and the raw parallel data adjusted with a demagnetizing factor to
approximate the perpendicular orientation ( ), where N, = 0.07 and N, = 0.84.


209








5
10 E0-
b (a) 4 (c)
5 E-

0-
1p.5 E 2-
r -. (b) : a
1.0 1
S0.0 _.0 =_ _-

0 50 100 150 0 20 40 60 80 100 120 140
T (K) T (K)

Figure 6-45. Fitting magnetization of Ni-Cr thin films in high field. (a) The high field
volume susceptibility (raw from fitting experimental data without any additional
factor of 5) and (b) the high field constant magnetization used for modeling
magnetization with Equation 6.14. (c) The magnetization at H = 40 kG of a
thin film of Rbo.7Ni4.o[Cr(CN)6]2.9 nH20 in parallel (-i-) and perpendicular (-i-)
orientations is modeled with mean-field theory (k), an anisotropic
superexchange constant ( ), mean-field theory with a demagnetizing
factor ( ), and the raw parallel data adjusted with a demagnetizing factor to
approximate the perpendicular orientation ( ), where N, = 0 and N, = 1.


1.0 O 40cydeH (a -
0O.0 40 cyde H
S0.8 A 200cyde Hi

S0.6 V 200cyde H
H- 400cydeH11
110 400 cydle H
0.4 o000oo < 400 cycle Hi
d lo,. spin cast H
0.2 .6 "8g' 0 spin cast H

0.0

0 20 40 60 80 100 120 140
T(K)


NHI BS


Figure 6-46. Thickness dependence of thin films. (a) The thickness dependence of the
anisotropy measured in 100 G at 2 Kfor Rbo.7Ni4.o[Cr(CN)6]2.9 nH20 thin films.
(b) An AFM measurement showing powder-like features growing on the
surface of a 30 cycle, thin film. (c) An SEM measurement showing
powder-like features growing on the surface of a 200 cycle, thick film.


210










0.5 42.0
(a) 41 .. (b)
i 0.0 41.5
t 41.0
S-0.5 4
S.- 40.5
-1.0 0
-_ 40.0
S-1.5 39.5 --'39
-2.0 39.0
30 35 40 45 50 0 50 100 150
H (kG) Temperature (K)


Figure 6-47. EMR lines of Ni-Cr thin films and powder. (a) A superposition of 40 cycle
Rbo.7Ni4.o[Cr(CN)6]2.9-nH20 thin film EMR lines in parallel (orange) and
perpendicular (blue) orientations, and a bulk powder line. All data was taken
at 10 K and f ~ 116 GHz. (b) Observed peak positions for powder (h),
400 cycle film parallel to the applied field (m), and 400 cycle film perpendicular
to the applied field (m). Peak positions using the demagnetizing formalism for
the 400 cycle film parallel to the applied field (e), and 400 cycle film
perpendicular to the applied field (n).


211









CHAPTER 7
HETEROSTRUCTURES OF PRUSSIAN BLUE ANALOGUES

One of the most compelling features of molecular-magnets is their ability to be

rationally designed, giving both increased understanding of fundamental physical

phenomenon as well as new or enhanced physical effects. Prussian blue analogues

(PBAs) are the class of molecular-magnets in which metals are bridged by cyanide in

order to generate simple cubic structures.

The RbaCob[Fe(CN)6]c-nH20 (Co-Fe) PBA has been of particular interest

because it shows long-lived persistent photoinduced magnetism at temperatures below

nominally 100 K, with long-range magnetic order appearing below ~ 20 K [1].

Unfortunately, from a practical standpoint, 20 K is still undesirable because of the need

for expensive cryogenics to reach such cold temperatures. In attempt to address this

problem, Co-Fe was incorporated into heterostructures including other Prussian blue

analogues. First, studies of an atomically mixed ternary Prussian blue analogues

containing Co-Fe will be presented, and then thin film heterostructures containing layers

of Co-Fe will be presented. The results demonstrate a novel method to increase the

photoinduced ordering temperature and to tune the sign of the photoinduced

magnetization.

7.1 Solid Solutions of Cobalt Hexacyanoferrate

7.1.1 Introduction

The ability to purposefully tune magnetic properties of synthetic materials has

motivated progress in the area of molecule based magnets. A new class of magnetic

coordination compounds was opened when long-range magnetic order was discovered

in Prussian blue [61] [62] [63] [64], and when its atomic and magnetic structures were


212









elucidated [65] [55], leading to the notion that properties could be controlled by

changing transition metal ions within the parent cubic framework. Binary metal Prussian

blue analogues (PBAs), A M'[M(CN)6]- nH20 (where A is an alkali metal ion, M' and M

are transition metal ions, and the values of a, 3, and n depend upon the stoichiometry)

and similar materials, have been the subject of extensive research due to their diverse

and exciting range of magnetic properties [59] [60] [114]. Room temperature magnetic

order [54] [115-117], photoinduced magnetization [1] [57] [68] [81] [82] [87]

[90] [118-124], thermal charge transfer induced spin transitions (CTIST) [83] [84] [125],

photoinduced tuning of magnetic coupling [126], anisotropic photoinduced magnetism in

thin films [10] [100-103], and linkage isomerism [127-132], are among the

phenomena observed in this class of compounds.

The photoinduced magnetism, discovered by Hashimoto and coworkers in

Ko.2Co1.4[Fe(CN)6]-6.9H20, has proven to be one of the more fascinating features of

PBAs [1]. Briefly, at low enough temperatures, incident light can cause an electron to

transfer from Fe2+ (LS, S = 0) to Co3+ (LS, S = 0), yielding long-lived metastable Fe3+

(LS, S = 1/2)-CN-Co2+ (HS, S = 3/2) pairs that couple antiferromagnetically and give

rise to an observed increase in magnetization. An impressive body of work has

elucidated the details of the thermal and optical CTIST effects in this series of

compounds, AaCo[Fe(CN)6]p-nH20 (A = Na, K, Rb, Cs) [1] [57] [68] [81] [82] [87]

[90][118-124].

There has been previous interest in the so-called ternary metal PBAs of the form

AM"i-xM'x[M(CN)6]p-nH20 (where M" and M' occupy similar lattice sites as determined

by x) stemming from the additional effects that are sometimes observed, such as


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photoinduced magnetic pole inversion [133] [134], dilution of spin-crossover [135], and

magnets having different types of Neel order [136-138]. This ability to substitute

different transition metals in the compound is due to the similar lattice parameters of the

cubic binary PBAs. In addition to novel function, new insight into the underlying

physical properties of the compounds can be obtained through a study of these mixed

PBAs.

In this section, the ternary Prussian blue analogue of the form

NaaNil-xCox[Fe(CN)6]p-nH20 will be discussed. The proposed structure is one in which

a cubic iron sublattice interpenetrates a cubic sublattice containing a statistical mixture

of cobalt and nickel ions, Figure 7-1. The use of the sodium cation allows for clear

thermal hysteresis [84], and the presence of Ni2+ gives rise to ferromagnetic

superexchange pathways between Ni2+ and Fe3+ with an exchange constant that is

similar in magnitude to the Co2+-NC-Fe3+ exchange [139]. The role of the

ferromagnetic species in the photodecrease can be illustrated by considering the Ni rich

(x > 0) and Ni poor (x 5 1) substitution regimes, Figure 7-1 (a) and (b). Although a

similar mix of materials yielding Coo.75Nio.75[Fe(CN)6]-6.8H20 has already been reported

elsewhere [140], the thermally and optically induced bistabilities of the spin states are not

present due to the stoichiometry.

The studies reported in this section show NaaNil-xCox[Fe(CN)6]p-nH20 bulk

powder displays photoinduced magnetism that can be either positive or negative

depending upon the cobalt fraction, x = [Co]/([Co] + [Ni]), the applied magnetic field, and

the temperature. These observations are only the second report of a photoinduced

decrease in magnetization in this class of photoswitchable coordination compounds.


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For the first time, the sign of the photoinduced change in magnetization can be

controlled by tuning chemical composition. Additionally, an x dependence is found for

the ordering temperature, the coercive field, the amount of photoactive material, the

magnitude of the thermal CTIST, and the width of the thermal CTIST hysteresis loop.

Qualitative understanding of these results can be obtained through the use of simple

molecular field theories.

It is important to note that these materials are analyzed with the assumption of

antiferromagnetic interactions in the binary NaaCo[Fe(CN)6]p-nH20 material, as is

precedented by the majority of the literature. However, additional modeling of the

binary materials by the author suggests that the current assignment of ferrimagnetism is

ambiguous. As a result, the microscopic origin of the JCoFe may be due to other

interactions, such as single-ion effects on the nickel. Alternatively, the current

understanding of the material may be correct, and the JCoFe energy may indeed be due

to superexchange. This situation is one in which more experimental data is required,

and upcoming neutron scattering experiments are scheduled to address these issues.

This work was published, in part, in the Journal of the American Chemical Society [141].

Those sections contained within the JACS article are copyright of the ACS (copyright

release form in Appendix C).

7.1.2 Synthesis and Chemical Composition

Prussian blue analogues NaaNil_xCox[Fe(CN)6]p-nH20 were synthesized by Dr.

Justin E. Gardner by varying the relative cobalt fraction, [Co2+(aq)]/([Co2+(aq) + [i2+(aq)]),

present during synthesis from 0.0 to 1.0 in steps of 0.2 [93] [141]. Energy dispersive

x-ray spectroscopy (EDS) was performed on a JOEL 2010F instrument to establish


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transition metal composition. Samples were deposited as methanol suspensions onto

400 mesh copper grids with holey carbon support films, purchased from Ted Pella, Inc.

Combustion analysis to determine carbon, hydrogen, and nitrogen (CHN) percentages

was performed by the University of Florida Spectroscopic Services laboratory. The

resultant chemical formulas given in Table 7-1 were determined from EDS, FT-IR, and

CHN analyses. The Co, Ni, and Fe ratios were explicitly taken from the EDS results,

because the signals for these ions are clean and reproducible. The percentages of C, H,

and N were taken directly from combustion analyses. By using the combustion results

for the hydrogen content, the amount of oxygen was calculated by assuming all

hydrogen and oxygen are in H20 molecules.

7.1.3 Transmission Electron Microscopy

Transmission electron microscopy (TEM) was performed on a JOEL 2010F

instrument to establish particle size. Samples were deposited as methanol suspensions

onto 400 mesh copper grids with holey carbon support films, purchased from Ted Pella,

Inc. Particle sizes were determined from the TEM images by measuring the edge

length of more than 50 particles for each composition through the use of ImageJ

imaging software [73]. For identical synthesis protocols, excepting the ratio of Co2+ to

Ni2+, the equilibrium size of the particles evolves continuously, with particles becoming

smaller as more Ni2+ ions are introduced into the lattice, Figure 7-2. Finally, some

control over particle size for a given x value is possible by varying the concentration and

the amount of time that the particles are in solution before isolation. However, there are

no observable changes in the magnetization, for given values of x, as a function of size

within the regime studied [8] [72] [142].


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7.1.4 Infrared Spectroscopy

A Thermo Scientific Nicolet 6700 spectrometer was used to record Fourier

transform infrared (FT-IR) spectra, using KBr pellets or powder samples spread

between NaCI plates. The relative ratios of Fe2+ and Fe3+ were estimated by fitting and

subsequently integrating the cyanide stretching peaks in the FT-IR spectra associated

with each species. Extinction coefficients of the cyanide stretching bands of the ternary

Prussian blue analogue compounds were estimated from those measured for Ni2+Fe3+

and Ni2+Fe2+ Prussian blue analogue species [93] [141]. The FT-IR spectrum of the

pure cobalt hexacyanoferrate displays peaks at 2163, 2120, 2090, and 2040 cm-1,

corresponding to the cyanide stretches of the Co2+Fe3+ (HS), Co3+Fe2+ (LS), Co2+Fe2+

and linkage isomerized Co2+Fe2+ phases, respectively [75]. The FT-IR spectrum of the

pure nickel hexacyanoferrate displays peaks at 2160 and 2125 cm-1 corresponding to

the bridged and terminal cyanide of Ni2+Fe3+ Prussian blue analog, as well as, peaks at

2079 and 2043 cm-1 corresponding to the same assignments for the reduced Ni2+Fe2+

sites [143]. As the concentration of Ni2+ in the lattice is increased at the expense of

Co2+ ions, the intensities of the three peaks at 2120, 2090, and 2040 cm-1 decrease,

while that of the peak at 2163 cm-1 remains relatively unchanged and a peak at 2125

cm-1 emerges. These intensity changes indicate both the reduction in the number of

cobalt-iron pairs and the subsequent formation of nickel-iron pairs. FT-IR spectra and

the peak fitting results can be found in Figure 7-3.

7.1.5 X-Ray Diffraction

To investigate the lattice constants and crystal structure, a Philips APD 3720

powder diffractometer, housed in the Major Analytical Instrument Center at the

University of Florida, was used to perform room temperature x-ray diffraction (XRD)


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using a Cu Ka source. Between 10-20 mg of the same samples used for all other

characterizations including magnetometry, except for the x = 1.00 sample that only had

0.6 mg left by the time XRD was performed, were mounted on glass slides and pressed

onto squares of double-sided cellophane tape of ~ 2.3 cm2. The resulting

diffractograms, Figure 7-4 and Figure 7-5, were used to model the structure by a

Rietveld refinement using the EXPGUI [53] interface for GSAS [52]. In order to

approximate the complicated Prussian blue analogue structure, a single-phase model

with Fm3m (No. 225) space group symmetry was used. Specifically, the cobalt and

nickel atoms were forced to occupy the same site. Atomic occupancies were set by the

experimentally determined chemical formulas, excepting the oxygen atoms of the

interstitial waters that were allowed to vary as the samples may have dehydrated or

hydrated between synthesis and diffraction. The same site symmetries as in Prussian

blue were used, where the iron vacancies were replaced by the six coordinated oxygen

atoms of the ligand water molecules [65]. Placement of the oxygen atoms of the

interstitial water molecules at the 32fWyckoff position [68] and a relatively small

percentage at the 192/position was found to yield a robust local minima during

refinement procedure. As clearly displayed by the (4, 0, 0) reflection, the unit cell

constants change continuously when changing from x = 0 to x = 1, Figure 7-6.

7.1.6 Mean-Field Calculations

Using simple mean-field approximations, investigations of the possible effects to

be observed in the magnetization were performed first, and subsequently refined the

model after completing a series of experiments. In order to model the magnetic

interactions, an approximation in which superexchange energies act as effective


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magnetic fields, so-called Weiss fields, was employed in a manner akin to previous

works in similar materials [132-138]. In order to model the cooperative thermally active

CTIST event, a Bethe-Peierls-Weiss approximation to a phenomenological spin-

crossover Hamiltonian was implemented [97]. These numerical studies extend the

previous work of others by allowing the high-spin fraction, nHs, which is experimentally

controlled by irradiation and temperature, to vary along with the relative metal

concentration, x, which is dictated by the synthesis. Details of the calculations,

including how x and nHs are utilized to provide numerical results that are directly

comparable to the experimental data.

7.1.6.1 Low temperature magnetization in the mean-field

Calculations were performed to investigate effects on the low temperature

magnetic susceptibility due to the substitution of Ni atoms for Co in

NaaNil-xCox[Fe(CN)6]p-nH20. For simplicity, consider only superexchange of spins

associated with nearest transition metal neighbors, designated as n.n., under the

influence of an applied magnetic field, and thus the Hamiltonian has the form

T = -2 JijSi,-Si +gBH S 7.1
i,j=n.n i

where J is an exchange constant, g is the Lande factor, /B is the Bohr magneton, S is

the electronic spin, and H is the applied field. One method of investigating Equation 7.1

is the usual mean-field expansion of the spin operator.

For the system in question, three magnetic ions must be considered: (a) Fe3,

which can superexchange with either Ni2+ or Co2+, (b) Ni2+, which only superexchanges

with Fe3+, and (c) Co2+, which also only superexchanges with Fe3+. Two parameters, x


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and nHS, are introduced to keep track of the magnetic species present when calculating

the temperature dependence of the magnetization, where x is the molar fraction of Co,

x = [Co]/([Co] + [Ni]), and nHs tracks the amount of material that has undergone CTIST.

It is important to note that the high-spin fraction, nHS, is representative of the amount of

material that is actually magnetic. Expressions for the average spin polarization values

of the constituent ions can be derived by minimizing the free energy with respect to

simultaneous variation of the spin polarizations of the different sublattices, yielding,

(SNi) = SNiBs (gBSNiHext 2ZNiFeJNiFe NSFe) 7.2
kBT kBT I

A, (9\/BScoHext + 2ZCoFeJCoFe and 7.3
(Sco) = Sco-Bs T + kBT Sco-(SFe) and 7.3


10, \ (9 BS FeHext 2ZFeCo CoFe 2ZFeNiJ NiFe S JSAv 74
(SFe) = SFeBs SFeHext + 2ZFe FeSFe-(Sco) + 2ZFeNiJNiFeSFe'(S) 7.4
kBT kBT kBT

where Bs is the Brillouin function, kB is the Boltzmann constant, and (...) denotes an

average. Total spin numbers of SNI= 1 for Ni2+, SFe= 1/2 for Fe3+, and Sco= 3/2 for Co2+

were used, with other species being diamagnetic. There is an explicit dependence of Z

upon both x and nHs that can be resolved by considering statistical mixing and using the

chemical formula, that is to say, ZNiFe= 4.0(1 x) + 3.3x-nHS,

ZCoFe = 4.0(1 x) + 3.3x-nHs, ZFeCo = 2.7x + 3.3x-nHs, and ZFeNi= 6.0(1 x). Subsequent

to simultaneous solution of Equations 7.3-7.5, the magnetization can be calculated from

MN = 4.00(1 x)Ng{B(SNi) 7.5

Mco = (1.83x + 2.17xnHs)Ng/B(Sco) 7.6

MFe = (3.26xnHs + 4.00(1 x))NgSB(SFe) ,7.7


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and Mtotal = D(MNi+ Mco+ MFe) 7.8

where an additional parameter D accounts for the presence of magnetic domains after

the onset of long-range magnetic order. In practice, D is chosen to be 100, as

estimated by fitting the pure materials. At low temperature, if most of the orbital

momentum is quenched, values of g ~ 2 are reasonable for all species.

7.1.6.2 Mean-field magnetic susceptibility and spin-crossover

To understand how the magnitude and hysteresis of the thermally induced CTIST

are affected by the introduction of Ni atoms to the lattice, an analytical model was

employed [75]. This solution exploits the mapping between the extensively studied

Ising Hamiltonian and the phenomenological spin-crossover Hamiltonian to be

considered,


_= -J kBTIn +-A si 7.9
I-

where g+ and g are the degeneracies of the high-spin (HS) and low-spin (LS) states,

Jsco is the intermolecular interaction energy associated with the different spin states, A

is the octahedral splitting energy, s is a pseudo-spin keeping track of the spin state, T is

temperature, kB is the Boltzmann constant, and refers to a summation over nearest

neighbors.

In a manner similar to that employed by Hoo et al. [97], it is straightforward to

calculate the HS fraction. For these calculations, it is assumed that the entropy content

of the different states, the intermolecular interaction, and the energy difference between

the HS and LS states are unchanged as x is changed. A small effect on the onset of

the transition is expected to arise from the slightly varying Na content in the samples,

even while the width of the hysteresis loop is unchanged [84], but this effect is not


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considered. The number of nearest neighbors to be considered in the spin-crossover

event for the different samples is taken from the chemical formulas by taking the

minimum occupation necessary for the charge transfer. Specifically, zsco(x = 1.00) =

4.32, zsco(x = 0.87) = 3.8, and zsco(x = 0.66) = 4.0, each determined from the

switchable Fe content, zsco(x = 0.45) = 2.7, and zsco(x = 0.22) = 1.3, each determined

from the amount of Co, and zsco(x = 0.00) = 0 because there are no Co ions in this

sample. Values of Jsco = 150 K, In(g+/g_) = 250, and A = 550 K are used for all

calculations and are determined by fitting the x = 1.00 data. Simultaneous fitting of the

data above 250 K for all samples gives gco = 2.7, gFe = 2.2, and gvi = 2.3. These g > 2

values arise from the incomplete quenching of orbital moments on the ions at high

temperatures [5]. Mean-field fits for each sample can then be performed in order to

calculate nHs above and below the spin transition.

7.1.7 Magnetic Measurements

Magnetic measurements were performed using a Quantum Design MPMS XL

superconducting quantum interference device (SQUID) magnetometer. A room

temperature halogen light source (~1-2 mW) was used to introduce light into the sample

chamber of the SQUID through a bundle of ten optical fibers, ~270 pm OD (Ocean

Optics Model 200), for photomagnetic measurements. Powders were mounted on

pieces of cellophane tape around plastic drinking straws to increase the optical

cross-section for the photomagnetic studies. High temperature data (T > 100 K) were

taken using gelcaps as the sample holders to accommodate additional sample mass.

Backgrounds were subtracted from the data by using the measured mass susceptibility

of similar sample holders. The same demagnetizing protocol, during which the magnet


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field is oscillated to zero by successive ramps starting at 20 kG, was used for all low

field measurements in ~10 G. Additionally, the magnet was allowed to relax for more

than two hours subsequent to demagnetization and prior to data taking. By using a

commercial Hall sensor, an in-house calibrated Toshiba THS118E, it is likely that there

may be differences of up to ~1 G in the external fields applied to different samples, but

for each specimen, the field was not changed between light and dark states for the

temperature sweeps, ensuring any resulting effects are not a result of slight

perturbations of the external field.

7.1.7.1 Low temperature DC susceptibility

The time dependence of the DC magnetic susceptibilities, X = M/H, during

photoirradiation of the samples are shown in Figure 7-7 (a). The temperature

dependence of the DC magnetic susceptibilities, x(T), in ~10 G between 2 K and 30 K

for various x values, both before and after photoirradiation, are shown in Figure 7-7 (b).

A clear bifurcation of the field-cooled (FC) and zero-field-cooled (ZFC) curves, with a

peak in the XZFC versus T plots, is observed for all samples. The results of the Weiss

mean-field calculations, as described in Section 7.1.4, are shown in Figure 7-7 (c). For

the mean-field calculated susceptibilities of the dark states, the value of the high-spin

fraction, nHS, is dictated by the amount of material measured to undergo spin-crossover,

whereas the calculated susceptibilities of the photoirradiated states use nHs = 1, where

all available material is in the high-spin state by definition.

All samples with x > 0 show a change in magnetization due to applied light.

Strikingly, at 5 K and in 10 G, the x = 0.66 sample shows a clear decrease in

magnetization with photoirradiation in both the calculations and the experimental results.


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To a weaker extent, a photoinduced decrease in magnetization is also observed in the

x = 0.45 sample. Expanded photoirradiation versus time plots for the x = 0.66 and

x = 0.45 samples are shown in Figure 7-8.

7.1.7.2 DC magnetization

Magnetization as a function of the applied magnetic field was measured for all

compounds at T = 2 K and up to 70 kG, Figure 7-9. A scaling of the coercive fields with

the mixing fraction, x, was seen. In addition, at the high fields, all samples where seen

to have photoinduced increases in magnetization. A check for symmetry of the

hysteresis loops as a potential contributing factor to the change in the coercive field was

made, Figure 7-10.

7.1.7.3 High temperature DC susceptibility

The temperature dependence of the DC magnetic susceptibility temperature

product, XT, in 5 kG and between 100 K and 300 K for various x values are shown in

Figure 7-11 (a). To help ensure equilibrium during the spin-crossover, a sweep rate of

less than 0.5 K/min was employed. The combined Bethe-Peierls-Weiss spin-crossover

and Weiss mean-field magnetization calculations, as described in Section 7.1.4, are

shown in Figure 7-11 (b). For clarity, the calculated temperature dependence of the

high-spin fraction, nHs, is also shown, Figure 7-11 (c).

All samples with x > 0 appear to show thermally induced CTIST, as evidenced by

the abrupt reduction of the magnetic susceptibility upon cooling below ~170 K. These

CTIST events can be cycled with temperature and exhibit hysteresis that is

characteristic of the cooperativity of the transition. Additionally, an evolution of the

ferromagnetic slope in XT, characteristic of the NaaNi[Fe(CN)6]p-nH20 compound, can


224









be seen as more Ni is introduced to the lattice in both the numerical and experimental

studies. Furthermore, the samples show a decrease of the width of the thermal

hysteresis, as the amount of Co decreases, and a drastic decrease in the amount of

material that undergoes CTIST, when Ni is introduced into the lattice.

7.1.7.4 Physically mixed x = 0.66 compound

To make sure that the observed behavior is not due to a physical mixture of the

parent compounds on a macroscopic level, a manually mixed sample of separately

synthesized nickel hexacyanoferrate and cobalt hexacyanoferrate powders was

prepared and studied, Figure 7-12 and Figure 7-13. For this type of synthesis, the TEM

data reveal a bimodal distribution clearly associated with the two distinct sizes of the

NaaNi[Fe(CN)6]- nH20 and NaCo[Fe(CN)6]p nH20 powders. The magnetic orderings

of both binary species are clearly Visible in x(T), and the magnetization only increases

with irradiation, even though the chemical composition is the same as the x = 0.66

sample showing a photodecrease.

7.1.8 Discussion

In the following subsections, the three main results of the experimental and

numerical work performed on the ternary transition metal Prussian blue analog,

NaaNil_xCox[Fe(CN)6]p-nH20, are discussed. Highlighted are, first, the observation of a

photoinduced decrease in magnetization, second, the scaling of magnetic properties as

a function of x, and third, the dependence of the observed CTIST effect upon dilution of

the parent cobalt hexacyanoferrate material. Finally, aspects and potential future

extensions of the mean-field calculations are examined in light of the experimental

results.


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7.1.8.1 Photoinduced decrease in magnetization

The results of the mean-field calculations predict a decrease in magnetization

within the ordered state with increasing high-spin fraction for

NaaNil-xCox[Fe(CN)6]p-nH20 powders with enough ferromagnetic Ni2+ constituent ions.

These predictions are compared to low temperature magnetic susceptibility experiments

as a function of x. All samples show an increase in magnetization at high field, Figure

7-9, even those showing the photodecrease at low field, Figure 7-7. This increase in

magnetization at high field, regardless of x, proves that additional spins are being

generated, rather than destroyed, during photoirradiation. These results also indicate

that the mechanism for the photoinduced magnetization, present in all samples having

x > 0, is the same CTIST leading to the persistent long-lived metastable states seen in

the pure RbaCo[Fe(CN)6]p-nH20 material. The photoeffect can be reproduced and

reversed with thermal cycling above approximately 150 K. The ability of the mean-field

calculations to predict whether a material will have a photoincrease or decrease based

upon its composition, Figure 7-7, substantiates the claim that the observed

photoinduced decreases in magnetization can be understood as an interplay between

ferromagnetic and antiferromagnetic superexchange interactions, as was hypothesized

in Figure 7-1.

These points can be further elucidated by focusing on the x = 0.66 sample. This

sample has enough ferromagnetically interacting nearest neighbors to begin driving the

Fe sublattice parallel to the applied field, while it simultaneously possesses enough Co-

NC-Fe switchable pairs to still show an appreciable CTIST effect. In the low field limit,

newly photoexcited Co-NC-Fe pairs align antiparallel to the applied field due to the


226









antiferromagnetic superexchange between Co and Fe ions. Since the Fe ions are

already parallel to the field due to the presence of Ni, a net photodecrease in

magnetization is observed, Figure 7-14. If a sufficient external magnetic field is applied,

the energy reduction gained by aligning Co-NC-Fe pairs with the applied field is larger

than the superexchange, so a net photoincrease in magnetization is measured, Figure

7-14. The temperature dependence of both experimental and numerical magnetizations

in the low field limit show a decrease in the measured susceptibility below

approximately 12 K, above which an increase is observed since the thermal energy is

now able to populate the excited states having Co spins parallel to the applied field.

In addition, there is a time dependence of the photoinduced magnetic effect on

the scale of weeks. For example, when the samples were measured again after one

month in a freezer at T ~ 248 K, the photodecrease was found to be slightly stronger by

a few percent. This evolution of the magnetic properties may be due to an increase of

atomic mixing of the samples arising from solid state diffusion or to the stabilization of

the positions of interstitial counterions to regions more prone to induce bistabilities in the

spin states.

As a final point, the expectation of a photoeffect having the opposite sign

compared to the pure Prussian blue analogue material due to a mixing of ferromagnetic

and antiferromagnetic superexchange interactions is similar to the photoinduced

magnetic pole inversion reported for Fel-xMnx[Cr(CN)6] nH20 Prussian blue

analogue [134]. A fundamental difference, however, is that in the

Fel-xMnx[Cr(CN)6] nH20 system, the applied light is destroying exchange pathways,


227









whereas additional moments are being generated in the NaaNil-xCox[Fe(CN)6]p-nH20

system reported here.

7.1.8.2 Scaling of magnetic properties

All NaaNil-xCox[Fe(CN)6]p-nH20 samples studied show spin glass-like long-range

magnetic order, as evidenced by the bifurcation of the FC and ZFC curves, Figure 7-7

(b). The peak in the ZFC susceptibility is a fingerprint of the spin glass-like nature of the

order in both parent compounds [78] [146-150], the presence of which hints at the

complicated nature of the magnetism in the samples investigated. It is noteworthy that

local minima are present near x ~ 0.8 in the scaling of the magnetic ordering

temperature, the coercive field, and the absolute value of the Curie-Weiss temperature

as a function of x, Figure 7-15.

The observed scaling of magnetic properties in NaaNil-xCox[Fe(CN)6]p-nH20 can

be compared with previous work on ternary transition metal Prussian blue

analogues [136-138][149-151]. In ternary materials of Cu[CoxFel-x(CN)6],

Ni[CoxFel-x(CN)6] and Fe[CoxFel-x(CN)6], a clear monotonic scaling of the transition

temperature with x was observed, and these results are dominated by the changing

number of magnetic nearest neighbors, since Co3+ on the M site is LS and therefore

diamagnetic [149]. Similar monotonic scaling was observed in Ni[CrxFel-x(CN)6] and

Fe[CrxFel-x(CN)6] [150], where the substitution of Cr 3+ (S = 3/2) for Fe3+ (S = 1/2)

provides a threefold increase of the number of superexchange pathways, as the number

of unpaired electrons on the M site changes from (t2g)1 to (t2g)3. Finally, the most cogent

example is NixMnl-x[Cr(CN)6]-nH20, which displays a clear dip in the ordering

temperature and a peak in the coercive field on the background of a linear dependence


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interpolating between the values of the parent compounds as x was changed [136-138].

A few similarities are obvious when comparing NaaNi_-xCox[Fe(CN)6]p-nH20 and

NixMnl-x[Cr(CN)6] nH20, particularly in the context of simple empirical rules for

superexchange [38]. First, both contain a Ni2+ ion that has a ferromagnetic

superexchange pathway (eg to t2g). Second, the interaction between Mn2+ and Cr3+ is

analogous to the Co2+ interaction with Fe3+ in NaaNilxCox[Fe(CN)6]p-nH20, as there is a

competition between ferromagnetic and antiferromagnetic interactions, Figure 7-16 (a).

It is plausible that when two superexchange energies of opposite sign compete, the net

magnetic interaction is particularly susceptible to perturbation when the inter-ion

distance is changed. The non-monotonicities observed in the ordering temperatures, as

a function of metal substitution, may therefore be due to a net superexchange that

depends strongly on these small distance changes that are introduced with the

substitution. In order to reproduce the experimental data, it was necessary to introduce

a distance dependence to the Co2+(HS)-NC-Fe3+(LS) superexchange interaction,

Figure 7-16 (b).

The need to introduce distance dependence to the superexchange interaction in

order to reproduce the data may seem drastic, but other methods to reproduce the

scaling of the magnetic properties were unsuccessful. Two remarkable features are

present in the data: the dip in the ordering temperature near x = 0.8 and the

unexpectedly large ferromagnetic character of the mixed samples. This increase in

ferromagnetic character manifests itself in the lack of a compensation point for the

mixed ferro-ferrimagnetic system and in the high temperature slope of XT for the mixed

samples. Specifically, from 250 K to 300 K, xT for the x = 0.66 sample is increasing as


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temperature decreases, where a model using the binary magnetic interactions predicts

a clear decrease as temperature decreases. For powder samples containing ions in

similar environments to ours, the introduction of single-ion anisotropy and spin-orbit

coupling terms can only give rise to a decrease in XT as temperature

decreases [38] [153] [154]. Finally, there are precedents in the literature for such a

modification of the superexchange energy. In CsCo[Cr(CN)6]p-nH20 ferromagnetic

compounds having competing ferromagnetic and antiferromagnetic pathways, a

dependence of the superexchange energy on the lattice constant was found [155].

Additionally, an AaCo[Fe(CN)6]p. nH20 material was reported in which ferromagnetic

coupling, as opposed to the usual antiferromagnetic coupling leading to a ferrimagnet,

was inferred based upon the high-temperature inverse susceptibility [139].

7.1.8.3 Spin-crossover dilution

The width of the thermal hysteresis, as represented by Tup-Tdown, decreases

when the cobalt hexacyanoferrate material is diluted, and this trend is correlated with

the number of active CTIST nearest neighbors, zsco, Figure 7-17. As described in

Section 7.1.4.2, zsco can be calculated from the chemical formula, and the observed

narrowing of the hysteresis is an expected result when zsco decreases. It is worth

noting that while the changing number of nearest neighbors is a dominant effect,

additional perturbations due to the changing of the local environments of the active

species are also present. Experimentally, the dilution of spin-crossover species has

been investigated intensively after being first realized in

[FexZnl-x(2-pic)3]C2- EtOH [156] [157], where a gradual reduction in the width of the

hysteresis loop was attributed to the many-body elastic interactions innate to these


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transitions. With respect to CTIST in Prussian blue analogues, the field is not as mature

and studies are still ongoing. Recently, a CTIST diluted

Rbo.70CUo.22Mno.78[Fe(CN)6]o.86-2.05H20 sample was compared to its undiluted parent

compound Rbo.81Mn[Fe(CN)6]o.95-1.24H20, and no appreciable change in the width of

the hysteresis loop was observed [158].

Furthermore, there is a striking reduction in the amount of CTIST active material

once nickel is introduced into the lattice, Figure 7-18 (a). More specifically, the x = 1.00

material transitions 83% of the amount expected from the chemical formula when

sweeping from 300 K to 100 K, whereas the x = 0.87 material transitions 16%, and less

than 10% transitions in the remainder of the samples with lower x-values. The percent

of CTIST active material can be established by considering the chemical formula, the

room temperature FT-IR, and the change in XT as the samples are cooled. Although a

detailed investigation of the microscopic origins of the observed reduction in

spin-crossover active material is warranted, the author conjectures that the reduction is

related to a Ni-induced stabilization of Co-NC-Fe HS pairs arising from subtle

variations of the unit cell parameters, Figure 7-18 (b). The lattice constants are

observed to scale with x in a monotonic fashion that is consistent with changes seen in

other ternary metal Prussian blue analogues [136-138][149-151]. However, the

nonlinear nature of the scaling implies an actual changing of the bond energies in the

system as the different systems are mixed. The FT-IR data also provide evidence

supporting the stabilization of the coordination bond with the incorporation of Ni2+.

Using the cyanide stretch associated with the divalent metal to iron bond, plots of the

stretching frequency and an effective spring constant as a function of x can be made,


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Figure 7-19, implying a stabilization of the bond and an increased rigidity of the lattice

with the introduction of nickel ions. Therefore, it may no longer be energetically

favorable for Co-NC-Fe pairs in NaaNil-xCox[Fe(CN)6]p. nH20 to undergo CTIST due to

the added strain that would result for the Ni-NC-Fe bonds in the system. More

specifically, the LS phase of NaCo[Fe(CN)6]p. nH20 has a lattice constant of 9.9721 A,

whereas the HS phase has a lattice constant of 10.3033 A [68], which is comparable to

the x = 1.00 sample that has a lattice constant of 10.30(7) A. In contrast, the x = 0.00

nickel hexacyanoferrate species has a lattice constant of 10.23(9) A, which is

comparable to the previously reported value of 10.229 A for Ni3[Fe(CN)6]2 [139].

Comparisons of this observation of the reduction of CTIST active material to a

recent work studying the dilution of cobalt hexacyanoferrate by diamagnetic Zn2+ at the

divalent metal site or by diamagnetic Co3+ at the cyanometallate site [135] are useful. In

particular, a similar sensitivity of the CTIST effect with the substitution of metals is seen

by Cafun et al., [135] and the reduction in the magnitude of the effect in their samples is

also larger than expected by a simple reduction in the spin-crossover active species on

a molecule by molecule level. It was previously shown by Ksenofontov et al. that by

application of hydrostatic pressure to AaCo[Fe(CN)6]p-nH20 powders, a stabilization of

the LS phase could be induced in the samples [2]. This pressure sensitivity leads to the

obvious contention that the stabilization of the high-spin phase for

NaaNil-xCox[Fe(CN)6]p-nH20 may simply be due to an effective "negative pressure."

Cafun et al. [135] argue that the observed stabilization with metal substitution cannot be

due to such an effect because their starting material has a cell size of ~10.32 A, while

the 100% Zn2+ doped material should have a negative pressure due to its cell size of


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~10.40 A and the 100% Co3+(CN)6 doped material should have a positive pressure

owing to lattice constant of ~10.23 A. However, these room temperature values all deal

with the high-spin lattice constants, and with respect to the low-spin lattice constants,

the alien species are still larger than the LS Co-NC-Fe state, and in fact closer in size

to the HS Co-NC-Fe state than the LS Co-NC-Fe state, suggesting that chemical

pressure may still be a valid argument for the effect. Finally, more subtle effects on the

energy of the cobalt-iron charge transfer arising from the presence of neighboring nickel

ions may also be present.

7.1.8.4 Mean-field predictions versus observations

The mean-field calculations were able to predict whether the magnetization of the

chosen samples would increase or decrease with photoirradiation. Upon completion of

the experiments, the lack of a compensation point and subsequent negative

magnetization, as well as the general scaling of magnetic properties, was surprising.

However, a better agreement between calculations and experiment was found if a

distance dependence was introduced for the superexchange constant. In addition,

discrepancies between predictions and experiment may stem from the need to choose

the simplest Hamiltonian that could capture the spirit of the problem, having only

Zeeman and superexchange terms, in order to make the number of free parameters

tractable. Future studies with more parameters may be possible once neutron

spectroscopy is performed to fix the parameter values to experimental data.

Using the machinery described in the previous subsections, magnetization as a

function of temperature can be calculated. For the first attempt, the superexchange

constants for all materials were taken from the nickel to iron and cobalt to iron

interactions of the x = 0.00 and x = 1.00 samples, respectively. This method is


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appealing because it is predictive and limits the number of model parameters. However,

some discrepancies between model and experiment were found, Figure 7-20.

A few additional parameters were tried, with a distance dependent

superexchange energy being the most successful at capturing the features observed in

the magnetization. The calculations then took on a flavor of fitting the data, with the

predictive role already having been fulfilled. First, the susceptibility data from 250 K to

300 K were fit using the mean-field solutions described previously, yielding values for

and . The nickel to iron superexchange was fixed, and the cobalt to iron

superexchange was allowed to vary as a function of lattice constant due to the presence

of both antiferromagnetic and ferromagnetic superexchange pathways. The distribution

of superexchange energies was binned into two populations, CTIST active and CTIST

inactive, the fractions of both being determined from high temperature susceptibility

data (Figure 7-21). The CTIST active superexchange value was associated with the

population having the closest lattice constant to the x = 1.00 material.

7.1.9 Conclusions

It has been demonstrated that the ternary transition metal Prussian blue

analogue NaaNi_-xCox[Fe(CN)6]p-nH20 shows a photoinduced decrease in

magnetization for certain values of x, temperature, and applied magnetic field.

Furthermore, the NaaNil-xCox[Fe(CN)6]p-nH20 system is the first example of a

compound in which superexchange energies control whether incident light increases or

decreases CTIST magnetization. As a result, the sign of the photoeffect can be

changed by stoichiometry. Although a photoinduced decrease in magnetization while

increasing the number of spins has also been seen in ACo[Fe(CN)6]p-nH20 thin


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films [10], the microscopic origins are different. In addition, the width of the thermal

hysteresis of the CTIST is reduced upon dilution of the active spin-crossover species in

the ternary mixture. The origins of the experimental observations are nicely explained

using mean-field calculations.

7.2 Heterostructured Films Containing Cobalt Hexacyanoferrate

7.2.1 Introduction

Recently, there has been interest not only in three-dimensional systems, but also

in two-dimensional and quasi-two-dimensional structures, some of which have been

shown to display phenomenon different than that seen in the analogous

three-dimensional materials [10] [29] [76] [93] [94][98-112]. Work on thin films of

Co-Fe was originally motivated by the functional need to increase the light cross-section

of the photoactive material, but it is noteworthy that anisotropy in the photoinduced

magnetization is seen in the quasi-two-dimensional geometry [10].

Originally, the previous student in Professor Meisel's lab, Dr. Ju-Hyun Park, had

the idea that, by layering the lower ordering temperature Co-Fe photomagnet between

layers of a higher ordering magnet that is not photoactive, a transition between

two-dimensional and three-dimensional behavior in the heterostructure might be seen

after photoirradiation [76]. This transition was hypothesized to occur as Co-Fe spins

become magnetic and contribute additional exchange pathways between the higher

ordering temperature layers. Upon surveying the different possible high-Tc magnets,

nickel hexacyanochromate (Ni-Cr) was chosen because of its robustness and similar

lattice constant to Co-Fe [54] [56]. Heterostructured films may be generated with a

synthesis technique similar to that used for pure phase films, sequential adsorption

(Figure 7-22).


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Exploration of Co-Fe/Ni-Cr heterostructures in various geometries eventually led

to the realization of the desired effect, although in a different manner than originally

hypothesized. Heterostructures with sufficiently thick regions of unadulterated high-Tc

magnets and photoactive Co-Fe, so as to retain bulk-like features but sufficiently thin

layers so as to have strong inter-layer interactions, show photoinduced modification of

long-range magnet order up to the limit of the photoinduced structural transition of the

Co-Fe molecule, ~ 100 K. These high-Tc photoeffects are due to a propagation of

photoinduced structural changes in the Co-Fe layer propogating to the Ni-Cr layers. In

the following subsections, these discoveries will be discussed. First, the evolution of

studies of Co-Fe/Ni-Cr thin film heterostructures will be presented, leading up to the

structures having the largest photo-response. Second, the optimized heterostructure

will be dissected in detail, and the current understanding of the phenomenon will

presented. Finally, heterostructures with different chromate Prussian blue analogues

will be presented. Elements of the chapter have recently been published in JACS

communications [159] and elsewhere.

7.2.2 Synthesis

Synthesis of the desired heterostructure of Ni-Cr and Co-Fe is possible by a

sequential deposition method (SD), in which PBA films are generated by sequentially

dipping a solid-support into aqueous solutions containing the desired constituent metals.

One such dipping process will often be referred to as one "cycle" in the following

sections. The attractiveness of the SD approach comes from its combination of

simplicity with fine thickness control for generation of homogeneous films of arbitrary

PBAs with tunable chemical compositions on a variety of substrates. The synthesis of a

heterostructured film is made possible by alternating protocols for the SD films of the


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sought after component PBAs, the details of which are discussed in Dr. Franz Frye's

thesis [94]. The shorthand notation used to refer to samples first lists the repeat cycle,

then the total number of repeats, and finally the constituent magnets. For example, a

film with every other cycle of Co-Fe and Ni-Cr repeated 40 times will be referred to as

1/1 x 40 Co-Fe/Ni-Cr, and a film with 40 cycles of Ni-Cr followed by 40 cycles of Co-Fe

followed by 40 cycles of Ni-Cr will be referred to as 40/40/40 Ni-Cr/Co-Fe.

7.2.3 Magnetization of Nickel Hexacyanochromate and Cobalt Hexacyanoferrate
Heterostructures

Due to similar lattice constants and the high ordering temperature of the Ni-Cr

material, most of the heterostructured films studied were of Ni-Cr and Co-Fe layers. A

brief summary of all films to be presented can be found in Table 7-2.

7.2.3.1 Slow deposition multilayer films

A series of films having the same total number of cycles, but with different

numbers of cycles between the alternation of Co-Fe and Ni-Cr depositions, was studied

in the SQUID magnetometer. Unlike the rest of the samples to be presented, these

were generated using a "slow" deposition method that included extra washing of the film

after deposition [94]. Exemplary samples are a 1/1 x 40 Co-Fe/Ni-Cr, 5/5 x 20

Co-Fe/Ni-Cr, and 40/40 Co-Fe/Ni-Cr. The temperature dependence of the DC magnetic

susceptibilities, X = M/H, are shown in Figure 7-23 for temperatures between 2 K and

100 K and an external field of 100 G. The magnetic signals are expressed per cm2

A clear evolution of the magnetic order in the samples can be seen as a

progression is made from separate behaviors of the Co-Fe and Ni-Cr magnets to an

overall combined magnet that has the addition of superexchange between Co and Cr as

well as superexchange between Ni and Fe. The disparity between the changes in


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magnetization with photoirradiation can be explained by poor transfer ratios causing

less Co-Fe content in the film.

7.2.3.2 Stacked films

Simpler, stacked films of 10/10 Co-Fe/Ni-Cr and of 10/10 Ni-Cr/CoFe were

measured to investigate their magnetic and photomagnetic character. The temperature

dependence of the DC magnetic susceptibilities, X = M/H, for temperatures between 2 K

and 100 K, and AM versus time irradiated at 5 K and 100 G, are shown in Figure

7-24 (a), Figure 7-25 (a), Figure 7-26 (a), and Figure 7-27 (a). AM versus time

irradiated is shown in Figure 7-24 (b), Figure 7-25 (b), Figure 7-26 (b), and Figure

7-27 (b). The magnetic signals are all expressed per cm2 of sample.

The presence of the Ni-Cr component can clearly be seen by the ordering onset at

~70 K and the large magnetic anisotropy exists below this temperature. The increase of

magnetization with photoirradiation is clearly coming from the Co-Fe component. In

addition, two ordering temperatures can be seen for both films, with an additional onset

at ~10 K. Differences between the stacked films are also apparent, where the

anisotropy of the photoeffect is stronger for the 10/10 Co-Fe/Ni-Cr film, which has the

Co-Fe deposited first, as opposed to the 10/10 Ni-Cr/CoFe film, which has the Ni-Cr

deposited first. A slightly different ordering temperature is also observed for the Co-Fe

component in the films, with the 10/10 Co-Fe/Ni-Cr film having a lower ordering

temperature than the 10/10 Ni-Cr/CoFe film. These effects can be explained by low

transfer ratios for the first few cycles and by the proximity of the solid support being a

key factor in the sign of the photoeffect of the Co-Fe.


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7.2.3.3 Thin sandwiched films

A third set of films, in which a thin layer of Co-Fe PBA is deposited between two

thin layers of Ni-Cr PBA, is the so-called thin sandwich geometry. Two different

heterostructures are presented, 10/5/10 and 10/10/10, where both are

Ni-Cr/Co-Fe/Ni-Cr, so the additional nomenclature for constituent makeup will be

dropped to ease discussion. The temperature dependence of the DC magnetic

susceptibilities, 7 = M/H, are shown in Figure 7-28 (a), Figure 7-29 (a) and Figure 7-30

for temperatures between 2 K and 100 K and an external field of 100 G. For the film

10/5/10, AM versus time irradiated is shown in Figure 7-28 (b) for H parallel, and Figure

7-29 (b) for H perpendicular, both in fields of 100 G. AM versus time irradiated at 5 K

and fields of 100 G and 1 kG is shown in Figure 7-31 and Figure 7-32 for both parallel

and perpendicular orientations of the 10/10/10 film with respect to the applied magnetic

field. The magnetic signals are expressed per cm2 of sample.

The susceptibility data again show the strong presence of Ni-Cr magnetism in the

samples as well as photoinduced magnetism from the Co-Fe moeities. However, the

most striking feature of the data for the sandwich films is that for the 10/10/10 film, and

to a lesser extent for the 10/5/10 film, a clear decrease in magnetization with

photoirradiation is seen in an applied field of 100 G in both film orientations. When

going to higher external fields of 1 kG, the 10/10/10 film no longer shows a decrease

with photoexcitation, but rather an increase for both orientations.

7.2.3.4 Thick sandwiched films

At this point, it is worth mentioning that a novel new effect had been seen in the

thin sandwiched films, and to help understand the effect, a more easily modeled, solid


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solution with a similar set of constituents, NixCol-x[Fe(CN)6]3/4 nH20, was studied [141].

Since intimate mixing of foreign species with Co-Fe strongly damped the photomagnetic

bistability, the sandwich geometry was reinvestigated with thicker layers that were

conjectured to possess a larger photoeffect. A larger photoeffect was observed and, in

fact, photoinduced changes could be observed as a function of field and temperature in

the heterostructures for the first time. Three different heterostructures are presented

here, 40/40/40, 20/40/20, and 40/20/40, where all are Ni-Cr/Co-Fe/Ni-Cr, so the

additional nomenclature for the constituent makeup will be dropped to ease discussion.

For the 40/40/40 film, the temperature dependence of the DC magnetic

susceptibilities, X = M/H, are shown for parallel and perpendicular orientations in Figure

7-33 (a) and Figure 7-33 (b), respectively, for temperatures between 2 K and 75 K and

an external field of 100 G. Field dependence of the magnetization is shown for parallel

and perpendicular orientations in Figure 7-33 (c) at 2 K. While kinks in the

temperatures sweeps can be associated with ordering of the pure Co-Fe and Ni-Cr

phases, the heterostructures show two striking features not observed in the

homogeneous phases. First, there is a significant increase in the temperature, from 18

K to 70 K, at which persistent photoinduced changes in the magnetically ordered state

are observed. Second, like the thin sandwich heterostructures, the magnetization

decreases with light, in contrast to the normal photoinduced increases known for pure

Co-Fe compounds. The increase in magnetization at high field is an indication that

there is the usual diamagnetic to magnetic transition of the Co-Fe spins. In addition,

aside from an overall scale factor between parallel and perpendicular orientations, the

photoinduced effects are the same.


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For the 20/40/20 film, the temperature dependence of the DC magnetic

susceptibilities, X = M/H, are shown for parallel orientation in Figure 7-34 (a), for

temperatures between 2 K and 75 K and an external field of 100 G. Field dependence

of the magnetization is shown for parallel orientation in Figure 7-34 (b) at 2 K. These

films behave similarly to the 40/40/40 films, except that the relative photoinduced

decrease is less, so much so that when the Co-Fe is in the ordered state, an overall

increase is observed on the background of the decrease, even in 100 G.

For the 40/20/40 film, the temperature dependence of the DC magnetic

susceptibilities, X = M/H, are shown for parallel orientation in Figure 7-35 (a), for

temperatures between 2 K and 75 K and an external field of 100 G. Field dependence

of the magnetization is shown for parallel orientation in Figure 7-35 (b) at 2 K. Again,

these films behave similarly to the 40/40/40 films, but here the photoinduced decrease

at low fields and increase at high fields are relatively smaller.

7.2.4 40/40/40 Heterostructure

Even better than the thin sandwich films showing a novel photoeffect, the thick

sandwich films showed a novel photoeffect clearly resolvable at temperatures much

higher than in the pure material, which is particularly clear in the 40/40/40 film, Figure

7-36. Therefore, to glean the underlying nature of the effect observed in all

heterostructures, the optimized 40/40/40 film was studied in further detail. These

studies include additional measurements in a SQUID magnetometer, Figure 7-37.

Transmission electron microscopy was performed to resolve the nanostructure of the

sample, Figure 7-38. Nanometer resolved high-resolution inelastic x-ray scattering,

EDS, was performed to resolve the chemical makeup as a function of film position,


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Figure 7-39. Elastic x-ray powder diffraction resolved the lattice constants present in

the heterostructure, Figure 7-40. Fourier transform infrared spectroscopy of the cyanide

stretches in the heterostructure, compared to the stretches in the pure constituent

materials, also provides evidence for the structure and chemical content of the

heterostructures, Figure 7-41.

7.2.4.1 40/40/40 film, 10 kG temperature sweeps

To test the nature of the photoinduced effect, temperature sweeps were performed

in high fields of 10 kG for both the light and dark states, Figure 7-37 (a). Difference

plots between the light and dark states show that 10 kG is sufficient to overcome the

photoinduced decrease when the temperature is less than approximately 60 K, Figure

7-37 (b).

7.2.4.2 40/40/40 film, transmission electron microscopy

To investigate the structure of the 40/40/40 Ni-Cr/Co-Fe/Ni-Cr heterostructure,

samples were microtomed and mounted on a transmission electron microscope.

Contrast differences in the transmission can be assigned to the different Prussian blue

analogue lattices, Figure 7-38 (a) and (b). While discrete regions are clear, interfacial

surfaces have roughnesses on the order of 20 nm. It is worth mentioning that additional

experiments were done, where the microtome process incited a fracture at the interface

between the Co-Fe and Ni-Cr layers, presumably due to the high strain induced by the

lattice mismatch, Figure 7-38 (c).

7.2.4.3 40/40/40 film, energy dispersive x-ray spectroscopy

While transmission electron microscopy provides clear evidence for the proposed

structure, the chemical composition as a function of the height of the film can provide

additional details about the heterogeneous atomic makeup. Energy dispersive x-ray


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spectroscopy (EDS) were performed on a JOEL 2010F super-probe. By line-scanning

an electron beam across the microtome heterostructure, position dependence of the

Co-Fe fraction can be plotted and directly compared to the model structure, Figure 7-39.

Practically, the Co-Fe fraction is found by integrating Co and Fe peaks together and

integrating Ni and Cr peaks together. The Co-Fe fraction is then the total amount of Co

and Fe divided by the total amount of Co, Fe, Ni and Cr.

7.2.4.4 40/40/40 film, x-ray powder diffraction

A Philips APD 3720 powder diffractometer was used to perform room

temperature x-ray diffraction (XRD) using a Cu K, source with a primary wavelength of

1.5418 A. Despite a large background due to the Melinex from the solid support, two

clear peaks can be seen, Figure 7-40. Two important conclusions can be drawn from

these data. First, the x-ray powder diffraction provides additional microscopic evidence

of the existence of both Prussian blue analogues in the heterostructure, in proportions

consistent with other microscopic measurements. Second, unlike substitutional solids

of Co-Fe [141], the heterostructures possess two distinct lattice constants, showing that,

while bonded at the interface, the majority of the constituents remain in structures

similar to their pure states.

7.2.4.5 40/40/40 film, infrared spectroscopy

Fourier transform spectroscopy was performed on the 40/40/40 heterostructure, as

well as the constituent pure materials, Figure 7-41. The heterostructure shows discrete

peaks corresponding to the cyanide stretches of the constituents.

7.2.5 Capping Layers of Cobalt and Chromium Hexacyanochromates

In order to further explore the novel photoeffect, most prominently present in the

40/40/40 Ni-Cr/Co-Fe/Ni-Cr heterostructure, different capping layers of


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RbaCob[Cr(CN)6]c-nH20 (Co-Cr) and RbaCrb[Cr(CN)6]c-nH20 (Cr-Cr) were used. The

Co-Cr analogue is known to be a ferromagnet with an ordering temperature near

30 K [155], and the Cr-Cr is a ferrimagnet with an ordering temperature near

200 K [116]. Hexacyanoferrate based capping layers were not used in order to avoid

additional charge transfer between the Fe in the Co-Fe layer and Fe in the capping

layer.

7.2.5.1 Magnetization of cobalt hexacyanochromate and cobalt
hexacyanoferrate sandwich heterostructures

Two Co-Cr/Co-Fe/Co-Cr heterostructures are presented, a 40/40/40 and 40/60/40

layering scheme. The temperature dependence of the DC magnetic susceptibilities,

X = M/H, are shown for 40/40/40 and 40/60/40 Co-Cr/Co-Fe/Co-Cr in Figure 7-42 (a)

and Figure 7-42 (b), respectively, for temperatures between 2 K and 50 K and an

external field of 100 G oriented parallel to the plane of the films. Time dependence

during irradiation is shown in the inset of 7.20 (b), showing clear increase below the

ordering temperature of Co-Fe and decrease above the ordering temperature of Co-Fe.

Sharp increases in the temperature sweeps can be associated with ordering of the pure

Co-Fe and Co-Cr phases, at ~10 K and ~30 K, respectively. Clear photoinduced

magnetization can be observed well above the ordering temperature of the Co-Fe, and

up to the ordering temperature of the Co-Cr. The photoeffect is seen to be negative,

excepting when dominated by ordered Co-Fe magnetization.

7.2.5.2 Magnetization of chromium hexacyanochromate and cobalt
hexacyanoferrate sandwich heterostructures

Two Cr-Cr/Co-Fe/Cr-Cr heterostructures are presented, a 40/40/40 and 60/40/60

layering scheme. The temperature dependence of the DC magnetic susceptibilities,

X = M/H, are shown for 40/40/40 and 60/40/60 Cr-Cr/Co-Fe/Cr-Cr in Figure 7-43 (a) and


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Figure 7-43 (b), respectively, for temperatures between 2 K and 300 K and an external

field of 100 G oriented parallel to the plane of the films. The difference between the

irradiated and dark sample for the 60/40/60 Cr-Cr/Co-Fe/Cr-Cr film is shown in the inset

of Figure 7-43 (b), showing detectable changes in the magnetization at temperatures

well above the liquid point of nitrogen. Sharp increases in the temperature sweeps can

be associated with ordering of the pure Co-Fe and Cr-Cr phases, at ~10 K and ~230 K,

respectively. The photoeffect is again observed to be negative.

7.2.6 Discussion

This chapter describes the characterization of cyanometallate Prussian blue

analogue heterostructured films, specifically with photomagnetic Co-Fe as a constituent.

The heterostructure geometry leads to two striking new behaviors, an increase in the

ordering temperature of the photomagnetic effect compared to Co-Fe and a change in

the sign of the photomagnetic effect compared to Co-Fe. The synthesis technique is

elegant in its simplicity, allowing fine control over thickness and constituents, while

using room temperature and pressure wet chemistry. The magnetism data presented

suggest a new mechanism for PPIM, whereby photoinduced changes in one lattice alter

the magnetic response of the other.

Many samples were studied to arrive at the current understanding of the effect.

Slow deposition multilayer films formulated with slow deposition techniques showed too

much intermixing of the lattices and insufficient material transferred to the solid supports.

The problems with slow deposition led to fast deposition techniques to be tried, with

stacked films showing promise due to a modification of the photoeffect, which remained

a small fraction of the total magnetization. A small breakthrough came with a sandwich

geometry, in which clear decreases in susceptibility were seen for films, although the


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effect was small on the background of the total magnetization. The big break came with

the thick sandwich geometry, where the effect was found to be large enough to be

resolved as a function of temperature and field. In fact, thick sandwich structures were

the first example showing a clear increase of the photoinduced modification of long-

range magnetic order, much higher than the pure Co-Fe material. These geometries

were engineered to give the optimal effect, observed in a 40/40/40 Ni-Cr/Co-Fe/Ni-Cr

heterostructure. This sample was studied with additional probes, to explore the

nanostructure and atomic character. Finally, different capping layers, of Co-Cr and

Cr-Cr, were fabricated and studied for their photomagnetic effects.

The structural probes clearly display the multi-layer character of the 40/40/40

Ni-Cr/Co-Fe/Ni-Cr heterostructure. The lattice constants and cyanide stretches of the

heterostructure are consistent with those observed in the homogeneous precursor Ni-Cr

and Co-Fe materials, and EDS line scans show an evolution of the chemical formula

with film height. Finally, TEM images show difference in contrast that can be associated

with the different layers of Co-Fe and Ni-Cr.

All studies come back to the novel photoeffect observed in the heterostructures

and the search to understand the fundamental origins of the photoeffect. To this end,

the well documented photoeffect in Co-Fe must first be considered. To begin, the

mechanism of PPIM in bulk Co-Fe PBA involves light induced electron transfer from

Fe2+ (LS, S = 0) to Co3+ (LS, S = 0), yielding long-lived metastable Fe3+ (LS, S = 1/2)-

CN-Co2+ (HS, S = 3/2) pairs that couple antiferromagnetically, giving rise to a net

increase in magnetization in the ferrimagnetic state below 18 K [1] [78] [81]. The

electron transfer and change in spin state also lead to a change in lattice constant,


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increasing ~0.2 A upon transitioning from the low-spin state to the high-spin state,

Figure 7-44 [160] [161].

When Co-Fe layers are fabricated to be in intimate contact with another analogue

that is not photoactive, these structural distortions propagate through the

heterostructure, Figure 7-45. The hexacyanochromate based networks have been

shown to have a dependence of the magnetic susceptibility upon pressure, with the

divalent nickel analogue have the greatest pressure dependence. For Ni-Cr, decreases

of the magnetization of ~ 50% can be induced by the application of hydrostatic pressure

of 0.8 GPa [162].

The photomagnetic response of the heterostructure indicates that the structural

change in the Co-Fe PBA layer couples to the M-Cr PBA (where M = Co, Cr, or Ni),

leading to the change in magnetization due to distortion of the divalent metal octahedral,

Figure 7-46. The interesting aspect exists because the other Prussian blue analogue

has a much greater ordering temperature compared to the Co-Fe. The dependence of

the high-Tc photoeffect in the heterostructures on capping layer can be clearly

correlated to the pressure dependence of the capping layer. The Ni-Cr has the most

pressure dependence, and therefore the photoeffect in the heterostructure is the most

dramatic. The end result is a meta-magnet, with long-range magnetic order, that

exhibits large changes in magnetization with the application of light, at unprecedented

temperatures for this class of compounds, due to photoinduced structural distortions in

the Co-Fe layer propagating to the previously non-photoactive capping layer.

Finally, it is tantalizing that, in materials with capping layers containing

antiferromagnetic exchange pathways (Co-Cr and Cr-Cr), small modifications of the


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ordering temperature can be seen, suggesting a photoinduced modification of the

exchange coupling in the samples. Unfortunately, an ideal candidate to further probe

this idea, a 40-40-40 Mn-Cr/Co-Fe/Mn-Cr heterostructure, was unable to be synthesized,

presumably due to the large lattice mismatch between the two materials and different

space groups [163]. In the future, an interesting set of experiments could study the Mn-

NC-Fe molecule as opposed to the Co-NC-Fe, and therefore synthesize a

Mn-Cr/Mn-Fe/Mn-Cr heterostructure and look for photoinduced changes in the magnetic

coupling of the Mn-Cr layer.

7.2.7 Conclusion

In summary, heterostructured films consisting of two different Prussian blue

analogues, one with a high-Tc and the other photoactive, have been fabricated for the

first time, and this novel arrangement leads to persistent photoinduced changes in

magnetization at elevated temperatures. The new behavior is not seen in either pure

phase and requires the unique heterostructure arrangement that generates an interface

between them. Simple mixing of ions in a three-dimensional lattice does not give the

same result, and in fact, serves to greatly suppress the amount of Co-Fe material that is

bistable [141] [135]. Heterostructures based on coordination polymers are largely

unexplored, and these results provide an example of new phenomena arising from

engineered coordination polymer based structures that may motivate the rational design

of further systems with new applications.


248









(b) Photoinduced Decrease in Magnetization


e
O i O 0 Co" or Co"'
ONi"
o 0..*-.
S* Fe"' or Fe"
o o ft Na

C16 0 H20
SCN NC H2
~10A


Hext
Hext


p-
4$ ^
ci --- d


(c) Photoinduced Increase in Magnetization


Hext 0
I Spair= 0 ~Sr-


Figure 7-1. The NaaNil-xCox[Fe(CN)6]p-nH20 material. (a) Structural model for
NaaNil-xCox[Fe(CN)6]p-nH20. (b) A decrease in the magnetization with
photoexcitation of a Ni dominated material when there is atomic mixing, and
spins are in an ordered state dictated by the exchange interactions JNiFe > 0
and JCoFe < 0. (c) The usual increase in the magnetization with
photoexcitation of a sufficiently Co dominated material.


Table 7-1. Molecular formulas and unit cell parameters for NaaCoxNil-x[Fe(CN)6]p nH20.
[23]
x Proposed molecular formula Unit cell length
0.0 Nao.27Ni2+1.o[Fe3+(CN)6]0.73[Fe2+(CN)6]o0.2 5.0 H20 10.23(9)

0.22 Nao.31C02+0.22Ni2+ 0.8[Fe3+(CN)6]0.74[Fe2+(CN)60.03 4.4 H20 10.24(9)

0.45 Nao.34CO2+0.45Ni2+0.55[Fe3+(CN)6]o.71[Fe2+(CN)6]0.05 4.9 H20 10.25(6)

0.66 Na.33 C2+ 0.66Ni2+0.34[Fe3+(CN)6]o.67[Fe2+(CN)6]0.08 4.6 H20 10.26(8)

0.87 Na o.27C2+0.87Ni2+o.13[Fe3+(CN)6]o.63[Fe2+(CN)6]o.10 3.8 H20 10.28(9)

1.0 Nao.31Co2+1.0[Fe3+(CN)6o0.72[Fe2+(CN)6]0.04 4.4 H20 10.30(7)


249









U.4


Figure 7-2. Typical TEM micrographs for samples reported in Table 7-1 for different
values of x. All scale bars shown are 100 nm. A continuous increase in
equilibrium particle edge length is observed when cobalt ions are added to
the extended networks. The average particle sizes, in nanometers, from left
to right are: 15.6 3.4, 26.5 5.3, 28.7 6.9, 38.7 7.7, 117.2 22.7, and
237.8 40.1.


250


1.00


jr
























S-
3


U
-)
0


- .













2200
2200


2000


Wavenumber (cm1)


4 24


o (cm-)


4(co- r,+w2


x= 0.00 2163.2 24.7 30.7
2122.9 22.3 5.0
2043.0 26.9 1.4
2078.6 40.5 2.5
x= 0.22 2163.3 24.6 9.2
2120.1 20.6 1.0
2095.9 42.1 0.9
2044.8 38.1 1.2
x=0.45 2161.5 22.1 8.9
2119.6 26.6 1.3
2069.5 42.8 1.6
2041.0 25.2 0.7
x= 0.66 2160.7 23.7 6.3
2118.6 29.0 1.1
2070.3 48.1 1.9
2038 2 24.4 0.6
x=0.87 2161.2 31.8 5.7
2116.9 15.2 0.4
2094.8 38.3 2.3
2040.5 35.1 1.5
x= 1.00 2159.9 20.8 23.7
2114.5 12.3 0.9
2094.0 27.9 3.6
2056.8 41.6 5.5


Figure 7-3. FT-IR spectra and fitting parameters of NaaNil-xCox[Fe(CN)6]p.nH20 as a
function of x. Fits ( ) were done using four Lorentzian lines (orange).
Gaussian line-shapes were also tried, but the results with the Lorentzian fits
had smaller residuals. A standard Levenberg-Marquardt algorithm was
employed for simultaneous fitting of the four lines until no observable change
in X2 was detected.


251


2100


W (cm-1)


A (I cm-1)













x = 0.00 -
- I -








x=0.22 -









S.45
^ ^L UL0.4JL^


Space group Fm3m (No 225)

Wyckoff position g x y z
Na 8c 027 025 025 025
Ni 4b 1 05 05 05
Co 4b 0 05 05 05
o Fe 4a 075 00 00 00
0 C 24e 075 0196(4) 00 00
SN 24e 075 0296(5) 00 00
01 24e 025 0216(2) 00 00
OA 32f 023 0283(7) 0 283(7) 0 283(7)
OB 192/ 006 0243(3) 0076(9) 0189(6)
Na 8c 031 025 025 025
Ni 4b 078 05 05 05
Co 4b 022 05 05 05
Fe 4a 077 00 00 00
0 C 24e 077 0211(7) 00 00
SN 24e 077 0294(0) 00 00
01 24e 023 0205(0) 00 00
OA 32f 023 0283(2) 0283(2) 0283(2)
OB 192/ 006 0206(4) 0078(1) 0210(2)
Na 8c 034 025 025 025
Ni 4b 055 05 05 05
Co 4b 045 05 05 05
Fe 4a 076 00 00 00
0 C 24e 076 0204(1) 00 00
SN 24e 076 0290(6) 00 00
01 24e 024 0207(6) 00 00
OA 32f 023 0282(2) 0282(2) 0282(2)
OB 192/ 006 0206(4) 0078(2) 0210(2)
Na 8c 033 025 025 025
Ni 4b 034 05 05 05
Co 4b 066 05 05 05
SFe 4a 075 00 00 00
0 C 24e 075 0204(5) 00 00
SN 24e 075 0292(6) 00 00
01 24e 025 0218(0) 00 00
OA 32f 023 0282(1) 0282(1) 0282(1)
OB 192/ 006 0206(4) 0078(2) 0210(2)
Na 8c 027 025 025 025
Ni 4b 013 05 05 05
Co 4b 087 05 05 05
Fe 4a 073 00 00 00
0 C 24e 073 0197(0) 00 00
SN 24e 073 0279(0) 00 00
01 24e 027 0212(7) 00 00
OA 32f 023 0285(6) 0 285(6) 0 285(6)
OB 192/ 006 0178(6) 0070(2) 0232(0)
Na 8c 031 025 025 025
Ni 4b 000 05 05 05
Co 4b 1 00 05 05 05
o Fe 4a 076 00 00 00
C 24e 076 0197(0) 00 00
SN 24e 076 0279(0) 00 00
x
01 24e 027 0212(7) 00 00
OA 32f 023 0285(6) 0 285(6) 0285(6)
OB 192/ 006 0178(6) 0070(2) 0232(0)


10 20 30 40 50 60

20 (degrees)


Figure 7-4. Full XRD diffractograms of NaaNil-xCox[Fe(CN)6]p-nH20. XRD data
(black), fitting parameters and fits (orange), with residuals displayed
below the peaks (dark cyan), and the fitted background Visible as a


thin line (


). Organic parameters were not of a particular interest


and many degenerate solutions consisting of slight perturbations to the
quoted solutions are possible.


252


x = 0.87 -






















16.8 17.0 17.2
20 (degrees)


Wyckoff
position g x y z
Na 8c 0.31 0.25 0.25 0.25
Ni 4b 0.00 0.5 0.5 0.5
o
6 Co 4b 1.00 0.5 0.5 0.5
SFe 4a 0.76 0.0 0.0 0.0
SC 24e 0.76 0.197(0) 0.0 0.0
N 24e 0.76 0.279(0) 0.0 0.0
S01 24e 0.27 0.212(7) 0.0 0.0
I OA 32f 0.23 0.285(6) 0.285(6) 0.285(6)
S OB 192/ 0.06 0.178(6) 0.070(2) 0.232(0)


Figure 7-5. XRD of the x = 1 NaaNil_xCox[Fe(CN)6]p-nH20. Since only 0.59 mg of the
x = 1.00 sample was available for the XRD studies, as opposed to the
10-20 mg used for x-ray powder diffraction experiments on the other samples,
a much weaker signal to noise ratio resulted. In addition, the background
curve resulting from the sample holder is seen to be comparable to the signal,
Figure 7-4. For these reasons, additional data were acquired for the (2,0,0)
reflection near 17.10. While a value of 10.3051(9) A was obtained for the
original fit, a value of 10.3072(6) A was generated by fitting this single
reflection, with better statistics, using the parameters for the structure from
the original fit, and simply refining the unit cell. The fit peak (orange) is
displayed, with residuals displayed below the peaks (dark cyan), and the
fitted background is shown as a thin line (


34.0 34.5 35.0
20 (degrees)


35.5 36.0


Figure 7-6. Room temperature XRD reflection with background subtracted and intensity
normalized to show the continuous evolution with x. The peak position shifts
as x decreases and the line width broadens, reflecting the smaller particle
size of the pure Ni-Fe analogue.


253












x = 0.00 100 ,.. x=0.00 x x=0.00
(a) (b) "... 100 (c)
no effect 50 .
50

0--...... 0-
x =0.22 <= 0.22 x = 0.22
128 100 100

127
50 50
126 0

x =0.45 -. x=0.45 100 x = 0.45
:86.0
E 50
2 I ?- |5 |50 \
E85.5 50

85.0 0 .I 0

42.5 x =0.66 40 x= 0.66 x = 0.66
40
42.0 20 20
20

41.5- 0 AAA' V" 0
100x
x=0.87 x=0.87 100 x=0.87
80
60 50 50
40
20 0 0
130 x=1.00 x=1.00 x x=1.00
2 100 100 %

110 50 c *. 50

100 0o ----A 0
0 10 20 30 40 0 5 10 15 20 25 30 0 5 10 15 20 25 30
time (hours) T(K) T (K)


Figure 7-7. Photoinduced magnetization of NaaNilxCox[Fe(CN)6]p-nH20. (a) Molar
magnetic susceptibility as a function of time irradiated at 5 K and 10 G,
measured in a SQUID. Discontinuities in magnetization when the light is
turned on and off are due to a subtle heating effect from the applied light.
(b) Molar magnetic susceptibility as a function of temperature in both the dark
FC (m), ZFC (A) and photoirradiated states (0), measured in a SQUID at 10 G.
(c) Mean-field calculations of molar magnetic susceptibilities at 10 G as a
function of temperature in both the dark state (solid line), where the high-spin
fraction, nHs, is determined from fitting high temperature susceptibility, and
photoirradiated states (dashed line), where nHs = 1. The magnetic signals are
expressed per mole of sample using the chemical formulas in Table 7-1.


254












86.0 x = 0.45


85.5


-3 85.0

x = 0.66
42.2


42.0


-30 -20 -10 0 10 20 30 40 50 60
time (minutes)

Figure 7-8. Molar magnetic susceptibility of NaaNil-xCox[Fe(CN)6]p-nH20 as a function
of time irradiated at 5 K and 10 G, measured in a SQUID. The samples are
irradiated continuously for time > 0, where the step in X at t = 0 is associated
with a small heating effect. For the x = 0.66 sample, the magnetization has a
small initial increase for 10 minutes followed by the large photoinduced
decrease.


255












(a) x = 0.00


S. 0.4 (c)
0.2


85 5 -0.2 -=2
E -1.0 x =0.45 -0.4 x = 0.45
8 4 x =0x.66 Z .. .b 1.0 4
42.5
0.5 0.2 *
42.0 0.0 0.0
-0.5-
41.5 1.0 x=0.66 -04 0.66


60 05 00 ;
40 00 -0.2
20 x 0.87 -0.5 x=0.87 -0.4 x=0.87
130 05 j 0.4

120 50.2
00
_2.0


Sx= 1.00 -0.5 x =1.00 -0.4 4 x =1.00
100
0 10 20 30 40 -20 0 20 40 60 80 -4 -2 0 2 4
trne (hours) H (kG) H (kG)



Figure 7-9. Magnetization versus field for NaaNil-xCox[Fe(CN)6]p- nH20. (a) Molar
magnetic susceptibility as a function of time irradiated at 5 K and 10 G,
measured in a SQUID. Molar magnetization for both (b) high field (c) and low
field, as a function of magnetic field in both the dark (m), and photoirradiated
states (0), measured in a SQUID at 2 K. High field magnetization always
increases after photoirradiation, even for samples showing a photodecrease
at low fields. The smallest field on the field sweeps is 100 G.


256











0.75

0.50


o 0.25
E
5 0.00
E
o -0.25

-0.50


-0.75


1000


-500


0
H (G)


500 1000


Figure 7-10. Checking for asymmetry in the hysteresis loop as a possible explanation
of the reduction in Hc for the x = 0.66 sample. Field was swept from 100 G to
70 kG to -70 kG and back to 70 kG, all at 2 K. No asymmetry is observed
within experimental uncertainty.


257


x= 0.66
- *


S" +880 G
*
-890 G *


I-I II*
.-*
~ *
























2.
o
E

E
3.(
^ 3.C


1 r 1 11 02
100 150 200 250 300 100 150 200 250 300 100 150 200 250 300
T(K) T(K) T(K)


Figure 7-11. Thermal induced changes in magnetization of NaaNilxCox[Fe(CN)6]p-nH20.
(a) XT versus T as measured in a SQUID magnetometer with 5 kG applied
field. The results of mean-field calculations are shown for (b) XT versus T
and (c) the high-spin fraction, nHS, versus T. The magnetic signals are
expressed per mole of sample using the chemical formulas listed in Table 7-1.


258

























Figure 7-12. Microscopic versus macroscopic mixing. TEM images of the manually
mixed powder with x = 0.60 showing the presence of both (a) Co-Fe and
(b) Ni-Fe powders in the material with the same sizes reported for the
x = 0.00 and x = 1.00 materials, respectively (Figure 7-2). All scale bars
shown are 100 nm.


0 5 10 15 20 25 30
t (hours)


0.3

0.2

0.1


35 0


5 10 15 20
T (K)


Figure 7-13. Magnetization of macroscopically mixed NaaNil-xCox[Fe(CN)6]- nH20.
Susceptibility measurements of manually mixed powder with x = 0.60 in
~10 G showing (a) no decrease in the magnetization with photoirradiation
over time, but rather a clear increase and (b) two well defined ordering
temperatures present in the material as well as an overall increase in the
magnetization at all temperatures from the dark (m) compared to the
photoirradiated (E) states. The small change in the susceptibility above ~
17 K is due to the slightly different measuring fields arising from the fact that
these data predate the careful procedure described in the text ensuring the
same measuring field for photoirradiated and dark states.


259


(b)


.- 0


D
0

--


.m .m.
mmU_,_


25 30


















0
E
E -0.2

-0.4
H=10
-0.6
-1 0 1 2 3 4 5
time (hours)


Figure 7-14. Field dependence of photoinduced magnetization for
NaaNil-xCox[Fe(CN)6]p-nH20. Photoinduced change in susceptibility for the
x = 0.66 sample at T = 5 K measured at low field (H = 10 G) and high field (H
= 1 kG). Photoirradiation is continuous for time > 0.


260












30 (a)





10
O (b)
T 2

1 .

30
(c)
20
10
0
-10 -
-20
0.0 0.2 0.4 0.6 0.8 1.0
x


Figure 7-15. Scaling of magnetic properties of NaaNil-xCox[Fe(CN)6]p nH20. (a) The
magnetic critical temperature, Tc, (b) the coercive field, Hc, and (c) the
Curie-Weiss temperature, Ocw, for both "low-spin" (m) and "high-spin" (0)
states, as a function of x. Dashed lines are a mean-field interpolation
between the two pure materials. Solid lines are from mean-field fits of Ocw
that allow for a modification of exchange constants as the M-NC-M' distance
changes as a function of x; superexchange constants were empirically scaled
to 80 % of their fit value for comparison to the low temperature Tc values.
Coercive fields were obtained at T = 2 K after sweeping to 70 kG.
Curie-Weiss temperatures, Ocw, were obtained by fitting from 250 K to 300 K,
where nHs and [teff are essentially constant.


261











(a) (b) 6 -

Co"-NC Fe"'-CN Ni"-NC 4 x=l
J J 2_ J
-C- o -eFe -- -@- Ni 'E

@G --&-t2g CoA-@- -@- --t2gFe -M- -- t2gNi -2 x = 0
10.24 10.26 10.28 10.30
Lattice Constant (A)


Figure 7-16. Superexchange in NaaNi_-xCox[Fe(CN)6]p-nH20. (a) Energy levels and
diagram for the superexchange interactions considered in the material. The
Co (HS) ion notably has both ferromagnetic (JF > 0) and antiferromagnetic
(JAF > 0) superexchange interactions with Fe3+ (LS), in contrast with the Ni2+.
(b) Average values of the exchange constants, Jave, for the Ni2+-NC-Fe3+ (x)
and Co2+-NC-Fe3+ (+) exchange bonds used in order to reproduce the
scaling in Figure 7-15. The line is merely a guide for the eye.





40 '
30 (a)
S20 -
i 0 -

4
3 (b) *
o 2 -
02
Nt 16-:
0 -
0.0 0.2 0.4 0.6 0.8 1.0
x


Figure 7-17. Charge transfer induced spin transition parameters for
NaaNil-xCox[Fe(CN)6]p-nH20. (a) The width of the thermal hysteresis,
Tup-Tdown, and (b) the number of spin-crossover active nearest neighbors,
zsco, as a function of x. Here, Tup-Tdown is defined by the difference in the
temperature at which half of the spin-crossover active material is high-spin
when sweeping up in temperature, and the temperature at which half of the
spin-crossover active material is high-spin when sweeping down in
temperature.


262












100
> 80
t 60
S40
- 20
S 0
10.30
10.28
10.26
10.24


- I I I,'I I ', I I -
S (a)



-U
1 7
S I i I l I
- (b)
-0


-0
0.0 0.2 0.4 0.6 0.8 1.0


x


Figure 7-18. Amount of CTIST materials in NaaNil-xCox[Fe(CN)6]p-nH20. (a) The
percentage of CTIST active material, %CTIST active, and (b) the unit cell
lattice constant, a, as a function of x.


1.208
1.206
1.204
1.202
1.200
2164


- 2162
E

" 2160


0.0 0.2 0.4 0.6 0.8 1.0
x



Figure 7-19. FT-IR parameters in NaaNil-xCox[Fe(CN)6]p nH20. (a) The effective spring
constant and (b) FT-IR frequency for M -C-N-Fe3+ as a function of x.


263


(a) U






(b) -
I I I I I l












2 (a) x = 0.66 40 x = 0.66 (b)
3.2-
30
20

3.1 AA

S 1 -10 ."
-20 //
30.0
-40 --_
100 150 20 250 300 0 5 10 15 20 25 30
T(K) T(K)

Figure 7-20. Comparison of ferromagnetic versus antiferromagnetic Co-Fe components.
(a) Comparison of XT versus T for SQUID magnetometer data (]), mean-field
calculation ( ) using JCoFe and JNiFe values from the binary materials, and
mean-field calculation ( ) fitting from 250 K to 300 K without
modifying JNiFe. (b) Comparison of X versus T for SQUID magnetometer data
(0 = FC photoinduced, = FC dark, A = ZFC dark), mean-field calculation (-
= FC dark,--- = FC photoinduced) using JCoFe and JNiFe values from the binary
materials, and mean-field calculation (- = FC dark, --- = FC photoinduced)
fitting from 250 K to 300 K without modifying JNiFe.


264











1< FCoFe
CTIST active
t-
U
E CTIST inactive
JCoFe

S.CTISlactive
E CTIST inactive



Supere ..change energy

Figure 7-21. Modification of superexchange energy in NaaNil-xCox[Fe(CN)6]p-nH20. (a)
The ternary metal NaaNil-xCox[Fe(CN)6]p-nH20 compounds were found
experimentally to have CTIST active and CTIST inactive populations, as
approximated from the high temperature magnetic susceptibility and the room
temperature FT-IR data. (b) A sketch of the superexchange energy
distribution showing , which is determined by fitting XT versus T from
250 K to 300 K. The CTIST inactive portion, J'coe, can actually have
ferromagnetic character in some samples. For simplicity, it is assumed that
the electronic structure of the CTIST active portion is most similar to the pure
x = 1.00 material and thus the magnetic interactions, JcoFe, are similar.


265

















One Ni-Cr


I One C -e


lOne Ni-Cr cycle I


Figure 7-22. A scheme showing synthesis of a heterostructured thin film using multiple
sequential adsorption cycles.


266


5,* ~
r

...


it IElNi

|Nri fNi1

isi









Table 7-2. Nickel hexacyanochromate and cobalt hexacyanoferrate heterostructures
studied.
Number of cycles Order of deposition Deposition Speed

1/1 x 40 Ni-Cr/Co-Fe/Ni-Cr/Co-Fe/... Slow

5/5 x 20 Ni-Cr/Co-Fe/Ni-Cr/Co-Fe/... Slow

40/40 Ni-Cr/Co-Fe Slow

10/10 Ni-Cr/Co-Fe Fast

10/10 Co-Fe/Ni-Cr Fast

10/5/10 Ni-Cr/Co-Fe/Ni-Cr Fast

10/10/10 Ni-Cr/Co-Fe/Ni-Cr Fast

40/40/40 Ni-Cr/Co-Fe/Ni-Cr Fast

20/40/20 Ni-Cr/Co-Fe/Ni-Cr Fast

40/20/40 Ni-Cr/Co-Fe/Ni-Cr Fast

40/40/40 Ni-Cr/Co-Fe/Ni-Cr Fast


0 20 40 60
T(K)


15


E
3


80 100


0.0 0.5
time (hours)


Figure 7-23. Magnetization of slow deposition multilayer films. (a) Plots of FC and ZFC
DC magnetic susceptibilities of 1/1 x 40 Co-Fe/Ni-Cr (-), 5/5 x 20 Co-Fe/Ni-Cr
(-), and 40/40 Co-Fe/Ni-Cr (-) films. The lines are guides to the eye
connecting the data points, taken every 10 K. (b) AM versus time irradiated,
are shown for parallel orientations of films 1/1 x 40 Co-Fe/Ni-Cr (-), 5/5 x 20
Co-Fe/Ni-Cr (-), and 40/40 Co-Fe/Ni-Cr (-) with applied magnetic field of
100 G and at 5 K.


267








4.0
3.5
-3.0
E 2.5
S2.0
E

1.0
X0.5
0.0


0 20 40 60 80 100
T (K)


-1 0 1 2
time (hours)


Figure 7-24. Magnetization of 10/10 Co-Fe/Ni-Cr thin film oriented parallel. (a) Plots of
FC and ZFC DC magnetic susceptibilities versus temperature for the 10/10
Co-Fe/Ni-Cr thin film in H = 100 G parallel to the planes of the films are
shown. (b) AM versus time irradiated is shown at 5 K and 100 G parallel to
the film.


2.0

1.5
"E
1.0
E
U,
S0.5

0.0


.935


.930


.925


I ....... I 1.9201 -, I
0 20 40 60 80 100 -1 0 1
T (K) time (hours)


Figure 7-25. Magnetization of 10/10 Co-Fe/Ni-Cr thin film oriented perpendicular.
(a) Plots of ZFC DC magnetic susceptibility versus temperature for the 10/10
Co-Fe/Ni-Cr thin film in H = 100 G perpendicular to the planes of the films are
shown. (b) AM versus time irradiated is shown at 5 K and 100 G
perpendicular to the film.


268


3.95

3.94

3.93
E
a 3.92
(9
3.91

3.90


Fc (a) -
Co-Fe/Ni-Cr-





ZFC
Z-














6
E
U,
N-
Z5
1


0 20 40 60 80 100
T (K)


0 1 2
time (hours)


Figure 7-26. Magnetization of 10/10 Ni-Cr/Co-Fe thin film oriented parallel. (a) Plots of
FC and ZFC DC magnetic susceptibilities versus temperature for the
10/10 Ni-Cr/Co-Fe thin film in H = 100 G parallel to the planes of the films are
shown. (b) AM versus time irradiated is shown at 5 K and 100 G parallel to
the film.


1


E
C,
N-
0


S.. 2 .0 ,
Co--e ordering (a) (b)

S Ni-Cr/Co-Fe -



C |1.8 -


-ZFC
I I I 1.7
0 20 40 60 80 100 -1 0
T (K) time


1
(hours)


Figure 7-27. Magnetization of 10/10 Ni-Cr/Co-Fe thin film oriented perpendicular.
(a) Plots of FC and ZFC DC magnetic susceptibilities versus temperature for
the Ni-Cr/Co-Fe thin film in H = 100 G perpendicular to the planes of the films
are shown. (b) AM versus time irradiated is shown at 5 K and 100 G
perpendicular to the film.


269


20

015
E
a) 10
N-



0












"E
0
E
U,
(9
C


0 20 40 60 80 100
T (K)


-1 0 1 2
time (hours)


Figure 7-28. Magnetization of 10/5/10 Ni-Cr/Co-Fe/Ni-Cr thin film oriented parallel. (a)
Plots of FC and ZFC DC magnetic susceptibilities versus temperature of the
sandwich 10/5/10 Ni-Cr/Co-Fe/Ni-Cr PBA thin film in H = 100 G parallel to the
planes of the films are shown. (b) AM versus time irradiated is shown at 5 K
and 100 G parallel to the film.


E
2 EH
E

1
M ZFC

0 20 40 60 80 100
T (K)


-1 0 1 2
time (hours)


Figure 7-29. Magnetization of 10/5/10 Ni-Cr/Co-Fe/Ni-Cr thin film oriented
perpendicular. (a) Plots of FC and ZFC DC magnetic susceptibilities versus
temperature of the sandwich 10/5/10 Ni-Cr/Co-Fe/Ni-Cr PBA thin film in H
= 100 G perpendicular to the planes of the films are shown. (b) AM versus
time irradiated is shown at 5 K and 100 G perpendicular to the film.


270








12 -- 10/10/10
10 -
"EH and
o 8 Co-Fe ordering '
E6-
CO 4

S2
0

0 20 40 60 80 100
T (K
Figure 7-30. Magnetization of 10/10/10 sandwich film versus temperature. Plots of FC
DC magnetic susceptibility versus temperature for the 10/10/10 sandwich
Ni-Cr/Co-Fe/Ni-Cr PBA thin film in H = 100 G are shown for parallel (-) and
perpendicular (-) orientations of the plane of the film with respect to the
applied field.


271










4.


4.



4.


-1 0 1
time (hours)


(b)


(b) 1000 C
- 1.371-


l .37


-1.36-
- I i I
2 -1 0 1
time (hours)


Figure 7-31. Photoinduced magnetization of 10/10/10 film oriented perpendicular. AM
versus time irradiated is shown for perpendicular orientations of the 10/10/10
sandwich Ni-Cr/Co-Fe/Ni-Cr PBA thin film with respect to the applied
magnetic field of (a) 100 G and (b) 1 kG, at 5 K.


.686

.684

.682

.680


-1 0 1 2
time (hours)


(b)
1000 G
-r


-1 0 1
time (hours)


Figure 7-32. Photoinduced magnetization of 10/10/10 film oriented perpendicular. AM
versus time irradiated is shown for parallel orientations of the 10/10/10
sandwich Ni-Cr/Co-Fe/Ni-Cr PBA thin film with respect to the applied
magnetic field of (a) 100 G and (b) 1 kG, at 5 K.


272










15 (a)- 40- 0a I Sc)
15 E =.= E-
10 o --
0 10 20 30 40 50 60 7000 -200 0 200 4 204060
S5-3 o-
-b 2020
U? __ c- :0 -..
5- (b)--2---- .... J -o .. ,
O -40 -...400 -20 G. 2. 40
0 10 20 30 40 50 60 70 -60 -40-20 0 20 40 60
Temperature (K) H (kG)

Figure 7-33. Magnetization of 40/40/40 Ni-Cr/Co-Fe/Ni-Cr heterostructure. The x(T)
data, normalized to the area of the 40/40/40 sandwich Ni-Cr/Co-Fe/Ni-Cr PBA
film, are plotted when the externally applied field of 100 G is oriented (a)
parallel (black) and (b) perpendicular (grey) to the surface of the film. The
closed symbols represent the data prior to irradiation (i.e. dark state), and the
open symbols designate the data acquired after 5 hrs of irradiation with white
light, but with the light subsequently off, (i.e. PPIM state). (c) The
magnetization, M, versus magnetic field, H, loops at 2 K are shown when
H II film (black) and H I film (grey). The closed symbols are before irradiation
and the open symbols are after photoexcitation but with the light off. The
insets show an expanded region at low magnetic fields, and the coercive
fields, Hc, are 85 G for H II film and 140 G for H I film.


273












1.5k


0.5-


4



EO
a,
c -2

2-4


0 10 20 30 40 50 60 70
T (K)


(b) ..*



3

0
-1
-3 ....n i
S -400-200 0 200400
H ____ (G)
-60-40 -20 0 20 40 60
H (kG)


Figure 7-34. Magnetization of 20/40/20 Ni-Cr/Co-Fe/Ni-Cr heterostructure. (a) The x(T)
data, normalized to the area of the 20/40/20 sandwich Ni-Cr/Co-Fe/Ni-Cr PBA
film, are plotted when the externally applied field of 100 G is oriented parallel
to the surface of the film. The closed symbols represent the data prior to
irradiation (i.e. dark state), and the open symbols designate the data acquired
after 5 hrs of irradiation with white light, but with the light subsequently off, (i.e.
PPIM state). (b) The magnetization, M, versus magnetic field, H, loops at 2 K
are shown when H II film. The closed symbols are before irradiation and the
open symbols are after photoexcitation but with the light off.


274









12 : ..... 20 -
(a) -. .
10 "-"-. E
86~ 0 -

:6 E 0
EU A 6
? 4- -10-
D 1 -2 n
2--

-20
O l I I . ...
0 10 20 30 40 50 60 70 -60 -40 -20 0 20 40 60
T (K) H (kG)


Figure 7-35. Magnetization of 40/20/40 Ni-Cr/Co-Fe/Ni-Cr heterostructure. (a) The x(T)
data, normalized to the area of the 40/20/40 sandwich Ni-Cr/Co-Fe/Ni-Cr PBA
film, are plotted when the externally applied field of 100 G is oriented parallel
to the surface of the film. The closed symbols represent the data prior to
irradiation (i.e. dark state), and the open symbols designate the data acquired
after 5 hrs of irradiation with white light, but with the light subsequently off, (i.e.
PPIM state). (b) The magnetization, M, versus magnetic field, H, loops at 2 K
are shown when H II film. The closed symbols are before irradiation and the
open symbols are after photoexcitation but with the light off.


275









D state (a) 1.0 ruure 16
Lightstate 0.8 -A 14 5K
10 0.6A 12 45K
E E
(DQ 0.4 A 1l
0 10\ -
0.2 8 60 K
S0.0 Single phase 60
0 r I I I 0.0
0 20 40 60 80 0 20 40 60 80 6 ,
0 1 2 3
Temperature (K) Temperature (K) time (hrs)

Figure 7-36. Magnetization data of the 40/40/40 sandwich Ni-Cr/Co-Fe/Ni-Cr PBA film.
(a) The field-cooled magnetic susceptibility x(T) in 100 G oriented parallel to
the surface of the film, where x(T) is normalized to the area of the film. The
closed symbols represent the data prior to irradiation (i.e. dark state), and the
open symbols designate the data acquired after 5 hrs of irradiation with white
light, but with the light subsequently off, (i.e. PPIM state). (b) The absolute
value of the photoinduced changes of X, AX = x(dark) x(light), normalized to
the maximum value. The data for the heterostructure is from the left panel,
whereas the data for the single phase Co-Fe PBA thin film is taken from Ref.
[103]. (c) An expanded view of the temporal evolution of the magnetic
response is shown during irradiation at 5 K, 45 K, and 60 K, with H II film and
H = 100 G. The irradiation begins at time = 0.


276









5 (a) 30 (b)
E 4 E
3 20-

E 2 10 -

b 1 b -
0

0 20 40 60 80 100 0 20 40 60

T (K) T (K)

Figure 7-37. High magnetic field, 10 kG temperature sweeps of 40/40/40
Ni-Cr/Co-Fe/Ni-Cr. (a) The x(T) data of a 40/40/40 Ni-Cr/Co-Fe/Ni-Cr film
are plotted when the externally applied field of 10 kG is oriented parallel to the
surface of the film. The solid line represents the data prior to irradiation (i.e.
dark state), and the dashed line designates the data acquired after 5 hrs of
irradiation with white light, but with the light subsequently off, (i.e. PPIM state).
(b) Difference plots of the light state minus the dark state, AM, showing a
photoinduced increase at high fields, 10 kG, and temperatures.



(a) ri,4,.r,,rii.j nH : (b) (C) 1 .


-bO7C4[e(CN^


Rbi,- [[-'n,:"C.1 I niH: -



Figure 7-38. Transmission electron microscopy of the 40/40/40 Ni-Cr/Co-Fe/Ni-Cr
heterostructure. (a) A schema of the heterostructure using a shading gradient
between layers to illustrate regions at the interfaces where there can be
mixing of the two phases. (b) A TEM micrograph shows a cross-section of a
microtomed sample. The scale bar is 100 nm. The Melinex solid support is
located at the bottom in the image. (c) A TEM micrograph showing a fracture
at the Co-Fe/Ni-Cr interface. The scale bar is 100 nm.


277


11













RI.'li. ['JC 4[' IE i( i6.]n: rlH 20


E '[:, ,I .i, [,: rL' :- I : rHi ,-I


0.0 0.2 0.4 0.6 0.8 1.0 3
Co-Fe fraction

Figure 7-39. An energy dispersive x-ray line scan of the 40/40/40 heterostructure.
(left) A schema of the heterostructure using a shading gradient between
layers to illustrate regions at the interfaces where there can be mixing of the
two phases. (b) A linescan from the EDS provided the Co-Fe fraction across
the film, and the apparently high Co-Fe fraction deep into the bottom Ni-Cr
layer arises because the films become rougher with increasing thickness.
The solid support of Melinex is located at the top of the image and the free
surface of the heterostructure is located at the bottom.


278










4.4 -
4.2
4.0 -E E"
3.8 -
3.6
3.4
3.2
3.0
2.8
32.5 33.0


I I I I






Ni-Cr D v
33.5 34.0 34.5 35.0 35.5
3 3 3 Fe 3




33.5 34.0 34.5 35.0 35.5


\(Ni-Cr)
v(Ni-Cr)
cc(Ni-Cr)

y(Co-Fe)
v(Co-Fe)
cc(Co-Fe)


degrees (20)

Figure 7-40. X-ray powder diffraction of a 40/40/40 heterostructure. To investigate the
crystal structure of the heterostructures, reflections of the (4, 0, 0) plane were
measured. Experimental intensities are shown (n), as well as fits to
Lorentzian lines for Co-Fe (-) and Ni-Cr (-) lattices to extract the relative
intensities and positions. The peak at 34.80 corresponding to a cubic lattice
constant of 10.3 A is consistent with the Co-Fe analogue in the high spin
state [68], whereas the peak at 34.00 corresponding to a cubic lattice constant
of 10.5 A can be assigned to the Ni-Cr analogue.


279


352
0.5
34.0

200
0.4
34.8














0.10

0.05

0.00
0.15

0.10

0.05

0.00
0.15-

0.10

0.05

0.00
2200


2180 2160 2140 2120 2100 2080
wavenumber (cm-)


)c(CoFel)
w(CoFel)
A(CoFel)
c(CoFe2)
w(CoFe2)
A(CoFe2)



xc(NCr1)
w(Cr1)
A(Cr1)
x (NC2)
w(Cr2)
A(NC2)




xc(NC2)
w(Na2)
A(Na2)
xc(CoFel)
w(CoFel)
A(CoFel)
c(CoFe2)
w(CoFe2)
A(CoFe2)


2163.31
18.62
2.19
2118.01
38.58
4.85



2145.40
54.87
3.80
2174.04
21.18
2.58




2176.20
25.98
2.03
2161.44
14.71
0.14
2119.69
45.13
7.70


Figure 7-41. Cyanide stretching energies in the infrared. Spectra and fits are shown at
left and fitting parameters (the center position, xc, the width, w, and the area,
A) are shown at right. (a) The FT-IR spectrum of pure cobalt
hexacyanoferrate is known to display peaks at 2163, 2120, 2090, and 2040
cm- corresponding to the cyanide stretches of the Co2+-NC-Fe3 (HS), Co33
NC-Fe2+(LS), Co2+-NC-Fe and linkage-isomerized Co2-NC-Fe2 phases,
respectively [7]. The experimental data can be fit well with two lines, CoFel,
which is assigned to Co -NC-Fe3(HS) and CoFe2, which is assigned to
Co3-NC-Fe (LS). (b) The FT-IR spectrum of pure nickel
hexacyanochromate displays peaks at 2160 and 2125 cm-1 corresponding to
the bridged Ni2-NC-Cr + pairs and terminal cyanides. The experimental
data can be fit to two lines, NiCrl, which is assigned to Ni2+-NC-Cr3+, and
NiCr2, which is assigned to terminal cyanides. (c) In the heterostructured thin
film, discrete peaks corresponding to each of the constituents can be seen.
The experimental data fit well to three lines, CoFel (Co2+-NC-Fe3+(HS)),
CoFe2 (Co3+-NC-Fe2+(LS)), and NiCr2 (Ni2+-NC-Cr3+). The observation of
these peaks is further evidence for the proposed structure of the 40/40/40 film.


280











(a) 40/40/40 H
Co-Cr
CO-Fe
*'. co-cr I


A I
A
A *
A'O


10 20 30
Temperature (K)


40


E 2.0
- 9
| 1.5

L 1.0

^0.5


0.0
0


(b) 40/60/40 i 240
I235
230 -
S-1 0 1 2 3 4



Stime (hurs)
A fl time(hours)


A


10 20 30 ,
Temperature (K)


Figure 7-42. Magnetization data of Co-Cr/Co-Fe/Co-Cr heterostructures. (a) For a
40/40/40 Co-Cr/Co-Fe/Co-Cr heterostructure, the x(T) data, normalized to the
area of the film, are plotted when the externally applied field of 100 G is
oriented parallel to the surface of the film. The closed symbols represent the
data prior to irradiation (i.e. dark state), and the open symbols designate the
data acquired after 3 hrs of irradiation with white light, but with the light
subsequently off, (i.e. PPIM state). (b) Analogous x(T) data for a 40/60/40
Co-Cr/Co-Fe/Co-Cr heterostructure is shown. Insets are the time
dependence of the magnetization for temperatures above and below the
ordering temperature of the Co-Fe layer.


281


2.0


1.5

1.0

0.5

0.0-
0


A o
.o^P


a
anftnnr r










(a) 40/40/40 H 60/40/60
-002
6 crr ,6 004
Co-Fe
E Cr-Cr E
0 50 100 150 200 250 300
4 4 Temperature (K)
E B E

02 0) 0
o- ......... 31

0 50 100 150 200 250 300 0 50 100 150 200 250 300
Temperature (K) Temperature (K)


Figure 7-43. Magnetization data of Cr-Cr/Co-Fe/Cr-Cr heterostructures. (a) For a
40/40/40 Cr-Cr/Co-Fe/Cr-Cr heterostructure, the x(T) data, normalized to the
area of the film, are plotted when the externally applied field of 100 G is
oriented parallel to the surface of the film. The closed symbols represent the
data prior to irradiation (i.e. dark state), and the open symbols designate the
data acquired after 3 hrs of irradiation with white light, but with the light
subsequently off, (i.e. PPIM state). (b) Analogous x(T) data for a 60/40/60
Cr-Cr/Co-Fe/Cr-Cr heterostructure is shown. The inset shows the difference
between the photoinduced and dark states, AX, showing modification of the
magnetization at temperatures well above liquid nitrogen.


282






















Figure 7-44. Photoexcitation of Co-Fe. In these schema of a Co-Fe lattice, the
ferricyanide molecules are represented as red octagons and the cobalt ions
are represented by gray circles. Bonds between the atoms are represented
by gray lines. (a) The low-spin Co-Fe sample has a lattice constant of ~
10 A [160] [161]. (b) From EXAFS measurements [96] it was shown that
under photoirradiation, the structural change in the Co-Fe, increasing the unit
cell to ~ 10.3 A, takes place in the Co-N bonds, while the Fe-C bonds remain
rigid. Photoexcited bonds are represented as thicker, longer yellow lines.
This has the effect that Co ligand fields are found to be distorted in different
states of photoexcitation.



(a) (b) (c)









Figure 7-45. Distortions in Ni-Cr. In these schema of a Ni-Cr lattice, the
hexacyanochromate molecules are represented as blue octagons and the
nickel ions are represented by black circles. Bonds between the atoms are
represented by gray lines. (a) While in the bulk Ni-Cr, simple cubicity is the
most energetically favorable, (b) in thin films there may be a tetragonal
distortion of the Ni coordination. This distortion gives rise to the anisotropy
seen in the dark state of films and is only of ancillary interest to the discussion
of the photoeffect, as it was discussed in detail previously in Chapter 6. (c) In
heterostructures, where Ni-Cr is in intimate contact with Co-Fe, structural
distortions induce by photoirradiation in the Co-Fe can propogate to the Ni-Cr
lattice, distorting the Ni octahedra.


283











He (a) (b)
Hext























Figure 7-46. Anisotropy in Ni-Cr. The lower figures are analogous to those presented
in Figure 7-45. The upper figures are representing the anisotropy axes
present and the subsequent preferred orientation of the spin momentum in an
applied field of the same energy scale as the anisotropy. (a) For a sample
with correlated anisotropy, black arrows, macroscopic differences in the
magnetization, red arrows, can be seen for different orientations. At the
furthest left, a perpendicular orientation of the anisotropy axis and the applied
field is shown, with the tendency for spin momentum to lie along the
anisotropy axis, and not necessarily along the applied field. Just to the right,
a parallel orientation of the anisotropy axis and the applied field is shown,
showing the tendency of both energies to align the spins along the field axis.
(b) For a sample with random anisotropy, black arrows, there is no angular
dependence of the magnetization. However, a reduction of the magnetization,
red arrows, compared to the case of no anisotropy is expected for sufficiently
low fields, as some moments always prefer to point away from the applied
field.


284









CHAPTER 8
SUMMARY AND CONCLUSIONS

While each chapter of this thesis has been presented in such a way as to be

largely self-contained, a more general summary and set of conclusions will be made in

this chapter. The motivation of the preceding experimental and theoretical

investigations had two, main driving factors, the investigation of previously unknown

science and education of the author and all interested parties. These goals will be

explicitly considered as a final contemplation of the dissertation is made.

In Chapter 2, the experimental techniques are outlined for the purpose of

understanding the experimental data presented. Although most of the information is a

compilation of existing literature and "word-of-mouth" sources, the original contributions

of the author are included in detail. In fact, portions of apparatus development have not

been included, since these efforts did not pertain directly to the photoinduced

magnetism of Prussian blue nanostructures. One specific example not included is the

development of a low temperature, high pressure magnetometer, which was utilized for

tunnel diode penetration depth studies of superconductors and in high pressure

susceptibility measurements of a single molecule magnet [164]. These tunnel diode,

pressure cell, and other low temperature studies were exceedingly effective in

expanding the experimental and fabrication repertoire of the author.

The major apparatus development included in this thesis is related to bringing

Visible light to sample measurement spaces. First, a rotation probe with fiber optics

was designed and constructed for use with a commercial SQUID magnetometer, and

this device was essential for studying thin film magnetization and photoinduced

magnetization. It is particularly noteworthy that this custom probe represents the first


285









example of simultaneous rotation and photoirradiation in a SQUID magnetometer.

Furthermore, automation with a computer controlled stepper motor makes the design

tractable for the modern experimentalist. During the design process, extensive

mechanical and magnetic testing was performed on potential construction materials.

Second, a neutron scattering probe with a quartz light guide is still being further refined

as of the writing of this thesis. While previous groups have performed photoinduced

neutron scattering experiments on single crystals, the new neutron optical probe is the

first to attempt photoirradiation of large quantities of bulk powder for neutron studies. A

novel sample tumbler was designed in order to overcome the obstacle of opaque

powders limiting photoirradiation to surface of the sample. For the future, a high

pressure probe for use with the sensitive SQUID magnetometer may be made, as

reports of similar probes are available [165] [166].

In Chapter 3, the relevant theoretical background was detailed. Analogous to the

experimental techniques, the theoretical and numerical tools were preexisting

techniques that were applied to the materials of interest, but some modifications were

needed to model the photoinduced and bistable effects. While few extended HOckel

calculations made it into the thesis, many were performed. These quick calculations

yeilded invaluable insight into the nature of the bonding in the systems, as well as the

energy spectra. The author is currently in the process of applying more sophisticated

semi-empirical methods, as well as density functional theory, to problems that proved

intractable with the tight-binding extended HOckel theory.

Chapter 4 is more than just an extension of the theoretical methods because it

begins to apply the outlined theories to relevant materials in a concrete manner. Care is


286









taken to understand the transition from modeling to fitting experimental data, which is

important, especially since examples can be found in the literature where formulae have

been applied without sufficient rigor to provide meaningful results. A thought-provoking

result is the ambiguity of the sign of the superexchange interaction in

AjCok[Fe(CN)6]/nH20, based upon more sophisticated modeling than is presently

described in the literature. To help address this issue, neutron scattering experiments

are scheduled to be performed (May 2010) on AjCok[Fe(CN)6]/nH20 samples, with the

goal of searching for unequivocal evidence proving or disproving the existing

assignment of antiferromagnetic Co-Fe interactions. In the future, an extension of the

current codes to treat all higher level perturbations of magnetic and vibrational energies

would increase understanding of the molecular based systems studied. Furthermore, a

transparent, open source code for modeling magnetization of molecules and ionic

crystals would be immensely beneficial to scientists throughout the world when

experimental data is being analyzed.

The first set of novel materials is presented in Chapter 5, where slow cooled and

thermally quenched nanoparticles of AjCok[Fe(CN)6]/nH20 were studied. The first

material was RbjCok[Fe(CN)6]/nH20 nanoparticles protected by polyvinylpyrrolidone.

Photoinduced magnetization was present in all sizes studied, however the efficiency of

the effect, as well as the coercive fields and ordering temperatures, could be correlated

with the particle size. In addition, KjCok[Fe(CN)6]/nH20 nanoparticles showed that the

amount of photoswitchable material is drastically reduced with size. The observed size

dependence showed that, in addition to chemical formula, particle size is a necessary

parameter for understanding the magnetism of the AjCok[Fe(CN)6]/nH20 system.


287









Furthermore, the data suggest that a core-shell type of distribution of states is present,

with bulk-like bistable cores, and modified diamagnetic shells. In the future, a series of

probes to quantify the true microscopic nature of the distribution of states, and test the

hypothesis of a core-shell geometry, would be desirable.

In Chapter 6, thin films of Prussian blue analogues were discussed, with most

attention paid to the AjNik[Cr(CN)6]/nH20 material. These samples were prepared with

sequential adsorption techniques, which can deposit many layers, but introduce more

disorder than Langmuir-Blodgett techniques. All films studied showed magnetic

anisotropy, but the magnitude and sign of the anisotropy depended on the metal ions

used. The underlying source of anisotropy in Prussian blue analogue films can have

multiple sources, including dipolar, magnetocrystalline, and g-factor effects. In addition

to the fundamental science involved, the ability to induce magnetic anisotropy in the

systems by deposition onto a film is novel in itself and may have implications for

materials engineering problems and future technologies. Future researchers may

further refine the synthesis protocol by using flow-cells, and finally characterize the

domain structure of the samples by Lorentz microscopy or magnetic force microscopy.

The precise length scale of the domain structures is not yet known; however,

nanostructure studies imply they are greater than ~ 50 nm, and x-ray studies of the

Na-Co-Fe material have implied structural domains as large as ~ 1000 nm.

From a materials standpoint, the most interesting data arose from studying

heterostructured materials presented in Chapter 7. Two new types of materials were

synthesized and studied, photomagnetic solid solutions and photomagnetic thin film

heterostructures. Studies of solid solutions showed that the sign and the magnitude of


288









the photomagnetic effect could be tuned with chemical composition. In addition, these

studies allowed for investigation of the dilution effect of charge transfer induced spin

transitions. The experience and understanding gained by studying photomagnetic solid

solutions was central to achieving a break-through in understanding and engineering

photomagnetic thin film heterostructures. Specifically, the need to have large enough

unadulterated regions of photomagnetic material to observe appreciable effects was

clearly demonstrated by the solid solution studies. The appreciable photoeffect finally

achieved in the heterostructures was the subject of keen interest within the group, due

to the unprecedented ordering temperatures of photomagnetization in a Prussian blue

analogue. The largest effect was observed for a 40/40/40 Ni-Cr/Co-Fe/Ni-Cr material,

but many other heterostructures were studied and presented. These studies have

already spurred the study of photomagnetic core-shell nanoparticles with analogous

fundamental properties.

To conclude, this dissertation has provided new insight into Prussian blue

analogue nanoparticles, thin films, solid solutions, and thin film heterostructures.

Portions have already been published and cited in the scientific literature, and others

represent manuscripts in preparation. Finally, none of the studies in this thesis would

have been possible without the synthesis of samples and characterization of

compounds by collaborating chemists, who, through constant conversations, also

imparted invaluable insight to the investigations for our flirtation with the vast unknown.


289









APPENDIX A
UNITS

The system of units used throughout the majority of this thesis is based upon the

literature standard used in Kahn's book on Molecular Magnetism [38]. Although

magnetic susceptibility is traditionally intrinsic to volume, the molecule magnetism

community prefers a molar normalization that is not dimensionless; Xmoi has units of

emu mol-1. Taking [X] = emu mol-1 as a starting point, molar magnetization is expressed

in emu G mol-1 and XT has units of emu K mol-1. Finally, the H field is sometimes

expressed in units of Tesla for familiarity [167], but this convention is not universal [3].


SI units of physical constants [38]


Planck constant


Elementary charge
Electron mass
Proton mass
Avogadro number
Molar gas constant
Boltzmann constant
Bohr magneton
Nuclear magneton

c.q.s. emu units of
Planck constant

Elementary charge
Electron mass
Proton mass
Avogadro number
Molar gas constant
Boltzmann constant
Bohr magneton
Nuclear magneton


h
h/2e7
e
me
mp
N
R
kB
[tB
B,N


6.6260755 10-34 J s
1.05457267 10-34 J s
1.60217733 10-19 C
9.1093897 10-31 kg
1.6726231 10-27 kg
6.0221367 1023 mol-1
8.3145121 J mol-1 K1
1.3806580 10-23 J K-1
9.27401549 10-24 J T-1
5.05078947 10-27 J T-1


physical constants [38]


h
h/2e7
e
me
mp
N
R
kB
[LB
[LB,N


6.6260755 10-27 erg s
1.05457267 10-27 erg s
4.80320427 10-10 esu
5.4857990943 10-4 amu
1.0072764667 amu
6.0221367 1023 mol-1
8.3145121 107 erg mol-1 K1
1.3806580 10-16 erg K-1
9.27401549 10-17 erg T-1
5.05078947 10-20 erg T-1


Some mixed units as favored by Kahn [38]
Boltzmann constant kB 0.69503877 cm- K1
Bohr magneton [LB 4.66864374 cm-1 G-1
Mol Bohr magneton NLB 5585 cm3 G mol-1


290









APPENDIX B
LOW TEMPERATURE ROTATION PROBE DRAWINGS

This appendix contains the machine drawings used for the optical rotation probe

described in Section 2.3.1.

Assembly Drawings
Assembly Drawings detail
Slide Seal
Cell
Drive Rod
Probe Head Bracket
Probe Head Cap
Manual Bracket (Piece #1)
Manual Bracket (Piece #2)
Manual Bracket (Piece #3)
Motor Bracket
Probe Head (hidden view)
Probe Head wireframee view)
Yoke


291












































Assembly Drawings


292











































Assembly Drawings detail


293










cide Seal, Aluminum
I a.f fw pem. asff it


- 0.062


294


0.500


1.107 -
- -0.030









LUtLL, DIBCK Uelnn
II maw timm Is &fa'~ww a* CA


- 00.250

/- 00.124
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APPENDIX C
COPYRIGHT RELEASE FORMS


PewimsonltOPP
Sen by: JI Membrey
01/122o00 09:11


To dpajghvs Lfl.edu
cc
bce
Subject Re: Fw: copyright release?


-- Forwarded by Sarah FrickerAegalIOPP on 25/112009 17:24 --
Darll M Pajerowalm
To legal@iop.org
25/11/2009 17:05 cc
Subject copyright release?




Hello!

I am w-riTing my PhD dissertation and was hoping to use passages from papers ve authored.

Specifically,


D. M. Pajerowski and M. W. Meisel

Magnetometer probe with low remperanrme ramaion and optical fbers

J. Phys.: Conf Ser. JU (tl)i'9 0120341 4pp). doi:IO. 108.1l742-65961 i rO'I012034



Please advise.

Best Regads,
Daniel M Pajtowsid


PERMISSION TO REPRODUCE AS REQUESTED

IS GIVEN PROVIDED THAT:


-9) -th S 01.m- -I-44 meAine

(b) the source of the material including autxo/edilor.

title, date and publisher Is acknowledged.'


IOP Publishing Ltd

Dirac House


306
















Temple Back
BRISTOL t ... ,.... ,.
BS1 6BE Date Rights & Permissions


* Please include the IOP Copyright line, mention the journars homepage at:


wwwlop.orgl/ournat/ipcs


and provide a link back to the article's abstract on ourwebsile from the electronic
version of your thesis (if applicable).


Thank you


307












To. Permissions ,


i:...I.: Re: Permission request
;r'er. Daniel M Pajemrwsk Monday 08/02/2010 16:09



Dear Sarah Ryder;

Thank you for your reply to my email.

The data in

http://www.iop.org/EJ/abstract/-search-68992789.1/1367-2630/9/7/222

represent a large portion of one chapter of my thesis. Therefore I
would like permission to reproduce the plots and text describing them.
Specifically, Table 1, Figure 1, Figure 2, Figure 3, Figure 4, Figure 5,
and Figure A.1, While the bulk of the text describing these figures
will be different, as it is required by my University to fit the flow of
the overall thesis, there are certainly subsections that I have worked
hard on to reach the tightest possible wording, and rewording would not
only be additional work, but would actually be a step backwards in terms
of the quality of my thesis document. I hope this is sufficient detail
for your records (if not please let me know), and I look forward to your
reply.

Have a nice day,
Daniel Pajerowski


> *reproduction of article in thesis*

> *Daniel M Pajerowski to: njp
> 01/02/2010 23:47

> From: Daniel M Pajerowski cdpajiphys.ufl.edu>

> To: njp@iop.org

> -----------------__-------_^^--------------------------^---------------_--

> To Whom It May Concern;

> I am in the process of writing my PhD thesis and was hoping to copy
> large portions of text and figures from

> http://www.iop.org/EJ/abstract/-search=68992789.1/1367-2630/9/7/222

> which is an article I had authored and published in your Journal.

> Based upon my reading of the copyright agreement between NJP and its
> publishing authors, I believe the aforementioned usage is pursuant to
> the agreement, however I was hoping for confirmation of this assumption.

> I look forward to your reply.
>
> Best Wished,
s Daniel M Pajerowski

PERMISSION TO REPRODUCE AS REQUESTED
IS GIVEN PROVIDED THAT:


308


,. L 1~-_~ n ft4
















(b) the source of the material including author/editor,
title, date and publisher is acknowledged.


JOP Publishing Ltd
Dirac House
Temple Back
BRISTOL
BS1 6BE


D. .;. Right. ..s i _
Date Rights & Permissions











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i : ,] t i : r l- : L 5T r to
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_ 4':-, l.i r l'-.4-'
Feb 01, 2010
AmeBrican :ri-1i7. lo Society
journal of the Amerrcan chemical Society
Tuning the Sign of Photoinduced Changes-in Mlagnetization: Spin
Transitions Inthe Ternary Metal Prussian Blue Analague
NaoNil'- .:. IFi-.N~ I:clanH -L
Daniel M, PaRerowskI et al,
Sep 1,. 2D09
131
36
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LIST OF REFERENCES


[1] O. Sato, T. lyoda, A. Fujishima, and K. Hashimoto, Science, no. 272, p. 704,
1996.

[2] V. Ksenofontov, et al., Phys. Rev. B, no. 68, p. 024415, 2003.

[3] D. J. Griffiths, Introduction to Electrodynamics. New Jersey: Prentice Hall,
1999.

[4] D. J. Griffiths, Introduction to Quantum Mechanics. Englewood Cliffs: Prentice
Hall, 1995.

[5] B. N. Figgis and M. A. Hitchman, Ligand Field Theory and Applications. New
York: Wiley-VCH, 2000.

[6] J. G. Moore, E. J. Lochne, C. Ramsey, N. S. Dalal, and A. E. Stiegman,
Angew. Chem., no. 115, p. 2847, 2003.

[7] L. Catala, et al., Chem. Commun., p. 746, 2005.

[8] D. M. Pajerowski, F. A. Frye, D. R. Talham, and M. W. Meisel, New J. Phys.,
no. 9, p. 222, 2007.

[9] J.-H. Park, et al., J. Magn. Magn. Mater., no. 310, p. 1458, 2007.

[10] J.-H. Park, et al., Appl. Phys. Lett., no. 85, p. 3797, 2004.

[11] N. H. Balshaw, Practical Cryogenics. Oxon: Oxford Instruments
Superconductivity Limited, 1996.

[12] T. Flynn, Cryogenic Engineering, Second Edition, Re Vised and Expanded.
New York: CRC Press, 2004.

[13] F. Pobell, Matter and Methods at Low Temperatures. Berlin: Springer-Verlag,
1992.

[14] R. C. Richardson and E. N. Smith, Experimental Techniques In Condensed
Matter Physics At Low Temperatures. New York: Westview Press, 1998.

[15] J. Clarke and A. I. Braginski, SQUID Handbook. Berlin: Wiley-VCH, 2004.


314









[16] O. V. Lounasmaa, Experimental Principles and Methods Below 1K. New York:
Academic Press, 1974.

[17] MPMS User's Manual. San Diego, CA: Quantum Design, 2000.

[18] MPMS Hardware Manual. San Diego, CA: Quantum Design, 2000.

[19] (2010, May) Quantum Design, Inc., 2010. [Online]. http://www.qdusa.com

[20] H. Bubert and H. Jenett, Surface and thin film analysis : principles,
instrumentation, applications. Weinheim: Wiley-VCH, 2002.

[21] F. L. Holmes, Isis, no. 54, no. 1, p. 50-81, 1963.

[22] J. I. Goldstein, et al., Scanning Electron Microscopy and X-Ray Microanalysis.
Springer, 2003.

[23] D. C. Harris and M. D. Bertolucci, Symmetry and Spectroscopy: An Introduction
to Vibrational and Electronic Spectroscopy. New York: Oxford University Press,
1978.

[24] A. Montasser, Inductively Coupled Plasma Mass Spectrometry. Berlin: Wiley-
VCH, 1998.

[25] H. Kohl, Transmission Electron Microscopy : Physics of Image Formation.
New York: Springer-Verlag, 2008.

[26] R. C. Denney and R. Sinclair, Visible and Ultraviolet Spectroscopy, D.
Mowthorpe, Ed. New York: John Wiley and Sons, 1987.

[27] A. Guinier, X-Ray Diffraction in Crystals, Imperfect Crystals and Amorphous
Bodies. New York: W.H. Freeman, 1963.

[28] D. M. Pajerowski and M. W. Meisel, J. Phys.: Conf. Ser., no. 150, p. 012034,
2009.

[29] T. Yamamoto, Y. Umemura, O. Sato, and Y. Einaga, Chem. Mater., no. 16,
p. 1195, 2004.

[30] A. Schuhl, S. Maegawa, M. W. Meisel, and M. Chapellier, Phys. Rev. B, no. 36,
p. 13, 1987.


315









[31] (2010, May) Spectra Fiber Products Honeywell Advanced Fibers and
Products, 2010. [Online]. http://www51.honeywell.com/sm/afc/products-
details/fiber.html

[32] B. H. Bransden and C. J. Joachain, Quantum Mechanics, Second Edition ed.
New York: Prentice Hall, 2000.

[33] C. J. Ballhausen, Introduction to Ligand Field Theory. New York: McGraw-Hill
Book Company, Inc., 1962.

[34] E. U. Condon and G. H. Shortley, The Theory of Atomic Spectra, 2nd ed.
New York: Cambridge University Press, 1953.

[35] G. Racah, Phys. Rev., no. 61, p. 438, 1942.

[36] J. S. Griffith, The Theory of Transition Metal Ions. Cambridge: Cambridge
University Press, 1964.

[37] H. Bethe, Ann. Phys., no. 3, p. 133, 1929.

[38] O. Kahn, Molecular Magnetism. New York: Wiley-VCH, 1993.

[39] J. B. Goodenough, Magnetism and the Chemical Bond. New York:
Interscience-Wiley, 1963.

[40] H. Bruus and K. Flensberg, Many-Body Quantum Theory in Condensed Matter
Physics. New York: Oxford University Press, 2004.

[41] H. Ibach and H. Luth, Solid-State Physics. New York: Springer-Verlag, 2002.

[42] N. W. Ashcroft and N. D. Mermin, Solid State Physics. Harcourt College
Publishers, 1976.

[43] K. Yosida, Theory of Magnetism. New York: Spring-Verlag, 1996.

[44] S. Blundell, Magnetism in Condensed Matter. New York: Oxford University
Press, 2007.

[45] K. H. Fischer and J. A. Hertz, Spin Glasses, D. Edwards and D. Melville, Eds.
Cambridge: Cambridge University Press, 1999.

[46] A. P. Young, Ed., Spin Glasses and Random Fields. New Jersey: World
Scientific, 1997.


316









[47] K. 1. Ramachandran, G. Deepa, and K. Namboori, Computational Chemistry
and Molecular Modeling. Berlin: Springer-Verlag, 2008.

[48] N. J. Giordano, Computation Physics. New Jersey: Prentice Hall, 1997.

[49] K. Levenberg, Quart. Appl. Math., no. 2, p. 164, 1944.

[50] D. Marquardt, SIAM J. Appl. Math., no. 11, p. 431, 1963.

[51] H. M. Rietveld, J. Appl. Cryst., no. 2, p. 65, 1969.

[52] A. C. Larson. and R. B. V. Dreele, Los Alamos National Laboratory Report
LAUR, p. 86, 2000.

[53] B. H. Toby, J. Appl. Cryst, no. 34, p. 210 2001.

[54] V. Gadet, T. Mallah, I. Castro, M. Verdaguer, and P. Veillet, J. Am. Chem. Soc.,
no. 114, p. 9213-9214, 1992.

[55] P. Day, et al., Helv. Chim. Acta, no. 63, p. 148, 1980.

[56] M. Mihalik, et al., J. Phys.: Conf Ser., no. 200, p. 022035, 2010.

[57] G. Champion, et al., J. Am. Chem. Soc., no. 123, p. 12544, 2001.

[58] S. Blundell, Magnetism in Condensed Matter. New York: Oxford University
Press, 2007.

[59] K. R. Dunbar and R. A. Heintz, Progress in Inorganic Chemistry, K. D. Karlin,
Ed. New York: Wiley and Sons, 1997, no. 45, p. 283.

[60] M. Verdaguer and G. Girolami, Magnetism: Molecules to Materials V,
J. S. Miller and M. Drillon, Eds. Weinheim: Wiley-VCH, 2003, p. 283.

[61] D. Davidson and L. Welo, J. Phys. Chem., no. 32, p. 1191, 1928.

[62] A. N. Holden, B. T. Matthias, P. W. Anderson, and H. W. Lewis, Phys. Rev.,
no. 102, p. 1463, 1956.

[63] R. M. Bozorth, H. J. Williams, and D. E. Walsh, Phys. Rev., no. 103, p. 572,
1956.

[64] A. Ito, M. Suenaga, and K. Ono, J. Chem. Phys., no. 48, p. 3597, 1968.


317









[65] H. J. Buser, D. Schwarzenbach, W. Petter, and A. Ludi, Inorg. Chem., no. 16,
p. 2704, 1977.

[66] S. Ohkoshi and K. Hashimoto, J. Photochem. and
Photobio. C: Photochem. Rev., no. 2, p. 71, 2001.

[67] F. Varret, M. Nogues, and A. Goujon, Magnetism: Molecules to Materials I,
J. S. Miller and M. Drillon, Ed. Weinheim: Wiley-VCH, 2002, p. 257.

[68] M. Hanawa, et al., J. Phys. Soc. Jpn., no. 72, p. 987, 2003.

[69] T. Kawamoto, Y. Asai, and S. Abe, Phys. Rev. Lett., no. 86, p. 348, 2001.

[70] T. Uemura and S. Kitagawa, J. Am. Chem. Soc., no. 125, p. 7814, 2003.

[71] T. Uemura, M. Ohba, and S. Kitagawa, Inorg. Chem., no. 43, p. 7339, 2004.

[72] F. A. Frye, et al., Polyhedron, no. 26, p. 2273, 2007.

[73] (2010, May) W.S. Rasband, ImageJ. Bethesda, 1997-2009. [Online].
http://rsb.info.nih.gov/ij/

[74] Y. Xian, et al., Anal. Chim. Acta, no. 546, p. 139, 2005.

[75] C. W. Ng, J. Ding, and L. M. Gan, J. Solid State Chem., no. 156, p. 400, 2001.

[76] J.-H. Park, Ph.D. thesis, University of Florida, 2006.

[77] J. A. Mydosh, Spin Glasses: An Experimental Introduction. London: Taylor and
Francis, 1993.

[78] D. A. PejakoviC, J. L. Manson, J. S. Miller, and A. J. Epstein, Phys. Rev. Lett.,
no. 85, p. 1994, 2000.

[79] K. Yoshizawa, F. Mohri, G. Nuspl, and T. Yamabe, Phys. Chem. B, no. 102,
p. 5432, 1998.

[80] T. Kawamoto, Y. Asai, and S. Abe, Phys. Rev. Lett., no. 86, p. 348, 2001.

[81] A. Bleuzen, et al., J. Am. Chem. Soc., no. 122, p. 6648, 2000.

[82] C. C. dit Moulin, et al., J. Am. Chem. Soc., no. 122, p. 6653, 2000.

[83] V. Escax, et al., J. Am. Chem. Soc., no. 123, p. 12536, 2001.


318









[84] N. Shimamoto, S. Ohkoshi, O. Sato, and K. Hashimoto, Inorg. Chem., no. 41,
p. 678, 2002.

[85] V. Escax, et al., no. 44, p. 4798, 2005.

[86] Z. Salman, et al., Phys. Rev. B, no. 73, p. 174427, 2006.

[87] A. Goujon, F. Varret, V. Escax, A. Bleuzen, and M. Verdaguer, Polyhedron,
no. 20, p. 1347, 2001.

[88] H. Liua, et al., J. Magn. Magn. Mater., no. 322, p. 572, 2010.

[89] M. Arai, M. Miyake, and M. Yamada, J. Phys. Chem. C, no. 112, p. 1953, 2008.

[90] D. Li, et al., J. Am. Chem. Soc., no. 130, p. 252, 2008.

[91] S. Gawali, et al., J. Phys. Chem. B, no. 109, p. 8251, 2005.

[92] M. Castro, et al., Eur. Phys. Lett, no. 79, p. 27007, 2007.

[93] J. E. Gardner, PhD Thesis, University of Florida, 2009.

[94] F. A. Frye, PhD Thesis, University of Florida, 2007.

[95] T. Yokoyama, T. Ohta, O. Sato, and K. Hashimoto, Phys. Rev. B, no. 58,
p. 8257-8266, 1998.

[96] C. C. dit Moulin, et al., J. Am. Chem. Soc., no. 122, p. 6653-6658, 2000.

[97] B. Hoo, K. Boukheddaden, and F. Varret, Eur. Phys. J. B, no. 17, p. 449, 2000.

[98] T. Yamamoto, Y. Umemura, O. Sato, and Y. Einaga, Chem. Lett., no. 33, p.
500, 2004.

[99] T. Yamamoto, Y. Umemura, O. Sato, and Y. Einaga, Chem. Mater., no. 16,
p. 1195, 2004.

[100] J.-H. Park, et al., Polyhedron, no. 24, p. 2355, 2005.

[101] T. Yamamoto, Y. Umemura, O. Sato, and Y. Einaga, J. Am. Chem. Soc., no.
127, p. 16065, 2005.

[102] F. A. Frye, et al., Polyhedron, no. 26, p. 2281, 2007.


319









[103] F. A. Frye, D. M. Pajerowski, J.-H. Park, M. W. Meisel, and D. R. Talham,
Chem. Mater., no. 20, p. 5706, 2008.

[104] J. Gulp, J.-H. Park, D. Stratakis, M. W. Meisel, and D. R. Talham, J. Am. Chem.
Soc., no. 124, p. 10083, 2002.

[105] J. Gulp, J.-H. Park, I. Benitez, M. W. Meisel, and D. R. Talham, Polyhedron,
no. 22, p. 2125, 2003.

[106] J. Gulp, et al., Chem. Mater., no. 15, p. 3431, 2003.

[107] J. Gulp, J.-H. Park, M. W. Meisel, and D. R. Talham, Inorg. Chem., no. 42,
p. 2842, 2003.

[108] J. Gulp, et al., Coord. Chem. Rev., no. 249, p. 2642, 2005.

[109] G. Agusti, et al., Chem. Mater., no. 20, p. 6721, 2008.

[110] N. Bagkar, et al., Thin Film Solids, no. 513, p. 325, 2006.

[111] O. Sato, S. Hayami, Y. Einaga, and Z.-Z. Gu, Bull. Chem. Soc. Jap., no. 76,
p. 443, 2003.

[112] G. Torres, B. Agricole, P. Delhaes, and C. Mingotaud, Chem. Mater., no. 14,
p. 4012, 2002.

[112bis] M. Pregelj, A. Zorko, D. Arcon, S. Margadonna, K. Prassides, H. van Tol, L.C.
Brunel, and A. Ozarowski, J. Magn. Magn. Mater., no. 316, p. e680, 2007.

[113] S. V. Vonsovskii, Ferromagnetic Resonance. Oxford: Pergamon Press, 1966.

[113bis] A. Gomez and S. W. Kycia, private communication, April 2010.

[114] K. Pokhodnya, V. Dokukin, and J. Miller, Inorg. Chem., no. 47, p. 2249, 2008.

[115] S. Ferlay, T. Mallah, R. Ouahes, P. Veillet, and M. Verdaguer, Nature, no. 378,
p. 701, 1995.

[116] T. Mallah, S. Thiebaut, M. Verdaguer, and P. Veillet, Science, no. 262, p. 1554,
1993.

[117] M. Taliferro, M. Thorum, and J. Miller, Angew. Chem., Int. Ed., no. 45, p. 5326,
2006.


320









[118] C. C. dit Moulin, et al., J. Am. Chem. Soc., no. 122, p. 6653, 2000.

[119] A. Goujon, et al., Eur. Phys. J. B, no. 14, p. 115, 2000.

[120] A. Bleuzen, V. Escax, J.-P. Iti, P. Mensch, and M. Verdaguer, C. R. Chimie, no.
6, p. 343, 2003.

[121] J.-W. Yoo, et al., Phys. Rev. Lett., no. 97, p. 247205, 2006.

[122] D. Brinzei, et al., J. Am. Chem. Soc., no. 129, p. 3778, 2007.

[123] J.-W. Yoo, R. Edelstein, D. Lincoln, N. Raju, and A. Epstein, Phys. Rev. Lett.,
no. 99, p. 157205, 2007.

[124] A. Bleuzen, V. Marvaud, C. Mathoniere, B. Sieklucka, and M. Verdaguer, Inorg.
Chem., no. 48, p. 3453, 2009.

[125] W. Kosaka, K. Nomura, K. Hashimoto, and S.-i. Ohkoshi, J. Am. Chem. Soc.,
no. 127, p. 8590, 2005.

[126] S. Ohkoshi, Y. Einaga, A. Fujishima, and K. Hashimoto, J. Electroanal. Chem.,
no. 473, p. 245, 1999.

[127] D. F. Shriver, S. A. Shriver, and S. E. Anderson, Inorg. Chem., no. 4, p. 725,
1965.

[128] D. B. Brown, D. F. Shriver, and L. H. Schwartz, Inorg. Chem., no. 7, p. 77,
1968.

[129] D. B. Brown and D. F. Shriver, Inorg. Chem., no. 8, p. 37, 1969.

[130] J. E. House and J. C. Bailar, Inorg. Chem., no. 8, p. 672, 1969.

[131] E. Reguera, J. A. Bertran, and L. Nuiez, Polyhedron, no. 13, p. 1619, 1994.

[132] R. Martinez-Garcia, M. Knobel, and E. Reguera, J. Phys. Chem. B, no. 110,
p. 7296, 2006.

[133] S.-i. Ohkoshi and K. Hashimoto, J. Am. Chem. Soc., no. 121, p. 10591, 1999.

[134] S.-i. Ohkoshi, et al., Appl. Phys. Lett., no. 70, p. 1040, 1997.

[135] J.-D. Cafun, L. Londiniere, E. Riviere, and A. Bleuzen, Inorg. Chim. Acta, no.
361, p. 3555, 2008.


321









[136] S. Ohkoshi, T. lyoda, A. Fujishima, and K. Hashimoto, Phys. Rev. B, no. 56,
p. 11642, 1997.

[137] S.-i. Ohkoshi, O. Sato, T. lyoda, A. Fujishima, and K. Hashimoto, Inorg. Chem.,
no. 36, p. 268, 1997.

[138] S.-i. Ohkoshi and K. Hashimoto, Phys. Rev. B, no. 60, p. 12820, 1999.

[139] S. Juszczyk, C. Johansson, M. Hanson, A. Ratuszna, and G. Matecki, J. Phys.:
Condens. Matter, no. 6, p. 5697, 1994.

[140] A. Kumar and S. M. Yusuf, Physica B, no. 362, p. 278, 2005.

[141] D. M. Pajerowski, J. E. Gardner, D. R. Talham, and M. W. Meisel, J. Am.
Chem. Soc., no. 131, p. 12927-12936, 2009.

[142] Nanoparticles of nickel hexacyanoferrate were independently measured and no
appreciable size dependence as found between 15 and 35 nm.

[143] 0. Sato, J. Solid State Electrochem., no. 11, p. 773, 2007.

[144] D. A. Pejakovic, J. L. Manson, J. S. Miller, and A. J. Epstein, J. Appl. Phys.,
no. 87, p. 6028, 2000.

[145] D. A. Pejakovic, J. L. Manson, J. S. Miller, and A. J. Epstein, J. Synth. Met.,
no. 122, p. 529, 2001.

[146] D. A. Pejakovic, J. L. Manson, C. Kitamura, J. S. Miller, and A. J. Epstein,
Polyhedron, no. 20, p. 1435, 2001.

[147] D. A. Pejakovic, C. Kitamura, J. S. Miller, and A. J. Epstein, J. Mol. Cryst. Liq.
Cryst., no. 374, p. 289, 2002.

[148] P. K. Phu, T. N. Giang, and N. Minh, V. Commun. Phys., no. 18, p. 43, 2008.

[149] A. Widmann, et al., Inorg. Chem., no. 41, p. 5706, 2002.

[150] N. Bagkar, et al., Philos. Mag., no. 85, p. 3659, 2005.

[151] A. Widmann, H. Kahlert, H. Wulff, and F. Scholz, J. Solid State Electrochem.,
no. 9, p. 380, 2005.

[152] D. Schwudke, R. St6sser, and F. Scholz, Electrochem. Commun., no. 2, p. 301,
2000.


322









[153] F. Lloret, M. Julve, J. Cano, R. Ruiz-Garcia, and E. Pardo, Inorg. Chim. Acta,
no. 361, p. 3432, 2008.

[154] J. Baker and B. N. Figgis, Aust. J. Chem., no. 35, p. 265, 1982.

[155] S.-i. Ohkoshi and K. Hashimoto, Chem. Phys. Lett., no. 314, p. 210, 1999.

[156] M. Sorai, J. Ensling, and P. GCtlich, Chem. Phys., no. 18, p. 199, 1976.

[157] H. Spiering, E. Meissner, H. Koppen, E. W. MOller, and P. GCtlich, Chem.
Phys., no. 68, p. 65, 1982.

[158] T. T. A. Lummen, et al., J. Phys. Chem. C, no. 112, p. 14158, 2008.

[159] D. M. Pajerowski, et al., J. Am. Chem. Soc., no. 132, p. 4058-4059, 2010.

[160] 0. Sato, Y. Einaga, A. Fujishima, and K. Hashimoto, Inorg. Chem., no. 38,
p. 4405, 1999.

[161] C. C. dit Moulin, G. Champion, J.-D. Cafun, M.-A. Arrio, and A. Bleuzen,
Angew. Chem., Int. Ed., no. 46, p. 1287, 2007.

[162] M. Zentkova, et al., J. Phys.: Condens. Matter, no. 19, p. 266217, 2007.

[163] H. U. GCdel, H. Stucki, and A. Ludi, Inorg. Chim. Acta, no. 7, p. 121, 1973.

[164] D. M. Pajerowski, et al., Polyhedron, no. submitted, 2010.

[165] K. V. Kamenev, S. Tancharakorn, N. Robertson, and A. Harrison, Rev. Sci.
Instrum., no. 77, p. 073905, 2006.

[166] P. L. Alireza and G. G. Lonzarich, Rev. Sci. Instrum., no. 80, p. 023906, 2009.

[167] R. M. White, Quantum Theory of Magnetism. New York: McGraw-Hill, 1970.


323









BIOGRAPHICAL SKETCH

Daniel Matthew Pajerowski was born as the third and final son of Mary I.

Pajerowski and John T. Pajerowski, Jr. in Wilmington, Delaware. Daniel has attended

public schools throughout his education, graduating from Brandywine High School in

2000. In 2004, Daniel graduated from the University of Delaware with a Bachelor

degree in Science, with a major in physics and minors in sculpture and math. During

his stay, he worked for Professor Barry C. Walker in a femtosecond laser lab for 3 years.

After graduation, Daniel continued to work in the Walker lab in a limited capacity and

took additional coursework to be sure that pursuing a PhD would be a viable career

choice. In 2005, he began studies at the University of Florida and immediately joined

the low temperature laboratory headed by Professor Mark W. Meisel. After graduating

with his PhD in physics in the summer of 2010, he will try to continue on to a career as a

research scientist. Finally, at some point he will retire.


324





PAGE 1

1 PHOTOINDUCED MAGNETI SM IN NANOSTRUCTURES OF PRUSSIAN BLUE ANALOGUES By DANIEL MATTHEW PAJEROWSKI A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2010

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2 2010 Daniel Matthew Pajerowski

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3 To all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun

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4 ACKNOWLEDGMENTS My sincere thanks go out to those who helped me during the course of my PhD studies at the University of Florida. First, I would like to thank the talented chemists Dr. Franz A. Frye, Dr. Justin E. Gardner, Matthieu F. Dumont, and Matthew J. Andrus for generously providing valuable chemistry insights and samples. I appreciate the help afforded by my predecessor in Professor Meisels lab, Dr. Ju Hyun Park as well as students in the neighboring low temperature labs, Dr. Byoung Hee Moon and Dr. Pradeep Bhupathi from Professor Lees lab, and Kyle Thompson from Professor Ihas lab I would like to thank cryogenic engineers Greg Labbe and John Graham for running the helium liquefier and providing time and plumbing for trouble shooting cryogenic issues. Thanks also go out to el ectrical engineers Larry Phelps, Pete Axson, and Rob Hamersma of the UF Physics electronics shop for valuable insight, electronic components in a pinch and for fixing broken equipment. I would like to thank Brent Nelson and David Hansen from UF Physics I nformation T echnology. I would like to thank Don Brennan, Tim Noland, and Jay Horton for help with technical issues in the New Physics Building and around lab B133. I would like to thank program assis tants Dori Faust, Darlene Latimer, Kristin Nichola, Denise Carlton, and Carolyn Grider I would like to thank Mark Link, Ed Storch, Bill Malphurs and Mike Herlevich of the UF Physics instrument shop for incisive design critiques and superior craftsmanshi p when building custom apparatus. I would like to thank Ben Pletcher of the UF Major Analytical Instrumentation Center for the TEM images and EDS data. I would like to thank Professor Steve Hill, Dr. Andrew Ozarowski, and Dr. Saitti Datta of the NHMFL for help with the magnetic resonance studies. I would like to thank Dr. Steve Nagler, Dr. Jerel Zaretsky, Dr. V. Ovidiu Garlea, Dr. Lou Santodonato, Chris Redmon, and the

PAGE 5

5 entire sample environment team of the ORNL for help with the neutron scattering experi ments. I would like to thank the teachers of courses I took, as well as fellow students who were essential to the learning process. I would like to thank my thesis committee members, Professors Alan T. Dorsey, Hai Ping Cheng, Yoonseok Lee, Daniel R. Talham and Mark W. Meisel. I would like to thank Professor Talham for his time and insight, specifically during Chem Phys group meetings. Finally, I would like to thank Professor Mark W Meisel for his advice and his dedication to providing me with research and learning opportunities. This work was supported, in part, by the National Science Foundation ( NSF ) through DMR 0701400 and the NHMFL via cooperative agreement with NSF DMR 0654118 and the State of Florida. Th e r esearch at Oak Ridge National Laborator y's High Flux Isotope Reactor was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U. S. Department of Energy.

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................. 4 LIST OF TABLES .......................................................................................................... 11 LIST OF FIGURES ........................................................................................................ 12 LIST OF ABBREVIATIONS ........................................................................................... 19 ABSTRACT ................................................................................................................... 22 1 INTRODUCTION .................................................................................................... 24 1.1 Experimental Techniques ............................................................................... 25 1.2 Theoretical Methods ....................................................................................... 26 1.3 Quan titative Analysis of Magnetization in Prussian Blue Analogues .............. 27 1.4 Cobalt Hexacyanoferrate Nanoparticles ......................................................... 28 1.5 Thin Films of Prussian Blue Analogues .......................................................... 28 1.6 Heterostructures of Prussian Blue Analogues ................................................ 29 2 EXPERIMENTAL TECHNIQ UES ............................................................................ 31 2.1 Sample Environment ...................................................................................... 32 2.1.1 Vacuum Equipment ............................................................................. 32 2.1.1.1 Pu mps .................................................................................... 32 2.1.1.2 Pumping lines ......................................................................... 34 2.1.1.3 Vacuum gauges ...................................................................... 34 2.1.1.4 Oi l mist filters and fore line traps ............................................ 35 2.1.1.5 O rings .................................................................................... 35 2.1.2 Cryostats ............................................................................................. 36 2.1.2.1 Bath cryostats ......................................................................... 36 2.1.2.2 Co ntinuous flow cryostats and cryogenic inserts .................... 36 2.1.2.3 Closed cycle refrigerators ....................................................... 37 2.1.3 Superconducting Magnets ................................................................... 38 2. 1.3.1 Magnet construction ............................................................... 38 2.1.3. 2 Magnet operation .................................................................... 39 2.1.4 Light Guides ........................................................................................ 40 2.2 Detection Methods ......................................................................................... 41 2.2.1 SQUID magnetometer ......................................................................... 41 2.2.1.1 Superconducting quantum interference devices ..................... 41 2.2.1.2 Quantum Design MPMS XL magnetometer ............................ 42 2.2.1. 3 Remnant fields and degaussing the MPMS ............................ 43 2.2.2 Additional Methods Performed at UF ................................................... 44 2.2.2.1 Atomic force microscopy ......................................................... 44 2.2.2.2 Carbon, hydrogen, and nitrogen combustion .......................... 44

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7 2.2.2.3 Energy dispersive x ray spectroscopy .................................... 45 2.2.2.4 Fourier transform infrared spectroscopy ................................. 45 2.2.2.5 Inductively coupled mass spectrometry .................................. 46 2.2.2.6 Transmission electron microscopy .......................................... 47 2.2.2.7 Ultraviolet and visible spectroscopy ........................................ 48 2.2.2.8 X ray powder diffraction .......................................................... 48 2.2.3 National Laboratories .......................................................................... 49 2.2.3.1 Electron magnetic resonance at the NHMFL .......................... 50 2.2.3.2 HB1A neutron tripleaxis spectrometer at HFIR ...................... 51 2.2.3.3 HB2A neutron powder diffractometer at HFIR ........................ 52 2.2.3.4 Inelastic neutron scattering on SEQUOIA at SNS .................. 52 2.3 Custom Apparatus .......................................................................................... 53 2.3.1 SQUID Probe with Low Temperature Rotation and Optical Fibers ...... 53 2.3.1.1 Probe specifications and design ............................................. 54 2.3.1.2 Probe material properties ....................................................... 56 2.3.1.3 Operation ................................................................................ 56 2.3.1.4 Conclusions ............................................................................ 58 2.3.2 Neutron Scattering Probe for Photoinducing Opaque Powders ........... 59 3 THEORETICAL METHODS .................................................................................... 74 3.1 Quantum Mechanics of Transition Metal Ions ................................................ 75 3.1.1 Coulomb Interaction and the Multi Electron Ion ................................... 76 3.1.2 Ligand Field Theory ............................................................................. 79 3.1.3 Spin Orbit Coupling ............................................................................. 81 3.1.4 Zeeman Splitting .................................................................................. 82 3.1.5 Superexchange Interaction .................................................................. 82 3.1.6 MeanField Theory .............................................................................. 83 3.2 Tigh t Binding Approximations ........................................................................ 85 3.2.1 Extended Hckel Theory ..................................................................... 86 3.2.2 Ligand Field Theory ............................................................................. 88 3.2.3 Superexchange Interaction .................................................................. 88 3.2.4 Infrared Vibrational Spectroscopy ........................................................ 88 3.3 Fitting Algorithms ........................................................................................... 89 3.3.1 Least Squares ..................................................................................... 89 3.3.2 Levenberg Marquardt .......................................................................... 91 3.3.3 Rietveld Refinement ............................................................................ 92 4 QUANTITATIVE ANALYSI S OF MAGNETIZATION IN COMPLEX CYANIDES ... 101 4.1 Synthesis and Chemical Composition ............................................................. 102 4.2 Spectroscopy .................................................................................................. 102 4.3 Magnetic Susceptibility ................................................................................... 1 03 4.4 Microscopic Probe of Magnetization ............................................................... 104 4.5 Magnetization Fitting ....................................................................................... 105 4.5.1 K3 Cr(CN)6 .......................................................................................... 105 4.5.2 K3Fe(CN)6 .......................................................................................... 106

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8 4.5.3 Cs2.8Ni4[Cr(CN)6]4 nH2O .................................................................... 107 4.5.4 Co4[Fe(CN)6]3 nH2O .......................................................................... 108 5 COBALT HEXACYANOFERR ATE NANOPARTICLES ........................................ 114 5.1 Nanoparticles of Rubidium Cobalt Hexacyanoferrate ................................... 116 5.1.1 Introduction ........................................................................................ 117 5.1.2 Synthesis and Chemical Composition ............................................... 117 5.1.3 Structure ............................................................................................ 118 5.1.3.1 Transmission electron microscopy ........................................ 118 5.1.3.2 Fourier transform infrared spectroscopy ............................... 119 5.1.4 Magnetization .................................................................................... 119 5.1.5.1 DC susceptibility ................................................................... 119 5.1.5.2 DC magnetization ................................................................. 120 5.1.5. 3 AC susceptibility ................................................................... 120 5.1.5 Discussion ......................................................................................... 121 5.1.6 Conclusions ....................................................................................... 123 5.2 Nanopartic les of Potassium Cobalt Hexacyanoferrate ................................. 124 5.2.1 Introduction ........................................................................................ 124 5.2.2 Synthesis and Chemical Composition ............................................... 126 5.2.2. 1 Fourier transform infrared spectroscopy ............................... 126 5.2.3 Structure ............................................................................................ 126 5.2.3.1 Transmission electron microscopy ........................................ 127 5.2.3.2 X ray diffraction ..................................................................... 127 5.2.3. 3 Neutron diffraction ................................................................ 129 5.2.4 Magnetization .................................................................................... 130 5.2.4.1 Quenched high temperature DC susceptibility ...................... 131 5.2.4.2 Quenched low temperature DC susceptibility ....................... 131 5.2.4.3 Quenched low temperature magnetization ........................... 132 5.2.4.4 Isothermal relaxation ............................................................ 132 5.2.4.5 Photoinduced low temperature DC susceptibility .................. 132 5.2.4.6 Photoinduced low temperature magnetization ...................... 133 5.2.5 Discussion ......................................................................................... 133 5.2.5.1 Details of modeling ............................................................... 133 5.2. 5. 2 Size dependence of thermal quenching ................................ 136 5.2.5. 3 Photoinduced versus quenched states ................................. 137 5.2.5. 4 Resulting schema of bulk and nanoparticles ........................ 138 5.2.6 Conclusions ....................................................................................... 138 6 THIN FILMS OF PRUSSIAN BLUE ANA LOGUES ............................................... 159 6.1 Introduction .................................................................................................. 159 6.2 Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O thin films ............................................................ 160 6.2.1 Sample Characterization ................................................................... 161 6. 2.1.1 Chemical composition ........................................................... 161 6.2.1.2 AFM ...................................................................................... 161 6.2.1.3 FT IR .................................................................................... 162

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9 6.2.1.4 UV Vis .................................................................................. 162 6.2.2 Magnetization .................................................................................... 163 6.2.2.1 DC susceptibility in 100 G ..................................................... 163 6.2.2.2 DC magnetization in 40 kG ................................................... 163 6.2.2.3 DC magnetization field dependence ..................................... 164 6.2.2.4 Magnetization Process ......................................................... 164 6.2.2.5 DC magnetization angular dependence ................................ 165 6.2.3 Electron Magnetic Resonance ........................................................... 165 6.2.3.1 EMR temperature dependence ............................................. 165 6.2.3.2 EMR angular dependence .................................................... 167 6.2.3.3 EMR frequency dependence ................................................ 168 6.2.4 X ray Diffraction ................................................................................. 168 6.3 Additional Prussian Blue Analogue Thin Films ............................................. 168 6.3.1 Rb0.6Co4.0[Cr(CN)6]2.9nH2O Thin Films ............................................. 168 6.3.2 Rb0.7Cu4.0[Cr(CN)6]2.9nH2O Thin Films ............................................. 169 6.3.3 Rb0.3Zn4.0[Cr(CN)6]2.8nH2O Thin Films .............................................. 170 6.3.4 Rb0.9Ni4.0[Fe(CN)6]2.8nH2O Thin Films .............................................. 171 6.3.5 Rb0.7Co4.0[Fe(CN)6]2.8nH2O Thin Films ............................................. 172 6.3.6 Rb0.5Cu4.0[Fe(CN)6]2.7nH2O Thin Films ............................................. 172 6.3.7 Rb0.5Zn4.0[Fe(CN)6]2.8nH2O Thin Films ............................................. 173 6.4 Discussion .................................................................................................... 173 6.4.1 Rb0.7Ni4.0[Cr(CN)6]2.9nH2O Thin Films ............................................... 174 6.4.2 Additional Prussian Blue Analogue Thin Films .................................. 183 6.5 Conclusions .................................................................................................. 185 7 HETEROSTRUCTURES OF PRUSSIAN BLUE ANALOG UES ............................ 212 7.1 Solid Solutions of Cobalt Hexacyanoferrate ................................................ 212 7.1.1 Introduction ........................................................................................ 212 7.1.2 Synthesis and Chemical Composition ............................................... 215 7.1.3 Transmission Electron Microscopy .................................................... 216 7.1.4 Infrared Spectroscopy ....................................................................... 217 7.1.5 X Ray Diffraction ............................................................................... 217 7.1.6 Me anField Calculations .................................................................... 218 7.1.6.1 Low temperature magnetization in the meanfield ................ 219 7.1.6.2 Meanfield magnetic susceptibility and spincrossover ......... 221 7.1.7 Magnetic Measurements ................................................................... 222 7.1. 7.1 Low temperature DC susceptibility ....................................... 223 7.1.7.2 DC magnetization ................................................................. 224 7.1.7.3 High temperature DC susceptibility ....................................... 224 7.1.7.4 Physically mixed x = 0.66 compound .................................... 225 7.1.8 Discussion ......................................................................................... 225 7.1.8.1 Photoinduced decrease in magnetization ............................. 226 7.1.8.2 Scaling of magnetic properties ............................................. 228 7.1.8.3 Spin crossover dilution ......................................................... 230 7.1.8.4 Meanfield predictions versus observations .......................... 233 7.1.9 Conclusions ....................................................................................... 234

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10 7.2 Heterostructured Films Containing Cobalt Hexacyanoferrate ..................... 235 7.2.1 Introduction ......................................................................................... 235 7.2.2 Synthesis ............................................................................................. 236 7.2.3 Magnetization of Nickel Hexacyanochromate and Cobalt Hexacyanoferrate Heterostructures ................................................... 237 7.2.3.1 Slow deposition multilayer films ............................................ 237 7.2.3.2 Stacked films ........................................................................ 238 7.2.3.3 Thin sandwiched films .......................................................... 239 7.2.3.4 Thick sandwiched films ......................................................... 239 7.2.4 40/40/40 Heterostructure ................................................................... 241 7.2.4.1 40/40/40 film, 10 kG temperature sweeps ............................ 242 7.2.4.2 40/40/40 film, transmission electron microscopy .................. 242 7.2.4.3 40/40/40 film, energy dispersive x ray spectroscopy ............ 242 7.2.4.4 40/40/40 film, x ray powder diffraction .................................. 243 7.2.4.5 40/40/40 film, infrared spectroscopy ..................................... 243 7.2.5 Capping Layers of Cobalt and Chromium Hexacyanochromates ...... 243 7.2.5. 1 Magnetization of cobalt hexacyanochromate and cobalt hexacyanoferrate sandwich heterostructures ....................... 244 7.2.5.2 Magnetization of chromium hexacyanochromate and cobalt hexacyanoferrate sandwich heterostructures ....................... 244 7.2.6 Discussion ......................................................................................... 245 7.2.7 Conclusion ......................................................................................... 248 8 SUMMARY AND CONCLUSIONS ........................................................................ 285 APPENDIX A UNITS ................................................................................................................... 290 B LOW TEMPERATURE ROTA TION PROBE DRAWINGS .................................... 291 C COPYRIGHT RELEASE FO RMS ......................................................................... 306 LIST OF REFERENCES ............................................................................................. 314 BIOGRAPHICAL SKETCH .......................................................................................... 324

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11 LIST OF TABLES Table page 2 1 Magnetic response of candidate probe materials for optical rotator magnetization probe ........................................................................................... 69 4 1 Parameters to be used in modeling magnetization data for Cs2.8Ni4[Cr(CN)6]4nH2O, Co4[Fe(CN)6]3nH2O, K3Cr(CN)6,and K3Fe(CN)6......... 111 5 1 Synthesis and chemical composition of rubidium cobalt hexacyanoferrate nanoparticles. ................................................................................................... 140 5 2 Magnetic properties of rubidium cobalt hexacyanoferrate nanoparticles. ......... 144 5 3 The microscopic states relevant to KjCok[Fe(CN)6]l nH2O ................................ 145 5 4 The macroscopic states relevant to KjCok[Fe(CN)6]l nH2O ............................... 145 5 5 Metal oxidation states of bulk powder and nanoparticles, in addition to fitting parameters used for ......................................................................................... 146 5 6 Chemical composition and characteristic sizes of potassium cobalt hexacyanoferrate nanoparticles and bulk powder ............................................ 146 5 7 Magnetic properties of quenched potassium cobalt hexacyanoferrate nanoparticles and bulk powder ......................................................................... 157 5 8 Magnetic properties of photoinduced potassium cobalt hexacyanoferrate nanoparticles and bulk powder ......................................................................... 157 6 1 Molecular formulas of films measured and techniques used ............................ 201 7 1 Molecular formulas and unit cell parameters for NaCoxNi1 x[Fe(CN)6] nH2O 249 7 2 Nickel h exacyanochromate and c obalt h exacyanoferrate heterostructures studied .............................................................................................................. 267

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12 LIST OF FIGURES Figure page 1 1 Illustration of constrained geometries in nanostructures ..................................... 30 1 2 Illustration of designer heterogeneous geometries in nanostructures ................. 30 2 1 Illustrations showing operation of standard pumps that can be found in a cryogenic laboratory ........................................................................................... 60 2 2 Illustration of a superconducting solenoid magnet .............................................. 61 2 3 Illustration of a SQUID magnetometer circuit ..................................................... 61 2 4 Illustration of SQUID magnetometer pickup coil s ............................................... 62 2 5 Remnant fields and degaussing the MPMS ........................................................ 62 2 6 AFM schematic ................................................................................................... 63 2 7 A schematic of a combustion train for CHN analysis .......................................... 63 2 8 EDS schematic ................................................................................................... 63 2 9 FT IR schematic ................................................................................................. 64 2 10 ICP MS schematic .............................................................................................. 64 2 11 TEM schematic ................................................................................................... 65 2 12 UV Vis spectrometer schematic ......................................................................... 65 2 13 XRD schematic ................................................................................................... 66 2 14 Theoretical EMR schematic ................................................................................ 66 2 15 Experimental EMR schematic ............................................................................. 67 2 16 The triple axis spectrometer at HFIR on beamline HB1A ................................... 67 2 17 The neutron powder diffractometer at HFIR on beamline HB2A ......................... 68 2 18 The SEQUOIA inelastic time of flight spectrometer at SNS ............................... 68 2 19 Photographs of the optical rotation probe for use in a SQUID magnetometer .... 70 2 20 Magnetization versus field for potential optical rotation probe materials ............. 71

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13 2 21 A circuit diagram of the control board for the automated operation of the custom probe using a stepper motor .................................................................. 71 2 22 Magnetization versus rotation angle measured with the custom probe for two different magnetite samples at 300 K (a) and 2 K (b) ......................................... 72 2 23 Photoirradiation of powdered neutron scattering samples .................................. 72 2 24 Photoirradiation of powdered neutron scattering samples using tumbler probe 73 3 1 The energy differences between different d5 Fe3+ free ion terms arising from electronelectron repulsion as a function of the Racah repulsion parameter B ... 95 3 2 The octahedral coordination geometry ............................................................... 95 3 3 The energy of a molecular term as a function of the octahedral splitting parameter, for a d5 ion, such as Fe3+ .............................................................. 96 3 4 Energy shift plotted versus the tetragonal distortion parameter, ...................... 97 3 5 Energy splitting of the octahedral hexacyanoferrate 2T2g ground state ............... 98 3 6 The Zeeman splitting versus applied magnetic field for the spinorbit split 2T2glike ground state of hexacyanoferrate ......................................................... 99 3 7 The effect of superexchange on magnetization .................................................. 99 3 8 The cyanide molecule ....................................................................................... 100 4 1 The cubic complex cyanide Prussian blue analogue structure ......................... 110 4 2 UV Vis of Rb0.7Ni4.0[Cr(CN)6]2.9nH2O ............................................................... 110 4 3 Measurement of magnetic ordering temperature in Cs2.8Ni4[Cr(CN)6]4 nH2O, Co4[Fe(CN)6]3 nH2O, K3Cr(CN)6,and K3Fe(CN)6 .............................................. 111 4 4 Magnetic properties of K3Cr(CN)6 ..................................................................... 112 4 5 Magnetic properties of K3Fe(CN)6 .................................................................... 112 4 6 Magnetic properties of Cs2.8Ni4[Cr(CN)6]4 nH2O ............................................... 113 4 7 Magnetic properties of Co4[Fe(CN)6]3 nH2O ..................................................... 113 5 1 Prussian blue analogue structure ..................................................................... 139 5 2 A detail of the photoexcitation processes in K0.2Co1.4[Fe(CN)6] 6.9H2O ........... 139 5 3 TEM of RbjCok[Fe(CN)6]l nH2O nanoparticles .................................................. 140

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14 5 4 FT IR absorption spectra of RbjCok[Fe(CN)6]l nH2O nanoparticles .................. 141 5 5 The temperature dependences of the low field, 100 G, susceptibilities of RbjCok[Fe(CN)6]l nH2O nanoparticles ............................................................... 142 5 6 The T = 2 K magnetization versus magnetic field sweeps of RbjCok[Fe(CN)6]l nH2O nanoparticles ............................................................... 143 5 7 The temperature dependences of the real ( ) and imaginary ( ) AC susceptibilities of RbjCok[Fe(CN)6]l nH2O nanoparticles ................................... 144 5 8 Ordered magnetic components of the smaller batches .................................... 145 5 9 FT IR spectra of bulk and nanoparticles of K Co Fe ........................................ 146 5 10 TEM of K Co Fe. (left) Typical TEM images are shown .................................. 147 5 11 XRD of K Co Fe ............................................................................................... 1 48 5 12 Neutron scattering of K Co Fe .......................................................................... 149 5 13 Neutron diffraction as a function of temperature for K Co Fe ........................... 150 5 14 Magnetic neutron scattering in K Co Fe ........................................................... 151 5 15 Temperature dependent magnetic moment of quenched states for K Co Fe ... 151 5 16 M agnetic ordering of quenched states in K Co Fe ........................................... 152 5 17 Magnetization versus field of quenched states for K Co Fe ............................. 153 5 18 Relaxation of magnetization in quenched states of K Co Fe ............................ 154 5 19 Magnetic ordering of photoinduced states in K Co Fe ...................................... 155 5 20 Magnetization versus field of photoinduced states for K Co Fe ....................... 156 5 21 Linearization of modeling .................................................................................. 157 5 22 Ordering temperatures and coercive fields of batches in different macroscopic states ........................................................................................... 158 5 23 Microscopic schema based upon all data ......................................................... 158 6 1 Prussian blue analogue structure ..................................................................... 186 6 2 The multiple sequential adsorption method that can be used for generating thin films of Prussian blue analogues ............................................................... 187

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15 6 3 Different orientations of the magnetic thin films with respect to the applied magnetic field are expected to have different behavior. ................................... 187 6 4 AFM of thin films ............................................................................................... 188 6 5 Room temperature FT IR spectroscopy measurements of the cyanide stretches present in Ni Cr materials ................................................................. 188 6 6 UV Vis spectroscopy of Ni Cr materials ........................................................... 189 6 7 Temper ature dependent magnetization of Ni Cr materials ............................... 189 6 8 Temperature dependent magnetization of Ni Cr materials at high fields .......... 190 6 9 Field dependent magnetization of Ni Cr materials ............................................ 190 6 10 Magnetizing process of thin Ni Cr film .............................................................. 191 6 11 Magnetizing process of thick NiCr film ............................................................ 191 6 12 Magnetizing process of Ni Cr powder ............................................................... 192 6 13 Angle dependence of magnetization in Ni Cr materials .................................... 192 6 14 EMR lines of NiCr powder ............................................................................... 193 6 15 Results of fitting EMR lines of NiCr powder ..................................................... 193 6 16 EMR lines of NiCr thin film perpendicular ........................................................ 194 6 17 Results of fitting EMR lines of NiCr thin film perpendicular ............................. 194 6 18 EMR lin es of Ni Cr thin film parallel .................................................................. 195 6 19 Results of fitting EMR lines of NiCr thin film parallel ........................................ 195 6 20 EMR lines of NiCr thick film perpendicular ...................................................... 196 6 21 Results of fitting EMR lines of NiCr thick film perpendicular ............................ 196 6 22 EMR lines of NiCr thick film paralle l ................................................................ 197 6 23 Results of fitting EMR lines of NiCr thick film parallel ...................................... 197 6 24 EMR lines of NiCr thin film as a function of angle ............................................ 198 6 25 Results of fitting EMR lines of NiCr thin film as a function of angle ................. 198 6 26 EMR lines of NiCr thick film as a function of angle .......................................... 199

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16 6 27 Results of fitting EMR lines of NiCr thick film as a function of angle ................ 199 6 28 EMR lines of NiCr thick film as a function of angle in lower field ..................... 200 6 29 Res ults of fitting EMR lines of NiCr thick film as a function of angle in lower field ................................................................................................................... 200 6 30 Ligand field levels of CoCr ............................................................................... 201 6 31 Magnetic susceptibility of Co Cr thin film .......................................................... 202 6 32 Ligand field energies of CuCr .......................................................................... 202 6 33 Magnetic susceptibility of Cu Cr thin f ilm .......................................................... 203 6 34 UV Vis of Cu Cr thin film .................................................................................. 203 6 35 Ligand field energies of ZnC r .......................................................................... 204 6 36 Magnetization of ZnCr versus field .................................................................. 204 6 37 Ligand field energies of Ni Fe ........................................................................... 205 6 38 Magnetic susceptibility of NiFe thin films ......................................................... 205 6 39 Ligand field energy levels and rotational magnetism of CoFe ......................... 206 6 40 Ligand field energies of CuFe .......................................................................... 207 6 41 Magnetic susceptibility of Cu Fe thin films ........................................................ 207 6 42 Demagnetizing fields in films uniformly magnetized perpendicular to the surface ......................................................................................................... 208 6 43 Demagnetizing fields in films uniformly magnetized parallel to the surface ...... 208 6 44 Fitting magnetization of Ni Cr thin films in low field .......................................... 209 6 45 Fitting magnetization of Ni Cr thin films in high field ......................................... 210 6 46 Thickness dependence of thin film s .................................................................. 210 6 47 EMR lines of NiCr thin films and powder ......................................................... 211 7 1 The NaNi1 xCox[Fe(CN)6]nH2O material ........................................................ 249 7 2 Typical TEM micrographs for samples reported in Table 71 for different values of x ........................................................................................................ 250

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17 7 3 FT IR spectra and fitting parameters of NaNi1 xCox[Fe(CN)6]nH2O as a function of x ...................................................................................................... 251 7 4 Full XRD diffractograms of NaNi1 xCox[Fe(CN)6]nH2O .................................. 252 7 5 XRD of the x = 1 NaNi1 xCox[Fe(CN)6]nH2O ................................................. 253 7 6 Room temperature XRD reflection with background subtracted and intensity normalized to show the continuous evolution with x ......................................... 253 7 7 Photoinduced magnetization of NaNi1 xCox[Fe(CN)6]nH2O ........................... 254 7 8 Molar magnetic susceptibility of NaNi1 xCox[Fe(CN)6]nH2O as a function of time irradiated at 5 K and 10 G, measured in a SQUID .................................... 255 7 9 Magnetization versus field for NaNi1 xCox[Fe(CN)6]nH2O ............................. 256 7 10 Checking for asymmetry in the hysteresis loop as a possible explanation of the reduction in HC for the x = 0.66 sample ...................................................... 257 7 11 Thermal induced changes in magnetization of NaNi1 xCox[Fe(CN)6]nH2O .... 258 7 12 Microscopic versus macroscopic mixing ........................................................... 259 7 13 Magnetization of macroscopically mixed NaNi1 xCox[Fe(CN)6]nH2O ............. 259 7 14 Field dependence of photoinduced magnetization for NaNi1 xCox[Fe(CN)6]nH2O ............................................................................. 260 7 15 Scaling of magnetic properties of NaNi1 xCox[Fe(CN)6]nH2O ........................ 261 7 16 Superexchange in NaNi1 xCox[Fe(CN)6]nH2O ............................................... 262 7 17 Charge transfer induced spin transition parameters for NaNi1 xCox[Fe(CN)6]nH2O ............................................................................. 262 7 18 Amount of CTIST materials in NaNi1 xCox[Fe(CN)6]nH2O ............................. 263 7 19 FT IR parameters in NaNi 1 xCox[Fe(CN)6]nH2O ............................................ 263 7 20 Comparison of ferromagnetic versus antiferromagnetic CoFe components .... 264 7 21 Modification of superexchange energy in NaNi1 xCox[Fe(CN)6]nH2O ............ 265 7 22 A scheme showing synthesis of a heterostructured thin film using multiple sequential adsorption cycles. ........................................................................... 266 7 23 Magnetization of slow deposition multilayer films ............................................. 267

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18 7 24 Magnetization of 10/10 CoFe/NiCr thin film oriented parallel ......................... 268 7 25 Magnetization of 10/10 CoFe/NiCr thin film oriented perpendicular ............... 268 7 26 Magnetization of 10/10 Ni Cr/Co Fe thin film oriented parallel ......................... 269 7 27 Magnetization of 10/10 Ni Cr/Co Fe thin film oriented perpendicular ............... 269 7 28 Magnetization of 10/5/10 Ni Cr/Co Fe/NiCr thin film oriented parallel ............. 270 7 29 Magnetization of 10/5/10 Ni Cr/Co Fe/NiCr thin film oriented perpendicular ... 270 7 30 Magnetization of 10/10/10 sandwich film versus temperature .......................... 271 7 31 Photoinduced magnetization of 10/10/10 film oriented perpendicular .............. 272 7 32 Photoinduced magnet ization of 10/10/10 film oriented perpendicular .............. 272 7 33 Magnetization of 40/40/40 Ni Cr/Co Fe/NiCr heterostructure .......................... 273 7 34 Magnetization of 20/40/20 Ni Cr/Co Fe/NiCr heterostructure .......................... 274 7 35 Magnetization of 40/20/40 Ni Cr/Co Fe/NiCr heterostructure .......................... 275 7 36 Magnetization data of the 40/40/40 sandwich Ni Cr/Co Fe/NiCr PBA film ...... 276 7 37 High magnetic field,10 kG temperature sweeps of 40/40/40 NiCr/Co Fe/NiCr ............................................................................................. 277 7 38 Transmission electron microscopy of the 40/40/40 Ni Cr/Co Fe/ NiCr heterostructure ................................................................................................. 277 7 39 An energy dispersive x ray line scan of the 40/40/40 heterostructure .............. 278 7 40 X ray powder diffraction of a 40/40/40 heterostructure ..................................... 279 7 41 Cyanide stretching energies in the infrared ...................................................... 280 7 42 Magnetization data of CoCr/Co Fe/Co Cr heterostructures ............................ 281 7 43 Magnetization data of Cr Cr/Co Fe/Cr Cr het erostructures .............................. 282 7 44 Photoexcitation of CoFe .................................................................................. 283 7 45 Distortions in NiCr ........................................................................................... 283 7 46 A nisotropy in Ni Cr ........................................................................................... 284

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19 LIST OF ABBREVIATION S A Alkali cation AC alternating current AFM Atomic force microscopy AQ Aqueous solution B Magnetic field CHN Carbon hydrogen nitrogen CLB Chemistry Lab Building CN C yanide CTI ST Charge transfer induced spin transition DC Direct current EDS Energy dispersive spectroscopy EMF Electromagnetic field emu Electromagnetic unit EMR Electron magnetic resonance EXAFS Extended x ray fine structure FC Field cooled FCC Face centered cubic FT IR Fourier transform infrared FWHM Fullwidth half maximum G Gauss H Magnetizing field HB HFIR Beamline HC Coercive field HFIR High Flux Isotope Reactor

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20 HS High spin HT High temperature ICP MS Inductively coupled mass spectrometry K Kelvin lb Pound LCAO Linear combination of atomic orbitals LS Low spin LT Low temperature m meter M Transition metal Transition metal MAIC Major Analytical Instrumentation Center MPMS Magnetic Properties Measurement System NHMFL National High Magnetic Field Laboratory NMR Nuclear magnetic resonance NPB New Physics Building OD Outer diameter ORNL Oakridge National Laboratory Pa Pascale PBA Prussian blue analog PVP Polyvinylpyrrolidone RMS Ro ot mean squared S Spin quantum number SCO Spin crossover SEM Scanning electron microscopy

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21 SEQUOIA an inelastic diffractometer at SNS, after the Native American chief SNS Spallation Neutron Source SQUID Superconducting quantum interference device T Tesla or Temperature ( depending upon context) TC Magnetic ordering temperature TCTIST Temperature around which the thermal spin crossover centers, (Tup+Td ow n)/2 Tf Freezing temperature associated with spinglass materials Tup Temperature at which half of the spin crossover material has transitioned in the heating cycle Tdow n Temperature at which half of the spin crossover material has transitioned in the cooling cycle TEM Transmission electron microscopy UF University of Florida UV Vis Ultraviolet and Vis ible (spectroscopy) XRD X ray diffraction ZFC Zero field cooled Magnetic susceptibility XRD X ray diffraction

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22 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy PHOTOINDUCED MAGNETISM IN NANOSTRUCTURES OF PRUSSIAN BLUE ANALOGUES By Daniel Matthew Pajerowski August 2010 Chair: Mark W. Meisel Major: Physics A wide range of experimental and theoretical investigations have been made on nanostructures of Prussian blue analogues, and a variety of results have been obtained. Most notably, a novel photoeffect has been observed in RbjCok[Fe(CN)6]l nH2O / RbjNik[Cr(CN)6]l nH2O Prussian blue analogue heterostructured films, and this effect per sists up to ~ 75 K, which is an unprecedented high temperature for photocontrol in this class of compounds. This engineering of the highTC heterostructures was made possible by insight gained from studying other nanostructured Prussian blue analogues. S pecifically, solid solutions of NaNi1 xCox[Fe(CN)6] nH2O proved that the sign and the magnitude of photoinduced magnetization can be tuned with chemical formula. In addition, these solid solutions elucidated the effect that diluting the lattice has on the magnetic properties of the photomagnet. By studying nanoparticles of RbjCok[Fe(CN)6]l nH2O, a size dependent scaling of magnetic ordering temperatures and coercive fields was established, with bulk long range magnetic order occurring in particles larger than ~ 25 nm. In parallel work, nanoparticles of KjCok[Fe(CN)6]l nH2O showed a reduction in bistable material with decreasing particle size. The magnetic anisotropy of sequential adsorption Prussian blue analogue films proved to be

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23 complicated, and required a plethora of probes to tease out the inherent properties. Many analogues were studied, and magnetostatic, g factor, and magnetocrystalline effects were identified in different samples, depending upon the magnetic ions. In addition to a discussion about characterization of these nanostructured Prussian blue analogues, a review of the relevant theoretical and experimental techniques is presented. For example, the design and implementation of a custom magnetometer probe with fiber optics and automatic sample rotation is noteworthy and is discussed. Furth ermore, the design of a custom optic probe for use with photoinducible opaque powders in neutron scattering experiments is included. Theoretical tools and numerical investigations that were employed to understand the experimental results are overviewed, w ith specific attention given to properly parameterizing the photoinduced systems. Detailed models of Prussian blue analogue materials are presented. Specific examples showing the potential ambiguity of assigning superexchange constants based on magnetization in the presence of first order orbital angular momentum are also discussed, and one example is the photomagnetic RbjCok[Fe(CN)6]l nH2O. Finally, experimental techniques, which include AFM, CHN, EDS, EMR, FT IR, ICP MS, neutron scattering, SQUID magnetometry, TEM, UV Vis spectroscopy, and XRD, are reviewed for the purpose of understanding the data presented.

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24 CHAPTER 1 INTRODUCTION 1 asdf To begin, magnetism is central to all discussion in this thesis. From a purely scientific standpoint, magnetism fascinates because it is an inescapable manifestation of quantum mechanics. Among fundamental physical concepts, magnetism is of particular importance technologically, most especially for the storage of information in memory devices. Modern memory storag e is done with oxides or alloys that require high temperatures for synthesis, and memory is written using other magnetic materials. Currently, potential alternative materials are being investigated with much vigor, with the hopes of either having similar performance using cheaper manufacturing than industry standards or improved performance with limited increase in expense. Of the many alternative possibilities, molecule based complex cyanides are the focus of this work. Compared to metallurgical synthes is, the room temperature and pressure wet chemistry required for complex cyanides is cheap and user friendly. In addition, while standard methods may be used to write to these materials, the magnetization of the complex cyanides can also be changed by the application of external light, heat, and pressure [ 1 2 ] Possible benefits of the complex cyanides include potentially storing bits in individual nanometer sized molecules, as well as the fast, three dimensional write ability that would be awarded by usi ng photons, instead of magnetic field induced torque, to write. The problem can then be stated as the search to understand the magnetization of the complex cyanides and to see how practical the goals of an optically controlled cyanometallate memory storag e device may be. More specifically for this dissertation, the main thrust of the research has been studying the effects of incorporation of complex cyanides into nanostructures. The

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25 nanostructures studied include nanoparticles, thin films, and heterostruc tures of thin films, along with atomically mixed solid solutions of bulk materials. Novel photomagnetic effects of the nanostructures and phenomena related to the effects will be discussed in detail in the main chapters of the thesis. On a practical note, this thesis is written at a level so as to be accessible to someone with an undergraduate education in a physical science, with the possible need for additional reading to understand the details of certain subsections. No details will be given on the experimental and theoretical topics covered in typical curricula, and the interested reader is directed to standard texts on the subjects [ 3 4 ] In addition, the units will generally be c.g.s., except in cases where precedent dictates otherwise for ease of c omparison. When plotting magnetic fields, 0 will be suppressed for readability. Appendix A addresses the units used herein. The structure of the dissertation is such that Chapters 2 and 3 provide specific, pertinent background information that may be helpful to fully understand the new measurements and materials that are presented in Chapters 5, 6, and 7. Chapter 4 is somewhat between the experimental chapters and the background chapters, as it illustrates essential concepts and treats data already pres ent in the literature to a quantitative analysis based upon tools outlined in the background sections. 1.1 Experimental Techniques Experimental methods are presented in Chapter 2. While DC magnetization measurements make up the majority of data presented in t his thesis, complementary techniques are essential to achieve in depth understanding of magnetization results. In the modern world, many apparatus for probing solids have been honed to a high level of

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26 sophistication, and an adequately educated scientist c an either participate in collaborations or simply step in as a guest to use the desired equipment. Therefore, part of the experimental methods chapter consists of an encyclopedia of the experimental procedures utilized. As external lab facilities with well supported user programs were essential to some investigations, including national laboratories and other labs at the University of Florida, capabilities as well as the locations of the equipment have been documented. Furthermore, more substantial space is allotted for procedures where the author has made significant contributions. In particular, an artisanal rotation probe to study the angular dependence of magnetization in thin films is detailed. Also, as photoinduced magnetization has been one of the most exciting topics studied, advancements on magnetization optic probes and neutron scattering optic probes are also presented. 1.2 Theoretical Methods Physics is an experimental science. On the other hand, theoretical work is essential for potential predi ctive power and insight into experimental results. What can provide greater joy than finding the additional term necessary to not only accurately fit a puzzling data set but to actually open up a whole new avenue of experiments? While complicated first p rinciples theories can dazzle with their power, well parameterized Hamiltonians tractable to the average physicist are light and elegant at their best (but misleading and confusing at their worst). In Chapter 3, starting with a bare atom, the relevant int eractions will be introduced using the magnetically ubiquitous iron as a bellwether. In vacuum, the first order energy of an atom is determined by electronelectron Coulomb repulsion and the Pauli exclusion principle. Additional corrections, such as relati Vis tic effects and spin orbit coupling can provide additional

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27 structure to the energy spectrum. Next, if a molecule forms with a single magnetic ion, the bare ionic energies will be affected by the presence of the additional nonmagnetic atoms the socalled ligands. Dubbed ligand field theory, the changes in the electronic energy levels of the ions in the molecule have been found to be caused by both electrostatic and covalent interactions [ 5 ] The situation becomes increasingly complex as more magnetic ions are introduced to molecules and allowed to interact with one another. Magnetic interactions, mediated through the nonmagnetic ligands in a coordination network, further perturb the energy levels and can even lead to macroscopic correlation of magnetic moments throughout a compound. This long range magnetic order can have many exotic properties of its own, as well as technological implications for information storage. 1.3 Quantitative Analysis of Magnetization in Prussian Blue Analogues To provide a strong foundation for the chapters discussing even more complicated nanostructured materials, the magnetic properties of nickel hexacyanochromate and cobalt hexacyanoferrate, the two compounds most thoroughly studied, will be described in Chapter 4 using m achinery presented in the theoretical methods chapter. The detailing of two materials of nearly identical structures having different magnetic ions will hopefully reinforce the need to understand the quantum mechanical theories that can accurately describe this type of magnetism. To begin, the paramagnetic precursors, K3[Cr(CN)]6 and K3[Fe(CN)]6, will be analyzed, stressing the singleion properties of the molecules, which remain relevant in the more complicated energy spectra of the full complexes. Subs equently, an analysis of the Ni4.0[Cr(CN)6]3.0 nH2O and Co4.0[Fe (CN)6]3.0 nH2O materials will be presented along with how magnetization as a

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28 function of temperature and magnetic field may be explained as resulting from the magnetic energy levels of the ions. 1.4 Cobalt Hexacyanoferrate Nanoparticles The first set of new materials studied consist of nanoparticles of the photomagnetic RbjCok[Fe (CN)6]l nH2O Prussian blue analogue. Partially motivated by the room at the bottom approach to science, magnetic nanoparticles ( Figure 1 1 ) are relevant to the development of memory storage devices as memory media are made increasingly dense and finite size effects may help or hinder device performance. The photoinduced magnetism of K0.2Co1.4[Fe(CN)6] 6.9H2O was first discovered in a bulk powder, showing long range magnetic order that was modified with the application of light [ 1 ] More recently, researchers have been synthesizing photomagnetic nanoparticles, however, no long range order was observed [ 6 ] [ 7 ]. It was not until nanoparticles of RbjCok[ [Fe (CN)6]l nH2O were synthesized with fine size control that a size dependent study of photomagnetic nanoparticle magnetic properties was performed. This work showed modifications of the coercive fields and the ordering temperatures as a function of size, spanning the regimes from bulk like to superparamagnetic properties [ 8 ]. Further sizedependent studies were performed on KjCok[Fe (CN)6]l nH2O magnets, which can be trapped into different magnetic states by varying the cooling rates [ 9 ] Details of the experiments, including magnetization, x ray diffraction, neutron diffraction, AC susceptibility, infrared spectroscopy and energy dispersive x ray spectroscopy, will be presented in Chapter 5. 1.5 Thin Films of Prussian Blue Analogues While nanoparticles limit the size of a material in all three spatial dimensions, thin films are the result of limiting the size of only one spatial dimension. In effect, the thin

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29 film shape breaks symmetry along the shortened axis ( Figure 1 1 ), and one might expect this broken symmetry to be reflected in the material properties For both optical applications and memory device applications, thin films are important. Previously, the photoinduced magnetism of RbjCok[Fe (CN)6]l nH2O thin films were studied, finding an anisotropy of the photoinduced magnetization [ 10 ] This chapter focuses mainly on thin films of RbjNik[Cr (CN)6]l nH2O that do not possess photoinduced magnetism, but do have a high ordering temperature, between 60 and 90 K, and a simpler magnetic ground state. Additional films were also studied, substituting different transition metals and studying how the magnetic anisotropy is affected. The goal of the study was not only to characterize the specific films in questi on, but to provide insight into the general issue of the anisotropy in complex cyanide thin films. The current understanding of this phenomenon, as well as detailed experimental studies, including magnetization, magnetic microwave resonance, UV Vis spectr oscopy, infrared spectroscopy, x ray diffraction, scanning electron microscopy and atomic force microscopy, will be presented in Chapter 6. 1.6 Heterostructures of Prussian Blue Analogues Using the knowledge base compiled during the photomagnetic nanoparticl e and thin film studies, an entirely different class of heterostructured materials were synthesized and investigated. The main idea was to take the useful properties from two different materials and to put them together in a new metamaterial that possess both of the desirable properties of the constituents. It is interesting that when materials are combined, new unexpected properties can evolve, not native to either parent compound. Two different types of heterostructures were studied, solid solutions and multi layered thin films, Figure 1 2 Two exciting results were a NaCoxNi1 x[Fe (CN)6] nH2O powder,

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30 in which the sign of the photoinduced magnetism can be tuned with chemical formula, and a RbjCok[Fe (CN)6]l nH2O / RbjNik[Cr (CN)6]l nH2O heterostructured film, which has photomagnetic effects at unprecedented temperatures for Prussian blue analogues. The experimental magnetization, x ray diffraction, infrared spectroscopy, transmission electron microscopy, and energy dispersive x ray spectroscopy measurements will be presented in Chapter 7, along with descriptions of the present understanding of the underlying heterostructure properties. Figure 1 1 Illustration of constrained geometries in nanostructures. Schematic projections of (a) bulk, (b) thin films, and (c) nanoparticles are shown. Figure 1 2 Illustration of designer heterogeneous geometries in nanostructures. Schematic projections of (a) a thin film heterostructure and a (b) s olid solution heterostructure are shown. (a) (b) (a) (b) (c)

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31 CHAPTER 2 EXPERIMENTAL TECHNIQ UES 2 Perhaps experimental physics may be understood as the harnessing of well documented physical phenomenon to further investigate the less well known properties of matter. A certain level of understanding of the apparatus used is necessary for a rigorous analysis of results. This understanding not only allows the experimentalist to identify the correct piece of equipment to probe the property of interest, but immensely aids in the ability to recognize and avoid spurious results in the forthcoming data. This chapter seeks to survey the experimental techniques utilized in taking the data shown in later chapters. In Section 2.1, sample environments will be discussed, followed by a summary of the detection methods in Section 2.2. Methods are sorted according to the physical location where the apparatus is located, with those performed in Professor Meisels lab in the New Physics Building (NPB) Room B133 outlined in Subsection 2.2.1, those performed in Professor Talhams lab in the Chemistry Lab Building (CLB) Room 404, techniques at the Major Analytical Instrumentation Center (MAIC) at the University of Florida or other external labs in Su bsection 2.2.2, and, finally, probes located at national labs in Subsection 2.2.3. Within these detection sections, methods are ordered alphabetically. Finally, Section 2.3 will be devoted to custom probes designed by the author and used to collect data presented in the dissertation. Detailed machine drawings and additional photographs of the custom equipment are relegated to the appendices.

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32 2.1 Sample Environment Sample environment is a crucial aspect of experimental physics [ 1114]. For the data to be presented in the following chapter s, the three most relevant p arameters are temperature, magnetic field, and the application of light to the sample. Specifically, relevant vacuum technol ogy, cooling schemes superconducting magnets and photoirradiation methods will be described in the following subsections 2.1.1 Vacuum Equipment Vacuum equipment is necessary in cryogenic applications for many reasons The two most relevant examples are t he need to evacuate sample and insulation spaces, and to reduce th e vapor pressure over liquid helium in order to reach temperatures below 4.2 K. Proper pumping of all cold spaces is important because an atmosphere with significant proportions of gas will impede cooling power. 2.1.1.1 Pumps Rotary pumps are the workhors es of the cryogenic laboratory. The pumping mechanism is purely mechanical in nature, consisting of a series of vanes that force air from the inlet to the exhaust as the pump turns, Figure 2 1 (a). Routine base pressures of 102 or 103 mbar can be reach ed, with maximum working pressures of a few hundred mbar, or 1 bar in spurts. Rotary pumps are available in many different sizes, with throughputs ranging from less than 1 m3/hour to a few tens of m3/hour. Rotary pumps are often used as roughing pumps before more sophisticated vacuum technology is recruited, backing pumps for diffusion or turbo pumps, or for reducing the vapor pressure over a liquid helium bath. Roughing pumps are required in many instances as technologies with lower base pressures oft en have lower maximum operating pressures. Diffusion pumps must have a back pressure below 0.1 mbar or oil

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33 can back stream into the vacuum system ruining expensive equipment, while turbo pumps with too large of a pressure differential can bend a fin, perm anently ruining the pump. Sometimes it is desirable to arrange two rotary pumps of different sizes in stages in order to achieve base pressures down to 104 mbar. Most rotary pumps have a gas ballast. When opened, a gas ballast causes the pump to work extra hard, thereby heating the oil. This heat helps remove any water that may have condensed in the pumping system when pumping a cryostat that has been exposed to air or has been left unused for a long time. Most surfaces release adsorbed water vapor when the pressure is reduced, and without the proper use of a gas ballast, the long term base pressure of the system will suffer. Oftentimes, it is necessary to reach pressures lower than those achievable by rotary pumping technology for proper thermal insul ation of a cryostat. Oil d iffusion pumps are capable of reaching pressures down to 107 mbar, but as mentioned previously have maximum working pressures of about 0.1 mbar and must be backed with a rotary pump Air is moved by the use of heated oil vapors that create a highvelocity stream to guide air from the inlet to the exhaust This stream is created in practice by use of a resistive heating element at the base, and either air, water, or liquid nitrogen cooling of the walls, Figure 2 1 (b). Pu mps are generally fitted with a cold trap to help remove water from the vacuum space while simultaneously preventing pump oil contamination of the cryostat or sample space. The other main pumping technology for reaching high vacuum is the turbomolecular pump Figure 2 1 (c). Turbomolecular pumps are capable of reaching pressures down to 108 mbar, and must also be backed with a rotary pump. One

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34 important aspect of the turbo pump is the dependence of pumping speed and base pressure upon the mass of the g as being pumped. Therefore, heavy oil molecules are pumped especially well, but helium actually has one of the poorest ratios for turbo pumps and it is due to this mass effect that diffusion pumps can outperform turbo pumps in helium applications. 2.1.1.2 Pumping lines If pumps are analogous to the voltage source in a pumping circuit, then pumping lines are the resistive wires In order to achieve the maximum pumping power at the vacuum space, lines with minimal impedance are of the highest importance. I deally, lines are metal for high vacuum, as plastics can be permeable to helium gas Pumping lines should be free of adsorbed impurities, and therefore it is ideal to clean lines with a volatile substance like acetone. Finally, the cross section al area of the lines should be large enough to ensure that the displacement throughput of the lines is larger than that of the pump being used. 2.1.1.3 Vacuum g auges Vacuum in the range of 1 to 1000 mbar can easily be measured with a simple spring loaded dial gaug e However, for higher vacuum, pressure is generally measured with a Pirani gauge in the range of 10 mbar to 103 mbar, and a Penning gauge in the range 102 to 107 mbar Pirani gauges consist of a wire filament in contact with the atmosphere, and depending upon the gas concentration, different thermal conductivity in the gauge is measured. Penning gauges measure the ion conductivity across a large voltage drop. It is important to remember that the calibration of these high vacuum gauges is dependent on the type of gas used in the system.

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35 2.1.1.4 Oil m ist f ilters and f ore line t raps In order to remove the oil vapors present in the exhaust gas of a rotary pump, oil mist filters are used. This filtration is for three important reasons: first, there is a health hazard associated with the inhalation of oil mist; second, oil mist may contaminate plumbing, pressure gauges, and flow meters behind the pump; and third, since UF has a helium recovery system, it is desirable to limit the amount of oil that must be removed in the recovery laboratory. Filters can either be coalescing or centrifugal. Centrifugal filters are mainly utilized for price reasons but have the downside that they must be drained occasionally. Foreline traps are used on the inlet of a rotary pump to reduce the amount of oil back streaming up the pumping line. These must also be changed regularly as traps saturate with oil over time. 2.1.1.5 O rings Any t ime two pieces of vacuum equipment are joined together, a seal must be made. Ideally, joints would be soldered or welded, but oftentimes setups are dynamic, so temporary seals are used. These may be oring seals, or different varieties of a flare fitting For o rings, the material to be used is the most important aspect of the seal. Nitrile rubber is the ubiquitous oring material used in modern day vacuum technology, most simply because it is the best seal for the price. Nitrile has a working temperat ure range from 40 C to 120 C. Silicone rubber is has a higher temperature range of workability, 100 C to 250 C, however it is not used in the cryogenic apparatus because it is permeable to helium gas. Butyl rubber is another oring material that has a low gas permeability. It can be used down to 60 C, but is a little more expensive

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36 than nitrile. Fluoroelastomers, such as Viton, are used for higher temperatures but is more expensive and less ductile than nitrile. It has a working temperature range from 20 C to 200 C. Teflon can also be used to make seals, but one has to always be wary of the dodgy mechanical properties and large thermal contractions. For low temperature orings, indium seals are ideal because of the similar thermal expansion coefficient of the indium and the metal cryogenic apparatus. 2.1.2 Cryostats Helium is the mainstay refrigerant for any researcher looking to reach temperatures below 70 K Specifically, the more common 4He isotope is capable of reaching temperatures dow n to 1 K due to its thermodynamic phase diagram. Depending upon needs and availability liquid helium cryostats or closed cycle cryostats are used. 2.1.2.1 Bath c ryostats The simplest cryostat configuration is a bath cryostat, consisting of a large volum e of cryogen (tens of liters) thermally insulated from ambient temperature. More complicated setups may include internal structure, such as a continuous flow cryostat, to allow for greater control of sample temperature. Regardless of the internal structure, bath cryostats require substantial shielding to achieve the necessary thermal insulation for economical experiments. Shielding generally consists of a radiative shield thermally anchored to either a liquid nitrogen or helium gas cooled shield. Currently, the boil off rates are comparable for nitrogen or helium gas shielded systems. 2.1.2.2 Continuous f low c ryostats and c ryogenic i nserts While it is possible to pump on the entire bath to reduce the temperature below 4.2 K, it is common practice to thermally isolate a smaller volume, which has been

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37 equipped with radiation shielding, to be pumped on while leaving the bath at 4.2 K. These volumes are a type of continuous flow cryostat called variable temperature inserts that allow for separate temperatur e control of the bath and the sample space. This separation of sample and bath spaces allows for cooling without the need for large displacements of gas and reduces the ~35% boil off of liquid necessary to reach the lambda point (2.17 K) to a small fracti on of the bath [ 13] Of course, this convenience requires an additional level of complexity, in which a stream of super cooled gas in conjunction with a resistive heater are able to achieve stability over wide ranges of temperature. Helium is drawn from the bath through an impedance line (which is sometimes variable), and the temperature of the insert is controlled by using a resistive heater and pump to control the pressure within the insert Cooling is achieved as the cold gas flows over the sample and out through the exhaust to the pump. Additional stability can be achieved at the expense of a longer relaxation time constant when changing temperatures by putting a heat exchanger betw een the cold gas and the sample. 2.1.2.3 Closed c ycle r efrigerators While bath cryostats require an external liquefier and the transfer of liquid helium into the apparatus, closed cycle refrigerators offer an increasingly popular alternative. The most common type of refrigeration cycle used is one of the GiffordMcMahon t ype Grossly, they consist of a closed circuit of helium gas, a probe, and a compressor During operation, helium is alternatively compressed and allowed to expand at tens of Hertz using the entropy of the expansion to cool the sample. Depending upon t he type of shielding employed and the power of the compressor temperatures as low as 5 K can be routinely reached in these instruments. Potential drawbacks of closed cycle

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38 refrigerators are the coupling of the compressor vibrations to the experiment, mai ntenance of the compressor, and the comparatively high initial cost of the setup when compared to a continuous flow cryostat. 2.1.3 Superconducting Magnets Perhaps the highest impact application for superconductors is in the wire of superconducting magnets At the expense of refrigeration, magnets with superconducting wire are capable of magnetic fields in the neighborhood of 20 T without the need for high voltage power supplies. The most common winding geometr ies fo r superconducting lab magnets are a continuous solenoid or split pair Figure 2 2 While solenoids allow for the best field homogeneity, for neutron scattering it is necessary to have access to the sample perpendicular to the magnetic field. 2.1.3.1 Magnet c onstruction Due to the need for high current densities, most commercial magnets utilize type I superconductors. In order to increase the critical field, alloying is employed, as impurity sites act as local flux pinning minima. In addition, the wire is generally multi filamentary to further prevent dissipative flux jumping. The most common superconducting wire is the NbTi alloy, because it is cheap, ductile, and has a high supercurrent density even in strong fields. Using only NbTi, fields up to 9 T at 4.2 K or 11 T at 2.2 K can be routinely reached. To get in the neighborhood of 20 T, the magnet wire must operate at or below the lambda point (2.17 K), and the inner windings must be made from the more expensive and brittle Nb3Sn alloy. Magnetic field homogeneity is always a concern, and standard superconducting solenoids have flat magnetic field profiles over a centimeter at the center field up to one part per thousand. To achieve higher homogeneity, many modern systems utilize

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39 compensating coils to combat the field gradients produced fr om the finite nature of the wound solenoids. For finer adjustment, shim coils can tune the homogeneity of the center field to 1 part in 105 for series shims and as good as 1 part in 107 for tunable shims. Finally, counter wound cancellation coils may be fit on the ends of a magnet to help cancel stray fields at distances away from the center of the magnet. Overall, the case is slightly worse for split pair magnets, where field homogeneity is typically an order of magnitude lower for analogous setups. 2.1. 3. 2 Magnet o peration Perhaps the most important thing to remember when working around and using a superconducting magnet is the huge amount of energy held within the structure, of the order of a megajoule. This enanced awareness of magnetic forces is especially true in split pair setups, where large mechanical reinforcements are needed to stop the pairs from joining together. Due to their small resistances, superconducting coils are able to produce huge back EMFs when the currend is ramped up or down. This inductance forces users to ramp at sufficiently slow rates to avoid transitioning the wires to the normal state, thereby quenching the magnet and potentially damaging expensive equipment. If the wires do phase transition to the normal state, the l arge amount of energy stored in the circuit must then be dissipated. It is not uncommon to see a plume of helium from the magnet bath vent during a quench. All modern magnets have measures in place to avoid the most destructive consequences of a quench. The socalled superconducting winding protection circuit is, most simply, resistive elements put in parallel with the magnet windings, thereby shorting the magnet once a quench has occurred.

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40 Happily, unlike resistive magnets, superconducting magnets do not require the application of an external voltage to maintain current flow. Practically, this aspect is often exploited in superconducting magnets to limit helium boil off during operation by the use of a persistent magnet mode. The persistence mode is when there is still supercurrent in the coils, but the external power supply has been turned off. To allow for persistence as well as ramping of the field, a superconducting switch is wired in parallel to the main coil windings, and a small heater is plac ed near the switch. When charging the magnet, heat is applied, causing the switch to go normal and therefore acting effectively as a broken wire in the circuit, dropping all applied voltage over the magnet coils. For the persistent mode, heat is removed from the superconducting switch, thereby isolating the coils, so the external power supply can then be slowly ramped down and turned off, leaving a persistent supercurrent in the windings. It is important to remember that if the field is to be changed aft er entering the persistent mode, the power supply must be ramped to the correct voltage corresponding to the current in the magnet, and the superconducting switch must then be activated before any changes in the voltage across the magnet circuit can be made. Typical decay rates in persistent mode are 100 T/hour. 2.1.4 Light Guides For photoinduced studies of magnetization and structural changes, it is necessary to have a way to get light from a room temperature halogen light source, with typical powers of 1 or 2 mW, down into the cold space of a cryostat. For small scale probes, optical fibers from Ocean Optics, Model 200 UV Vis OD ~ 270 m were used. These fibers are ideal because they are flexible, are thermally insulating, and are easily

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41 arranged to direct light onto small samples. For larger scale probes, fiber optics are no longer ideal or even economically feasible. In these cases, solid quartz rods were used as light guides; although they are not flexible, much larger amounts of light can be gui ded down to the sample space. 2.2 Detection Methods With sample environment considerations in one hand, the other obvious element to an experimental study is the detection methodology itself. With the large amount of scientific infrastructure already in place, experimentation often comes down to keen identification of the correct probe and the conditions to extract the material properties of interest. In the following subsections, the Superconducting QUantum Interference Device (SQUID) magnetometer that w as used most heavily, auxiliary methods performed in other labs, and probes located at national lab facilities will be overviewed. 2.2.1 SQUID magnetometer 2.2.1.1 Superconducting qu antum i nterference d evices Superconducting QUantum Interference Devices (S QUIDs) are highly precise amplifiers, often utilized in magnetometers due to their sensitivity to magnetic fields weaker than 1014 T [ 15 ] [16 ] A clear example of the precision of SQUIDs is their ability to detect, and actually discover, that flux is quantized in units of 0 = h 2 e = 2 0678 1015 m2 2 1 w here h is Plancks constant, and is the charge quanta. SQUID devices are based upon Josephson junctions which consist of two superconducting regions connected by a weak link that allows quantum tunneling between the two regions without bulk transport.

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42 Josephson junctions are generally made from niobium or niobium alloy superconductors, such as NbSe2 or NbTi. While originally a point contact using a sharpended screw was used, modern day junctions are microbridges using patterned lithography. Current and voltage can then be measured across the weak link created at the point contact. The Josephson junction alone does not act as a magnetometer, so it must be included in a larger SQUID circuit, Fig ure 2 3 The basic aspects of a SQUID circuit are a transformer coil, which is large enough to interact with the sample, coupled to a signal coil, which is manufactured symmetrically with a radiofrequency detector coil. The signal coil and detector coil s are connected through a weak link, allowing flux coupling. Magnetic flux changes arising from a magnetic sample induce current in the transformer, and these changes couple directly to the detector coil via mutual inductance. Finally, the radiofrequenc y voltage across the detector coil can be measured, after going through a conditioning circuit, and fit to extract the field associated with the sample. 2.2.1.2 Quantum Design MPMS XL m agnetometer Two different commercial magnetometers from Quantum Design were used for the DCand AC SQUID measurements, an MPMS 5S and an MPMS XL [ 17 19]. The MPMS 5S, located in the New Physics Building Room B20, is the older of the two and is equipped with an AC detection board, a 5 T superconducting magnet equipped with a field reset, magnetic shielding, and a pumped 4He cryostat. The MPMS XL, located in the New Physics Building Room B133, is newer and has the added benefit of a 7 T superconducting magnet. An additional advantage of the MPMS XL is a low temperature impedance allowing for the continuous operation at base temperatures

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43 lower than 2 K, but the XL is not equipped with any of the other additional features of the 5S model. Powder samples were mounted in either diamagnetic gelcaps or on sticky tape to increase the optical cross section for photoinduced experiments. Commercial straws were used as a diamagnetic sample rod to allow translation of the sample through the SQUID magnetometer detector coils. In general, backgrounds were subtracted based upon the known m ass susceptibility of the sample holders, but in many cases, background contributions were insignificant. The Quantum Design magnetometers utilized a second derivative transformer coil to measure sample flux and to couple to the SQUID, Figure 2 4 The adv antage of the multiple coil setup is from the inherent background subtraction of any signal that is a longer wavelength than the ~ 4 cm long transformer. These noise sources will simply cause, for example, a positive voltage in the top coil, a negative v oltage in the second, a negative voltage in the third, and a positive voltage in the fourth coil, summing to zero [ 18] 2.2.1. 3 Remnant f ields and d egaussing the MPMS For probing magnetic systems with weak anisotropy, the use of small fields may be necessary. The fields can be measured using a custom Hall probe for the SQUID magnetometer (based on a Toshiba THS118E chip ) developed by the author. The use of the standar d oscillate option on the MPMS provides zero field of ~ 10 G, Figure 2 5 (a). However, this level of error uncertainty can be undesirable at times, when a manual degaussing sequence can be employed instead. The specific degaussing method depends on t he recent history of the magnet. An example of a degauss sequence would consist of high resolution charging between 40 kG, 30 kG, 20 kG,

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44 10 kG, 5 kG, 2.5 kG, 1 kG, 500 G, 250 G, 100 G, 75 G, 50 G, 25 G, 10 G, 10 G, 0, 5 G, 5 G, 0. The results of this sequence give a different field profile with a smaller magnitude of ~ 1 G, Figure 2 5 (b). It is also important to wait a sufficient amount of time for eddy currents to dissipate, since superconducting magnets have long time constants because of a large inductance combined with a small resistance. 2.2.2 Additional Methods Performed at UF 2.2.2.1 Atomic f orce m icroscopy Atomic Force Microscopy (AFM) is a highly sensitive scanning probe microscopy that was used to characterize the surface morphology of thin films [ 20 ] All AFM studies were performed on a Digital Instruments multimode scanning probe microscope in Professor Talhams Chemistry Lab in CLB Room 404. Roughly a square centimeter of film was cut and placed under the scanning tip. The scanning tip is a sharp point attached to the end of a cantilever, which in turn is connected to the feedback electronics that monitor the height of the tip over the surface to help prevent tip crashes onto the sample, Figure 2 6 Nanometer scale changes in the deflection of the tip are detected by a laser coupled to a photodiode. 2.2.2.2 Carbon, h ydrogen, and n itrogen c ombustion Carbon, Hydrogen, and Nitrogen combustion analysis (CHN) was utilized to determine light atom (with 2p electrons) concentrations for sel ected samples [ 21 ] All CHN was performed at the University of Florida Spectroscopic Services laboratory Roughly 35 milligrams of sample are used and destroyed in the measurement process To achieve controlled combustion, the sample is sealed in an oxygen atmosphere and external heat is applied. As atoms are released, they flow through a series of columns containing a water trap, a carbon dioxide trap, and a nitric oxide trap, Figure 2 7 By

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45 measuring the masses of the different traps after combust ion of the sample is complete, the analytical determination of the chemical makeup is possible with straightforward calculations. 2.2.2.3 Energy d ispersive x r ay s pectroscopy Energy Dispersive X ray Spectroscopy (EDS or EDX) was the primary analytical technique used for determining relative concentrations of heavy atoms (containing 3d electrons) for selected samples [ 22 ] All results reported herein were recorded on a JOEL 2010F Super Probe, housed at the Major Analytical Instrumentation Center (MAIC ) at the University of Florida (UF) Only micrograms of samples are necessary to perform the measurements, with mounting achieved by deposition of microliter quantities of sample containing solution on holey carbon TEM grids purchased from Ted Pella, Inc. Simplistically, the experimental apparatus required are an electron gun with proper magnetic lenses and an inelastic x ray detector, Figure 2 8 (a). In this method, a beam of electrons is focused on the sample, with a finite probability of incident elec trons ejecting bound electrons If the ejected electron was from an inner shell, the atom will seek the new ground state, emitting energy in the form of a photon in the process, Figure 2 8 (b). These photons can then be collected and analyzed by a detect or, revealing the electronic transitions present A typical spectrum consists of x ray counts as a function of energy As each atom has a unique electronic structure, the electronic transitions present in the experimental x ray spectrum are diagnostic of the chemical composition of the sample. 2.2.2.4 Fourier t ransform i nfrared s pectroscopy Fourier T ransform I nfra R ed (FT IR) spectroscopy in the middle of the spectrum, is the study of how light with wavelengths from 4,000 cm1 to around 400 cm1 interacts

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46 with matter [ 23] All measurements were performed on a mid infrared Nicolet 6700 spectrometer located in CLB Room 411. Powder samples were either mounted in pressed KBr pellets or sandwiched between two salt plates for the study of materials t hat were sensitive to pressure. Thin film samples were run with no additional modifications As with all spectroscopic methods, infrared spectroscopy is sensitive to transitions between discrete energy levels for the material being irradiated. These energy levels can be either vibrational or electronic in origin, and for this work vibrational modes were of primary accessibility and interest Interferometers are the basis of an infrared spectrometer, which consists of a broadband source, a beam splitte r, a fixed mirror, a movable mirror, a sample, and a detector, Figure 2 9 (a). Spectra are obtained as a function of moveable mirror position. Fourier transform spectroscopy is so called because it consists of sending a pulse of radiation with many frequ ency components through the sample to the detector, which registers the signal in the time domain (technically the moveable mirror position domain). Finally, a Fourier transform is performed to obtain the spectra in the frequency domain, Figure 2 9 (b). 2.2.2.5 Inductively c oupled m ass s pectrometry Inductively Coupled Plasma Mass Spectrometry (ICP MS) is an analytical chemical technique used to determine the concent rations of metals, and some nonmetals, with a hig h sensitivity. F or example, detection limits are less than a picogram per second for transition metals [ 24 ] ICP MS results were obtained using a Thermo Finnigan Element 2 spectrometer located at the UF Department of Geology Samples were prepared for ICP MS by dissolvi ng them i n trace metal grade nitric acid. This chemical dissolution is necessary in order to aerosolize the sample for when it is introduced into the plasma chamber This aerosol enters an argon environment of the

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47 chamber and is subsequently exposed to powerful radio frequency radiation, converting the gas into a plasma, Figure 2 10. Argon is chosen because of the much higher first ionization potential, co mpared to all elements except He, F, and Ne. It is the dynamics of the argon and sample plasma that provide for sample explosion and subsequent ionization. Once ionization is complete, the RF field also serves to delineate ions having different charge to mass ratios, and thus determine the elemental content of the sample. 2.2.2.6 Transmission e lectron m icroscopy Patterned after the more traditional light transmission microscopy, Transmission Electron Microscopy (TEM) allows for resolution of a few s, owing to the short deBroglie wavelength of the electrons [ 25 ] All results reported herein were recor ded on a JOEL 2010F Super Probe, housed at the Major Analytical Instrumentation Center (MAIC) at the University of Florida (UF) Only micrograms of samples are necessary to perform the measurements, with mounting achieved by the deposition of microliter q uantities of sample containing solution on various holey carbon TEM g rids purchased from Ted Pella, Inc. A representative setup consists of an electron gun, conditioning lenses, and an imaging screen, Figure 2 11 Electrons are generated at a thermionic electron gun. This source is then focused by the use of magnetic lenses, in which the trajectory of the charged particles is bent by the presence of the applied field. Electrons next travel through an aperture to avoid background from large angle particl es Subsequently, the focused electrons are scattered by the matter present in the sample, creating a negative image of the sample in the electron beam The objective optic serves to focus the image, and the objective aperture again cuts off highangle scatterers The image is then enlarged by the intermediate and projector lenses onto

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48 the imaging screen, where electrons interact with phosphor to produce light that can be recorded with standard camera techniques These methods have been utilized to obtain light field, dark field, and diffraction data on the samples discussed in the following chapter s. 2.2.2.7 Ultraviolet and v is ible s pectroscopy Spectroscopy in the UltraV iolet and Vis ible range ( UV Vis ) is useful f or studying coordination networks because the wavelengths of approximately 200 to 800 nm probe energy scales of 6.21 to 1.24 eV, which are comparable to the energies separating different multi electron magnetic energy levels [ 26] A typical spectrometer c onsists of a source, a monochromator, the sample space, a photodetector, and a computer interface, Figure 2 12. Two different machines located in the Chemistry Lab Building were used, a room temperature device, and a spectrometer equipped with a closecyc le cryostat for temperature dependent studies The source consists of two elements, a tungsten halogen for wavelengths above 320 nm and a deuterium arc lamp for wavelengths below 320 nm Monochromators are made up of a diffraction grating and a series of filters to remove higher order diffractions S amples are mounted using quartz slides for thin films and quartz cuvettes for solutions The detector is a silicon photodiode. Spectra are then recorded as ASCII delimited files. 2.2.2.8 X r ay p owder d iffra ction X Ray Diffraction (XRD) is used to find the average positions of heavy atoms in a wide range of samples [ 27] All samples studied were polycrystalline, so the XRD Philips APD 3720 2 powder diffracto meter located in MAIC Room 117 was used. A standard Cu K source is used, producing a dominant wavelength of 1.54 (933 eV)

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49 x rays by applying voltages near the ionization energy of the K (1s) to L3 (1p3/2) transition in Cu. For good signal to noise on organometallic compounds, tens of milligrams are desirable, but samples on the order of one milligram show clear, refinable Bragg peaks Samples were mounted on a 25 mm x 47 mm glass slide purchased from Ted Pella Inc. and immobilized using a 1 cm2 piece of doublesided sticky tape in the center of the slide In XRD, the lattice planes within ordered crystals satisfy the condition of constructive interference when the path length difference between x rays scattering from different lattice layers is an integer multiple of the inc ident wavelength, Figure 2 13. For randomly oriented polycrystalline samples, all lattice planes are effectively probed at the same time the disadvantage is that the experimental integration over crystalline angles reduces the amount of structural information. 2.2.3 National La boratories National Laboratory user facilities are an important resource for the modern research scientist and the author feels fortunate to have been able to Vis it a few different facilities. For the materials studied in this thesis, measurements were carried out at the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida, and the Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee. High field Electron Magnetic Resonance (EMR) studies were performed at NHMFL, Neutron Diffraction (ND) studies were performed at the HighFlux Isotope Reactor (HFIR) at ORNL on the tripleaxis spectrometer on beamline HB1A and the neutron powder diffractometer on beamline HB2A, and Inelastic Neutron Scattering (INS) was performed at the Spallation N eutron Source (SNS) on the fineresolution Fermi chopper spectrometer at beamline 17. Online at full power since 1966, the 85 megawatt HFIR

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50 source is unique because it has the highest flux, of the order of a million neutrons/cm2s on most beamlines, of any reactor based source of neutrons for condensed matter research in the United States. Neutrons at the HFIR are in the thermal spectrum, with a small portion of cold neutrons available. On the other hand, the SNS is still in the process of being commissi oned as of this writing, but is expected to have record neutron fluxes available for studying materials. Unlike the HFIR, the SNS uses spallation to create bursts of neutrons with a wide range of energies, and as such, the spectrometers must operate in ti me of flight mode. 2.2.3.1 Electron m agnetic r esonance at the NHMFL Electron Magnetic Resonance (EMR) experiments were performed in Professor Stephen Hills lab at the NHMFL, using a resonant cavity insert to a Quantum Design Physical Property Measurement System (PPMS) equipped with a 5 T magnet. For approximately a square centimeter of sample, Prussian blue analogue films require about 400 cycles, which is an arrangement that may also be achieved by using multiple films of less than 400 cycles. For powder resonance, approximately a milligram of a Prussian blue analogue magnet is required. Resonant absorption of external microwave radiation for electron systems can be easily understood in the context of energy splittings to be presented in the next chapter Briefly, if an electron is placed in a magnetic field, there is a difference in energy between the parallel and antiparallel orientations of the spin vector with respect to the field vector, Figure 2 14 (a). When the energy difference between the two orientations is in resonance with external radiation, spins may be excited from the lower energy level to the upper one, resulting in the absorption of the incident radiation, which may be measured experimentally, Figure 2 14 (b).

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51 The apparatus necessary for performing resonant absorption experiments is generalized in Figure 2 15. The first essential ingredient is a tunable external magnetic field that is large enough to split the energy levels of the system to separations excitable by an external radiation source, which is the second essential ingredient. Furthermore, the sample is often placed in a resonant cavity to further amplify the effects of absorption. To measure absorption, the ratio of the microwave intensity before the sample and after the sample can be compared. 2.2.3.2 HB1A n eutron t riple a xis s pectrometer at HFIR The FixedIncident Energy TripleAxis Spectrometer on beamline HB1A at HFIR (Figure 2 16) is ideal if the exact energies and momenta of interest in a sample are known. Approximately five grams of deuterated Prussian blue analogue powder are required for HB1A. Momentum transfers from 0.2 to 4.9 1 can be measured in elastic mode, and energy transfers from roughly 35 meV to 11 meV at q = 3 1 can be measured in inelastic mode. In the best case, energy resolutions of ~0.5 meV are possible. This beamline has one of the most intense, with a flux at the sample of ~2 x 107 neutrons/cm2s, and c leanest beams at the reactor, due in part to the pyrolitic graphite monochromator system, which fixes the incident energy at 14.6 eV. In addition to the monochromator, HB1A has a variety of analyzers to help condition the beam, with analyzer angles able t o be ranged from 60 to 120, as well as the option to place a sapphire filter in the beam before the monochromator. A variety of collimators are available at different portions of the beam, before the monochromator the collimation is 48' between the mo nochromator and the sample, collimators of 10' 20 30 or 40' may be used, between the sample and the analyzer, collimators of 10' 20 30 or 40 may be used, and finally between the analyzer and the detector, collimators of 70' or 140' are

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52 available. The fixed detectors give access to scattering angles from 5 to 120 Finally, the maximum beam size on HB1A is 40 mm x 150 mm. 2.2.3.3 HB2A n eutron p owder d iffractometer at HFIR The Neutron Powder Diffractometer (NPD) o n beamline HB2A at HFIR (Figure 2 17) is optimized to take powder patterns, as opposed to the tripleaxis machine. Approximately three grams of deuterated Prussian blue analogue powder are required for HB2A. This spectrometer is useful to refine crystal and magnetic structures. The detectors are 44 movable 3He tubes setup to detect intensities in a DebyeScherrer geometry. As the detectors are situated, they give access to 0 to 150 scattering angles, although the lower end may be plagued by air scattering and the high end is weak due to the structure factors of samples. The monochromator is germanium and is capable of providing three different incident wavelengths, 1.54 2.41 or 1.12 The collimator may be left out of the beam, giving 12' collimation, or 16' 21 or 31' collimators may be used, at the expense of intensity. The beam is optimized for samples with a 25 x 25 mm2 crosssectional area, but it may be masked with borated plastics if smaller samples are necessary. Finally, the maximum resolution is about 0.2 o r 2 x 103 d/d, where d is the real space distance between lattice planes. 2.2.3.4 Inelastic n eutron s cattering on SEQUOIA at SNS Inelastic Neutron Scattering (INS) was performed on the SEQUOIA spectrometer (Figure 2 18) at SNS. Approximately five grams of deuterated Prussian blue analogue powder are required for SEQUOIA. Operating in timeof flight mode, two choppers are utilized, a T0 chopper to block the unwanted highenergy neutrons resulting from spallation, and a Fermi chopper to choose the inciden t energy range. This setup allows for an incident energy range from 102000 meV, but it may be possible to increase the

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53 upper limit. The energy resolution varies from 1% to 5%, getting worse in the higher energy limit. This fine energy resolution is due to the 5.56.3 m sample to detector distance. 2.3 Custom Apparatus 2.3.1 SQUID Probe with Low T emperature Rotation and Optical Fibers In order to study materials with magnetic anisotropy as well as photo induced magnetism, a new probe has been developed [ 28 ] In situ sample rotation is a valuable tool to measure the angular dependence of the magnetization. Additionally, there is an ongoing research effort to study materials that show changes in magnetization with applied light. Specifically, samples showing photoinduced magnetization as well as magnetic anisotropy have been identified [ 10] [29 ] Therefore, an experimental setup that is able to measure magnetizations down to low temperatures, while affording in situ sample irradiation and rotation, is beneficial. There are a few inherent difficulties one has to be aware of when setting up such a system. A probe that is suitable for use with commercial SQUID magnetometers was designed because signals can be quite small, i.e. 10 6 emu and less. Although using a commercial setup was considered most promising, it puts significant size and weight constraints on the probe and, most importantly, on the sample space. Also, because of the small signals involved, a minimization of the background signal from the probe is sought. Finally, care has to be taken that the system can operate at temperatures below the boiling point of 4He. Thermal contraction and expansion of parts must be taken into account, as well the need to keep parts movable while minimizing heating. As an important part of developing the experimental setup, possible construction materials have been characterized. A probe that meets the desired specifications has

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54 been built and tested. Probe specifications, design, materials data, and operation will be presented and discussed. 2.3.1.1 Probe s pecifications and d esign The probe setup is shown in Figure 2 19. Detailed machine drawings of the probe can be found in Appendix B Sample rotation is uniaxial about an axis perpe ndicular to the applied magnetic field. Rotation is done with a line connecting the low temperature sample holder to a cylinder at ambient temperature, with an additional line for resetting. Operation can be completely manual, manually controlled by a st epper motor, or automated by using computer controls for the stepper motor. Commands can be initiated in commercial software by using control data bits that are insignificant. For example, scan length parameters ranging from 4.00001 cm to 4.001 cm can be mapped onto 100 different commands and subsequently read by the stepper motor control program. A fiber optic cable allows for irradiation. The head of the probe must be light enough to accommodate the servo that translates the entire probe vertically to move the sample through the SQUID coils. The housing is aluminum because of its density, strength, and machinability. The drive shaft is beared by a slideseal assembly consisting of o rings and plastic spacers. Vacuum is achieved by the use of rubber orings for the drive shaft seal, the connection of the probe head to the shaft, and for the two additional access ports, one on the top and one on the side. The orings themselves to not keep the vertical position of the probe head static, so a quick conn ect of Teflon ferrules is attached to the probe head and can be tightened at the desired height; this variability in height can be quite useful to accommodate additional slack that may be introduced by the different thermal

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55 contraction of the long sections between room temperature and low temperature. Stycast 2850GT black epoxy seals the clear holes made for the optical fiber. The shaft of the probe is constrained to be 0.12" (0.305 cm) OD for use in a commercial QD MPMS SQUID magnetometer, allowing the co mmercial shaft seal assembly to be used. Most of the shaft is stainless steel for strength, but the bottom portion is quantalloy to minimize the background signal. The shaft is attached to the low temperature end with epoxy. For the drive lines, fishing materials were of prime consideration because they are nonmetallic, thin, and strong. To decide between monofilament or braided lines, two exemplary products were studied: Spiderwire 8 lb monofilament (mono line) and Spectra PowerPro 15 lb (braidedline). The braidedline was chosen for its larger Youngs modulus, since stretching of the lines can lead to errors in sample angles. Although there is a larger magnetic signal associated with the braidedline, the amount near the SQUID coils is only 10 m illigrams. Magnetic and mechanical properties of the lines are summarized in the next subsection. For the low temperature end of the probe, the main pieces are a yoke and a rotatable sample stage. Delrin was chosen for its compromise of strength and smal l magnetic signal. The yoke is long enough to be locally symmetric with respect to translations vertically through the detector coils, minimizing its flux contributions. The rotation stage is a hollow cylinder beared by plastic on plastic, with all but 90 open for accessibility during irradiation. The drive strings are attached to the rotation stage via nylon set screws. The magnetic properties of black DelrinTM acetal polymer, brown VespelTM polyimide, and white nylon are summarized in the next subsection.

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56 2.3.1.2 Probe m aterial p roperties During the design process, several candidate materials were investigated. Magnetic properties were measured in a Quantum Design MPMS XL SQUID magnetometer, and the results are summarized in Table 2 1 and Figure 2 20. All samples were mounted in uniform straws using press fits, so no background signal was subtracted. Annealing of the Delrin was attempted in case additional magnetism was coming from free bonds [ 30] but no obvious change in the susceptibility was observed. Temperature sweeps were done at 100 G, 1 kG, and 1 T between 2 K and 300 K. Field sweeps were done at 2 K, 10 K, and 100 K for fields up to 7 T. The magnetic susceptibility results could be wel l fit to a semiempirical formula, = C T + + D 2 2 Additionally, for the drive lines, some mechanical properties were investigated. Force constants were examined at room temperature to test the line deformation in response to an applied force. The monoline has a diameter of 0.010 in (0.254 mm) and a measured Youngs modulus of 1.4 GPa which is lower than 2.3 GPa reported to us by Berkley Fishing in a private communication. The braidedline has a diameter of 0.007 in (0.1778 mm ) and a measured Youngs modulus of 68 GPa, which is close to the 73 GPa range reported by Honeywell for different Spectra fibers [ 31 ] 2.3.1.3 Operation Use of the custom rotation probe is more complicated than simply using a standard MPMS sample rod. First, the user must mount the sample at the bottom of the probe, while being careful not to let grease touch the axles of the rotation cell. Next, if the fiber optic cable is desired, the side head having the fiber optic cable must be in

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57 place rather than a blank. The user must decide what form of angular control is to be used, manual, manual with the stepper motor, or automated with the stepper motor. For simple manual control, the dial will be mounted on the probe head, but for stepper motor control, the dial is removed and the stepper motor mounted in its stead. Good practice consists of testing rotation on the bench at room temperature before operation. Extreme care must be taken as to the extent to which the probe is rotated in one direction or the other, as without such care, the user may over rotate the probe and cause damage. Generally, one should completely load one end of the spool to prepare f or rotation in the opposite direction. Insertion of the probe into the SQUID should utilize the custom counterbalance and preset weight, at which point standard practices should be followed. Once the probe has been cooled to the desired temperatures, car eful tensioning of the control line should be checked, since slack may be introduced upon cooling. Tensioning may be adjusted by the set of ferrules attaching the probe head to the shaft. By the same token, if slack was taken up at cold temperatures, the user must remember to add slack to the probe before warming. If only one direction of rotation is desired, the issue of slack is less important. If manual operation is to be used, the probe has now been completely prepared for use. If stepper motor cont rol is to be utilized, additional steps are necessary. The driver board (Figure 2 21) must be set up, consisting of the 12 V power supply for the motor, the 5 V power supply for the logic, and cables connecting to the probe and (if desired) the computer parallel port. To begin controlling the probe via boardonly (i.e. no computer control) automated operation, make sure that switch SW3 is up. Set SB1 1, SB1 3 and SB1 5 to up; SB12 and SB14 can be changed at the discretion of

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58 the user. Whenever chang es are made to the boards inputs, depressing SW1 (the master reset) may be necessary. To begin controlling the probe with computer automated operation, make sure switch SW3 is in the down position. Set SB11, SB1 2, and SB15 to down; SB13 and SB14 can be changed at the discretion of the user. Connect the computer controller cable to CN3, making sure that position 1 is placed in the GND terminal and position 4 is placed on the CLK terminal. The bits can then be written to by the parallel port using the computers logic power. The Stepper Motor Control.vi on the SQUID computer can now be used for automated control of the motor and simultaneous data acquisition with MPMS MultiVu. To verify successful probe operation, a piece of magnetite with the magnetic axis aligned perpendicular to the axis of rotation was measured without any applied field. Sample rotation was tested through greater than 360 and at temperatures down to 2 K, Figure 2 22 (a). Sample irradiation was also tested us ing thin films of AjCok[Fe(CN)6]lnH2O oriented parallel to an applied magnetic field of 100 G at 5 K, as seen in Figure 2 22 (b). 2.3.1.4 Conclusions The ability to photoirradiate and rotate samples in situ while using the convenient setup of a commercial magnetometer has been demonstrated and represents a combination of previously unreported features. Probe materials and design have been presented with the hopes of providing insight to others who are investigating the photomagnetic properties of new materials. Future improvements may be made to the probe by more carefully etching the materials to remove possible magnetic impurities introduced by the machining process.

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59 2.3.2 Neutron Scattering Probe for Photoinducing Opaque Powders One final project is the photoinducing of opaque powders for neutron scattering. As this endeavor is a new experimental undertaking, a probe must be developed to provide the necessary sample environment. As of this writing, the development of a second generation of prototypes for the neutron light experiments are underway. The novel part of the sample design consists of a low temperature tumbler that allows for opaque particles to be exposed to light as a function of time, without the problem of surface particles blocking light from the rest of the sample. A schematic of the problem and proposed solution can be seen in Figure 2 23 and Figure 2 24. The expertise gained in rotating samples at low temperature with the custom SQUID probe described in the above section, as well as more standard photoinduced magnetism probes for the SQUID is invaluable in the development of such a probe.

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60 Figure 2 1 Illustrations showing operation of standard pumps that can be found in a cryogenic laboratory. (a) A schematic cutaway showing the main design elements of a rotary pump, (i) the rotor, ( ii) the stator, (iii) the exhaust valve, (iv) the exhaust outlet, (v) the pump inlet, (vi) the vanes, and (vii) the pump oil bath. (b) A schematic cutaway showing the main design elements of an oil diffusion pump, (i) the heater, ( ii) the hot oil, ( iii) cooling elements, (iv) the low pressure inlet, (v) different compression stages of the oil vapor jets, and (vi) the exhaust. Oil is represented by dashed lines and the pumped molecules by small circles. A schematic cutaway showing the main design elements of a turbomolecular pump, including the (i) exhaust, ( ii) motor, ( iii) turbines, and (iv) low pressure intake. These images were generated by the author and inspired by standard texts on the subject [ 1114 ]. (i) (ii) (iii) (iv) (v) (vi) (vii) (i) (ii) (iii) (iv) (v) (vi) (i) (ii) (iii) (iv) (a) (b) (c)

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61 Figure 2 2 Illustration of a superconducting solenoid magnet. A schematic cutaway showing the main design elements of a superconducting solenoid, including (a) a nitrogen jacket, (b) the magnetic coils, (c) the helium bath, (d) the magnet bore, and (e) the vacuum isolation space. This image was generated by the author and inspired by standard texts on the subj ect [ 11] [ 14 ]. Figure 2 3 Illustration of a SQUID magnetometer circuit. A circuit set up to detect flux changes resulting from a magnetic sample, utilizing the high sensitivity afforded by a SQUID amplifier. (a) (b) (c) (d) (e)

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62 Figure 2 4 Illustration of SQUID magnetometer pickup coils. The superconducting second derivative pickup coil used in the MPMS magnetometer and the voltage induced as a function of the position of a magnetic sample within the coils. Figure 2 5 Remnant fields and degaussing the MPMS. (a) The field in the magnet after a standard oscillate to zero protocol, with center field ~ 8 cm. (b) The field in the magnet after a manual degaussing sequence, with center field ~ 10 cm. 0 5 10 15 20 0 2 4 6 8 10 H (Gauss)distance in magnet (cm) 0 5 10 15 20 25 -6 -4 -2 0 2 4 6 H (Gauss)distance in magnet (cm) (b) (a)

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63 Figure 2 6 AFM schematic. The samp le tip scans over a material, revealing details of the surface morphology. Figure 2 7 A schematic of a combustion train for CHN analysis. Figure 2 8 EDS schematic. (a) Experimental setup of EDS (b) Microscopic effect showing incident electron (green square) hitting bound electron (red triangle) causing it to be ejected The atom then relaxes down to the ground state by filling in electrons, one example is displayed (yellow circle), and emitting x rays to conserve energy.

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64 Figure 2 9 FT IR s chematic. (a) A schematic of a typical Fourier transform infrared spectrometer. (b) A schematic showing the how the highspectral content pulse is modified after passing through the sample, and subsequently is Fourier transformed to give a spectrum in fr equency space. Figure 2 10. ICP MS schematic. The sample dissolved in acid is aerosolized in a nebulizer and introduced into an argon environment, in which it flows into an ionizing RF field, and the ions are subsequently detected via mass spectrometr y. detector source mirror moveable mirror beam splitter

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65 Figure 2 11. TEM schematic. Electrons travel from the top to the bottom, with the waist of the beam represented by the solid lines, and magnets represented by boxes with exes in them. Figure 2 12. UV Vis spectrometer schematic. More complicated setups may include conditioning optics, multiple sources and references beams, for example.

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66 Figure 2 13. XRD schematic. A schematic showing how incident x rays gain an extra path length of 2dsin( ) when scattering off of evenly spaced planes a distance d apart. Figure 2 14. Theoretical EMR schematic. (a) The simplest en ergy level splitting of an S = 1/2 spin species as a function of magnetic field. The horizontal lines indicate the microwave energy, and the dashed red line indicates the resonance condition. (b) The increase in cavity absorbance when the resonance condi tion is met. x ray tube sample detector d d (a) (b) magnetic field energy magnetic field (a) (b)

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67 Figure 2 15. Experimental EMR schematic. The key features are a microwave source, a resonant cavity, a magnet, and a detector. Figure 2 16. The triple axis spectrometer at HFIR on beamline HB1A. Here, the red lines show the neutron beam. Frequency counter/ power meter Microwave source Circulator Resonant cavity Reference arm with attenuator Detector diode Signal Phase sensitive detector Modulator Modulation coils Magnet

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68 Figure 2 17. The neutron powder diffractometer at HFIR on beamline HB2A. Here, the red lines show the neutron beam. Figure 2 18. The SEQUOIA inelastic time of flight spectrometer at SNS. Here, the red line is the neutron beam.

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69 Table 2 1 Magnetic response of candidate probe materials for the optical rotator magnetization probe. Remnant magnetizations (MREM) are in emuG/gram, C is emuK/gram, L is emu/gramK and D is emu/gram. T (K) H (T) mono line braided line Vespel Delrin nylon C 2 300 1e 2 9.1068e 07 1.0671e 05 5.3498e 07 7.2737e 07 1.5198e 07 L 2 300 1e 2 4.2817e 10 4.1188e 09 5.9675e 12 8.7623e 11 8.5243e 11 D 2 300 1e 2 1.3229e07 2.5228e 06 8.2524e 07 8.2636e 08 3.3978e07 C 2 300 1e 1 1.8156e 07 6.9705e 06 5.1355e 07 2.6522e 07 9.1798e 08 L 2 300 1e 1 1.0635e10 2.6259e10 1.1519e 10 6.5593e12 1.1522e11 D 2 300 1e1 4.6009e07 3.0526e07 6.0252e08 3.8065e07 3.9787e07 C 2 300 1 1.6334e 07 6.2278e 06 4.8105e 07 2.3244e 07 8.1238e 08 L 2 300 1 9.9114e11 5.5029e10 8.5564e11 3.8665e 11 1.5503e 11 D 2 300 1 6.0505e07 4.6172e 07 3.8356e07 5.0794e07 4.0962e07 M REM 2 7 0 5.4541e 05 2.3989e 04 1.5669e 04 4.7316e 05 6.2121e 06 M REM 10 7 0 4.3157e 05 2.2723e 04 1.0041e 04 2.1461e 05 1.3669e 05 M REM 100 7 0 1.5032e05 1.0743e 04 3.3839e 06 6.8093e 06 4.1010e 06

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70 Figure 2 19. Photographs of the optical rotation probe for use in a SQUID magnetometer. (a) There are three main sections of the probe: (i) a low temperature end that sits within the SQUID coils and magnet bore and houses the rotating sample stage, ( ii) a shaft that seats within an oring for movement of the probe through the SQUID coils that connects the high and low temperature spaces, and ( iii) a head that contains the active rotation element and other probe elements (b) A photograph of the low temperature end of the probe displays the (i) drive lines, ( ii ) the optic fiber, and ( iii) the sample rotation cell (c) A pho tograph of the top end of the probe shows (i) the drive spool, clearly Vis ible through a clear plastic seal, and ( ii) the manual dial for angular control. (ii) (i) (iii) (iii) (ii) (i) (ii) (i) (a) (b) (c)

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71 Figure 2 20. Magnetization versus field for potential optical rotation probe materials. Magnetization as a function of field measured at 100 K (green ), 10 K (red ) and 2 K (black ) for the monoline (a), braided line (b), Vespel (c), Delrin (d), and nylon (e). Figure 2 21. The schematic of the circuit control board for the automated operation of the custom probe using a stepper motor. Circuit elements use standard shorthand notation for, C for capacitors, D for diodes, R for resistors, CN for plugs, SB for switch banks, SW for switches.

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72 Figure 2 22. Magnetization versus rotation angle measured with the custom probe for two different magnetite samples at 300 K (a) and 2 K (b). (b) Magnetization versus time for thin films of AjCok[Fe(CN)6]ln H2O. Light was introduced to the sample at t = 0 minutes and turned off at t = 90 minutes. Figure 2 23. Photoirradiation of powdered neutron scattering samples. Because the powders to be photoinduced are opaque, the top layer of th e powder may be photoexcited, but the majority of the sample does not become photoexcited because it does not receive any radiation. The dark state of the sample is represented by dark blue and the photoexcited state is represented by yellow.

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73 Figure 2 24. Photoirradiation of powdered neutron scattering samples using tumbler probe. To mitigate the problem of opacity, powders are instead mounted in a quartz tumbler cell that may be rotated about one axis. As irradiation starts, the same problem as a fi xed cell is encountered, with the top layer blocking light from reaching the majority of the sample. However, the cell can rotate and tumble the previous top layer to be on the bottom. After a sufficiently long time, all powder within the cell will be ph otoexcited and measured. The dark state of the sample is represented by dark blue and the photoexcited state is represented by yellow.

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74 CHAPTER 3 THEORETICAL METHODS 3 While this thesis is experimentally driven, the interpretation of experimental results is eternally intertwined with the theoretical methods that seek to explain them This desire for more fundamental explanations of data is the main reason that an experimentalist must delve into the realm of theory, where even simple models can be predictive and further drive the experimental research. The ideology behind the theoretical applications employed in this work is not always to obtain precise quantitative expl anations of results, but often to glean in formation from results that is not otherwise obvious using semi empirical, transparent methods The different experimental techniques introduced in Chapter 2 require varying degrees of post processing in order to extract the desired information, and while methods such as microscopy provide information even to the untrained eye, spectroscopy and magnetization data can be exceedingly complex and can require detailed modeling. As the studies undertaken are explicitly of the photoinduced magnetism of a coordi nation network, the theories presented seek to provide additional insight into this problem. First, in Section 3.1, the general machinery of the quantum mechanical interpretation of transition metal ions within a localized picture is presented, with the different relevant energies in the system introduced one at a time. Next, Section 3.2 consists of a more ab initio approach that is still accessible to experimentalists, the tight binding set of theories. Finally, in Section 3.3, the ubiquitous fitting r outines based upon least squares type methods are discussed.

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75 3.1 Quantum Mechanics of Transition Metal Ions The discussion will take place in a building block mode, with examples and asides inserted where convenient Of particular interest is the calculat ion of energy levels, and how these energies change with the application of an external magnetic field, as these two pieces of information can be probed directly by experiment. Rough ly, one can begin with a single, free ion and work from the simple hydrog enlike picture, study ing the electronelectron interactions as additional electrons are added to build up a multielectronic wavefunction. Next, it is necessary to invoke the socalled ligand field theory, as the electrostatic interaction and covalency o f the ligands with the magnetic ion will add additional structure to the energy levels. The familiar spinorbit coupling and Zeeman splitting terms from quantum mechanics are then discussed in the context of transition metal ion energy levels. From here, interacting ions are considered, via the superexchange interaction, and the many body ground state is approximated, via mean field theory. Finally, the motivating interpretation of experimental electron magnetic resonance, inelastic neutron scattering, U V Vis spectroscopy, and magnetization measurements are put forward. Of high interest is the magnetization example, in which the quantum mechanical treatment discussed is applied to a novel piece of experimental data, namely the field dependence of the mag netic moment in potassium ferricyanide. While these methods are old, the advent of modern computers allows for simultaneous diagonalization of many interactions without the need to invoke perturbation theory, allowing old problems to be reVis ited with mo re power and to provide more insight.

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76 3.1.1 Coulomb Interaction and the Multi Electron Ion The hydrogen atom is a standard model, even for more complicated atomic systems, such as the transition metals [ 32] However, even within this framework, building m ultielectron wavefunctions is nontrivial. Therefore, the standard practice is to learn the empirical set of Hund's rules to formulate the terms that constitute a multielectron ion [ 4 ] These rules can be summarized as follows. (1) Within an electron configuration, the ground state is the term with the maximum multiplicity, and, strictly speaking, the maximum value of the spin quantum number. (2) Within a given spin configuration, the ground state is the term with the largest value of the angular momentum quantum number. (3) Finally, for less than half filled valence shells, the ground state is the term with the least total angular momentum, and for more than half filled shells, the ground state is the term with the most total angular momentum. However, these e mpirical rules represent the obfuscated surface of the underlying Coulombic interactions and wavefunction antisymmetrization that govern the energy levels of multi electron ions on a more fundamental level [ 33] To treat the problem using the machinery of quantum mechanics, one must start with the Coulomb repulsion term, Ve e( i j ) = e2r12 n i > j = 1 3 1

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77 where e is the charge of the electron, r12 is the distance between two electrons, and the sum is over all interactions within the ion. As this potential represents a pair wise electronelectron interaction, the interaction integral can be expressed as AB e2r12 CD = A ( r1 ) B ( r2 ) e2r12 C ( r1 ) D ( r2 ) dV1d V2 3 2 where is a single electron wavefunction of a hydrogenlike atom the subscripts denote distinct orbitals, and the integral is over all space. The treatment discussed here only considers the valence electrons of the ion, and ignores higher order effects, such as potential 4s 3d electron interactions Practically, the integral in Equation 3 2 is calculated by expanding Ve e(i,j) in terms of the natural basis of the wavefunctions, which are spherical harmonics After a few lines of calculus and the application of angular momentum selection rules, AB e2r12 CD = ms A, ms C ms B, ms D ml A+ ml B, ml C+ ml D ck lAml A, lCml C ck lDml D, lBml B Rk( ABCD ) k = 0 3 3 where the ck terms are the angular integrals and the Rk terms are the radial integrals, and detail s of these calculations can be found in Condon and Shortleys book [ 34 ] It is worth noting that the diagonal elements are defined as J ( A B ) = AB e2r12 AB 3 4 the Coulomb integral, and

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78 K ( A B ) = AB e2r12 BA 3 5 the exchange integral. For equivalent electrons Rk(ABCD) = Fk, where the Fk terms are referred to as the Condon and Shortly electron repulsion parameters. The ck integrals are standard spherical harmonic overlaps, and these can be calculated. Practically, at this point, one has arrived at a singleion Hamiltonian where these Fk terms are parameters that can be used to fit to experimental data to learn about the physics and chemistry of a given ion. Precedents show that the inclusion of only k = 2 and k = 4 terms is sufficient to parameterize the interaction [ 34] A different treatment by Racah is more general and provides a slightly different parameterization of the electronelectron interactions [ 35 ] While the derivation itself is more complicated, t he Racah parameters themselves are slightly more pleasing because e nergy differences between terms of the same multiplicity within a given configuration only depend upon the Racah parameter B, while in the CondonShortley scheme, two parameters are used. However, separations between terms of different multiplicities involve both Racah parameters B and C The electron electron repulsion parameters in the two frameworks have a simple linear relationship, B = F 2 5 F 4 and 3 6 C = 35F 4 3 7 In practice, the energies and wavefunctions corresponding to the different states of the multi electron singleion can be solved within the matrix formulation of quantum mechanics on a personal computer For the free Fe3+ ion, B = 1,029 cm1 and C/B = 4.1 (giving C ~ 4,220 cm1) [ 5 ] An interaction diagram showing the splitting of the terms as

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79 electronelectron repulsion is turned on is shown in Figure 3 1 It is worth noting that when the Fe3+ forms the hexacyanoferrate complex sign ificant to this thesis, B = 535 cm1 and C = 4,219 cm1 [ 5 ] 3.1.2 Ligand Field Theory While spin orbit coupling is a logical next single ion energy to be considered, for the 3d transition metals, the energy shifts due to interactions with surrounding ions in a m olecule provide a larger perturbation, and therefore they will be considered [ 5 ] [ 36] While originally, the interactions between the ion in question and the surrounding ligands were treated in the framework of an electrostatic interaction [ 37] quantita tive analysis has shown that most (but not all!) energy shifts are in fact due to covalency between the ions. Since both treatments are not usually performed ab initio but rather semi empirically, it is a matter of taste as to which approach provides the most personal insight. Herein, the picture where energy shifts are due to wavefunction overlap, the angular overlap model [ 5 ] will be employed. Octahedral coordination is relevant to the networks considered in this thesis, Figure 3 2 so a brief outline of such an interaction will be undertaken. Taking atomic wavefunctions on ligand and ion sites, overlap integrals can be generated for each metal ligand interaction, giving a generic angular overlap interaction matrix for an orthoaxial molecule, ligand = 3 4 [e ( 1 ) + e ( 2 ) + e( 3 ) + e( 4 ) ] 3 4 [e ( 1 ) + e ( 3 ) -e( 2 ) -e( 4 )0 0 0 3 4 [e( 1 ) + e( 3 ) -e( 2 ) -e( 4 ) ] e( 5 ) + e( 6 ) + 1 4 [e( 1 ) + e( 2 ) + e( 3 ) + e( 4 ) ]0 0 0 0 0ex( 1 )+ex( 4 )+ey( 2 )+ey( 3 )0 0 0 0 0ey( 1 )+ey( 6 )+ex( 5 )+ex( 3 )0 0 0 0 0ex( 2 )+ex( 6 )+ey( 5 )+ey( 4 ) 3 8

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80 where a basis of dx2y2, dz2, dxy, dxz, and dyz was used, with e and e denoting overlap of the ion with and orbitals of the i th ligand. The x and y subscripts denote directions of the overlap of the ligand assuming a right handed coordinated system with the z axis vector from the ligand to the ion. For bonding that is symmetric with respect to rotations about the metal to ligand bond axis these overlap energies are degenerate. For Oh symmetry, the standard crystal field splitting parameter of the t2g and eg strong field limit orbitals is simply = 3 4 In practice, a Kronecker tensor product of the singleelectron ionligand Hamiltonian, Hligand, must be made for each electron to be considered. Finally the eigen problem can be solved on a standard personal computer. For a d5 Fe3+ ion in a symmetric octahe dral field, the splitting of the freeion states due to the ligand interaction can be calculated, Figure 33 It is interesting to note that under the influence of a sufficiently strong metal ligand interaction, the ground state actually comes from the 2I term, rather than the freeion ground state 6S term, for example, carbon ligated Fe3+. In addition to the splitting shown in Figure 33 due to a symmetrical octahedral field, lower symmetries further lift the degeneracies. For example, a tetragonal distortion serves to separate states of different total angular momentum within a given ligand field multipl et. For the 2T2 g Fe(CN)6 3 ground state term, such a distortion is exemplified in Figure 3 4. Lower symmetries of the six fold ligand field are also possible, such as a trigonal distortion. However, in practice these distortions may be small, or they may serve to overparameterize the pr oblem in such a way as to render the meaning of the high order distortion parameters unclear. For the molecules under

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81 consideration in this thesis, a tetragonal distortion is sufficient to capture the salient features of the energy spectrum. 3.1.3 SpinOr bit Coupling Spin orbit coupling arises from the magnetostatic interactions of the inherent spin angular momentum of an electron with the orbital angular momentum [ 4 ] The hydrogenlike single electron Hamiltonian single, S O = l s 3 9 where is the single electron spinorbit coupling parameter, l is the angular moment, and s is the spin moment. The single electron spinorbit coupling parameter scales linearly with the effective charge of the nucleus, and therefore becomes more important the heavier the ion is. For multiple electron ions, the spinorbit interaction energy becomes S O = ( i ) l i s i i = L S 3 10 where is the multi electron spinorbit coupling parameter, and Stephens reduction factor has been introduced to take into account the quenching of orbital angular momentum due to the ligands. The angular momentum reduction factor can vary between zero and one, depending upon the local environment, and it is often somewhere in between. The m ultiple electron spinorbit parameter is related to the single electron parameter as = / 2 S 3 11 with a plus sign for less than half full shells, and the minus sign for greater than half full shells, as for the latter, the picture is one of pos itively charged holes [ 5 ] [ 33]

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82 In practice, the solution of this spinorbit Hamiltonian is analogous to those posed in the previous sections. For a d5 Fe3+ ion in a symmetric octahedral field of cyanides, it was shown that the ground state is a well separated 2T2g, Figure 33. The effect of the spin orbit coupling on this term can be calculated as the interaction is turned on, and for different values of th e reduction parameter, (Figure 3 5). 3.1.4 Zeeman Splitting The response of a magnetic ion to an external magnetic field is called the Zeeman effect [ 4 ] T he Hamiltonian for t his effect can be written as =BH ( Lz+ 2 Sz ) 3 12 where B is the Bohr magneton, H is the applied magnetic field, is the orbital reduction parameter, Lz is the component of the angular momentum along the magnetic field, and Sz is the component of the spin angular momentum along the magnetic field. Experimental ly, this term is the smallest yet considered, with maximum interaction energies of ~10 cm1 for fields less than 10 T. However, this term is essential when considering the magnetization of a sample, since changes in energies with applied magnetic field ar e detected with a magnetometer. The effect of this term on spinorbit split 2T2glike ground state of the hexacyanoferrate ion can be calculated for different values of the orbital reduction factor, Figure 36. 3.1.5 Superexchange Interaction Superexchang e is a result of secondorder perturbation theory. It is the most relevant ionion interaction for coordination compounds, like those studied in this thesis. The superexchange interaction arises from a mutual wavefunction overlap with a shared ligand and two ionic centers. While the connectivity can be greater than twofold and

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83 span an entire lattice, superexchange is still a two body exchange interaction. This interaction can give rise to a splitting between the singlet and triplet states of a dimer, e ffectively aligning the spins parallel or anti parallel. Using the molecular orbital theory model put forward by Hoffman [ 38] the leading terms in the superexchange energy can be written in terms of the integrals between dimer A and dimer B, such that Et riplet Esinglet = JAB superexchange = 2 KAB + ( 2 hAB) 2 JAAJAB 3 13 where K and J are the familiar exchange and Coulomb integrals previously introduced (Equations 3 4 and 3 5 ), and hAB is di fference of ionization potentials of the dimer A and dimer B magnetic orbitals. Since the exchange integral is always positive, the first term in Equation 3 13 leads to ferromagnetic interaction, and the second term in Equation 3 13 leads to antiferromagnetic interaction. For multiple magnetic orbitals, the con tributions can be summed, to first order. This model effectively reproduces the empirical Goodenough Kanamori rules for the sign of superexchange, where the interactions are ferromagnetic for orthogonal orbitals and antiferromagnetic for orbitals with substantial overlap [ 39 ] 3.1.6 Mean Field Theory If the superexchange interaction is present to an appreciable degree, the problem becomes a many body system of all ions in the lattice. Many body problems cannot be solved exactly, but by invoking a meanfie ld approximation to the free energy, many features of the many body ground state can be reproduced [ 40 ] [ 41] [ 42] [43 ] For simplicity, consider ing only the superexchange of spins associated with nearest transition metal neighbors, designated as n.n. und er the influence of an applied magnetic field, the Hamiltonian has the form

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84 = 2 JijSi Sj i j = n n + gBH Si i 3 14 where J is an exchange constant (not to be confused with the Coulomb integral, please note context) g is the Land factor, B is the Bohr magneton, S is the electronic spin, and H is the applied field. The mean field ex pansion of the spin operator give s MF = 2 Jij Si Sj 2 JijSi Sj i j = n n + 2 Jij Si Sj i j = n n + gBH Si i i j = n n 3 1 5 where S denotes an average spin polarization value. For the case of a spatially independent average spin polarization, one can further simplify the problem to a diagonal Hamiltonian, MF = 2 2 Z J0 S Si i + 2 Z J0N S 2 + gBH Si ,i 3 16 where Z is the number of nearest neighbors, J0 is the scalar exchange constant, and N is the total number of spins Expressions for the average spin polarization can be derived by minimizing the free energy with respect to variation of the spin polarization, yi elding, S = BS g B S H extkBT + 2 Z J kBT S 3 17 where BS is the Brillouin function [ 44 ] kB is the Boltzmann constant, and denotes an average. Equation 3 17 is transcendental and therefore cannot be solved exactly, but must done numerically. For the specific example of a lattice of Fe3+ ions with spin orbit split 2T2glike ground states and fully quenched angular momentum, the resulting solutions for values of gBHext/kB = 0.01 and 2Z/kB = 1 are shown in Figure 37

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85 It should be mentioned that disorder and random effects can give rise to complicated and highly degenerate microstates, such as the spinglass (relevant to Prussian blue analogues), but a detailed discussion o f these effects cannot be simply understood by any low energy theories, and are therefore beyond the scope of this thesis [ 45] [46 ] It is worth noting that external stimuli, such as sufficiently strong magnetic fields, can tune a disordered system away f rom a spinglass like state towards more standard magnetic states. 3.2 Tight Binding Approximations While the previous section outlining semi empirical methods of quantum mechanics is useful, an additional level of understanding can be gained by a more fun damental approach to calculation of energy levels. Because the coordination networks studied can be understood in terms of a perturbed molecular orbital picture, as opposed to an itinerant electron picture, tight binding approximations are appropriate to approximate the energy levels of the systems. The most common tight binding approximation is using a linear combination of atomic orbitals (LCAO), and a further approximation is the extended Hckel theory [ 47] While quantitative results are not expected with these methods, oftentimes qualitative features of the systems can be reliably reproduced. Specifically, tight binding methods are useful for understanding the nature of the chemical bonding in a system. In the following, a simple example of the car bon and nitrogen interaction leading to a CN molecule will be outline first. Next, specific examples of tight binding calculations to approximate ligand fields, superexchange interactions, and forcefields between atoms will be discussed. This final poin t is relevant when interpreting infrared vibrational spectroscopy data, often

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86 used to delineate between different types of heterobinuclear moieties present in Prussian blue analogues. 3.2.1 Extended Hckel Theory The Schr dinger equation can often be written down for a complex system, but rarely solved exactly. Three standard approximations are employed in solving the Schr dinger equation within the Hckel formalism. First, the BornOppenheimer approximation assumes that electrons move in a field of fixed nuclei, due to the disparate masses of the two types of particles. Second, the independent particle approximation makes the assumption that the total many electron wave function can be written down as a product of the singleelectron wave functions. Third, only the valence electrons are considered in the calculation, as the electrons in filled orbitals are mostly inert. Aside from specific fitting of experimental data, tight binding calculations of molecules act as a sandbox from which valuable chem ical intuition may be extracted. Each molecular orbital is given as a linear combination of atomic orbitals. The basis set used is spherical harmonics for the angular part of the wavefunction, and Slater type orbitals for the radial part of the wave func tion. Slater type orbitals are a further approximation to the hydrogenlike orbitals in which R ( r ) = N rn 1er 3 18 where N is a normalization factor, n is the principle quantum number, and z is a semi empirical parameter that characterizes the diffuseness of the orbital. For more diffuse orbitals, such as the 3d set, a doublezeta expansion is used consisting of a

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87 linear combination of two Slater type orbitals. The problem is then reduced to an eigen problem for the coefficients of the atomic orbitals in the basis set, C = SCE 3 19 where is a square matrix containing the one electron energy integrals (analogous to those discussed in Section 3.1.1 on electron repulsion within the Condon and Shortley regime), and C is the coefficient matr ix, S is the matrix of overlap integrals, and E is the diagonal matrix of orbital energies. In practice, the coefficient matrix is found by the variational method to minimize the total energy of the system. The core matrix elements of the single electrons are given by atomic energies, and for the off diagonal elements, by the Wolfsberg and Helmholz approximation, ij = 1 2 K Hii+ Hjj Sij 3 20 where S is the overlap integral, and K is a scaling parameter introduced to account for the increased overlap in molecules. While the fundamental calculation of K is not done, experimental studies of ethane by Hoffman showed that K = 1.75 is suitable [ 47 ] A somewhat straightforward example involving the bonding of carbon and nitrogen to for the cyanide molecule, which is highly relevant to the cyanobridged networks studied in this thesis, will now be presented. The calculation involves the four valence atomic orbitals of carbon C(2s), C(2px), C(2py), and C(2pz), and the four valence atomic orbitals of nitrogen N (2s), N(2px), N(2py), and N(2pz). Therefore the cyanide molecule will have a basis set of 8 atomic wave functions, and Equation 3.18 will require the inversion of 8 x 8 matrices. The solution to Equation 3.18 gives both the molecular orbitals and their e nergies, which are shown in Figure 3 8 The lowest anti bonding orbital, the *, is especially important to the magnetization of the Prussian blue

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88 analogues, because it acts as a strong electron acceptor on the carbondominated molecular orbitals and medi ates the superexchange between metal ions. 3.2.2 Ligand Field Theory While the parameters used in ligand field theory are best utilized as fitting parameters to experimental data, it is also possible to calculate them from fundamental principles. However, calculating from first principles is a difficult problem. It is not surprising that qualitative methods such as LCAO methods are unable to provide quantitative predictions for the desired parameters, and more complicated methods such as Density Function Theory (DFT) are not, themselves, parameterized in such a way to make the ligand field theory parameters assignable from these calculations. However, it is desirable to glean qualitative trends from first principles to help explain experimental trends, such as the relative ligand field strengths of different atoms and the effects of distortions on the ligand field. Two specific examples will be briefly introduced: (1) the difference in the octahedral ligand field splitting parameter for a Co(NC)6 4 molecule compared to the Fe(CN)6 3 molecule, and (2) the effect of tetragonal distortions on the energy levels of a Ni(NC)6 4 and Cu(NC)6 4 molecules. 3.2.3 Superexchange Interaction The superexchange interaction introduced in Section 3.1.5 was again dealt wit h completely as an empirical parameter. Like the ligand field splitting parameters, the qualitative changes in the superexchange constant may be investigated with extended Hckel theory. 3.2.4 Infrared Vibrational Spectroscopy Quantitative theoretical analysis of the cyanide stretching frequencies in Prussian blue analogues is still lacking. This situation is unfortunate, given the high degree to

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89 which scientists use cyanide stretching frequencies as diagnostic tools to assign oxidation states and coordination numbers in cyanometallate compounds. Although a quantitative solution should be possible using modern density functional theories, such a study is beyond the scope of this thesis, and instead a tight binding analysis was attempted in order to understand the trends observed. Unfortunately, the tight binding approximations are inadequate for this level of structure determination and no meaningful information could be extracted from the studies. 3.3 Fitting Algorithms More often than not, experimental scientists are forced to deal with overdetermined systems. The classic example of an overdetermined system is a data set with many points that a researcher would like to fit to a function having fewer parameters than ther e are data points. This situation happens all the time, daily for many people. All is not lost, especially if one has access to a math machine, such as a standard personal computer. The ubiquitous technique employed is a method of least squares. In Sect ion 3.3.1, the general procedure of least squares fitting is overviewed. In Section 3.3.2, the Levenberg Marquardt algorithm that was extensively employed for fitting functions in this thesis is discussed. In Section 3.3.3, the specific example of least squares fitting used to fit all diffraction data in the thesis, the Rietveld method, is overviewed. 3.3.1 Least Squares The process of fitting data comes down to minimizing the difference between the function that is fit and the data. These differences ar e called the residuals of the fit. The name least squares refers to the fact that, in this method, a square of the

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90 difference between the data and the fitting function is used [ 48 ] The squared difference is used instead of the absolute difference for the simple reason that this allows the residuals of the fit to be treated as a continuous differentiable quantity. An important thing to remember when using this method is that the squaring of the residuals effectively weights outliers stronger than other data. Therefore, if data points are known to be erroneous they should be excluded, or one can additionally weight the residuals by the known experimental errors involved. A straightforward derivation can illustrate these points. The least square fit par ameter, S, is defined as the sum of the square of the residuals S = ri 2 n i = 1 3 21 where there are n discrete data points. Explicitly, these residuals are the difference between the experimental data and the fitting function, r i = y i f ( x i ) 3 22 which depends upon the independent experimental variable and the fitting parameters. This parameter may be minimized by setting the derivative with respect to changes in the function parameters equal to zero. j = 2 ri ri j n i = 1 = 0 j = 1 m 3 23 where there are m parameters. By substituting Equation 3 22 into Equation 3 23 one gets

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91 2 ri ( xi, ) j n i = 1 = 0 j = 1 m 3 24 If the function f ( xi, ) depends linearly upon the fit parameter, f ( xi, ) = jFj ( xi ) m j = 1 3 25 the derivative is straightforward ( x i ) j = Fj ( xi ) 3 26 If a tensor is defined such that X ij F j ( x i ) 3 27 The solution to the linear least square problem becomes clear, namely = XTX 1 XTy 3 28 3.3.2 LevenbergMarquardt While the linear least squares case is straightforward, complicating issues arise when the function in question is nonlinear, and a more complicated approach must be used. Here the Levenberg Marquardt algorithm can be utilized [ 49] [ 50] In order to find the minimum of a function F ( x ) that is a sum of squares of nonlinear functions, fi( x ) i.e. F ( x ) = 1 2 [ fi ( x ) ] 2 m i = 1 3 29 Let the Jacobian of fi( x ) be denoted Ji( x ) then the Levenberg Marquardt method searches in the direction given by the solution to the equations

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92 Jk TJk+ kI pk = Jk Tfk 3 30 where k are nonnegative scalars and I is the identity matrix 3.3.3 Rietveld Refinement Rietveld refinement was used to interpret powder diffraction patterns from neutron and x ray scattering experiments [ 51] Specifically, the GSAS [ 52 ] and EXPGUI [ 53] computer programs were used for all refinements. This technique refers to the use of least squares fitting of experimental data with theoretical models, and it is called refinement because it can only modify parameters within a given test model, rather than predict the appropriate model a priori In essence, the refinement consists of minimizing the function which depends upon the square of the difference between the fit and the data, such that F ( R ) = Wi ( experiment cmodel ) 2 i 3 31 where Wi is the statistical weight and c is an overall scale factor. Because of this short coming, crystallographers must test many different models to arrive at a solution. Practically, most of the information that goes into a Rietveld refinement comes from the Bragg condition for constructive interference of scattered waves Q = 4 sin ( ) n 3 32

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93 where Q is the momentum transfer, is the scattering angle, n is the order of the reflection, and is the incident wavelength. Or, conversely, the Bragg condition can be reformulated as the Laue condition k K = 1 2 K 3 33 where k is the incidence wave vector, and K is the momentum transfer ( K k' k where k' is the final wave vector). For a powder pattern, the Bragg reflections can be thought of in terms of the Ewald sphere [ 42] The Ewald sphere is the surface generated by rotating the incident wavevector through the origin and testing to see if a recipro cal lattice point lies on the surface of the sphere at a distance K from the origin, thus satisfying the Laue condition. The random distribution in a powder sample averages over all possible scattering angles, so that each reciprocal lattice vector gener ates a sphere of radius K This powder scattering sphere will intercept the Ewald sphere to create a circle, so long as K is less than 2 k The vector between a point on the intersecting circle and the end of the incident wave vector is the final momentum k' that satisfies the Laue scattering condition. Formally, K = 2 k sin 2 3 34 therefore, by measuring the angular dependence of the scattered intensities of a powder, information about all Bragg reflections corresponding to reciprocal lattice vec tors shorter than 2 k is available.

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94 While the gross features are captured by the positions of the reflections, the intensities of the reflection contain important information about the structure of the atoms within a unit cell. The geometric structure fact or modulates the intensities by S ( K ) = ei K dj n j = 1 3 35 where S ( K ) is the geometric structure factor, and dj are the positions of the atoms within the unit cell. This structure factor can diminish observed Bragg peaks due to interference between waves scattered within a given unit cell. For neutron refinements, an additional magnetic factor due to the spin scattering must be added to the atomic scattering. Finally, aside from the position and intensities of the Bragg reflections, information is also contained in the shape of the lines. Crystallite size can serve to broaden the lines, as a departure from infini te space symmetry occurs. This final point is particularly relevant to nanostructured materials.

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95 Figure 3 1 The energy differences between different d5 Fe3+ free ion terms arising from electronelectron repulsion as a function of the Racah repulsion parameter B. The vertical line corresponds to the standard value for free Fe3+ [ 5 ] Figure 3 2 The octahedral coordination geometry. An octahedrally coordinated smiley metal ion is shown, with an overlap between a lobe of the centr al, red, 3d orbital with an s wave, green, ligand orbital shown for site 2. The numbers are relevant to the Hamiltonian angular overlap parameters discussed in the text. 0 500 1000 1500 2000 -200,000 -150,000 -100,000 -50,000 0 50,000 energy (cm-1)B (cm-1) 2S 2P 2Dp 2Dm 2D 2Fa 2Fb 2Ga 2Gb 2H 2I 4P 4D 4F 4G 6S Fe3+ ion C/B = 4.1 2 S 2 P 2 D + 2 D 2 D 2 F a 2 F b 2 G a 2 G b 2 H 2 I 4 P 4 D 4 F 4 G 6 S

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96 Figure 3 3 The energy of a molecular term as a function of the octahedral splitting parameter, for a d5 ion, such as Fe3 +. A typical value for Fe(CN)6 3 is denoted by the vertical line, showing a 2T2g ground state separated from the next excited state by roughly 10,000 cm1. 0 10 20 30 40 50 0 10 20 30 40 50 60 70 80 90 Energy/B /B 6A1g(S) 4T1g(G) 4T2g(G) 4A1g/4Eg(G) 4T1g(P) 4T2g(D) 4Eg(D) 2T2g(I) 2A2g(I) 2T1g(I) 2T2g(I) 2Eg(I) 2A1g(I) 2T2g(I) 2Eg (I) 2T1g(I) Fe3+ in Ligand field B = 859 cm-1C/B = 4.48 6 A 1g (S) 4 T 1g (G) 4 T 2g (G) 4 A 1g / 4 E g (G) 4 T 1g (P) 4 T 2g (D) 4 E g (D) 2 T 2g (I) 2 A 2g (I) 2 T 2g (I) 2 T 2g (I) 2 E g (I) 2 A 1g (I) 2 T 2g (I) 2 E g (I) 2 T 1g (I)

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97 Figure 3 4 Energy shift plotted versus the tetragonal distortion parameter, Specifically, for a d5 2T2 g Fe(CN)6 3 ground term. 0 100 200 300 400 500 600 -400 -300 -200 -100 0 100 200 4 states Lz 2 = 0 E (cm-1) (cm-1) Lz 2 = 1 tetragonal distortion of Fe3+ 2T2g2 states

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98 Figure 3 5 Energy splitting of the octahedral hexacyanoferrate 2T2 g ground state. Energies are shown for = 1 (solid lines), the = 0 totally quenched (green line), and a partially quenched = 0.5 state (dashed lines). In the presence of spin orbit coupling, the six fold degeneracy of t he 2T2g state is lifted for different values of the total angular momentum, j. The vertical line at 460 cm1 is the free ion value of spin orbit coupling [ 5 ] Clearly if the orbital moment is completely quenched, spinorbit coupling has no effect. 0 100 200 300 400 500 600 700 -1000 -800 -600 -400 -200 0 200 400 2 states j = 1/2 E (cm-1) (cm-1) j = 3/2 4 states spin-orbit coupling of Fe3+ 2T2g state no splitting 6 states

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99 Figure 3 6 The Zeeman splitting versus applied magnetic field for the spinorbit split 2T2glike ground state of hexacyanoferrate. Energies are shown for (a) = 0, with no angular momentum, (b) = 0.5, the angular momentum is partially quenched, and t he magnetic field splits the j = 3/2 into a quartet and the j = 1/2 into a doublet, and (c) = 1, with no quenching of the angular moment, the j = 3/2 term is robust and the j = term is split into a doublet. Figure 3 7 The effect of superexchange on magnetization. (a) The effect of superexchange on the average spin value along the magnetic field above the magnetically ordered state is shown for an S = 1/2, Fe3+ ion for both ferromagnetic (J > 0) and antiferromagnetic interactions (J < 0), Equation 3 17 (b) The average spin value along the magnetic field shows sharp increases corresponding to long range magnetic order, with increasing ordering temperatures for increasing values of the superexchange parameter, J. Here only ferromagnetic examples ar e shown, as antiferromagnetic samples have no net magnetic moment in their ordered state. 0 1 2 3 4 5 6 7 -4 -3 -2 -1 0 1 2 3 4 Sz = -1/2 E (cm-1)H (Tesla) Sz = +1/2 0 1 2 3 4 5 6 7 -350 -348 -346 -344 -342 168 170 172 174 176 +3/2 -3/2 +1/2 -1/2 -1/2 +1/2 E (cm-1)H (Tesla) 0 1 2 3 4 5 6 7 -694 -692 -690 -688 -686 340 342 344 346 348 350 E (cm-1)H (Tesla) +1/2 -1/2 (a) (b) (c) 0 2 4 6 8 10 0.0 0.1 0.2 0.3 0.4 0.5 J = 20 J = 5T (K) J = 10 100 150 200 250 300 0.00244 0.00246 0.00248 0.00250 0.00252 0.00254 0.00256 J = -10 J = -5 J = 0 J = 5 T T (K) J = 10 (a) (b)

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100 Figure 3 8 The cyanide molecule. (a) An extended Hckel interaction diagram shows how carbon and nitrogen are able to lower their energy by forming a cyanide molecule. (b) An illustration of the p* and s+ p* orbitals of the CN molecule relevant for the formati on of extended cyanobridged coordination networks. -40 -35 -30 -25 -20 -15 -10 -5 157 158 159 160 Energy (eV) C N CN-2s 2p 2p 2s p* sp* s* + pps + p* s* p s + p p p (a) (b)

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101 CHAPTER 4 QUANTITATIVE ANALYSI S OF MAGNETIZATION IN COMPLEX CYANIDES 4 Using the experimental and theoretical machinery described in the previous two section s, a quantitative analysis of the magnetization in select Prussian blue analogues and their paramagnetic precursors can be made. The two magnets of particular interest to this thesis are Cs2.8Ni4[Cr(CN)6]4 nH2O, which has a high ordering temperature of 60 to 90 K [ 54] and Co4[Fe(CN)6]3 nH2O, which can display photoinduced magnetism when proper proportions of interstitial ion s are included in the lattice [ 1 ] In addition, paramagnetic precursors of these materials, K3Cr(CN)6 and K3Fe(CN)6, will be presented for comparison. A well parameterized Hamiltonian, whose components may be determined by experimental measurements is sought Specifically, electron spectroscopy, magnetic neutron diffraction, and magnetization measurements are suffic ient to fully determine the relevant Hamiltonian for magnetization in these systems. As for most problems, many paths to a possible solution may exist, and the best approach may depend upon personal taste and the availability of resources. One recipe of the many possible is outlined in this chapter The author would like to stress the dangers of using improper, incomplete, or misunderstood recipes when modeling the magnetization of the systems in question. For example, the change in magnetic susceptibili ty as a function of temperature may be due to multiple effects, including spinorbit coupling, structural distortions, and superexchange. If the model is underdetermined, it may be impossible to separate the different contributions. In the same vein, the use of simple equations without a full understanding of their derivation should be avoided at all costs. A common culprit is the Curie Weiss formula, = C/(T ), where the assumption that all deviations from high

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102 temperature Curie like behavior come onl y from superexchange is often assumed by the unaware researcher, even in systems with first order orbital angular momentum. This issue, and others, may be avoided if proper companion measurements to magnetization are performed, where the Hamiltonian to be considered contains only well understood parameters. In Section 4.1, spectroscopy experiments designed to determine the single ion parameters are discussed. These measurements help in determining possible spin and orbital states for ions. In Section 4. 2, magnetization measurements performed to detect the presence of magnetic order are presented. In Section 4.3, microscopic probes of the magnetic structure are presented. Finally, in Section 4.4, known parameters are summarized and a fit of the magnetiz ation data is presented. 4.1 Synthesis and Chemical Composition The precursor materials were used without modification after purchase from Acros. The powders of Cs2.8Ni4[Cr(CN)6]4 nH2O and Co4[Fe(CN)6]3 nH2O were synthesized by Matthieu F. Dumont C hemical formulae were determined using ICP MS for the relative ratios of of all elements excepting hydrogen, which was determined by multiplying the oxygen content by two. The ICP MS was performed by Complete Analysis Laboratories, Inc. (www.calilabs.com). 4. 2 S pectroscopy Even before beginning a spectroscopy experiment, it is useful to have knowledge of the general atomic environment and structure of the material. For Prussian blue analogues, a good starting point is always the simple cubic structure that was assigned after single crystal diffraction studies of Prussian blue [ 55 ] The structure consists of metal ions linked by cyanides in a unit cell with two metals, Figure 4 1 From this space

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103 model, the point symmetry and local environments of the magnetic atoms can then be determined. Two different types of spectroscopy may be applied to the materials in question, INS for excitations from ~25 meV to ~1 eV ( spanning the energies of spin orbit coupling and structural distortions) and UV Vis for excitations from ~1 eV to ~6 eV ( for which ligand field interactions and electron repulsion are relevant energies ). Inelastic neutron scattering has not yet been exploited for complex cyanides, but at the end of May 2010, a collaboration between UF and ORNL should remedy this lack of data Ultraviolet and Vis ible spectroscopy, on the other hand, has been performed on Cs2.8Ni4[Cr(CN)6]4 nH2O, Co4[Fe(CN)6]3 nH2O, K3Cr(CN)6,and K3Fe(CN)6. Results are conclusive for determining the ligand field splittings of nickel and chromium ions Figure 4 2 However, iron has transitions from the of the CNto the d levels of the metal in the same energy range. The ligand to metal transitions have much larger probabilities than dd transitions, rendering the standard UV Vis spectroscopic determination of iron ligand field states with standard UV Vis spectrometers difficult. 4. 3 M agnetic S usceptibility The next experiment consists of a temperature dependent magnetic susceptibility measurement down to the lowest temperatures available. The point of this measurement is to look for anomalies in the shape that may be characteristic of magnetic phase transitions. The magnetization data for K3Cr(CN)6 and K3Fe(CN)6 do not show any anomalies, whereas the Prussian blue analogues, Cs2.8Ni4[Cr(CN)6]4 nH2O and Co4[Fe(CN)6]3 nH2O, show divergence of t he magnetic susceptibility at ~90 K and ~ 13 K, respectively, Figure 4 3 These transition

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104 temperatures may be used to estimate the magnitude of the superexchange parameter using mean field theory, TC = ZAZB | JAB | 3 kB SA ( SA+ 1 ) SB ( SB+ 1 ) 4 1 where TC is the ordering temperature, Z is the number of nearest neighbors, J is the superexchange constant, S is the spin quantum number, and kB is the Boltzmann constant. It is important to note that Equation 4 1 can only give the magnitude of the superexchange parameter, but not the sign. 4. 4 M icroscopic Probe of Magnetization While magnetization or specific heat measurements can determine the magnitude of the superexchange constant, true determination of the magnetic structure requires a microscopic probe. One possible probe is magnetic neutron diffraction. Neutron diffractio n of Cs2.8Ni4[Cr(CN)6]4 nH2O has been performed, and a ferromagnetic structure was found [ 56 ] For Co4[Fe(CN)6]3 nH2O, an x ray magnetic circular dichroism measurement has been reported [ 57 ] however the electronic transitions present in the spectra are n ot well enough understood to be quantitatively model ed, and macroscopic magnetic susceptibility data was relied upon for analysis of the microscopic data. For this reason, it is crucial to perform a study of the magnetic structure of Co4[Fe(CN)6]3 nH2O us ing a more easily modeled probe, such as neutron diffraction, where the relevant interactions are understood. The proposal to perform such an experiment has been accepted and is scheduled to be performed at ORNL on the HB2A beamline of HFIR in May 2010.

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105 4 5 Magnetization Fitting At this point, a parameter set is available for the different compounds, Table 4 1 Here, certain parameters could not be determined from the available measurements, but plausible estimates can be made based upon spectroscopic and nephelauxetic series [ 5 ] Once the ground term has been assigned, the first order relevant energy scales are spinorbit coupling and superexchange. Magnetization [ 3 ] [ 58 ] is one of the more complex quantities to critically understand because it does not just depend upon the energy levels of a system, but rather how those energy levels respond to an applied magnetic field M = dE d H 4 2 One can use Boltzmann statistics to calculate the average magnetization resulting from the thermal population of a set of calculated energy levels, Ei, < M > = d E i d H ie E i / k B T e E i / k B T i 4 3 For the magnetization calculations in this section, the well separated, ligandfield ground term will be used as a starting point. 4.5.1 K3Cr (CN)6 First, consider the magnetization of the K3Cr(CN)6 mat erial. The parameters in Table 4 1 suggest that a good Hamiltonian to calculate magnetization between 2 K and 300 K may be = B H ( 2 S z ) 4 4

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106 where B is the Bohr magneton, H is the external magnetic field, and Sz is the spin moment along the field direction. The agreement between model calculations, using Equation 4 4 and experiments is astonishing, as no fitting parameters are used, Figure 4 4 The slight deviations between experiment and model may be due to errors in background subtraction, or second order spinorbit coupling effects coming from mixing with excited states of similar symmetry. The decrease in the T values at low temperatures is real, present in both the model and the experiment al and is due to a violation of the H /T << 1 limit for Curie behavior. 4.5.2 K3Fe(CN)6 The next material, K3Fe(CN)6, has unquenched orbital angular momentum that gives an additional level of complexity. In light of this orbital contribution, a plausible Hamiltonian is then = BH ( Lz + 2 Sz ) + (L S ) 4 5 where B is the Bohr magneton, H is the external magnetic field, is the spinorbit coupling constant, Lz is the orbital moment along the field direction, Sz is the spin moment along the field direction, and is the orbital reduction parameter. While structural distortions might exist (and in the most rigorous case, should be included) for simplicity thes e effects may be approximated by the parameter. It is also noteworthy that the use of powder data makes additional parameterization suspicious. The experimental data of K3Fe(CN)6 may be compared to a model without orbital contributions ( Equation 4 4 ) and one with orbital contributions ( Equation 4 5 ), Figure 4 5 The failure of the spinonly model to reproduce the magnitude of the magnetization as a function of field and temperature is striking for this material, in contrast with K3Cr(CN)6.

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107 The fact t hat the moment is anomalously large at high temperatures and small at low temperatures is a suggestive sign ature of the presence of orbital moments and spin orbit coupling. With spinorbit coupling parameters of ~ 100 cm1 for the iron series ions, the width of the energy multiplet is nearly equal to the thermal difference between 300 K and 2 K. Discrepancies of the fit can be attributed to the known structural transition at 70 K and the lack of a structural distortion parameter in the fitting Hamiltonian. These higher order effects go beyond the scope of the current discussion and analysis These data clearly illustrate two important lessons for fitting magnetic data of organometallic compounds. First, a simple application of = C/(T ) would yield ~ 40 K, although spin correlations of that magnitude can be completely ruled out by the absence of a divergence in the susceptibility in the relevant temperature range. Second, a simple application of a spinonly Hamiltonian with a scaled g factor cannot reproduce the data, unless an appreciable temperature dependence of the effective g factor was introduced. 4.5.3 Cs2.8Ni4[Cr(CN)6]4 nH2O The next material to be considered is Cs2.8Ni4[Cr(CN)6]4 nH2O. This material posses ses two magnetic ions, Ni2+ and Cr3+, that both have orbital singlet ground states. Therefore, to first order, orbital angular momentum should not affect the magnetization. However, the observed divergence of the magnetic susceptibility and subsequent neutron diffraction studies showed s trong spinspin correlations associated with superexchange. Therefore, a reasonable Hamiltonian should be = BH ( 2 Sz ) 2 JijSi Sj i j = n n 4 6

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108 where B is the Bohr magneton, H is the external magnetic field, is the spinorbit coupling constant, Sz is the spin moment along the field direction, and J is the superexchange parameter. The many body portion of Equation 4 6 can be approximated with a meanfield solution, as described in Section 3.1.6 In addition, demagnetizing effects will be present i n the low field magnetically ordered data and the phenom en ological approach described in Section 6.4.1 that scales the magnetization by a domain factor will be used. Good global agreement of the magnitude and shape of the model calculation compared to th e experiment al data is found, Figure 4 6 Deviations near the onset of magnetic order are expected due to the approximate nature of the meanfield approach. Additional differences may be due to 2nd order orbital momentum effects, notorious for these ions [ 36] 4.5.4 Co4[Fe(CN)6]3 nH2O Co4[Fe(CN)6]3 nH2O is the final, most complicated material to be considered. As seen by the parameters in Table 4 1 Co4[Fe(CN)6]3 nH2O has both spinspin correlations and first order orbital angular momentum. A reasonable Hamiltonian to try modeling the system is = B H ( L z + 2 S z ) + ( L S ) 2 J ij S i S j i j = n n 4 7 where B is the Bohr magneton, H is the external magnetic field, is the spinorbit coupling constant, Lz is the orbital moment along the field direction, Sz is the orbital moment along the field direction, J is the superexchange parameter and is the orbital reduction parameter. Like the Cs2.8Ni4[Cr(CN)6]4 nH2O, the many body interaction is solved using mean field theory, and the low field ordered state is expected to be multidomain. The author is currently in the process of debugging his fullfledged code

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109 for this Hamiltonian, so at this point only high temperature expansion fits are made. Similar t o the paramagnetic K3Fe(CN)6 precursor that has one of the chromophores present in Co4[Fe(CN)6]3 nH2O, first an illustrative attempt to fit the data using a spin only formula is made, Figure 4 7 For all fits, both signs of J will be used in order to see which one produces smaller residuals. While the antiferromagnetic J can reproduce the shape of the susceptibility as a function of temperature quite well without needing to include orbital moments the magnitude of the magnetization is terribly wrong, Figu re 4 7 This disagreement is good reason to include orbital moments, as in the ori ginally hypothesized formula, Equation 4 7 The magnitude of the magnetization after including first order angular momentum is in much better agreement with experimental da ta Perhaps surprisingly, the shape can now also be reproduced with a ferromagnetic J, as a decrease in the susceptibility with temperature now also takes place because of the depopulation of high orbital momentum states, just as in K3Fe(CN)6. Preliminar y fits of the field dependence yield similar conclusions. One tell tale sign of orbital contributions in these compounds is the finite slope of magnetization at high magnetic fields, which is absent in systems with first order orbital singlets. In the end, the Co4[Fe(CN)6]3 nH2O fitting is the most complicated, but most intriguing as well. Although T decreases with temperature, an observation that has lead scientists to conclude antiferromagnetic interactions based upon application of the Curie Weiss law, a full fit that properly concludes orbital contributions actually suggests ferromagnetic interactions may be present Whichever the case, these questions

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110 should be answered with the inelastic neutron scattering and magnetic diffraction experim ents scheduled at ORNL at the end of May 2010. Figure 4 1 The cubic complex cyanide Prussian blue analogue structure. (a) A representation of the crystal structure of a generic Prussian blue analogue, AjM1k[M2 (CN)6]l nH2O. (b) The M1 (NC)6 molecular subunit to be considered for single ion excitations. (c) The M2(CN)6 molecular subunit to be considered for singleion excitations Figure 4 2 UV Vis of Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O (a) Room temperature UV Vis spectroscopy measurements of the dd transitions present in 10 mM Cr(CN)6 precursor ( ---), and a 10 mM Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O powder with background subtracted by using the functional form of a diamagnetic Rb0.5Zn4.0[Fe(CN)6]2. 8 nH2O ( ). (b) Using the transitions shown in Figure 4 2 (a) a multiplet calculation can be performed to show the electronic energy levels for the Ni2+ and Cr3+ ions in the Prussian blue network Chromium energy levels are shown for (i ) no spinorbit coupling and ( ii) using reported values of spin orbit coupling for the free ion [ 5 ] Nickel energy levels are shown for ( iii) no spinorbit coupling and (iv ) using reported values of spinorbit coupling for the free ion [ 5 ] 0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000 NiII(NC)6 (iii) (iv) (ii)Energy (cm-1)(i)CrIII(CN)60 2 4 6 8 10 12 1A1g 1T2g 1Eg 1T1g 3T1g 1A1g 1T2g 3T1g 1Eg 3T2g3A2g2Eg 2T2g 2T1g 2T2g 2T1g 4T1g 2Eg 2A2g 2T2g 2T1g 2E2T1g 2T2g 2A1g 4T1g 4T2g 2T2g 2T1g 2Eg4A2g Energy (eV) A M1 M2 C N H2O (b) (c) (a) (a) (b)

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111 Figure 4 3 Measurement of magnetic ordering temperature in Cs2.8Ni4[Cr(CN)6]4 nH2O, Co4[Fe(CN)6]3 nH2O, K3Cr(CN)6,and K3Fe(CN)6. A derivative of the DC magnetic susceptibility, in an applied field of 100 G, as a function of temperature is shown to display magnetic transitions, from paramagnetic to either ferromagnetic or ferrimagnetic states. The K3Fe(CN)6 and K3Cr(CN)6 materials are seen to be paramagnetic from 2 K to 300 K. The Cs2.8Ni4[Cr(CN)6]4 nH2O and Co4[Fe(CN)6]3 nH2O materials show divergence of the magnetic susceptibility at ~13 K and ~ 90 K respectively. Table 4 1 Parameters to be used in modeling magnetization data for Cs2.8Ni4[Cr(CN)6]4 nH2O, Co4[Fe(CN)6]3 nH2O, K3Cr(CN)6,and K3Fe(CN)6. is the octahedral splitting parameter, B and C are electron repulsion parameters, is the spin orbit coupling parameter, is the orbital reduction factor, J is the superexchange parameter, and the ground states have been calculated based upon the previous parameters. Units are all in cm1, unless specified. 0 50 100 150 200 250 300 -400 -200 0 d /dT (emu / mol K)Temperature (K) K3Fe(CN)6 K3Cr(CN)6 Cs0.7Ni4[Cr(CN)6]4nH2O Co4[Fe(CN)6]3nH2O complex e e B C J ground state Co 2+ (NC) 6 11,000 0 3,667 885 4,253 515 0 1.5 13 K 4 T 2g Fe 3+ (CN) 6 35,000 1,000 10,333 535 4,219 460 0 1 2 T 2g Ni 2+ (NC) 6 11,000 0 3,667 1,044 4,594 630 +90 K 3 A 1g Cr 3+ (CN) 6 26,000 1,000 7,333 933 3,732 275 4 A 1g

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112 Figure 4 4 Magnetic properties of K3Cr(CN)6. (a) T versus T and (b) magnetization versus field for K3Cr (CN)6, from a SQUID magnetometer experiment ( ), and model calculations ( ). Figure 4 5 Magnetic properties of K3Fe(CN)6. (a) T versus T and (b) magnetization versus field for potassium ferricyanide, from a SQUID magnetometer experiment ( ), a simple model with only Zeeman splitting of the 2T2g ground state ( ), and a model including both spinorbit interaction and Zeeman splitting of the ground state ( ). A reduction parameter of 3/4 was found to fit best. It is clear that even qualitative features of the data cannot be fit without spin orbit coupling. 0 50 100 150 200 250 300 1.7 1.8 1.9 2.0 T (emu K/mol)T (K) 0 10 20 30 40 50 60 70 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 M (emuG/mol)H (kG) (a) (b) 0 50 100 150 200 250 300 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 T (emu K/mol)T (K) 0 10 20 30 40 50 60 70 0 1000 2000 3000 4000 5000 6000 M (emuG/mol)H (kG) (a) (b)

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113 Figure 4 6 Magn etic properties of Cs2.8Ni4[Cr(CN)6]4 nH2O (a) Magnetization versus temperature, (b) T versus temperature, and (c) magnetization versus field for Cs2.8Ni4[Cr(CN)6]4 nH2O from a SQUID magnetometer experiment ( ), a mean field model taking into account interacting spins ( ), and a model neglecting spinspin interactions ( ). Figure 4 7 Magnetic properties of Co4[Fe(CN)6]3 nH2O (a) Magnetization versus temperature, (b) T versus temperature, and (c) magnetization versus field for Co4[Fe(CN)6]3 nH2O (actually normalized to Co1.5[Fe(CN)6] nH2O to compare directly to literature), from a SQUID magnetometer experiment ( ), a model without orbital moments( ), and different models having degrees of quenching of the orbital moments, with full moments on C o and Fe ( ), reduced orbital momentum on the Fe ( ), reduced orbital momentum on the Co ( ), and reduced orbital moments on both ions( ). Here, bold lines correspond to ferromagnetic models and thin lines correspond to ferrimagnetic models. 0 20 40 60 80 100 120 0.0 0.2 0.4 0.6 0.8 1.0 1.2 M (103 emu G / mol)T (K) 100 150 200 250 300 0 2 4 6 8 10 12 14 16 18 M T (emu K / mol)T (K) 0 10 20 30 40 50 60 70 0 10 20 30 M (103 emu G / mol)H (kG) 0 5 10 15 20 25 30 0.0 0.5 1.0 1.5 2.0 2.5 M (103 emu G / mol)T (K) 100 200 300 3 4 5 6 7 M T (emu K / mol)T (K) 0 10 20 30 40 50 60 70 0 2 4 6 8 10 12 14 16 M (103 emu G / mol)H (kG)

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114 CHAPTER 5 COBALT HEXACYANOFERR ATE NANOPARTICLES 5 Nanotechnology represents an exciting new frontier of science, and the study of nanoparticles is bound to uncover additional scientifically and technologically relevant phenomenon in the coming years Nanoparticles are materials between a few nanometers and a few hundred nanometers, and often behave differently from microparticles and bulk crystals of the same material For this reason, nanoparticles of the photoactive Prussian blue analogue cobalt hexacyanoferrate were studied and presented in this thesis. Different interstitial cations were used to investigate different material properties, as the choice of rubidium, potassium, or sodium has subtle effects on the magnetization of the host material These results for the nanoparticles are best understood within the greater context of past studies on similar bulk materials. The investigation of the magnetism of Prussian blue, Fe4[Fe(CN)6]3nH2O, and related analogues has a rich history [ 59 ] [60 ] dating back to 1928 [ 61 ] Measurements down to liquid helium temperatures identified the transition in Prussian blue to be long range ferromagnetic order [ 62 ] [ 63 ] [ 64 ][ 6264], but an understanding of the magnetic interactions remained elusive until the 1970s, when x ray [ 65] and neutron [ 55 ] diffraction data identified the crystal structure and the spin transfer from highspin Fe3+ to low spin Fe2+. Subsequent to this identification, the class of isostructural, face centered cubic cyanometallates were dubbed Prussian blue analogues, Fig ure 51 Interest in Prussian blue analogues was renewed by the 1996 discovery of long lived, photoinduced magnetism in K0.2Co1.4[Fe(CN)6] 6.9H2O [ 1 ] A flurry of experimental and theoretical research has elucidated the fundamental nature of the phenomena in threedimensional bulk materials [ 66 ] [67 ] In the following a brief

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115 summary of the photoinduced magnetization mechanism elucidated by these studies will be presented. Magnetic transitions in cobalt hexacyanoferrate systems depend strongly on the single ion parameters introduced in Section 3.1. Iron is in a strong ligand field of carbon and is therefore found to be either Fe3+ (LS, S = 1/2) or Fe2+ (LS, S = 0). Cobalt is in an intermediate ligand field of nitrogen and is found to be either Co2+(HS, S = 3/2) or Co3+(LS, S = 0 ) The nomenclature of the following is such that pairs displaying defined local minima in configuration space with respect to energy have a 0 appending their label and short lived metastable states are appen ded with a prime. Co3+Fe2+(SCo = 0, SFe = 0, Stot = 0) is the diamagnetic LS0 pair and Co2+Fe3+(SCo = 3/2, SFe = 1/2, Stot = 1) is the ferrimagnetic HS0 p air with a superexchange through the bridging cyanides. Co3+Fe2+(SCo = 1, SFe = 0, Stot = 1) is the intermediate HS pair and Co2+Fe3+(SCo = 1/2, SFe = 1/2, Stot = 0) is intermediate the LS p air Other pairings, appearing to be oxidized and reduced species when compared to HS0 and LS0, exist mainly at the boundary of HS and LS pairs and at surfaces where metal coordination numbers change. T here are also structural differences between the different pairs wi th LS0 being the shortest (~10 Co Co) and HS0 being the longest (~10.2 Co Co) [ 68] The ability to increase magnetization with blue light and decrease it with red light can be understood as changing the relative populations of LS0 and HS0 in the sample, Figure 5 2 To in crease the magnetization in K0.2Co1.4[Fe(CN)6] 6.9H2O an LS0 state is irradiated with blue light, exciting an elect ron from Fe to Co, giving LS The L S state then transitions to the meta stable HS0 state, separated from LS0 by an energy barrier

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116 of the order of 1 eV [ 1 ] [ 69] To decrease the magnetization in K0.2Co1.4[Fe(CN)6] 6.9H2O, HS0 regions are irradiated with red light, exciting an electron from Co to Fe, giving HS The HS state then transitions to the metastable LS0 state, separated from HS0 by an energy barrier of the order of 1 eV [ 1 ] [ 69 ] In this chapter, starting with Section 5.1, nanoparticles of rubidium cobalt hexacyanoferrate will be discussed, with specific attention to finite size effects on the photoinduced magnetism and magnetically ordered states. In the subsections of 5.1, synth esis and chemical composition (5.1.2), structure (5.1.3), and magnetization (5.1.4) will be presented and discussed (5.1.5). Second, Section 5.2 compares nanoparticles and bulk powder of cobalt hexacyanoferrate. This potassium cation system is interesting because it allows for clear thermal transitions and thermal quenching of magnetic states in addition to photoinduced magnetization. Within Section 5.2, synthesis and chemical composition (5.2.2), structure (5.2.3), and magnetization (5.2.4) are to be pr esented and discussed (5.2.5). Finally, in Section 5.3, sodium cobalt hexacyanoferrate nanoparticles showing large thermal hysteresis are discussed to show the size dependence of this effect. 5.1 Nanoparticles of Rubidium Cobalt Hexacyanoferrate Nanopar ticles of rubidium cobalt hexacyanoferrate (RbjCok[Fe(CN)6]l nH2O ) were synthesized using different concentrations of the organic ligand polyvinylpyrrolidone (PVP) to produce four different batches of particles with characteristic diameters ranging from nominally 3 to 13 nm. Upon illumination with white light at 5 K, the magnetization of these particles increases. In addition, the magnetic properties, namely the long range ferrimagnetic ordering temperature ( TC) and

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117 the coercive field ( HC), of the nanopar ticles evolve with preparation protocol. At 2 K, particles with diameters less than 10 nm are in the superparamagnetic limit. This work was published, in part, in the New Journal of Physics ( http://www.iop.org/EJ/journal/NJP ). Those sections contained w ithin the NJP article are copyright of the IOP (copyright release form in Appendix C ) and the online abstract can be found at http://www.iop.org/EJ/abstract/13672630/9/7/222/ [ 8 ] 5.1.1 Introduction Numerous efforts to synthesize nanoparticles of Prussian blue analogues have been made, but only a few examples of photoinduced magnetism have been reported, including work that isolated KjCok[Fe(CN)6]ln H2O particles, with typical diameters of 8 10 nm, within a silica xerogel [ 6 ] and other research producing 11 nm 70 nm nanorods of Mo(CN)8Cu2 protected by polyvinylpyrrolidone (PVP) [ 7 ] In each case, although photoinduced magnetism was observed, the particles did not exhibit long range order. This section reports RbjCok[Fe(CN)6]l nH2O nanoparticles, protected by PVP, that exhibit photoinduced magnetism for all sizes and that possess long range ordering, with coercive fields ranging between 0.25 1.5 kG, in the larger particles. From the data, the superparamagnetic limit at 2 K is identified, and the magnetic signal generated from particles in this limit can be estimated for the different batches of particles. 5.1.2 Synthesis and Chemical Composition The nanopar ticles were synthesized by Dr. Franz Frye, modifying the procedure described by Uemura and coworkers [ 70] [ 71 ] The Prussian blue analogue powder is encapsulated in a polyvinylpyrrolidone polymer (PVP) during synthesis. By varying the

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118 amount of PVP ( Tabl e 5 1 ), the protocol produced specimens with different sizes and distributions. After 30 minutes of stirring, the solution was allowed to sit for one week. To isolate the particles, three volumes of acetone were added to the synthesis solution, which was centrifuged, and then further washed with acetone and dried under vacuum. Chemical analysis was obtained from a combination of CHN combustion analysis and inductively coupled plasma mass spectroscopy (ICP MS), and the resulting formula are listed in Tabl e 5 1 along with the ratio of the PVP repeat unit per cobalt. The concentration of H2O was estimated by considering the measured Fe vacancies and charge balance. 5.1.3 Structure In order to characterize the nanoscopic nature of the samples transmission electron microscopy (TEM) and fourier transform infrared (FT IR) spectroscopy was performed. Other techniques that would provide analysis of large sample sizes such as dynamic light scattering of samples in suspension, were tried but lacke d the requisite resolution for the size regimes and distributions of interest 5.1.3.1 Transmission e lectron m icroscopy For the transmission electron microscopy (TEM) studies, a 50 L aliquot of the suspension was diluted 2000 times, and 8 L of the dilute d suspension was placed on a holey carbon grid. Selected area electron diffraction was compared to powder x ray diffraction patterns to confirm the structure [ 72 ] Using Image J imaging software [ 73 ] the TEM images were analyzed to obtain the particle s ize distributions shown in Figure 5 3 similar size distributions have been obtained for Prussian blue nanoparticles

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119 protected by PVP [ 74] These data were fit to a log normal function that yielded the characteristic diameters shown in Table 5 1 5.1.3.2 Fourier t ransform i nfrared s pectroscopy FT IR was performed on samples analogous to those studied in more detail in the following subsections, Figure 5 4 Similar synthesis protocols yielded batches with characteristic diameters, d, of d ~ 240 nm (a bulk powder ) d ~ 13 nm ( nanoparticles analogous to Batch D ) and d ~ 3 nm nanoparticles ( analogous to Batch A ). It was necessary to synthesize new samples as these studies were performed after the original samples had degraded over time. The FT IR spectrum of the pure cobalt hexacyanoferrate displays peaks near 2163, 2120, 2090, and 2040 cm1, corresponding to the cyanide stretches of the Co2+Fe3+ (HS), Co3+Fe2+ (LS), Co2+Fe2+ (reduced) and linkage isomerized Co2+Fe2+ phases, respectively [ 75] As the size of the particle is reduced, the intensity of the HS peak near 2163 cm1 decreases, while that of the LS and reduced peaks near 2120 and 2090 cm1 emerge and grow. 5.1.4 Magnetization The quantity of central interest is magnetization. To this end, a standard commercial SQUID magnetometer was employed. The samples were mounted to commercial transparent tape and could be irradiated with light from a room temperature, halogen source by using a homemade probe equipped with a bundle of optical f ibers [ 76 ] Background contributions from the holder and tape are independently measured and subtracted from the data. 5.1.5.1 DC s usceptibility The temperature dependences of the DC magnetic susceptibilities, ( T ), of the four batches of particles are shown in Figure 5 5 The magnetic signals are expressed

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120 per mole of Prussian blue analogue (PBA), Table 5 1 The dark state ZFC data were obtained after cooling in zero applied field from 300 K, while the dark stat e FC data were taken after cooling in 100 G from 300 K. The light state was established after field cooling the samples from 300 K to 5 K in 100 G and subsequently irradiating with light for 5 hours. The light state FC data were obtained after cooling fr om 30 K in 100 G. All samples reported show a photoinduced increase in their magnetic signals and ordering temperatures, and the strength of the change is correlated with the size of the particles. The differences between the FC susceptibilities of the li ght and dark states, = light FCdark FC, are plotted in the insets of Figure 55; finite values can only arise from the photoinduced magnetism. 5.1.5.2 DC m agnetization The field dependences of the DC magnetization, M( H ), of the four batches of particles are shown in Figure 5 6 The magnetic signals are expressed per mole of Prussian blue analogue (PBA), Table 5 1 The dark and light states are the same as for the DC susceptibility measurements. All samples reported s how a photoinduced increase in their coercive fields and saturation magnetizations, and the strength of the change is correlated with the size of the particles. 5.1.5.3 AC s usceptibility The temperature dependences of the real ( ) and imaginary ( ) AC susceptibilities of the ZFC dark states, i.e. dark states, of all four batches are shown in Figure 5 7 The phenomenological parameter for frequency dependence of the peak is = T fTf ( log ) 5 1

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121 where Tf is the freezing temperature given by the cusp in ( T ) and is the angular frequency, is 0.024 0 .004 for batches C and D, and this observation is consistent with spin glass or cluster glass behavior [ 77] [ 78 ] 5.1.5 Discussion It is important to recall that the photoinduced magnetic properties of the Prussian blue analogues depends upon a spin crossover effect and the presence of vacancies that allow crystalline flexibility [ 57] [ 7 9 8 5 ]. [ 79] [ 80 ] [ 81 ] [ 82 ] [ 83 ] [ 84 ] [ 85 ]More specifically for the RbjCok[Fe(CN)6]l nH2O nanoparticles, the spins can exist in either of two arrangements. The low spin state consists of Fe2+ ( S = 0 ) and Co3+ ( S = 0 ), while the highspin state possesses Fe3+ ( S = 1/2) and Co2+ ( S = 3/2). Depending on the local chemical environment due to the values of j k, l and n in the chemical formula, the Fe and Co spins can be locked into either their highspin or low spin states for all accessible temperatures. Alternatively, with the proper tuning of constituent elements, the Fe and Co ions can exist in highspin states at room temperature and then experience a crossover to their low spin states at approximately 150 K. This spin crossover phenomenon prepares the system for the possibility of experiencing photoinduced magnetism. However, since the spin crossover effect may not be 100% efficient, some regions remain locked in their highspin states, giving rise to the magnetic signals observed for the dark state of the particles. When irradia ted, the low spin regions are photoinduced to the highspin magnetic state, resulting in a growth of the magnetic domain. This scenario is supported by the frequency dependent AC susceptibility studies, and by the local probe investigations of others [ 68] [ 86 ]

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122 Two main features, onset of long range magnetic order and increasing net magnetization, are important when considering the evolution of material properties due to the increasing average size of the separate batches. The ordering temperatures of the particles and the magnetic susceptibility are both seen to decrease as particles become smaller, Figure 55. This scaling of magnetization is linked to an increased diamagnetic surface to magnetically active volume ratio at smaller particle sizes. This contention is supported by the FT IR spectra, which show an increase in the LS and reduced content as particle size decreases, although the precise location of these moieties cannot be gleaned from such methods. At low temperatures, the highspin Fe and C o ions interact antiferromagnetically, giving rise to a ferrimagnetic transition at TC [ 87] For the magnetic data shown in Figure 55 and Figure 58, the onset of this transition can be estimated, and these macroscopic temperatures are listed in Table 5 2 Therefore, it is plausible that particles larger than a critical size will allow domains large enough to approach bulk like magnetic properties. Conversely, smaller particles may put limits on allowed domain size, suppressing the ordering temperature. Microscopically, if the size of the magnetic domains is less than or of the order of the magnetic coherence length, then a spectrum of TC values can be expected until the superparamagnetic limit is achieved. Consider the magnetic properties of the sampl es presented in conjunction with the TEM analysis. Batch A has no observed coercivity and follows Curielike behavior. Batches B and C show a combination of Curietail and partial ordering with a reduced Tc (Figure 5 5 and 58), as well as finite coercive fields. Finally, the active sites in Batch D are almost entirely ferrimagnetically ordered with the largest coercive field of all batches

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123 presented. In addition, the differences between the FC and ZFC data for the dark stat e in batch D is consistent with spin glass or cluster glass behavior [ 77 ] [78 ] in accord with the presence of large magnetic domains. Based upon fitting the macroscopic magnetization data available in the Curielike contributions at low temperature, the s uperparamagnetic contribution for each of the four batches of nanoparticles (smallest to largest) is 100%, 90%, 50%, and 10%. Consequently, at least down to 2 K, nanoparticles with sizes below 10 nm are in the superparamagnetic limit. These interpretati ons are consistent with the M versus H measurements performed at 2 K, where significant coercive fields, HC, and remnant magnetization values are observed for the two largest sets of particles but not for the two smallest sets of particles, Figure 5 6 and Table 5 2 It is interesting to note that even at 70 kG, there is still a finite slope to the magnetization, and this slope actually gets larger with particle size. Two plausible explanations for this behavior exist: (1) there is local flipping of minor Fe spins to be aligned with the increasing field, without a clear spinflip field due to the disordered nature of the magnetism, and (2) there is a field dependent magnetic moment on either the cobalt or iron sites due to the first order angular momentum not being completely quenched by the crystal environment (as detailed in Chapter 4). 5.1.6 Conclusions In conclusion, four different sizes of RbjCok[Fe(CN)6]l nH2O nanoparticles protected by PVP were synthesized and characterized. Each batch of particles is photoinducible, but the strength of this effect, as well as other global properties, e.g. TC and HC, are correlated with the intrinsic particle size distributions of each batch. The

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124 combinati on of photoinduced magnetism and nanosized Prussian blue analogue particles with finite coercive fields is unique and establishes a length scale limit of 10 nm diameter for these properties. Since publication, these findings have been used to understand the results of additional nanoparticle studies [ 88 ] [ 89 ] 5.2 Nanoparticles of Potassium Cobalt Hexacyanoferrate Nanoparticles and bulk powder of potassium cobalt hexacyanoferrate (KjCok[Fe(CN)6]l nH2O ) were synthesized by changing the density of reactants in solution, without using surfactant, excluding additional experimental uncertainties from estimating the amount of surfactant present. Bulk powder with characteristic diameters, d, of d ~ 200 nm and nanoparticles of d ~ 27 nm were studied. Both sizes show photoinduced magnetization and the ability to trap magnetic states by thermal quenching. However, the magnetic properties are modified in the nanoparticles, showing less total magnetization, greater magnetic coercivities, and longer isothermal relaxation constants. In addition, macroscopic differences between photoinduced and thermally quenched low temperature magnetic states of KjCok[Fe(CN)6]l nH2O are presented for the first time. Magnetic data is complemented by infrared spectroscopy, transmissio n electron microscopy, x ray powder diffraction, and temperature dependent neutron diffraction. This wor k is expected to be submitted for publication at a later date 5.2.1 Introduction While there are reports of photoswitchable nanoparticles by other groups [ 6 ] [7 ] as well as a study of rubidium cobalt hexacyanoferrate nanoparticles that the author co authored [ 8 ] these studies focused mainly on the low temperature magnetically ordered state. However, charge transfer induced spin transitions (CTIST) between the high spin (HS) and low spin (LS) states can also be incited by changing the

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125 temperature [ 1 ] The relevant states to be considered for these transitions are summarized in Table 5 3 and Table 5 4 In the high temperature limit (~300 K), most Co NC Fe pairs will be in a HS (S = 1) state. Upon reaching a sufficiently low temperature (~200 K), Co NC Fe pairs begin transitioning to the LS state, and the process can be summarized as Co3+ ( S = 0) Fe2+ ( S = 0) Co2+ ( S = 3/2) Fe3+ ( S = 1/2) ( low temperature) (high temperature). The K cation samples are of particular interest because it was shown that in addition to photoinducing, thermal quenching can trap variable amounts of HS pairs [ 9 ] [ 76 ] In the extreme limit of cyanide bridged molecular cobalt iron, it was found that quenched and photoinduced states are identical [ 90] While the quenched and photoinduced states in Na cation, bulk powders have been studied by x ray diffraction [ 68 ] and quenching has been studied with magnetic susceptibility, M ssbauer spectroscopy [ 91] and specific heat [ 92 ] no systematic study has been presented on the macroscopic measurements of such a material or of how finite size effects might play a role and furt her elucidate the fundamental mechanisms involved. This thesis chapter reports on KjCok[Fe(CN)6]l nH2O (henceforth K Co Fe) nanoparticles, synthesized without surfactant, that exhibit photoinduced magnetism and thermal quenching of magnetic states for ch aracteristic diameters, d, of d ~ 27 nm and d ~ 200 nm batches of nanoparticles. Clear modifications of the magnetic properties

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126 with size are observed and macroscopic differences between the photoinduced and thermally quenched magnetic states are detected. 5.2.2 Synthesis and Chemical Composition Bulk K Co Fe Prussian blue analog ue powders were prepared by Dr. Justin E. Gardner and Matthew J. Andrus, using procedures previously described by Shimamoto and coworkers [ 84] N anoparticles of K Co Fe Prussian blue analog ues were synthesized Dr. Justin E. Gardner and Matthew J. Andrus, by modifying the procedures previously reported by Yamada and coworkers [ 93] In order to estim ate the chemical formulae, EDS and CHN were performed on the samples and the results are part of Table 5 5 5.2.2. 1 Fourier t ransform i nfrared s pectroscopy Infrared spectroscopy of cobalt hexacyanoferrates are typically performed in the energy region of the cyanide stretch of the compound, where the structure evolves due to th e changing oxidation states of the coordinating metals of the cyanide, Figure 59. The first peak at 2160 cm1 represents the Fe3+(LS) CN Co2+(HS) stretch, while the second broad peak is a combination of the Fe2+(LS) CN Co3+(LS) and Fe2+(LS) CN Co2+(HS) stretches which appear at 2115 cm1 and 2090 cm1, respectively. Nanosized powders are known to exhibit spectra with a smaller Fe3+/Fe2+ ratio [ 8 ] [ 94 ] Results of fitting the FT IR spectra are detailed in Table 5 5 5.2.3 Structure The macrostructure o f the samples were characterized with HR TEM, in order to estimate the average particle size. XRD at room temperature was performed to show the structure of the heavier elements. Temperature dependent neutron diffraction from 5 K to 300 K was performed on the d ~ 27 nm nanoparticles to probe the structural

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127 transition and to look for magnetic scattering. While structural transitions correlated to changes in the magnetic states have been observed in the bulk, they have yet to be reported for samples showing finite size effects. 5.2.3.1 Transmission e lectron m icroscopy For the transmission electron microscopy (TEM) studies, a 50 L aliquot of the suspension was diluted 2000 times, and 8 L of the diluted suspension was placed on a holey carbon grid. The TEM images were analyzed by printing on standard 8.5 in x 11 in paper and measuring particles by hand with a digital caliper to obtain the particle size distributions shown in Figure 5 10 and similar size distributions have been obtained for Prussian blue nanoparticles protected by PVP [ 8 ] [ 74] This size analysis method was found to yield the same results as the previous method in which a computer program was used, however the paper method involved less eyestrain and the ability to perform the measurements while enjoying Florida sunshine. These data were fit to a log normal function that yielded the characteristic diameters shown in Table 5 6. 5.2.3.2 X r ay d iffraction To investigate the lattice constants and crystal structure, a Philips APD 3720 powder diffractometer, housed in the Major Analytical Instrument Center at the Univesity of Florida, was used to perform room temperature x ray diffraction (XRD) using a Cu K source. It is worth noting that two lines, 1.54056 and 1.54441 are present for th e K edge of Cu, and stripping of the weaker K 2 line was performed, however the line widths are so large for these samples that such an analysis is not necessary. Diffraction studies on similar compounds have assigned the HS unit cell to ~10.3 and

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128 the LS unit cell to ~10.0 [ 68] As extended x ray fine structure (EXAFS) measurements have shown that the CoN distance is most sensitive to the changing oxidation states of a Co NC Fe pair [ 95 ] [ 96] the Fe3+(LS) CN Co2+(HS) and Fe2+(LS) CN Co2+(HS) moietie s should have similar lengths near ~10.3 Between 1020 mg of the same samples used for all other characterizations were mounted on glass slides and pressed onto squares of doublesided cellophane tape of ~ 2.3 cm2. The room temperature x ray powder diffractograms, shown in Figure 5 11, were used to model the structure by a Rietveld refinement using the EXPGUI [ 53 ] interface for GSAS [ 52 ] In order to approximate the complicated Prussian blue analogue structure, a single phase model w ith 3 (N o. 225) space group symmetry was used. Specifically, the cobalt and nickel atoms were forced to occupy the same site. Atomic occupancies were set by the experimentally determined chemical formulas, excepting the oxygen atoms of the interstitial waters t hat were allowed to vary as the samples may have dehydrated or hydrated between synthesis and diffraction. The same site symmetries as in Prussian blue were used, where the iron vacancies were replaced by the six coordinated oxygen atoms of the ligand wat er molecules [ 65] Placement of the oxygen atoms of the interstitial water molecules at the 32 f Wyckoff position [ 68] and a relatively small percentage at the 192l position was found to yield a robust local minima during refinement procedure. The bulk pow der shows one phase, with a lattice constant of 10.35 and a FWHM of 0.12 for the (2, 2, 0) reflection. The nanoparticles show two phases, with lattice constants of 10.31 and 10.07 and a FWHM of 0.5 for the (2, 2, 0) reflection.

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129 5.2.3.3 Neutron d i ffraction As compared to x ray diffraction, neutron diffraction is more sensitive to light elements and may also scatter off local magnetic moments in a lattice. In order to probe the structure of the quenched phase in nanoparticles, as compared to the bulk materials that have already been studied [ 68 ] 5 grams of deuterated K Co Fe were used. A full neutron powder diffraction pattern of the nanoparticles gives the same unit cell parameters as x rays, Figure 5 12. Quenching the sample by quickly inserting into a liquid helium filled cryostat ( T/ time ~ 100 K/min) shows clear changes in the diffraction pattern, corresponding to the thermally quenched state (Q). Upon warming to 200 K, a transition of the unit cell to the low temperature ground state (G) c an be seen, Figure 5 13 (a). Conversely, by slowly cooling the sample from room temperature at ~1 K/min, a transition from the high temperature equilibrium phase (RT) to the G state is observed, Figure 5 13 (b). These peaks can be fit with a three phase model analogous to that used for the infrared measurements, with the HS, LS, and reduced components taken into account. Unfortunately, the neutron data is more convoluted due to the HS and reduced phases having the same lattice constant and the overlap of the lines due to the structural disorder in the samples. To mitigate this issue, the LS component was fit with a temperature independent lattice constant, and this assumption is corroborated by a set of fits done with free parameters showing no clear trend in the LS unit cell parameter. On the other hand, the HS unit cell parameter shows a clear dependence on the high spin fraction and must be allowed to vary. Although both a Lorentzian and Gaussian character is present in the peaks, and in fact the Gaussian nature is

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130 somewhat stronger, a Lorentzian shape was used to fit the temperature dependence to avoid insurmountable variable covariance as the HS/ reduced and LS lines get close at lower temperatures. The results of fitting the lines yield the temperature dependences of the lattice constants, Figure 5 13 (c) and the amount of scattering associated with each unit cell, Figure 5 13 (d). The full width half maximum values did not shown any clear trends above the noise. Finally, an attempt was made to observe magnetic scattering of the nanoparticles, in order to elucidate the magnetic structure. The expected nature of the scattering was modeled by positioning point spins on the nuclear positions of the metals and comparing the magnetic signal for parallel and antiparallel alignment, Figure 5 14 (a). A main result of the model calculations is that the relative intensity of the first two peaks is different for the ferrimagnetic versus ferromagnetic states. However, any magnetic scattering present, as potentially evidenced by the difference between the spectra at 5 K in the magnetically ordered state and 30 K in the paramagnetic state, was smaller than the experimental resolution of the setup used, Figure 5 14 (b). In addition, no difference was obs erved between 0.5 T and 5 T at 5 K. 5.2.4 Magnetization M agnetization is a key parameter to further probe the quenched state (Q), low temperature ground state (G), and photoinduced states (P), which are defined explicitly in Table 5 4 To this end, a standard commercial SQUID magnetometer was employed. For photoinduced measurements, the samples were mounted to commercial transparent tape and could be irradiated with light from a room temperature, halogen source by using a homemade probe equipped with a bu ndle of optical fibers [ 76] Background contributions from the holder and tape are independently measured and

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131 subtracted from the data. For the quenching measurements, in order to achieve better signal to noise in the paramagnetic states, gelcaps were packed with powder and mounted on a standard sample holder. Backgrounds from the gelcaps were subtracted based upon the mass susceptibility of an analogous gelcap. 5.2.4.1 Quenched h igh t emperature DC s usceptibility The temperature dependences of the DC mag netic susceptibility temperature product, ( T ) T in the paramagnetic limit are shown in Figure 515. The magnetic signals are expressed per mole, Table 56. Quenching was achieved by stabilizing the cryostat temperature to 100 K, and quickly inserting t he sample. While infinitely slow cooling will reach the true G state, this was realized experimentally by quenching to 100 K, warming to 200 K at less than 1 K/min and subsequently cooling again. Both samples show a clear trapping of magnetic states with thermal quenching, and the ability to trap the magnetic states is correlated with the size of the particles. In fact, the room temperature HS content of the samples, nHS, is strongly affected by t he particle size. 5.2.4.2 Quenched l ow t emperature DC s usceptibility The temperature dependences of the DC magnetic susceptibilities, ( T ), in the ordered state before and after thermal quenching are shown in Figure 5 16. The magnetic signals are express ed per mole, Table 56. Quenched states were achieved by stabilizing the cryostat temperature to 100 K, and inserting the sample. The G state was reached by quenching to 100 K, warming to 200 K at less than 1 K/min, and subsequently recooling. The ZFC data were obtained after cooling in zero applied field from 100 K, while the dark state FC data were taken after cooling in 100 G from 100 K.

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132 Both samples show an increase in their magnetic signals as more spins are thermally trapped by quenching, and th e strength of the change is correlated with the size of the particles. Insets of Figure 5 16 show a weak increase in the magnetic ordering temperature for the Q states compared to the G states. 5.2.4.3 Quenched l ow t emperature m agnetization The temperatur e dependences of the DC magnetization, M( H ), before and after thermal quenching are shown in Figure 5 17. The magnetic signals are expressed per mole, Table 56. The history of the cooling is the same as for the DC susceptibility measurements. Both samples show clear increases in their high field magnetization with quenching, and a weak increase in the coercive fields. The ~27 nm nanoparticles have a much larger coercive field than the ~200 nm nanoparticles. 5.2.4.4 Isothermal r elaxation The time dependences of the DC magnetic susceptibility temperature product, ( T ) T of the quenched states in the paramagnetic limit are shown in Figure 5 18. The magnetic signals are expressed per mole, Table 56. Thermal quenching was achieved by stabilizing the cr yostat temperature to 100 K, and quickly inserting the sample. Three different isothermal relaxation temperatures were chosen for each sample. Both samples show nonexponential relaxation of the metastable Q state at elevated temperatures, and the nanoparticles show elongated relaxation times. 5.2.4.5 Photoinduced l ow t emperature DC s usceptibility The temperature dependences of the DC magnetic susceptibilities, ( T ), in the ordered state before and after photoirradiation are shown in Figure 5 19. The magnetic signals are expressed per mole, Table 56. The light state was established after field cooling the samples from 300 K to 5 K in 100 G and subsequently irradiating with light

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133 for 5 hours. The ZFC data were obtained after cooling in zero ap plied field from 100 K, while the dark state FC data were taken after cooling in 100 G from 100 K. The light state FC data were obtained after cooling from 30 K in 100 G. Both bulk and nanoparticle samples showed a dramatic increase in the magnetic orderi ng temperature. The nanoparticles show a reduced ordering temperature compared to the bulk powder due to finite size effects. The differences between the FC susceptibilities of the P and G states are plotted in the insets of Figure 5 19. 5.2.4.6 Photoinduced l ow t emperature m agnetization The temperature dependences of the DC magnetization, M( H ), are shown in Figure 5 20. The magnetic signals are expressed per mole of Prussian blue analogue (PBA), Table 5 6 The history of the cooling is the same as for the DC susceptibility measurements. Both samples show clear increases in their high field magnetization with quenching, and a weak increase in the coercive fields. The ~27 nm nanoparticles have a much larger coercive field than the ~200 nm nanopa rticles. 5.2.5 Discussion First, details of the models used to interpret the data will be introduced. Second, the size dependence of the quenching effect will be discussed. Third, the difference between the photoinduced and quenched states, and particularly how the nanoparticles add to this understanding, will be given. Finally, a schema of the particles that can qualitatively reproduce the previous points will be presented. 5.2.5.1 Details of m odeling While it is possible to model the complicated system of temperature dependent moments and superexchange interactions using a combination of ligand field theory and mean field theory, a more transparent and simple approach will be applied for the

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134 analysis of the magnetization data presented. A few assumptions are necessary for the simple model and are (1) T versus T is assumed to be linear in the range from 100 K to 300 K, (2) the oxidation states at room temperature are taken from the infrared spectroscopy measurements, (3) the ground state in the bulk reached by quenching and subsequently slow cooling, is assumed to have transitioned all switchable Fe ions into the low spin state, and (4) a linear dependence of the susceptibility on the highspin fraction is assumed. The validity of assumption (1) is clear, to a high degree, by inspection of the data. Assumption (2) is only an assumption in the sense that the exctinction coefficients of the different moieties are not exactly known. Assumption (3) is the biggest conjecture, however, microscopic pr obes on analogous bulk samples show that complete transition is plausible [ 68 ] The final assumption, the linearity of the effective moment with respect to the highspin fraction, may not be obvious. However, this temperature dependence can be justified from mean field theory predictions by using a plausible set of parameters and plotting the effective moment as a function of the highspin fraction, Figure 5 21, where the approximate linearity becomes clear. The results of the aforementioned assumptions, is a semi empirical expression for T as a function of the highspin fraction

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135 T ( nHS ) = ( 1 996 + 2 591 n HS ) 1000 T + ( 0 407 + 5 002 nHS ) 5 2 In order to study the bi stability in the paramagnetic region, an Ising like model for the charge transfer induced spin transition is used. For this approach, the Hamiltonian is = J sisj < i j > T 2 ln g +g si i 5 3 where < i,j> is the sum over nearest neighbors, kB is the Boltzmann constant, T is the temperature, s is the pseudo spin that keeps track of the CTIST state of a pair, takes into account the configuration interaction due to electronelectron interaction and the ligand field, g+ is the degeneracy of the HS state and g is the degeneracy of the LS state. Later, for co nvenience, g will be defined as the ratio Using a Bethe Peierls Weiss approximation, this equation can be solved to get the highspin fraction, nHS, and the relative population of the mixing of the states, nH L [ 97 ] In order to fit the magnetization curves, various constraints can be applied. To begin, the equilibrium temperature of the transition is well defined, Teq = 2 k ln ( g ) 5 4 Equation 5.4 can be used to constrain the parameters of Equation 5.3. First, Teq can be fixed accor ding to the T versus T data (~225 K), and the relative degeneracy of the states can be taken from specific heat measurements on similar materials (ln(g) ~ 12) [ 92 ] These constraints give a value for of 1315 K. From here all parameters can be fit simultaneously to give an activation energy of 5000 K, and an

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136 interaction energy of 90 K, with a fundamental relaxation rate of 1/ 0 = 1/(0.12 x 1012 minutes). Aside from scale, the temperature dependence of the magnetization is identical in the nanoparticles and the bulk powder. Based upon this similarity, it is logical to assume that the phenomenological equilibrium parameters are the same, rather than coincidentally modified in such a way as to give nearly identical macroscopic equilibrium states. Two models were tested by studying the relative homogeneity of the state, nH L. In the first model, the amount of highspin material was distributed through a nanoparticle volume, and in the second, the nanoparticle acted essentially as regions of a bulk powder and regions of locked pairs in reduced or LS states at all temperatures. It was found that the second model, having discrete regions of trapped and bistable material, had better fits to experimental data. However, in order to more properly fit the nanoparticle relaxation data, an additional temperature dependent interaction term had to be included in the fits to account for the extended relaxation times. This interaction is proposed to be between the bistable pairs and the locked reduced and LS regions in the sample. This association is consistent with the clear temperature dependence of the strain that was detected in lattice constants from the neutron scattering data. 5.2.5. 2 Size d ependence of t hermal q uenching To begin, the TEM images show clear differences in the sizes of the bulk and nanoparticle samples. Although the chemical formulae are similar, the FT IR displays the decrease in HS material in the nanoparticles at the expense of reduced and LS fractions Neutron powder diffraction is a microscopic probe that delineated between the HS/reduced phases and LS phase in the nanoparticle sample as a function of

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137 temperature and provided clear evidence of the strain induced on the lattice for different degrees of thermal quenching. From the paramagnetic susceptibility, nHS could be approximated, and the similarity of the equilibrium temperatures for the bulk and nanoparticles is evidence that the local chemical formula is similar for both samples. Although dramatic increases in the magnetic moment are seen with quenching, only small modifications of the coercive fields and ordering temperatures are observed, Table 5 7 It is interesting to note that the nanoparticles actually have a significantly larger coercive field, roughly 10 times, although their magnetic ordering tem perature is reduced. Isothermal relaxation experiments of the dynamics in the quenched state provide additional evidence for the similarity between the bulk and nanoparticle samples, while also displaying subtle differences due to the strain between the bistable and locked pairs in the nanoparticles. 5.2.5. 3 Photoinduced versus q uenched states Although the photoinduced and quenched states are identical on the molecular level [ 90] the resulting many molecule state in a lattice turns out to be different fo r quenched or photoinduced pai r s due to the interactions between molecular units. This difference can clearly be seen in the macroscopic magnetization, and the manner in which it changes under photoexcitation as compared to thermal quenching, Table 5 8 While the quenched states show little dependence of the ordering temperature and coercive field on the degree of quenching, photoexcitation brings dramatic changes to these parameters. Within the context of a meanfield picture, these differences may be d ue to better interconnectivity of the lattice in the photoinduced as opposed to quenched states. Also, in the photoinduced state, the difference in the ordering temperature of the bulk and nanoparticles becomes more pronounced, suggesting that the magneti c

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138 domains in the nanoparticles are being limited by structural constraints for the photoinduced states, but not in the quenched states. This observation is consistent with the current microscopic picture of smaller domains in the quenched as compared to p hotoinduced states for bulk materials [ 68] Furthermore, the nanoparticle data implies that the photoinduced domains in the bulk are larger than ~27 nm. In a similar way, the increased ordering temperature of the quenched nanoparticles implies the finite size is constraining trapped spins to be more interconnected in the nanoparticles. 5.2.5. 4 Resulting s chema of b ulk and n anoparticles Based upon the microscopic data already discussed, as well as the macroscopic magnetization data taken as a whole, Figur e 5 22, a qualitative model for the bulk and nanoparticles in the quenched and photoinduced states can be formulated, Figure 5 23 This model is consistent with the quenched and photoinduced structural domains proposed based upon microscopic x ray diffraction experiments [ 68], where the magnetism in quenched states is less connected than in the photoinduced states. A consistent extension of the previous bulk model is made for the nanoparticle samples, in which size effects and surface effects now play a role in how the macroscopic magnetization manifests itself based upon the microscopic domains. 5.2.6 Conclusions In conclusion, bulk powder and nanoparticles of KjCok[Fe(CN)6]l nH2O were synthesized and characterized. The experimental data was anal yzed using semi empirical methods to show clear trends suggesting that nanoparticles consist of a core shell type distribution of states. While the particles are large enough to display long range magnetic order, the amount of photoswitchable material is dramatically

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139 reduced. The coercive fields of the nanoparticles are enhanced more than ten times compared to the bulk. Fig ure 5 1 Prussian blue analogue structure. Prussian blue analogues have a chemical formula of AjM1k[M2(CN)6]l nH2O, j k, l and n are constrained by charge balance. Cations (A = Cs+, Rb+, K+, Na+) are incorporated based upon the number of M2 vacancies. M2 vacancies are coordinated by water as shown. T he distance between M1 and M2 is generally ~5 Figure 5 2 A detail of the photoexcitation process es in K0.2Co1.4[Fe(CN)6] 6.9H2O Strong field eg and t2g orbitals are used for simplicity. M1 M2 A H 2 O N C C N e g t 2g Co 3+ ( S = 0) Fe 2+ ( S = 0) LS0 e g t 2g Co 3+ ( S = 1) Fe 2+ ( S = 0 ) HS e g t 2g Co 2+ ( S = 3/2) Fe 3+ ( S = 1/2) HS0 e g t 2g Co 2+ ( S = 1/2) Fe 3+ ( S = 1/2) LS e g t 2g h a Intersystem crossing Intersystem crossing h b d xy d xz d yz d z 2 d x 2 y 2 oct

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140 Table 5 1 S ynthesis and chemical composition of rubidium cobalt hexacyanoferrate nanoparticles. Batch Starting PVP (g) Resulting chemical formula PVP:Co ratio Diameter (nm) A 1.0 Rb1.9Co4[Fe(CN)6]3.2 2O 360 3.3 0.8 B 0.5 Rb1.8Co4[Fe(CN)6]3.2 2O 200 6.9 2.5 C 0.2 Rb1.7Co4[Fe(CN)6]3.2 2O 60 9.7 2.1 D 0.1 Rb0.9Co4[Fe(CN)6]2.9 2O 20 13.0 3.2 Figure 5 3 TEM of RbjCok[Fe(CN)6]l nH2O nanoparticles. (left) Typical TEM images. (right) The particle distributions, normalized to the largest bin, versus diameter for the four batches of particles, see Table 5 1 The total number of particles for each distribution, smallest to largest, is 44, 27, 53, and 62, respectively. The solid lines are the results of log normal fits that provide the characteristic diameters shown for each distribution. 0 5 10 15 20 0.0 0.5 1.0 0.0 0.5 1.0 3.3 nm ( + 0.8) 0.0 0.5 1.0 6.9 nm ( + 2.5) 0.0 0.5 1.0 Normalized Number of Particles9.7 nm ( + 2.1) 13.0 nm ( + 3.2) Diameter (nm) Batch A Batch D 5 nm 5 0 nm Batch A Batch B Batch C Batch D

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141 A w / 2 e2 x xc w 2 (a) d ~ 240 nm bulk (b) d ~ 13 nm Batch D (c) d ~ 3 nm Batch A x c (cm 1 ) 2157 2122 2088 2156 2116 2088 2088 2120 2160 w (cm-1) 17. 6 18.41 22.3 15.7 35.9 45.2 42.8 38.6 15.4 A (arb. Units) 0.72 0.04 0.24 0.22 0.23 0.52 0.28 0.60 0.12 Figure 5 4 FT IR absorption spectra of RbjCok[Fe(CN)6]l nH2O nanoparticles. (a) d ~ 240 nm bulk powder, (b) d ~ 13 nm nanoparticles analogous to Batch D, and (c) d ~ 3 nm nanoparticles analogous to Batch A. Fits to Gaussian lines are shown for HS ( ), LS (), reduced ( ), and total intensity ( ), and the values of the fitting parameters are tabulated. 2200 2150 2100 2050 2000 Absorbance (arb. units)wavenumber (cm-1) 2200 2150 2100 2050 2000 Absorbance (arb. units)wavenumber (cm-1) 2200 2150 2100 2050 2000 Absorbance (arb. units)wavenumber (cm-1) (a) (b) (c)

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142 Figure 5 5 The temperature dependences of the low field, 100 G, susceptibilities of RbjCok[Fe(CN)6]l nH2O nanoparticles. The zerofield cooled (ZFC) dark (), field cooled (FC) dark (), and FC light () states of each batch produced are shown. The insets display the differences between the FC light and dark states, as described in the text. Finite values for this difference can only arise from photoinduced ma gnetism. 0.0 0.1 0.2 0.3 0 5 10 15 20 25 0.00 0.01 0.02 0.03 0 5 10 15 20 25 0.00 0.05 0.10 0.15 0 5 10 15 20 25 0.0 0.5 1.0 1.5 0 5 10 15 20 25 0 2 4 6 8 T (K) (emu / mol)0.0 0.2 0.4 0 2 4 0 5 10 15 20 25 30 0 10 20 30 (emu / mol)Temperature (K) (emu / mol) T (K) (emu / mol) T (K) (emu / mol) T (K) Batch A Batch B Batch C Batch D

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143 Figure 5 6 The T = 2 K magnetization versus magnetic field sweeps of RbjCok[Fe(CN)6]l nH2O nanoparticles. Here Batch C for (a) high fields and (b) the field region relevant to the hysteresis in both light () and dark () states are shown. In addition, Batch D for (c) high fields and (d) the field region relevant to the hysteresis in both light ( ) and dark ( ) states. The coercive fields, HC, for the light and dark states for each batch are listed in Table 5 2 and the lines are guides for the eyes. -60 -40 -20 0 20 40 60 -40 -20 0 20 40 -1.0 -0.5 0.0 0.5 1.0 -10 -5 0 5 10 -60 -40 -20 0 20 40 60 -40 -20 0 20 40 -10 -5 0 5 10 -20 -10 0 10 20 M (103 emu G / mol)H (kG) M (103 emu G / mol)H (kG) M (103 emu G / mol)H (kG) M (103 emu G / mol)H (kG) (a) (b) (c) (d) Batch C Batch D

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144 Figure 5 7 The temperature dependences of the real ( ) and imaginary ( ) AC susceptib ilities of RbjCok[Fe(CN)6]l nH2O nanoparticles. Zero field cooled dark states were measured with no applied stat ic field and an alternating field of 4 G, except for batch D, which was measured in 1G The frequency dependenc e w as studied at 1 Hz (), 10 Hz (), 100 Hz (), and 1 kHz () for all batches, except for batch D, which has an additional measurement a t 333 Hz () Arrows are guides for the eyes and pass through the peaks. Table 5 2 M agnetic properties of rubidium cobalt hexacyanoferrate nanoparticles. Batch Diameter (nm) TC dark (K) TC light (K) HC dark (G) HC light (G) A 3.3 0.8 < 2 < 2 < 10 < 10 B 6.9 2.5 ~10 ~13 ~15 ~30 C 9.7 2.1 13 17 250 330 D 13.0 3.2 19 22 1000 1500 0.0 0.1 0.0 0.1 0.0 0.1 0 5 10 15 20 0.0 0.5 1.0 1.5 0.0 0.1 0.2 0.3 0.0 0.5 1.0 0.0 0.2 0.4 0 5 10 15 20 0 5 10 15 Batch A Batch B Batch C '' (emu / mol)Batch D Temperature (K)Batch A Batch C (emu / mol) Batch B Batch D Temperature (K)

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145 Figure 5 8 Ordered magnetic components of the smaller batches. The ZFC () and FC () dark states show a signal associated with long range order for the two batches (Batch B and Batch C) displaying a mix of magnetic behavior after subtracting the Curielike contribution. Table 5 3 The microscopic states relevant to KjCok[Fe(CN)6]l nH2O microstate shorthand oxidation states bond length ( ) Co spin Fe spin HS Co 2+ NC Fe 3+ 10.3 3/2 1/2 LS Co 3+ NC Fe 2+ 10.0 0 0 reduced Co 2+ NC Fe 2+ 10.3 3/2 0 Table 5 4 The macroscopic states relevant to KjCok[Fe(CN)6]l nH2O macrostate shorthand Meaning realization expected microstates Q Quenched rapid cooling to 100 K at ~100 K/min mostly HS, some LS P Photoinduced photoirradiation at temperatures below 100 K mostly HS, some LS G low temperature Ground state slow cooling at ~1 K/min to 100 K LS 0.00 0.02 0.04 Temperature (K) 0 5 10 15 20 0 1 2 Curie ( emu/mol) Batch B Batch C

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146 Figure 5 9 FT IR spectra of bulk and nanoparticles of K Co Fe. The nanoparticles show a reduction in the amount of switchable material, the leftmost peak. Table 5 5 Metal oxidation states of bulk powder and nanoparticles, in addition to fitting parameters used for Figure 5 9 Fits are to a Gaussian function for the lines, i.e. A w / 2 e2 w 2. Sample Oxidation States 0 (cm-1) W (cm-1) A (I cm-1) 2159.4 0.2 19.78 0.3 21.6 0.3 bulk Co3+ 0.25Co2+ 0.75 [Fe2+(CN)6]0.18[Fe3+(CN)6]0.56 2125. 0 1.1 18.7 2.2 5. 9 1.0 2095. 9 0.8 32.1 1.3 2 5. 0 1.0 2162 .4 3.7 32.7 8.8 13.0 9.3 nano Co3+ 0.07Co2+ 0.93[Fe2+(CN)6]0.32[Fe3+(CN)6]0.36 2123.2 1.3 36.5 9.8 37.5 18.8 2096.7 2.9 45.7 5. 5 33.0 10.3 Table 5 6. Chemical composition and characteristic sizes of potassium cobalt hexacyanoferrate nanoparticles and bulk powder. Sample Chemical Formula Diameter (nm) bulk K0.39 Co 3+ 0.25Co 2+ 0.75 [Fe 2+ (CN)6]0.18[Fe 3+ (CN)6]0.56 3.30 H2O 200 3 8 nano K0.32Co 3+ 0.07Co 2+ 0.93[Fe 2+ (CN)6]0.32[Fe 3+ (CN)6]0.36 3.30 D2O 27.4 5.7 2200 2150 2100 2050 2000 0.0 0.5 1.0 0.0 0.5 1.0 nanoparticles wavenumber ( cm-1)Normalized Absorbance bulk

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147 Figure 5 10. TEM of K Co Fe. (left) Typical TEM images are shown. (right) The particle distributions, versus diameter for the bulk and small particles, see Table 5 6 The solid lines are the results of log normal fits that provide the characteristic diameters shown for each dis tribution. 0 50 100 150 200 250 300 350 0 5 10 15 20 ~200 nm 'bulk' number of particlesedge length (nm) ~27 nm nanoparticles 5 0 nm 100 nm

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148 Figure 5 11. XRD of K Co Fe. Room temperature x ray powder diffractograms of (a) bulk and (b) nanoparticles of K Co Fe. The wavelength is 1.54 The nanoparticles clearly show two peaks, a larger peak accounting for 65% of the scattering near 10. 313 corresponding to the HS and reduced phases, and a smaller peak accounting for the remaining 35% of the structured peak near 10.068 corresponding to the LS phase. Experimental counts are shown in black, with Rietveld ref inements in red, and residuals of the fit below the data in blue. Peak nomenclature is described in Table 5 3 10 20 30 40 50 60 -400 -200 0 200 400 600 800 1000 photons (counts)2 (degrees) 20 30 40 50 60 -100 0 100 200 300 400 500 photons (counts)2 (degrees) (a) (b)

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149 Figure 5 12. Neutron scattering of K Co Fe. Room temperature neutron powder diffractograms of nanoparticles of K Co Fe are shown, for = 1.54 The nanoparticles clearly show two peaks, a larger peak accounting for 73% of the scattering near 10. 312 corresponding to the HS and reduced phases, and a smaller peak accounting for the remaining 27% of the structured peak near 10.06 1 corre sponding to the LS phase. (inset) The (4, 0, 0) reflection showing the structure of the reflections, experimental counts are shown as white circles, with a HS/ reduced peak in blue and the LS peak in red. Peak nomenclature is described in Table 5 3 20 40 60 80 100 120 0 1000 2000 3000 34 35 36 500 1000 1500 2000 neutrons (counts)2 (degrees) Aluminum from cell (4, 0, 0)

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150 Figure 5 13. Neutron diffraction as a function of temperature for K Co Fe The evolution of the structure is seen by tracking the (4,0,0) reflection for (a) warming after quenching to 100 K and (b) warming after slowly cooling at ~1 K/min. Results of fitting these reflections can be seen in the temperature dependence of (c) the unit cell size and (d) the fraction of high spin and reduced material, nHS + nreduced, having the larger lattice constant. Quenching is shown in green, slow cooling in blue, and room temperature as a red star for data taken at HFIR on HB2A ( ~ 1.54 ) data taken slow cooling on HB1A ( ~ 2.36 ) is black. The different horizontal axes on the raw scattering data is due to slightly different ways of counting the angle on the experimental detector bank, and these angles have been kept as they were recorded by the machines. 0 50 100 150 200 250 300 0.5 0.6 0.7 0.8 0.9 nreduced + nHST (K) 10.1 10.2 10.3 HS and 'reduced' peak A ()LS peak 34 35 36 37 6 8 10 12 14 16 200 KNeutrons (102 counts)2 (degrees) 100 K -58 -56 -54 4 6 8 10 12 14 16 18 20 22 24 26 300 KNeutrons (102 counts)2 (degrees) 140 K (b) (a) (c) (d)

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151 Figure 5 14. Magnetic neutron scattering in K Co Fe. (a) Calculated magnetic scattering for K Co Fe predicting different intensities for ferromagnetic () and ferrimagnetic ( ---) structures are shown for an incident wavelength of 1.54 (b) Experimental observed magnetic scattering contribution taken as the difference between the magnetically ordered state at 5 K and the magnetically disordered state at 30 K. Figure 5 15. Temperature dependent magnetic moment of quenched stat es for K Co Fe. The temperature dependences of T are shown in the paramagnetic state at 5 kG for Q (), intermediately quenched (), and G states () of (a) ~200 nm particles and (b) ~27 nm particles. A clear reduction in the magnetism in the quenched state can be seen for the smaller, ~27 nm nanoparticles. Solid lines are fits to extract the HS fraction ( nHS), and the details of the fits are described in the text. 12 14 16 18 20 22 24 26 28 30 nuetrons (arb. units)2 (degrees) 12 14 16 18 20 22 24 26 -30 -20 -10 0 10 20 30 magnetic scattering (neutrons)2 (degrees) (a) (b) 100 150 200 250 300 1 2 3 4 nHS ~ 0.18 nHS ~ 0.34 nHS ~ 0.48 nHS ~ 0.56T (emu K / mol)T (K) 100 150 200 250 300 1 2 3 4 nHS ~ 0.23T (emu K / mol)T (K) nHS ~ 0.28 (a) (b)

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152 Figure 5 16. Magnetic ordering of quenched states in K Co Fe. The temperature dependences of the FC and ZFC DC magnetization, M are shown in the ordered state at 100 G for Q (), intermediately quenched (), and G states () of (a) ~200 nm particles and (b) ~27 nm particles. A clear reduction in the magnetism in the quenched state can be seen for the smaller, ~27 nm nanoparticles. The ordering temperatures of both samples are similar. (inset) The FC susceptibility normalized to the low temperature limit is shown to more clearly display the changing ordering temperatures. 5 10 15 20 25 30 0.0 0.5 1.0 1.5 2.0 5 10 15 20 0.0 0.5 1.0 M (103 emu G / mol)T (K) M / M (T = 5 K)T (K) 0 5 10 15 20 25 30 35 40 0.0 0.5 1.0 1.5 2.0 5 10 15 20 0.0 0.5 1.0 M (103 emu G / mol)T (K)M / M (T = 5 K)T (K) (a) (b)

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153 Figure 5 17. Magnetization versus field of quenched states for K Co Fe. The T = 2 K magnetization versus magnetic field sweeps are shown in the ordered state at 100 G for Q (), intermediately quenched (), and G states () of (a) ~200 nm particles and (b) ~27 nm particles. The coercive fields, HC, strongly depend upon the size of the particle, as shown in the expanded views for (c) ~200 nm particles and (d) ~27 nm particles. 0 1 2 3 4 5 6 7 0 2 4 6 8 10 12 M (103emu G / mol)H (T) 0 1 2 3 4 5 6 7 0 2 4 6 8 10 12 M (103emu G / mol)H (T) -0.2 -0.1 0.0 0.1 0.2 -3 -2 -1 0 1 2 3 M (103emu G / mol)H (T) -0.4 -0.2 0.0 0.2 0.4 -3 -2 -1 0 1 2 3 M (103emu G / mol)H (T) (a) (b) (c) (d) g = 2, S = 1 g = 2, S = 1

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154 Figure 5 18. Relaxation of magnetization in quenched states of K Co Fe. (a) The time dependences of T in 5 kG is shown for the Q state of the ~200 nm particles for 1 2 0 K ( ),1 3 0 K ( ), and 1 4 0 K ( ) (b) The time dependences of T in 5 kG is shown for the Q state of the ~27 nm particles for 12 0 K ( ),100 K ( ), and 16 0 K ( ) Using the T data, the active HS fraction, nHS nHS 0, can be extracted and fit ( ) to extract the HS LS mixing parameter, nHL ( ) and the semi empirical spin crossover parameters, which are shown for (c) ~200 and (d) ~27 nm particles. In order to fit the ~27 nm particle data, an additional parameter for the strain due to reduced states must be added to reproduce the relaxation. Details of models used for the fits are explained in the text. 0.0 0.5 1.0 1.5 2.0 1.5 2.0 2.5 3.0 140 K 130 KT (emu K / mol)Time (Days) 120 K 0.0 0.2 0.4 0.6 0.8 1.0 1.9 2.0 2.1 2.2 120 K 140 KT (emu K / mol)Time (Days) 160 K 0.0 0.5 1.0 1.5 2.0 0.0 0.1 0.2 0.3 nHS nHS0 nHLTime (Days) 0.0 0.2 0.4 0.6 0.8 1.0 0.00 0.02 0.04 0.06 0.08 nHS nHS0 nHLTime (Days) Jstrain (a) (b) (c) (d)

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155 Figure 5 19. Magnetic ordering of photoinduced states in K Co Fe. The temperature dependences of the FC and ZFC DC susceptibility, are shown in the ordered state at 100 G for ground states ( ) and photoinduced ( ) of (a) ~200 nm particles and (b) ~27 nm particles. Clear increases in the magnetic signals after photoirradiation are present. The arrows point out photoinduced and ground state magnetic signals, most likely due to inhomogeneity of photoirradiation. (inset) The FC susceptibility normalized to the low temperature limit is shown to more clearly display the changing ordering temperatures. 5 10 15 20 25 30 0.0 0.2 0.4 0.6 5 10 15 20 25 0.0 0.2 0.4 0.6 M (10-3 emu G/mol)T (K) 0 5 10 15 20 25 30 35 40 0.0 0.5 1.0 1.5 2.0 5 10 15 20 25 0.0 0.5 1.0 M (10-3 emu G / mol)T (K)M / M (T = 5 K)T (K)

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156 Figure 5 20. Magnetization versus field of photoinduced states for K Co Fe. The T = 2 K magnetization versus magnetic field sweeps are shown in the ordered state at 100 G ground states ( ) and photoinduced ( ) of (a) ~200 nm particles and (b) ~27 nm particles. The coercive fields, HC, strongly depend upon the size of the particle, and to illustrate this clearly zoomed plots are shown for (c) ~200 nm particles and (d) ~27 n m particles. Lines are guides for the eyes. 0 1 2 3 4 5 6 7 0 2 4 6 8 10 12 M (103emu G / mol)H (T) 0 1 2 3 4 5 6 7 0 2 4 6 8 10 12 M (103emu G / mol)H (T) -0.4 -0.2 0.0 0.2 0.4 -3 -2 -1 0 1 2 3 M (103emu G / mol)H (T) -0.4 -0.2 0.0 0.2 0.4 -3 -2 -1 0 1 2 3 M (103emu G / mol)H (T) (a) (b) (c) (d)

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157 Figure 5 21. Linearization of modeling. The value of T as a function of the highspin fraction ( nHS), calculated using meanfield theory and ligand field theory and a plausible set of parameters for 100 K and 300 K shows approximately linear behavior. Table 5 7 Magnetic properties of quenched potassium cobalt hexacyanoferrate nanoparticles and bulk powder. Batch Diameter (nm) T C G (K) T C I (K) T C Q (K) H C G ( G ) H C I ( G ) H C Q ( G ) bulk 200 38 12.3 12.4 13.8 130 165 200 nano 27.4 5.7 12.4 15.0 1500 2200 Table 5 8 Magnetic properties of photoinduced potassium cobalt hexacyanoferrate nanoparticles and bulk powder. Batch Diameter (nm) T C G (K) T C P (K) H C G ( G ) H C P ( G ) bulk 200 38 12.3 19.2 130 690 nano 27.4 5.7 12.4 15.7 1500 2440 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 100 K T (emuK/mol)nHS300 K

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158 Figure 5 22. Ordering temperatures and coercive fields of batches in different macroscopic states. (a) The dependence of the coercive field upon HS, showing clear increase in coercivities for the nanoparticles, as well as an increase in the coercivity of the P state as compared to the Q state. (b) The dependence of the magnetic ordering temperature upon HS, showing an increase in the ordering temperature of the P state as compared to the Q state, as well as a suppression of the magnetic ordering temperature in the nanoparticles. Figure 5 23. Microscopic schema based upon all data. A plausible microscopic picture is shown of the different low temperature ground state ( G ), quenched state ( Q ) and photoinduced state ( P ) in (a) the bulk powder and (b) the nanoparticles, where in the latter, the states are prefixed with n. In the bulk, the similar ordering temperatures of the G and Q states imply similar domain structure, as opposed to the increased ordering temperature of the P state, which implies a larger domain structure. In the nanoparticles, the nG versus nQ states are analogous to the bulk G and Q states, however in the nP state, the domains are larger gi ving an increased ordering temperature, but one that is reduced compared to the bulk P state. These schematics can be compared to those by other workers [ 68 ]. G n G Q P n Q n P (a) (b)

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159 CHAPTER 6 THIN FILMS OF PRUSSI AN BLUE ANALOGUES 6 6.1 Introduction From a technological standpoi nt, magnetic thin films are of a high interest due to their applications in memory storage. While modern devices utilize mainly metals and alloys, t he study of magnetization in thin films of coordination compounds is continuing to provide new twists on magnetic thin films [ 10 ] [29 ] [ 76] [ 93 ] [ 94] [ 9 8 112 ]. [ 98 ] [ 99 ] [ 100 ] [ 101] [ 102] Aside [ 103] [ 104 ] [ 105 ] [ 106 ] [ 107 ] [ 108] [ 109] [ 110] [ 111 ] [ 112 ] from the benign conditions necessary for synthesis, compared to metallurgy, the ability for magnetic properties to be tuned with external stimuli makes these systems especially attractive. For example, Prussian blue analog ues (PBAs), AjM1k[M2(CN)6]l nH2O ( Fig ure 6 1 ) have been shown to display interesting behavior when in thin film geometries [ 10 ] [ 76] [ 93 ] [94 ] [ 100 ] [ 102 ] In this chapter a close look is taken at the magnetic properties of PBA thin films generated by a sequential adsorption technique, Figure 6 2 This synthesis technique is attractive due to fine thickness control and the ability to change transition metal centers in a str aightforward fashion. Previously, Park et al had seen anisotropy in photoinduced magnetization for thin films of a RbjCok[ Fe (CN)6]l nH2O Prussian blue analogue and arrived at a plausible schematic explanation for the effect [ 100] However, the RbjCok[ Fe (CN)6]l nH2O is complicated due to the ability for iron and cobalt ions to have multiple stable oxidation states, as well as the orbital angular moment contribution to the net magnetization. Therefore, additional systems should be studied with microscopic probes to understand these films on a more fundamental level. The hypothesis of this work is that when films are oriented at different directions with respect to an applied magnetic field, Figure 6 3 different magnetic susceptibilities

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160 will be observed d ue to the geometry of the samples. To this end, a series of nickel hexacyanochromate films were studied in the greatest detail, but other transition metal analogues were also studied. The main probe utilized was a SQUID magnetometer, however to get a mor e detailed microscopic understanding of the effects observed in the magnetization, UV Vis FT IR, EMR, and XRD, among other probes, were employed. The results of these studies are to be presented in the following chapter. Portions of this work have already been published in Dr. Justin E. Gardners thesis [ 93] and the main results are to be submitted for future publication. In Section 6.2, experimental work on thin films of Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O thin films is presented, including magnetization and magnetic resonance experiments. In Section 6.3, experimental data resulting from studying additional Prussian blue analogue films are given. The source of anisotropy in the different systems, as well as ion dependence, is discussed (Section 6.4) and possible future experiments are mentioned. 6.2 Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O thin films The material chosen for an indepth study of the thin film geometry, having a Melinex solid support, was the rubidium ni ckel hexacyanochromate Prussian blue analogue, RbjNik[ Cr (CN)6]l nH2O The reasons for choosing this material are threefold. First, the samples are robust for months after synthesis, with little detectable changes in their materials properties. Second, RbjNik[ Cr (CN)6]l nH2O has a convenient magnetic ordering temperature that is quite high for coordination networks, varying between 60 and 90 K, depending upon the stoichiometry of the sample [ 54 ] Finally, the single ion

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161 ground states are well defined, having stable oxidation states and no first order orbital angular momentum. 6.2.1 Sample Characterization To begin, thin films of nickel hexacyanochromate were characterized with scanning electron microscopy to determine the chemical formula, atomic force microscopy to determine the thickness and surface morphology, infrared spectroscopy to determine the cyanide stretching frequencies, and ultraviolet and Vis ible electron spectroscopy to determine the electronic structure of the ions. Results of these studies will be presented in the following sections, to set a frame of reference for magnetization, resonance, and x ray scattering experiments. 6.2.1.1 Chemical c omposition Chemical composition was determined using scanning electron microscopy (SEM). Films w ere cut into squares of 1 cm2, mounted onto a metal puck, and coated with a thin film of carbon to enhance conductivity. The results gave chemical formulas of Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O for the films, and additional details can be found in Dr. Justin E. Gardners thesis [ 93] 6.2.1.2 AFM Detailed atomic force microscopy (AFM) studies of thin films of different Prussian blue analogues were performed by Dr. Justin E. Gardner, as detailed in his thesis [ 93 ] Briefly, the nickel hexacyanochromate analogue shows a linear dependence of thickness upon the number of deposition cycles, and an approximately linear dependence of the root mean square (RMS) roughness upon the number of deposition cycles, Figure 6 4 The curious dip in roughness at 20 cycles is repr oducible, and likely

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162 dependent upon the inherent structural coherence of the material under the given growth conditions. 6.2.1.3 FTIR Fourier transform infrared (FT IR) spectroscopy measurements provide information about the structure of cyanometallate networks, based upon the energies of the cyanide stretches. Room temperature FT IR was performed on a K3Cr(CN)6 precursor, an 80 cycle Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O thin film, and a Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O powder. While the films could be mounted directly, the powders were measured in diluted KBr solid solutions The cyanide stretches associate with the Ni2+ NC Cr3+ unit of the bulk powder and thin films were observed to be similar, Figure 6 5 The sharp peak in the precursor also appears to be present in both the thin films and the powder samples. 6.2.1.4 UV Vis Electronic spectroscopy in the Vis ible and ultraviolet (UV Vis ) provides important information about the electronic structure of a K3Cr(CN)6 prec ursor, an 80 cycle Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O thin film and a Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O powder. Room temperature UV Vis spectroscopy shows d d transitions associated with the molecular building blocks of the materials, Figure 6 6 namely Cr(CN)6 and Ni (N C )6. Ligand field multiplet calculations were performed using the methods described in Chapter 3. Two clear transitions are present in all samples, one for Ni2+ at 17,100 cm1 corresponding to a 3A2g(F) 3T1g(F) type transition and one for Cr3+ at 26, 600 cm1 corresponding to a 4A2g(F) 4T2g(F) type transition. In the precursor, an additional transition at 32,600 cm1 can be resolved. In the PBA film and powder, a less resolvable 3A2g(F) 3T2g(F) type transition for Ni2+ can be seen near 10,000 cm1. There is no clear

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163 evolution from precursors, to bulk, to film possibly due to the large line widths. All samples show ground states consistent with quenched orbital angular momentum. 6.2.2 Magnetization A commercial magnetometer from Quantum Design w as used for the DC SQUID measurements. Powder samples were mounted in diamagnetic gelcaps, thin film samples were either stacked in boxes or measured individually in a straw without additional mounting. Commercial straws were used as a diamagnetic sample rod to allow translation of the sample through the SQUID magnetometer detector coils. 6.2.2.1 DC s usceptibility in 100 G The low field magnetization as a function of temperature for the bulk Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O and thin films of Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O are shown in Figure 6 7 with both parallel and perpendicular orientations with respect to the applied magnetic field shown for the film samples. Bulk material is expressed per mole and film samples are expressed per unit area. The ZFC data wer e obtained after cooling in zero applied field from 300 K, while the FC data were taken after cooling in 100 G from 300 K. All samples show a change in the inflection of the magnetization versus temperature, indicative of the three dimensional magnetic or der known to exist between 6090 K, depending upon stoichiometry [ 54 ] The thin film samples show clear anisotropy between the susceptibility in the parallel and perpendicular orientations. 6.2.2.2 DC m agnetization in 40 kG The temperature dependences of the high field magnetization for the bulk Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O and thin films of Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O are shown in Figure 6 8 with both parallel and perpendicular orientations with respect to the applied magnetic field shown for the film samples. Bulk material is expressed per mole and film

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164 samples are expressed per unit area. For these measurements in H = 40 kG, the ZFC and FC data were found to lie on top of each other. This measuring field is particularly relevant for comparison with f ~ 116 GHz microwave magnetic resonance experiments. All samples show the usual increased ordering temperature and a more gradual slope of the onset compared to the low field measurements. Even at these high fields, the thin film samples show clear anisotropy between the susceptibility in the parallel and perpendicular orientations. 6.2.2.3 DC m agnetization f ield d ependence The field dependences of the DC magnetization at T = 2 K for bulk Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O and thin films of Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O are shown in Figure 6 9 with both parallel and perpendicular orientations with respect to the applied magnetic field shown for the film samples. Bulk material is expressed per mole and film samples are expressed per unit area. All samples show clear saturation indicative of ferromagnetic interactions in the complex. Thin films show anisotropy persisting up to the largest available field of 70 kG. 6.2.2.4 Magnetization Process The field dependence during the magnetization process, H = 0 to H = 2 .5 kG, was mapped at many temperatures, from 10 K to 70 K in 10 K increments. This series of scans was performed on the bulk powder of Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O ( Figure 6 10), 40 cycle Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O thin films ( Figure 6 11), and 400 cycle Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O thin films ( Figure 6 12), with both parallel and perpendicular orientations with respect to the applied field measured for the films.

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165 6.2.2.5 DC m agnetization a ngular d ependence In order to determine the functional form of the angular dependent magnetic anisotropy, insitu rotation measurements were performed in a SQUID magnetometer using a custom made rotation probe [ 28 ] Measurements were performed on 40 cycle and 400 cycle thi n films of Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O at T = 10 K and H = 40 kG, Figure 6 13. These field and temperature values were chosen to be compared to f ~ 116 GHz microwave electron magnetic resonance experiments. Both films show clear anisotropy, with an inplane easy axis. 6.2.3 Electron Magnetic Resonance To further probe the magnetic transitions in the Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O magnets, and particularly the anisotropic thin films, electron magnetic resonance experiments were performed on powders and film s. Continuous wave absorption measurements were performed using a resonant cavity technique at the NHMFL in Tallahassee. Temperature dependence, angular dependence and frequency dependence of the microwave absorption will be presented in the following sections. Previously, EMR researchers at the NHMFL studied powders of the charge transfer Mn Fe Prussian blue analogue [ 112 bis]. 6.2.3.1 EMR t emperature d ependence The temperature dependence of the microwave absorption for a Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O powder was measured at a continuous frequency of ~116 GHz, Figure 6 14 One clear absorption peak can be seen, with an effective g factor of 2.05. The full width half maximum of the linewidth, the position, and the area of the peak all show dependences on t he magnetic order parameter, Figure 6 15 More specifically, the change in the shape of the curves, near 90 K, is the temperature

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166 at which the magnetization curves also have a change in shape, which has been assigned to the development of long range magnetic order in the samples. The temperature dependence of the microwave absorption for a Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O 40 cycle film was measured perpendicular to the applied magnetic field at f ~ 116 GHz, Figure 6 16. In contrast with the powder, two clear a bsorption peaks can be seen, with effective g factors of 1.97 and 2.05 at 10 K. These shifts in effective g factors (and those in the following paragraphs) may not actually be associated with changing g factors, but are merely stated to give an additional frame of reference for the magnitude of the shifts of the lines. The full width half maximum of the line widths, the position of the peaks, and the area of the peaks all show dependences on the magnetic order parameter, Figure 6 17 Just as in the powde r resonance experiment, the lines are showing changes in behavior near 90 K, which is the magnetic ordering temperature of the sample for external fields of 40 kG. The temperature dependence of the microwave absorption for a Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O 40 cycle film was measured parallel to the applied magnetic field at f ~ 116 GHz, Figure 6 18. Similar to the perpendicular orientation, two clear absorption peaks can be seen. However, effective g factors of 2.11 and 2.05 are observed at 10 K. The ful l width half maximum of the line widths, the position of the peaks, and the area of the peaks all show dependence on the magnetic order parameter, Figure 6 19 with a change in shape of the temperature dependence happening near the magnetic ordering temper ature of 90 K. The temperature dependence of the microwave absorption for a Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O 400 cycle film was measured perpendicular to the applied

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167 magnetic field at f ~ 116 GHz, Figure 6 20. Similar to the 40 cycle film measured perpendicularly, two peaks are seen, with effective g factors of 1.97 and 2.05 at 10 K. The full width half maximum of the line widths, the position of the peaks, and the area of the peaks all show dependence on the magnetic order parameter, Figure 6 21, in the same manner as the thinner films. The temperature dependence of the microwave absorption for a Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O 400 cycle film was measured parallel to the applied magnetic field at f ~ 116 GHz, Figure 6 22. Similar to the 40 cycle film measured par allel to the field, two peaks are seen, with effective g factors of 2.11 and 2.05 at 10 K. The full width half maximum of the line widths, the position of the peaks and the area of the peaks all show dependence on the magnetic order parameter, Figure 6 23, in the same manner as the thinner films. 6.2.3.2 EMR a ngular d ependence The angular dependence of the microwave absorption was measured for 40 cycle thin films of Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O in ~116 GHz radiation at a temperature of 10 K, Figure 6 24, wh ich is well within the magnetically ordered state for the material. Angle dependence of the peak position and linewidth show a planar anisotropy, Figure 6 25 (a) and (b), which cannot be resolved above the ordering temperature, Figure 6 25 (c). The angular dependence of the microwave absorption was measured for 400 cycle thin films of Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O in ~116 GHz radiation at a temperature of 10 K, Figure 6 26, which is well within the magnetically ordered state for the material. Anal ogous to the angle dependence of the 40 cycle films, the 400 cycle films show planar anisotropy in the peak position and linewidth, Figure 6 27 (a) and (b), which cannot be resolved above the ordering temperature, Figure 6 27 (c).

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168 6.2.3.3 EMR f requency d e pendence The angular dependence of the microwave absorption was measured for 400 cycle thin films of Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O in 50 GHz radiation at a temperature of 10 K, Figure 628, which is well within the magnetically ordered state for the material The observed behavior is reminiscent of the f ~ 116 GHz data, again showing planar anisotropy in the peak position and linewidth, Figure 629 (a) and (b), with similar splitting of the lines at both frequencies, Figure 629 (c). 6.2.4 X ray Diffraction X ray diffraction experiments are currently underway. After room temperature experiments were performed at MAIC with the standard setup, samples were sent to Professor Stefan Kycia at the University of Guelph, and his research group is in the process of looking for structural distortions as a function of temperature. 6.3 Additional Prussian Blue Analogue Thin Films In addition to the detailed study of Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O thin films, an array of 3 d block transition metal Prussian blue analogue thi n films were studied. Films were synthesized with the sequential adsorption technique, using Co2+, Ni2+, Cu2+ and Zn2+ for the divalent nitrogen coordinated metal site and hexacyanometallic Cr3+ and Fe3+ for the trivalent, carbon coordinated site of the network, Table 6 1 All thin films synthesized display magnetic anisotropy, which can be tuned depending upon the constituent metals. Details of magnetization studies on each material, as well as additio nal modeling and experimental probes will be discussed in the following sections. 6.3.1 Rb0.6Co4.0[Cr(CN)6]2. 9 nH2O Thin Films Thin films of Rb0.6Co4.0[Cr(CN)6]2. 9 nH2O generated with 200 sequential adsorption cycles on a Melinex solid support were charac terized by SEM to obtain their chemical

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169 formula [ 93 ] Similar to the nickel based films, these materials can be thought of as consisting of Co(NC)6 and Cr(CN)6 molecular building blocks. The singleion ground states of these building blocks are important to understand the magnetization of the sample, and be calculated using ligand field multiplet calculations with known spectroscopic and nephelauxetic parameters, Figure 6 30 [ 5 ] The results show a Co2 + ground state with S = 3/2 and unquenched orbital angular momentum and a Cr3+ ground state with S = 3/2 and no orbital angular momentum contributions. The low field magnetization as a function of temperature for thin films of Rb0.6Co4.0[Cr(CN)6]2. 9 nH2O ar e shown in Figure 6 31 in both parallel and perpendicular orientations with respect to the applied magnetic field. The ZFC data were obtained after cooling in zero applied field from 300 K, while the FC data were taken after cooling in 100 G from 300 K. The films show a change in the inflection of the magnetization versus temperature, indicative of the three dimensional magnetic order known to exist near 23 K, depending upon stoichiometry [ 60 ] In addition, a clear anisotropy in the low field susceptibi lity is present below the magnetic ordering temperature. 6.3.2 Rb0.7Cu4.0[Cr(CN)6]2.9 nH2O Thin Films Thin films of Rb0.7Cu4.0[Cr(CN)6]2.9 nH2O generated with 200 sequential adsorption cycles on a Melinex solid support were characterized by SEM to obtain t heir chemical formula [ 93 ] The singleion ground states have calculated using ligand field multiplet calculations with known spectroscopic and nephelauxetic parameters, Figure 6 32 [ 5 ] The results show a Cu2 + ground state with S = 1/2 and unquenched or bital angular momentum and a Cr3+ ground state with S = 3/2 and no orbital angular momentum contributions.

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170 The low field magnetization as a function of temperature for thin films of Rb0.7Cu4.0[Cr(CN)6]2.9 nH2O are shown in Figure 6 33 in both parallel and perpendicular orientations with respect to the applied magnetic field. The ZFC data were obtained after cooling in zero applied field from 300 K, while the FC data were taken after cooling in 100 G from 300 K. The films show a change in the inflection o f the magnetization versus temperature, indicative of the three dimensional magnetic order. In addition, a clear anisotropy in the low field susceptibility is present below the magnetic ordering temperature. Temperature dependent ultraviolet and Vis ible s pectroscopy of an 80 cycle Rb0.7Cu4.0[Cr(CN)6]2.9 nH2O thin film on a quartz solid support was performed to study the electronic structure of the Cr3+ ion. The highly transmitting quartz was used instead of Melinex, which has many UV Vis transitions that make transmission experiments impossible with the available setups. An absorption peak at ~24,000 cm1 may be assigned to a 4A2g(F) 4T2g(F) type transition on the chromium ion. A shift and sharpening of the absorption line can be seen in the background subtracted spectra of the film, Figure 6 34 (a), and even more clearly when taking differences between room temperature and low temperature scans, Figure 6 34 (b). 6.3.3 Rb0. 3Zn4.0[Cr(CN)6]2. 8 nH2O Thin Films Thin films of Rb0. 3Zn4.0[Cr(CN)6]2. 8 nH2O generated with 200 sequential adsorption cycles on a Melinex solid support were characterized by SEM to obtain their chemical formula [ 93 ] The singleion ground states have calculated using ligand field multiplet calculations with known spectroscopic an d nephelauxetic parameters, Figure 6 35 [ 5 ] The results show a Zn2 + ground state with S = 0 and no orbital angular momentum and a Cr3+ ground state with S = 3/2 and no orbital angular momentum contributions.

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171 The field dependences of the DC magnetization at T = 2 K for thin films of Rb0. 3Zn4.0[Cr(CN)6]2. 8 nH2O are shown in Figure 6 36, with both parallel and perpendicular orientations with respect to the applied magnetic field shown for the film samples These films show anisotropy persisting up to the largest available f ield of 70 kG. As these materials are paramagnetic, the standard low field susceptibility plots were not made, because the background of the Melinex solid support is the same order of magnitude as the paramagnetic Prussian blue analogue s ignal. 6.3.4 Rb0.9Ni4.0[Fe(CN)6]2.8 nH2O Thin Films Thin films of Rb0.9Ni4.0[Fe(CN)6]2.8 nH2O generated with 200 sequential adsorption cycles on a Melinex solid support were characterized by SEM to obtain their chemical formula [ 93 ] Similar to the hexacyanochromate based films, these materials can be thought of as being made up of Ni(NC)6 and Fe(CN)6 molecular building blocks. The single ion ground states of these building blocks are important to understand the magnetization of the sample, and be c alculated using ligand field multiplet calculations with known spectroscopic and nephelauxetic parameters, Figure 6 37 [ 5 ] The results show a Ni2 + ground state with S = 1 and no orbital angular momentum and a Fe3+ ground state with S = 1/2 and unquenched orbital momentum contributions. The low field magnetization as a function of temperature for thin films of Rb0.7Ni4.0[ Fe (CN)6]2.9 nH2O are shown in Figure 6 38, in both parallel and perpendicular orientations with respect to the applied magnetic field. The ZFC data were obtained after cooling in zero applied field from 300 K, while the FC data were taken after cooling in 100 G from 300 K. The films show a chan ge in the inflection of the magnetization versus temperature, indicative of the three dimensional magnetic order. In addition, a

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172 clear anisotropy in the low field susceptibility is present below the magnetic ordering temperature. 6.3.5 Rb0.7Co4.0[Fe(CN)6]2.8nH2O Thin Films Thin films of Rb0.7Co4.0[Fe(CN)6]2.8 nH2O have already been studied in detail, both for their photoinduced anisotropy [ 10] and for the thickness dependence of that photoinduced anisotropy [ 103] The photoinduced magnetism in the cobalt iron analogue is due to bistabilities of oxidation states in the material, which may be changed with external stimuli Therefore, two oxidation states for each ion must be considered when interpreting experimental data, and when performing ligand field m ultiplet calculations, Figure 6 39 (a). Known values of spectroscopic and nephelauxetic parameters give energy level schemes for the Co3+/Fe2+ dark state and the Co2+/Fe3+ light state. Results give well separated diamagnetic ground states in the Co3+/Fe2+ dark state and Co2+ S = 3/2, Fe3+ S = 1/2 ground states with unquenched orbital angular momentum for the Co2+/Fe3+ light state. Although parallel and perpendicular magnetization and photoinduced magnetization has already been reported, the development of a SQUID magnetometer probe that is capable of performing in situ photoexcitation and sample rotation allowed for a new twist on the previous results [ 28 ] Rotation of films modifies the magnetization of the photomagnetic state, Figure 639 (b) and (c). 6.3.6 Rb0.5Cu4.0[Fe(CN)6]2.7nH2O Thin Films Thin films of Rb0.5Cu4.0[Fe(CN)6]2.7 nH2O generated with 200 sequential adsorption cycles on a Melinex solid support were characterized by SEM to obtain their chemical formula [ 93 ] The singleion ground states have calculated using ligand field multiplet calculations with known spectroscopic and nephelauxetic parameters, Figure 6 40 [ 5 ]

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173 The results show a Cu2 + ground state with S = 1/2 and unquenched orbital angular momentum and a Fe3+ ground state with S = 1/2 and unquenched orbital angular momentum contributions. The low field magnetization as a function of temperature for thin films of Rb0.5Cu4.0[Fe(CN)6]2.7 nH2O are shown in Figure 6 41 in both parallel and perpendicular orientations with respect to the applied magnetic field. The ZFC data were obtained after cooling in zero applied field from 300 K, while the FC data were taken after cooling in 100 G from 300 K. The films show a, upturn in the magnetization versus temperature, in dicative of the three dimensional magnetic order. In addition, a clear anisotropy in the low field susceptibility is present in the magnetically ordered state. 6.3.7 Rb0.5Zn4.0[Fe(CN)6]2.8 nH2O Thin Films This film is expected to be diamagnetic, and various measurements have shown that to be true. It is useful to have a diamagnetic sample available if magnetically quiet capping layers are to be used for heterostructured films. 6.4 Discussion The principle result of the thin film studies is that films of P russian blue analogues show magnetic anisotropy not present in the bulk solid state This anisotropy manifests itself in magnetization and microwave spectroscopy experiments H owever, no clear trends are observed in vibrational infrared and UV Vis electron spectroscopy measurements The functional form of the fundamental source of the anisotropy has been determined, and good candidates for the fundamental source of the anisotropy are presented. The state of what is known and unknown about the new found anisotropies will be discussed in the following.

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174 6.4.1 Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O Thin Films Since the Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O Prussian blue analogues are of primary interest due to their high ordering temperature and their large magnetic anisot ropy, they were studied in the most detail Four types of anisotropy are possible in the Prussian blue analogue films, namely magnetostatic, superexchange, magnetocrystalline, and g factor [ 113 ] Frequency dependent magnetic resonance experiments have ruled out g factor anisotropy, Figure 629 (c). Temperature dependent x ray diffraction has shown no departure from cubic structure, even in the ordered stated, making magnetocrystalline effects highly unlikely [ 113 bis] Superexchange anisotropy cannot be present as no change in the ordering temperature is observed for different film orientations, a fact that will be reiterated in the following discussion section. Consequently, anisotropy magnetostatic effects, especially those associated with demagnetizat ion within magnetic domains, will be the focus of the present discussion of the anisotropy in the Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O films. T he low field temperature dependence of the magnetization is where the magnetic anisotropy of the films was first observed as a difference between the magnetic susceptibility when the surface of the films wa s oriented parallel or perpendicular to the applied magnetic fields. The simplest Hamiltonian f o r a magnetic system with superexchange coupling is = 2 JijSi Sj i j = n n + gBH Si i 6 1 where J is the superexchange parameter, S is an electron spin, g is the parameter that scales the magnetic field dependence of the energy, B is the Bohr magneton, and H is

PAGE 175

175 the applied field Anisotropy may be introduced into Equation 6.1 through either the first or second term, however, changing the superexchange parameter, J changes the ordering temperature, which is not experimentally observed. On the other hand, a modification of the internal field due to demagnetizing ef fects may scale the magnetization without modifying the ordering temperature. A demagnetizing field consists of modifying the applied field to be Heff = HlabNM 6 2 where Heff is the effective field, Hlab is the applied field, M is the magnetization, and N is the demagnetizing factor. Immediately this functional form is attractive because it is observed that once magnetic order begins to take place in the films while cooling, an anisotropy develops that depends upon the macrosc opic magnetization. To begin, the magnetic fields produced by a 1 cm x 1 cm x 2 m film, analogous to the 400 cycle nickel hexacyanochromate film, may be considered from a theoretical perspective. Using the magnetic charge formalism [ 3 ] calculations o f internal fields are straightforward for both parallel and perpendicular orientations of films In the absence of electric current, Maxwells equations for magnetostatics have a reduced number of degrees of freedom that allow all of the information for m agnetic fields to be tabulated in the same way as electric fields so that vector quantities are simply gradients of scalars. Thus, Hz = d dz M 6 3 where M is the magnetic potential, which is determined from an analogue of Poissons equation, and magnetic dipoles may be modeled as dumbbells with a positive magnetic

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176 charge on one end and a negative magnetic charge on the other. First, the potential of the oppositely, magnetically charged upper and lower surfaces of the film is calculated, and then a derivative is taken to find the field along the z axis, so Hz = d dz 0M0dxdy 4 0 x2 + y2 + z + c 2 2 a / 2 a / 2 b / 2 b / 2 0M0dxdy 4 0 x2+ y2+ z c 2 2 a / 2 a / 2 b / 2 b / 2 6 4 w here for parallel orientations, a = 1 cm, b = 1 m, and c = 1 cm, and for perpendicular orientations, a = 1 cm, b = 1 cm, and c = 1 m with the permanent magnetization along the z axis for both cases These calculations are performed using a permanent magnetization with no applied field, although applied fields are easily considered using the superposition principle. ( N ote: for this subsection calculations will use SI units by which [B] = T, [H] = A/m, and [ 0] = 4 x 107, and volume susc eptibility for clarity, and the units are explained in Appendix A .) In the perpendicular orientation, the field produced is small, Figure 6 42 (a) and (b) and the H field is actually negative within the sample and equal to M0, Figure 642 (c). Therefore, the perpendicular orientation is equivalent to N = 1 in Equation 6.2. The integral and subsequent differentiation for the parallel orientations is less w e ll behaved and was computationally more expensive, so less data were calculated. However, it is clear that the B fields produced are much larger in the parallel orientation compared to the perpendicular orientation, Figure 6 43 (a), and that there is no demagnetizing H field in the parallel orientation Figure 6 43 (b). Therefore, in the parallel orientation, N = 0 in Equation 6.2 and there is no modification of the external field.

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177 The effect of the demagnetizing fields on the magnetization can be seen by assuming a functional form for the magnetization in 100 G, namely, M = low H eff 6 5 where low is the low field volume susceptibility. This assumption of linearity is customary for fields that are small compared to temperature, and is clearly justified by the magnetization data, Figures 610, 611, and 612. Plugging Equation 6. 5 int o Equation 6.2 yields M = low ( Hlab NM ) 6 6 Equation 6. 6 can be solved self consistently to give an expression for the magnetization in the presence of a demagnetizing field, M = low H lab1 + low N 6 7 Therefore, to calculate the magnetization in the presence of a demagnetizing field, one needs to estimate low and N The low field susceptibility may be found by using low = M Heff 6 8 to fit the magnetization data between 20 G (2 mT) and 100 G (10 mT) and N may be kept as a fitting parameter. Practically, to estimate the low field susceptibility the powder data may be used, under the assumption that the magnetization at 70 kG ( 7 T ) and 2 K reaches the theoretical saturation value of MsatV = g BS V = 2 05 9 27 10 24 J/ T ( 4 1 + 3 1 5 ) 10. 33 1010 m 3 = 1 47 105A m2 6 9

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178 First attempts to fit the low field magnetization data to N = 0 for the parallel orientation and N = 1 for the perpendicular orientation failed. In retrospect, this is not surprising as domains are present in low fields. Therefore, a more general spheroid may be considered [ 113 ] where the demagnetization of the principle axes are related by Nx + Ny + Nz = 1 6 .10 or for Nx= Ny, as in the case of the film geometry, 2 N + N = 1 6 .11 With these conditions i mpressive agreement of calculated and measured magnetization of the films at low field is seen for N = 0.07 and N = 0.8 6 Figure 644 In a similar way as for the H = 100 G temperature dependence, the H = 40 kG temperature dependen t magnetization may be modeled. First, i t is clear that the increase in the ordering temperature is reproduced in the mean field model due to increased spinspin correlations in a high magnetic field, Figure 6 4 5 An important difference between the two regimes is the functional form of the field dependence of the magnetization. For the high field limit, M = M0 + highHeff 6 1 2 where M0 and high are empirical parameters, and the introduction of a demagnetizing factor, Equation 6.2, gives M = M0+high ( Hlab NM ) 6 1 3 and solving self consistently as before yields M = M 0 + high H lab 1 + high N 6 1 4

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179 To find M0 and high, the magnetization of the powder data scaled to the theoretical maximum (Equation 6. 9 ) was fit between 20 kG and 40 kG, Figure 6 45 (a) and (b). The magnetization data for the thin films in the high field limit can then be fit using N = 0 and = 1 Figure 6 45 (c) however an additional scaling of the high by a factor of 5 is necessary to achieve the magnitude of magnetic anisotropy observed, which may simply be experimental uncertainty in the determination of high. It is also worth noting the effect of film thickness on the anisotropy of the low field susceptibilities, Figure 6 4 6 (a), where the anisotropy persists even in the thickest films, albeit at slightly reduced levels Perhaps even more startling is that anisotropy is present even in the caustically manufactured spin cast films [ 93] The reduction in anisotropy may be attributed to a departure from an ideal thin film geometry, since films of increasing thickness also have an increasing presence of powder like features on the surface, Figure 646 (b) and (c) [ 93 ]. The complex cyanides are ideal candidates for resonance experiments because their high resistivity eliminates complications coming from potential skin effects that plague metallic magnets. For the resonance data, the powdered sample is a logical place to begin as it has the simplest spectrum The presence of only one line in the powder spectrum of Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O indicates that only one species is in resonance, which might be a well cou ple d Ni2+ and Cr3+ line, a single Cr3+ line, or a Ni2+ line T he possibility that the signal is due to an impurity can be ruled out because the area of the line as a function of temperature, which i s proportional to the magnetization, tracks the measured magnetization in the SQUID magnetometer at 40 kG In addition, the fullwidth half maximum of the absorption peak decreases as the temperature is

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180 lowered, until the magnet becomes ordered, where the line width increases. Thes e changes can be understood q uite well in terms of the simple expression H = 2 Heff 6 1 5 where is the damping parameter and Heff is the effective field seen by the spin [ 113 ] The general decrease above the ordering temperature may be attributed to reduc tion in spin lattice relaxation as the number of phonons decreases with temperature. The increase in width of the line from the onset of magnetic order and down to the coldest temperatures may be associated with an increased spinspin relaxation as the sample enters more deeply into the magnetically ordered state. The position of the peak as a function of temperature may be analyzed in a similar demagnetization formalism as the magnetization data was analyzed. To begin, the same functional form of the demagneti zation effects is assumed Hx = Hlab, x + NxMx Hy = Hlab, y + NyMy Hz = Hlab, z + NzMz 6 1 6 where for resonance experiments all principle axes of the magnet and magnetic field are important. Using the equations of motion for a spin in an applied magnetic field along the z axis, the resonance condition may be written [ 113 ] 0 2 = g20 2 Hlab, z + Ny Nz Mz Hlab, z + ( Nx Nz ) Mz 6 1 7 For a perfect sphere, Nx = Ny = Nz = 1/3, so the resonance condition should be isotropic and have no magnetization dependence, 0 sphere = g 0Hlab, z 6 1 8 In practice, there may be small deviations from spherical symmetry for the powder, and the resonance condition may be written as

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181 0 powder = g 0 Hlab, z Mz 6 1 9 where takes care of dev iations from spherical symmetry. F or the powder data of Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O a shift of ~100 G is observed in the fully magnetized state compared to the paramagnetic state, and this is consistent with a value of ~ 0.05. C ompared to the powder, things begin to get more complicated in the resonance experiments of the thin films The variation of the line thickness with temperature is reminiscent of the bulk powder, however, t he most striking feature is the evolution of two resonance conditions in the films when cooled below the magnetic ordering temperature. The two lines, most clearly seen in the parallel and perpendicular orientations, cannot reproduce the powder line when integrated over the angular degree of freedom as can be clearly demonstrated by observing that the lines in the powder are actually sharper than the lines in the films Figure 6 47 (a). It is also worth noting that the magnitudes of the absorption lines for different orientations may be different due to the complicated coupling term between the modes of the cavity and the sample. More clearly than the magnetic susceptibility measurements, this experimental fact confirms that there is magnetic anisotropy in the films that is not present in the bulk Restated, the thin film samples have a magnetic anisotropy that is induced by their thin film character A quantitative treatment of the resonance in the thin films can be carried out using the demagnetization formalism used to analyze the magnetization data of the thin films and the general resonance condition in Equation 6.17 For a film oriented perpendicul ar to an applied field, Nx = Ny = 0 and Nz = 1 in the limit that the film is uniformly magnetized, and the resonance condition for the perpendicular orientation is

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182 0 perp = g 0 Hlab, z Mz 6 20 For the parallel orientation of the film Nx = Nz = 0 and Ny = 1 in the limit that the film is uniformly magnetized, and the resonance condition for the parallel orientation is 0 par = g 0 Hlab, z Hlab, z + Mz 1 2 6 21 Just as in the modeling of the magnetization data, Mz will be taken from the powder magnetization scaled to reach the theoretical value at 2 K and 70 kG (7 T). The results of these calculations can be seen in Figure 6 47 (b). The quantitative agreement between model and the position of the largest absorption line is striking. One may a rgue that qualitative agreement breaks down above approximately 100 K, and this may be explained as a breakdown of the applicability of the model. One potential problem as temperature increases is the loss of the singledomain state, which gave Nx = Ny = 0 and Nz = 1 in the perpendicular orientation and Nx = Nz = 0 and Ny = 1 in the parallel orientation. Another potential discrepancy between the model and experimental data is the existence of two absorption lines in the resonance experiments of the films. While demagnetization may produce inhomogeneous internal fields for some samples, giving rise to such a behavior, calculations of the internal field showed a high degree of homogeneity for the chosen geometries Figure 642 and Figure 643. However, the same powder like component that explains the observed thickness dependence of the magnetization can also explain the smaller absorption line that does not shift. A portion of the Prussian blue analogue material in the film actually exists as a powder lik e phase

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183 on the surface, and therefore has a weak temperature dependence in the resonance experiments that is different than the temperature dependence of the film like material. The angular dependence of the resonance line provides further confirmation of the uniaxial ~ sin2( ) nature of the anisotropy that was deduced from the angular dependent magnetization measurements. Just as in the temperature sweeps, two lines are seen, one that moves with angle and another that remains still. The ang ular dependence of the line width shows wider lines in the perpendicular compared to the parallel orientations, consistent with previous resonance experiments on thin films [ 105 ]. Finally, t he resonance data for the thin films looks similar whether 40 cycle films or 400 cycle films were used. One key difference, however, is that i n the thin films, a Lorentzian fits the line shape better, while in the thicker films, a Gaussian fits the line shape better This observation implies that the disorder increases as the films become thicker and is consist ent with the magnetization data. 6.4.2 Additional Prussian Blue Analogue Thin Films Taking what has been learned from the Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O films as a fixed point for understanding magnetostatic effects in these materials, the other films may be analyzed to see if additional anisotropy may be present. The Rb0.6Co4.0[Cr(CN)6]2.9 nH2O film shows a large anisotropy in the low field magnetization, reminiscent of the Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O film. While both films are ferromagnets, one difference between them is that Ni2+ is a L = 0, S = 1 ion, while Co2+ is a L = 1, S = 3/2 ion (in the unquenched limit). If the anisotropy was magnetostatic one would expect a larger anisotropy in the Rb0.6Co4.0[Cr(CN)6]2.9 nH2O compound compared to the Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O as the moments in

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184 Rb0.6Co4.0[Cr(CN)6]2.9 nH2O are larger. In fact, the nickel based film is found to have a larger angular dependence of susceptibility. In addition, Co2+ is known to be a highly anisotropy ion, due to the large amount of orbital momentum. Therefore additional measurements may be required to fully understand the cobalt film. The Rb0.7Cu4.0[Cr(CN)6]2.9 nH2O film is in a different class than the Ni or Co films, as a tetragonal struc ture has been resolved in x ray diffraction measurements [ 106 ]. This distortion from simple cubicity is understood within the confines of the classic JahnTeller distortion that lowers the electronic energy by elongating or compressing the coordinating oc tahedral, also relieving the orbital degeneracy of the upper crystal field split doublet [ 5 ] In light of this distortion, it is not surprising that thin films would be anisotropic, as both single ion anisotropy of the Cr3+ ions, and g factor anisotropy o f the Cu2+ are expected to be first order effects in such a compound, in addition to magnetostatic effects Further evidence changes in the electron energy levels in the ordered state are shown in the temperature dependent UV Vis spectroscopy studies. The final hexacyanochromate material studied was the zinc analogue. As Zn2+ has a full dshell, the Rb0.3Zn4.0[Cr(CN)6]2.8 nH2O material remains paramagnetic down to the lowest temperature measured, which was 2 K. Anisotropy is observed, which becomes enhanced at higher fields when the magnetization is greater, showing the familiar dependence upon the sample magnetization. Therefore, the Rb0.3Zn4.0[Cr(CN)6]2.8 nH2O film anisotropy is probably magnetostatic. In addition to chromate cyanides, iron cyanides were also studied. The magnetization of Rb0.9Ni4.0[Fe(CN)6]2.8 nH2O thin films is actually quite similar to Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O thin films. The only difference, besides the ordering

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185 temperature, is the magnitude of the difference between the parall el and perpendicular low field susceptibilities. The Rb0.7Co4.0[Fe(CN)6]2.8 nH2O thin films actually add confusion instead of enlightenment. It was found with the new probe setup that rotation in field causes a change in the magnetization, the mechanism of which is still unclear. The Rb0.5Cu4.0[Fe(CN)6]2.7 nH2O films are also different than the other films studied, and of acute interest for this reason. Unlike all other analogues, the Rb0.5Cu4.0[Fe(CN)6]2.7 nH2O has the less common out of plane anisotropy. Although it has yet to be observed, the Rb0.5Cu4.0[Fe(CN)6]2.7 nH2O analogue is expected to have a tetragonal structure analogous to the Rb0.7Cu4.0[Cr(CN)6]2.9 nH2O However, since both copper and iron ions are S = 1/2, no zerofield splitting s are possible. Therefore, the only first order effects should be g factor anisotropy, which may explain the out of plane anisotropy. It is much more difficult to explain out of plane anisotropy with demagnetization as the source. One possibility would be if the Rb0.5Cu4.0[Fe(CN)6]2.7 nH2O film had a spirelike structure on the surface. However, atomic force microscopy measurements actually show Rb0.5Cu4.0[Fe(CN)6]2.7 nH2O films to be the smoothest of all studied [ 93 ] 6.5 Conclusions In conclusion, all Prussian blue analogue thin films studied show magnetic anisotropy. The Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O films have been studied in the most detail and show a uniaxial, sin2( ) dependence to the anisotropy that can be well understood within the context of a demagnetization model Other n oteworthy results are an out of plane anisotropy in Rb0.5Cu4.0[Fe(CN)6]2.7 nH2O and a field rotation induced change in susceptibility in Rb0.7Co4.0[Fe(CN)6]2.8 nH2O It is important that each

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186 transition met al film be examined in detail, as multiple sources or magnetic anisotropy are likely to be present. Additional probes of the films might include Lorentz microscopy to analyze the domain structure of the films, or dispersive x ray spectroscopy to look for anisotropy of the fundamental parameters, specifically in spin orbit coupling. Fig ure 6 1 Prussian blue analogue structure. Prussian blue analogues have a chemical formula of AjM1k[M2(CN)6]l nH2O j k, l and n are constrained by charge balance. Cations (A = Cs+, Rb+, K+, Na+) are incorporated based upon the number of M2 vacancies, which are coordinated by water as shown. M1 M2 A H 2 O N C C N

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187 Figure 6 2 T he multiple sequential adsorption method that can be used for generating thin films of Prussian blue analog ues. For example, synthesis of a Ni Cr analogue would consist of using a solid support (typically Melinex) and immersing it in an aqueous solution of hexacyanochromate, and into separate solutio n of Rb+ and Ni2+ ions, thus depositing approximately one layer of Ni CN Cr After each cycle, a simple washing with water is essential to remove excess ions This process can be iterated to yield films of varying thicknesses and morphologies. Figure 6 3 Different orientations of the magnetic thin films with respect to the applied magnetic field are expected to have different behavior. H = 0 = 45 = 90 HPARHPERPH = 0 = 45 = 90 HPARHPERP x cycles M e l i n e x Ni 2+ ( aq ) Cr 3+ (CN) 6( aq ) & Rb + Ni Cr Ni Cr Ni Cr Cr Ni Cr Ni Cr Ni Ni Cr Ni Cr Ni Cr Rb Rb

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188 Figure 6 4 AFM of thin films. (a) Multiple sequential adsorption film roughness generally increases with number of cycles, but the 20 cycle film is smoothest This minimum in the roughness is due to the inherent structural coherence of the material and is reproducible (b) The thickness of the multiple sequential adsorption films is directly proportional to the number of cycles The red line is a fit yielding 5.7 nm/cycle. Figure 6 5 Room temperature FT IR spectroscopy measurements of the cyanide stretches present in Ni Cr materials. Cyanide stretches are seen in the K3Cr(CN)6 precursor ( ---), Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O powder ( ), and a 80 cycle thin film of Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O ( The K3Cr(CN)6 sample shows one clear stretch at 2131 cm1. The thin film and powder samples show peaks at 2173 cm1 and 2131 cm1 with the latter being associated with free cyanides at the surface and coordinating vacancies. 0 20 40 60 0 25 50 75 100 RMS Roughness (nm)Deposition Cycles 0 50 100 150 200 0 400 800 1200 Film Thickness (nm)Deposition Cycles 2300 2250 2200 2150 2100 2050 0.0 0.2 0.4 0.6 0.8 1.0 Normalized AbsorbanceEnergy (cm-1) (a) (b)

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189 Figure 6 6 UV Vis spectroscopy of Ni Cr materials. (a) Room temperature UV Vis spectroscopy measurements of the dd transitions present in 10 mM Cr(CN)6 precursor ( ---), 10 mM Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O powder with background subtracted by using the functional form of a diamagnetic Rb0.5Zn4.0[Fe(CN)6]2.8 nH2O ( ), and a n 80 cycle thin film of Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O with background subtracted by using the functional form of a diamagnetic Rb0.5Zn4.0[Fe(CN)6]2.8 nH2O film ( (b) Using the transitions shown in Figure 6 6 (a) a multiplet calculation can be performed to show the electronic energy levels for the Ni2+ and Cr3+ ions in the Prussian blue network Chromium energy levels are shown for (i ) no spinorbit coupling and ( ii) using reported values of spinorbit coupling for the free ion [ 5 ] Nickel energy levels are shown for ( iii) no spinorbit coupling and ( iv ) using reported values of spinorbit coupling for the free ion [ 5 ] Figure 6 7 Temperature dependent magnetization of Ni Cr materials. The temperature dependences of the low field, 100 G, magnetizations are shown for Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O (a) powder, zerofield cooled (ZFC) ( ) and field cooled (FC) ( ) (b) 400 cycle thin film parallel ZFC ( ), parallel FC ( ), perpendicular ZFC ( ) and perpendicular FC ( ), (c) 40 cycle thin film, parallel ZFC ( ), parallel FC ( ), perpendicular ZFC ( ) and perpendicular FC ( ) Connecting lines are guides to the eye. 0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000 NiII(NC)6 (iii) (iv) (ii)Energy (cm-1)(i)CrIII(CN)60 2 4 6 8 10 12 1A1g 1T2g 1Eg 1T1g 3T1g 1A1g 1T2g 3T1g 1Eg 3T2g3A2g2Eg 2T2g 2T1g 2T2g 2T1g 4T1g 2Eg 2A2g 2T2g 2T1g 2E2T1g 2T2g 2A1g 4T1g 4T2g 2T2g 2T1g 2Eg4A2g Energy (eV) (a) (b) 0 20 40 60 80 100 120 140 0 1 2 3 4 5 6 M (103 emuG/mol)T (K) 0 20 40 60 80 100 120 140 0.0 0.5 1.0 1.5 2.0 2.5 3.0 M (10-2 emuG/cm2)T (K) 0 20 40 60 80 100 120 140 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 M (10-3 emuG/cm2)T (K) (a) (b) ( c )

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190 Figure 6 8 Temperature dependent magnetization of Ni Cr materials at high fields. The temperature dependences of the high field H = 40 kG magnetizations are shown for Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O (a) powder ( ), (b) 400 cycle thin film parallel ( ) and perpendicular FC ( ), (c) 40 cycle thin film, parallel ( ) and perpendicular FC ( ) Lines connecting discrete data points are guides to the eye. Figure 6 9 Field dependent magnetization of Ni Cr materials. The field dependences of the low temperature, 2 K magnetizations are shown for Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O (a) powder ( ), (b) 400 cycle thin film parallel ( ) and perpendicular FC ( ), (c) 40 cycle thin film, parallel ( ) and p erpendicular FC ( ) The negative high field slope of the magnetization in the 40 cycle film can be attributed to the diamagnetic substrate. Lines connecting discrete data points are guides to the eye. 0 20 40 60 80 100 120 140 0 2 4 6 8 10 M (103 emuG/mol)T (K) 0 20 40 60 80 100 120 140 0 1 2 3 4 5 M (10-2 emuG/cm2)T (K) 0 20 40 60 80 100 120 140 0.0 0.2 0.4 0.6 0.8 1.0 M (10-2 emuG/cm2)T (K) (a) (b) ( c ) 0 20 40 60 0.0 0.5 1.0 1.5 M (104 emuG/mol)H (kG) 0 20 40 60 0 1 2 3 4 M (10-2 emuG/cm2)H (kG) 0 20 40 60 0.00 0.25 0.50 0.75 1.00 M (10-2 emuG/cm2)H (kG) (a) (b) ( c )

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191 Figure 6 10. Magnetizing process of thin Ni Cr film. Magnetization as a function of external field for 40 cycle film of Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O in (a) parallel and (b) perpendicular orientations with respect to the applied magnetic field. Here the field dependence of the low field data is fit to a linear model, M = H where = M / H In a similar manner, the high field data can be fit to M = M0 + E H These fits are relevant to understanding the magnetization process and more specifically to understand the potential roles of demagn etizing fields in the materials. Figure 6 11. Magnetizing process of thick Ni Cr film. Magnetization as a function of external field for 400 cycle film of Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O in (a) parallel and (b) perpendicular orientations with respect to the applied magnetic field. The meaning of the fitting lines are described in the figure caption of Figure 6 10. 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.2 0.4 0.6 0.8 1.0 M (10-2 emuG/cm2)H (kG)10 K 20 K 30 K 40 K 50 K 60 K 70 K "M0" M/H 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.2 0.4 0.6 0.8 1.0 M (10-2 emuG/cm2)H (kG)10 K 20 K 30 K 40 K 50 K 60 K 70 K M/H (a) (b) 0.0 0.5 1.0 1.5 2.0 2.5 0 1 2 3 4 M (10-2 emuG/cm2)H (kG)10 K 20 K 30 K 40 K 50 K 60 K 70 K "M0" M/H 0.0 0.5 1.0 1.5 2.0 2.5 0 1 2 3 4 M (10-2 emuG/cm2)H (kG)10 K 20 K 30 K 40 K 50 K 60 K 70 K M/H (a) (b)

PAGE 192

192 Figure 6 12. Magnetizing process of Ni Cr powder. Magnetization as a function of external field for Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O bulk powder. The meaning of the fitting lines are described in the figure caption of Figure 6 10. Figure 6 13. Angle dependence of magnetization in Ni Cr materials. (a) Magnetization as a function of angle, at T = 10 K and H = 40 kG, for (a) 400 cycle Rb0 7Ni4.0[Cr(CN)6]2. 9 nH2O thin films and (b) 40 cycle Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O thin films. Both data sets are raw magnetization from the SQUID, without any additional processing. A detail of the 400 cycle film data shows the discrete 1.5 steps used during rotation, as data was taken continuously. The difference in the signal to noise for the two measurements may due to the different amount of sample present in a 40 versus 400 cycle film. 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.3 0.6 0.9 1.2 1.5 M (104 emuG/mol)H (kG)10 K 20 K 30 K 40 K 50 K 60 K 70 KM/H 0 90 180 270 360 3.95 4.00 4.05 4.10 4.15 M (10-2 emuG/cm2) (degrees) 0 90 180 270 360 0.90 0.95 1.00 1.05 M (10-2 emuG/cm2) (degrees) 140 141 142 143 144 145 146 147 148 4.065 4.070 4.075 4.080 M (10-2 emuG/cm2) (degrees) (a) (b) ( c )

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193 Figure 6 14. EMR lines of NiCr powder. The field dependence of the microwave transmission through the resonance cavity as a function of temperature, measured at a constant frequency of ~116 GHz, for Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O powder. Here the data have been offset to more clearly display the evolution of the line, and the data have not been adjusted to reflect the temperature dependent resonance in the cavity. Figure 6 15. Results of fitting EMR lines of NiCr powder. Fitting the temperature dependence in Figure 6 14 to a Lorentzian line yields (a) the full width half maximum (FWHM) of the line, (b) the center position of the absorption line and (c) the relative area of the absorption peak as a function of temperature. 30 35 40 45 50 10 K 15 K 20 K 25 K 30 K 35 K 40 K 45 K 50 K 55 K 60 K 65 K 70 K 75 K 80 K 85 K 90 K 95 K 100 K 105 K 110 K 115 K 120 K 125 Kf ~ 116 GHzCavity Transmission (Arb. units, offset)H (kG) 0 20 40 60 80 100 120 140 500 600 700 800 900 1000 1100 1200 1300 FWHM (Gauss)T (K) 0 20 40 60 80 100 120 140 40260 40280 40300 40320 40340 40360 40380 40400 40420 position (Gauss)T (K) 0 20 40 60 80 100 120 140 Area (Arb. units)T (K) (a) (b) ( c )

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194 Figure 6 16. EMR lines of NiCr thin film perp endicular. The field dependence of the microwave transmission through the resonance cavity as a function of temperature, measured at f ~ 116 GHz, for Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O 40 cycle films perpendicular to the applied field. Here the data have been o ffset to more clearly display the evolution of the line, and the data have not been adjusted to reflect the temperature dependent resonance in the cavity. Figure 6 17. Results of fitting EMR lines of NiCr thin film perpendicular. Fitting the temperature dependence in Figure 6 16 to Lorentzian lines yield (a) the fullwidth half maximum (FWHM) of the lines, (b) the center positions and (c) the relative area of both peaks as a function of temperature. 30.0 35.0 40.0 45.0 50.0 150 K 130 K 115 K 100 K 85 K 70 K 55 K 40 K 25 K 10 Kf ~ 116 GHz HPERPCavity Transmission (Arb. units, offset)H (kG) 0 20 40 60 80 100 120 140 160 500 1000 1500 2000 2500 3000 FWHM (Gauss)T (K) big line little line 0 20 40 60 80 100 120 140 160 40000 40500 41000 41500 42000 Peak Position (Gauss)T (K) big line little line 0 20 40 60 80 100 120 140 160 Area (Arb. units)T (K) (a) (b) ( c )

PAGE 195

195 Figure 6 18. EMR lines of NiCr thin film pa rallel. The field dependence of the microwave transmission through the resonance cavity as a function of temperature, measured at f ~ 116 GHz, for Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O 40 cycle films parallel to the applied field. Here the data have been offset to more clearly display the evolution of the line, and the data have not been adjusted to reflect the temperature dependent resonance in the cavity. Figure 6 19. Results of fitting EMR lines of NiCr thin film parallel. Fitting the temperature dependence in Figure 6 18 to Lorentzian lines yield (a) the fullwidth half maximum (FWHM) of the lines, (b) the center positions and (c) the relative area of both peaks as a function of temperature. 30.0 35.0 40.0 45.0 50.0 f ~ 116 GHz HPARCavity Transmission (Arb. units, offset)H (kG) 150 K 130 K 115 K 100 K 85 K 70 K 55 K 40 K 25 K 10 K 0 20 40 60 80 100 120 140 160 200 400 600 800 1000 1200 1400 1600 FWHM (Gauss)T (K) big line little line 0 20 40 60 80 100 120 140 160 Area (Arb. units)T (K) (a) (b) ( c )

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196 Figure 6 20. EMR lines of NiCr thick film perpendicular. The field dependence of the microwave transmission through the resonance cavity as a function of temperature, measured at f ~ 116 GHz, for Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O 400 cycle films perpendicular to the applied field. Here the data have been offset to more clearly display the evolution of the line, and the data have not been adjusted to reflect the temperature dependent resonance in the cavity. Figure 6 21. Results of fitting EMR lines of NiCr th ick film perpendicular. Fitting the temperature dependence in Figure 6 20 to Gaussian lines yield (a) the fullwidth half maximum (FWHM) of the lines, (b) the center positions and (c) the relative area of both peaks as a function of temperature. 35.0 40.0 45.0 HPERPf ~ 116 GHz Cavity Transmission (Arb. units, offset)H (kG) 10 K 15 K 20 K 25 K 30 K 35 K 40 K 45 K 50 K 55 K 60 K 65 K 70 K 75 K 80 K 85 K 90 K 95 K 100 K 110 K 120 K 130 K 140 K 150 K 35.0 40.0 45.0 0 20 40 60 80 100 120 140 160 500 1000 1500 2000 FWHM (Gauss)T (K) big line little line 0 20 40 60 80 100 120 140 160 39500 40000 40500 41000 41500 42000 Peak Position (Gauss)T (K) big line little line 0 20 40 60 80 100 120 140 Area (arb. units)T (K) (a) (b) ( c )

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197 Figure 6 22. EMR lines of NiCr thick film parallel. The field dependence of the microwave transmission through the resonance cavity as a function of temperature, measured at f ~ 116 GHz, for Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O 400 cycle films parallel to the applied field. Here the data have been offset to more clearly display the evolution of the line, and the data have not been adjusted to reflect the temperature dependent resonance in the cavity. Figure 6 23. Results o f fitting EMR lines of NiCr thick film parallel. Fitting the temperature dependence in Figure 6 22 to Gaussian lines yield (a) the fullwidth half maximum (FWHM) of the lines, (b) the center positions and (c) the relative area of both peaks as a function of temperature. 35.0 40.0 45.0 HPARf ~ 116 GHz Cavity Transmission (Arb. units, offset)H (kG) 10 K 15 K 20 K 25 K 30 K 35 K 40 K 45 K 50 K 55 K 60 K 65 K 70 K 75 K 80 K 85 K 90 K 95 K 100 K 105 K 110 K 115 K 120 K 130 K 140 K 150 K 0 20 40 60 80 100 120 140 160 400 600 800 1000 1200 1400 FWHM (Gauss)T (K) big line little line 0 20 40 60 80 100 120 140 160 39000 39500 40000 40500 Peak Position (Gauss)T (K) big line little line 0 20 40 60 80 100 120 140 160 Area (arb. units)T (K) (a) (b) ( c )

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198 Figure 6 24. EMR lines of NiCr thin film as a function of angle. The field dependence of the microwave transmission through the resonance cavity as a function of angle, measured at f ~ 116 GHz and T = 10 K, for Rb0.7Ni4.0[Cr(CN)6]2 .9 nH2O 40 cycle films. Here the data have been offset to more clearly display the evolution of the line. The vertical lines delineate the extremum values of the big line, labeled by the numbers near each line in units of kG. Figure 6 25. Results of fitting EMR lines of NiCr thin film as a function of angle. Fitting the temperature dependence in Figure 6 24 to Lorentzian lines yield (a) the center positions and (b) the full width half maximum (FWHM) of the lines. (c) The angular dependence above the ordering temperature. Here the data have been offset to more clearly display the evolution of the line. The vertical line delineates the peak position, labeled by the number at the top in units of kG. 30 35 40 45 50 f ~ 116 GHz T = 10 K 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 -2039.1 41.8 Cavity Transmission (Arb. units, offset)H (kG) 0 30 60 90 120 39000 39500 40000 40500 41000 41500 42000 Peak Position (Gauss)Angle ( ) big line little line 30 35 40 45 50 f ~ 116 GHz T = 150 K 39.9 Cavity Transmission (Arb. units, offset)H (kG) 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 (a) (b) ( c )

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199 Figure 6 26. EMR lines of NiCr thick film as a function of angle. The field dependence of the microwave transmission through the resonance cavity as a function of angle, measured at f ~ 116 GHz and T = 10 K, for Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O 400 cycle films. Here the data have been offset to more clear ly display the evolution of the line. The vertical lines delineate the extremum values of the big line, labeled by the numbers near each line in units of kG. Figure 6 27. Results of fitting EMR lines of NiCr thick film as a function of angle. Fitting the temperature dependence in Figure 6 26 to Gaussian lines yield (a) the center positions and (b) the full width half maximum (FWHM) of the lines. (c) The angular dependence above the ordering temperature. Here the data have been offset to more clearly display the evolution of the line. The vertical line delineates the peak position, labeled by the number at the top in units of kG. 30.0 35.0 40.0 45.0 f ~ 116 GHz T = 10 K 39.0 41.8 Cavity Transmission (Arb. units, offset)H (kG) 90 80 70 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 -90 -60 -30 0 30 60 90 38500 39000 39500 40000 40500 41000 41500 42000 Peak Position (Gauss)Angle ( ) big line little line 30.0 35.0 40.0 45.0 50.0 40.1 f ~ 116 GHz T = 150 K Cavity Transmission (Arb. units, offset)H (kG) -45 -55 -65 -75 -85 -95 -105 -115 -125 -135 -145 -155 -165 -175 -185 -195 -205 -215 -225 -235 (a) (b) ( c )

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200 Figure 6 28. EMR lines of NiCr thick film as a function of angle in lower field. The field dependence of the microwave transmission through the resonance cavity as a function of angle, measured at f ~ 50 GHz and T = 10 K, for Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O 400 cycle films. Here the data have been offset to more clearly display the evolution of the line. Figure 6 29. Results of fitting EMR lines of NiCr thick film as a function of angle in lower field. Fitting the temperature dependence of the 400 cycle film shown in Figure 6 28 to Lorentzian lines yield (a) the center positions and (b) the fullwidth half maximum ( FWHM) of the lines. (c) The difference between parallel and perpendicular resonance as a function of frequency shows the lack of field dependence for the splitting of the line. This lack of field dependence rules out g factor anisotropy as the source of the observed splitting. The dotted line is a guide to the eye. 10.0 15.0 20.0 25.0 f ~ 50 GHz T = 10 K 16.2 19.1 Cavity Transmission (Arb. units, offset)H (kG) 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 -90 -60 -30 0 30 60 90 16000 16500 17000 17500 18000 18500 19000 19500 Peak Position (Gauss)Angle ( ) big line little line -120 -90 -60 -30 0 30 60 90 1000 1200 1400 1600 1800 2000 FWHM (Gauss)Angle ( ) big line little line 0 20 40 0 500 1000 1500 2000 2500 3000 3500 Splitting of Line (Gauss)Resonance Field (kG) (a) (b) ( c )

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201 Table 6 1 Molecular formulas of films measured and techniques used. constituent ions chemical formula measurements Co 2+ (S = 3/2) Cr 3+ (S = 3/2) Rb 0.6 Co 4.0 [Cr(CN) 6 ] 2.9 nH 2 O magnetization Cu 2+ (S = 1/2) Cr 3+ (S = 3/2) Rb 0.7 Cu 4.0 [Cr(CN) 6 ] 2.9 nH 2 O magnetization, UV Vis Zn 2+ (S = 0) Cr 3+ (S = 3/2) Rb 0.3 Zn 4.0 [Cr(CN) 6 ] 2.8 nH 2 O magnetization Ni 2+ (S = 1) Fe 3+ (S = 1/2) Rb 0.9 Ni 4.0 [Fe(CN) 6 ] 2.8 nH 2 O magnetization Cu 2+ (S = 1/2) Fe 3+ (S = 1/2) Rb 0.5 Cu 4.0 [Fe(CN) 6 ] 2.7 nH 2 O magnetization, thickness dependence, EMR Co 2+ (S = 3/2) Fe 3+ (S = 1/2) Rb 0.7 Co 4.0 [Fe(CN) 6 ] 2.8 nH 2 O magnetization, photoinduced magnetization, in situ rotation Zn 2+ (S = 0) Fe 2+ (S = 0) Rb 0.5 Zn 4.0 [Fe(CN) 6 ] 2.8 nH 2 O magnetization Figure 6 30. Ligand field levels of CoCr. A multiplet calculation can be performed to show the el ectronic energy levels for the Co2+ and Cr3+ ions in the Prussian blue network On the left, chromium energy levels are shown for (i ) no spinorbit coupling and ( i i ) using reported values of spinorbit coupling for the free ion [ 5 ] On the right, cobalt energy levels are shown for ( iii) no spinorbit coupling and (iv ) using reported values of spinorbit coupling for the free ion [ 5 ] 0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000 4A2gCoII(NC)6 (iii) (iv) (ii)Energy (cm-1)(i)CrIII(CN)60 2 4 6 8 10 12 2T2g 2Eg 2T1g 2A2g 2T2g 2Eg 2T1g 2T2g 2Eg 2T1g 2T2g 2A1g 2T1g 4T1g 2T2g 2T1g 4T2g 2Eg4T1g2Eg 2T2g 2T1g 2T2g 2T1g 4T1g 2Eg 2A2g 2T2g 2T1g 2E2T1g 2T2g 2A1g 4T1g 4T2g 2T2g 2T1g 2Eg4A2g Energy (eV)

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202 Figure 6 31. Magnetic susceptibility of Co Cr thin film. The temperature dependences of the low field, 100 G, magnetizations are shown for 200 cycle Rb0.6Co4.0[Cr(CN)6]2. 9 nH2O thin film s, parallel ZFC ( ), parallel FC ( ), perpendicular ZFC ( ) and perpendicular FC ( ). Here connecting lines of orange and blue are guides to the eye. Figure 6 32. Ligand field energies of CuCr. A multiplet calculation can be performed to show the el ectronic energy levels for the Cu2+ and Cr3+ ions in the Prussian blue network On the left, chromium energy levels are shown for (i ) no spinorbit coupling and ( ii) using reported values of spinorbit coupling for the free ion [ 5 ] On the right, copper energy levels are shown for ( iii) no spinorbit coupling and (iv ) using reported values of spin orbit coupling for the free ion [ 5 ] 0 10 20 30 40 0.0 2.5 5.0 7.5 10.0 12.5 M (10-2 emuG/cm2)T (K) 0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000 CuII(NC)6 (iii) (iv) (ii)Energy (cm-1)(i)CrIII(CN)60 2 4 6 8 10 12 2T2g2E2g2Eg 2T2g 2T1g 2T2g 2T1g 4T1g 2Eg 2A2g 2T2g 2T1g 2E2T1g 2T2g 2A1g 4T1g 4T2g 2T2g 2T1g 2Eg4A2g Energy (eV)

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203 Figure 6 33. Magnetic susceptibility of Cu Cr thin film. The temperature dependences of the low field, 100 G, magnetizations are shown for 200 cycle Rb0.7Cu4.0[Cr(CN)6]2.9 nH2O thin film s, parallel ZFC ( ), parallel FC ( ), perpendicular ZFC ( ) and perpendicular FC ( ). Here connecting lines of orange and blue are guides to the eye. Figure 6 34. UV Vis of Cu Cr thin film. (a) Temperature dependent UV Vis spectroscopy measurements of an 4A2g(F) 4T2g(F) type transition on the Cr3+ ion of an 80 cycle Rb0.7Cu4.0[Cr(CN)6]2.9 nH2O thin film on a quartz solid support. (b) A difference plot of the UV Vis absorption, displaying the temperature dependent shift and sharpening of the line. 0 20 40 60 80 100 0.0 0.5 1.0 1.5 M (10-4 emuG/cm2)T (K) 20,000 22,000 24,000 26,000 2.48 2.73 2.98 3.22 E ( eV )Absorbance (arb. units)E ( cm-1) 300 K 175 K 100 K 75 K 50 K 25 K 12 K 20,000 22,000 24,000 26,000 2.48 2.73 2.98 3.22 E ( eV )Absorbance (arb. units)E ( cm-1) 300 K 175 K 100 K 75 K 50 K 25 K 12 K

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204 Figure 6 35. Ligand field energies of ZnCr. A multiplet calculation can be performed to show the el ectronic energy levels for the Zn2+ and Cr3+ ions in the Prussian blue network On the left, chromium energy levels are shown for (i ) no spinorbit coupling and ( ii) u sing reported values of spinorbit coupling for the free ion [ 5 ] On the right, zinc energy levels are shown for ( iii) no spinorbit coupling and (iv ) using reported values of spinorbit coupling for the free ion [ 5 ] Figure 6 36. Magnetization of ZnC r versus field. The magnetic field dependences of the low temperature, 2 K magnetizations are shown for Rb0. 3Zn4.0[Cr(CN)6]2. 8 nH2O thin films in parallel ( ) and perpendicular ( ) orientations. Here connecting lines of orange and blue are guides to the eye. 0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000 ZnII(NC)6 (iii) (iv) (ii)Energy (cm-1)(i)CrIII(CN)60 2 4 6 8 10 12 1Ag2Eg 2T2g 2T1g 2T2g 2T1g 4T1g 2Eg 2A2g 2T2g 2T1g 2E2T1g 2T2g 2A1g 4T1g 4T2g 2T2g 2T1g 2Eg4A2g Energy (eV) 0 20 40 60 0.0 0.1 0.2 0.3 0.4 0.5 M (10-2 emuG/cm2)H (kG)

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205 Figure 6 37. Ligand field energies of Ni Fe. A multiplet calculation can be performed to show the el ectronic energy levels for the Ni2+ and Fe3+ ions in the Prussian blue network On the left, iron energy levels are shown for (i ) no spinorbit coupling and ( ii) using reported values of spin orbit coupling for the free ion [ 5 ] On the right, nickel energy levels are shown for ( iii) no spinorbit coupling and (iv ) using reported values of spinorbit coupling for the free ion [ 5 ] Figure 6 38. Magnetic susceptibility of NiFe thin films. The temperature dependences of the low field, 100 G, magnetizations are shown for 200 cycle Rb0.9Ni4.0[Fe(CN)6]2.8 nH2O thin film s, parallel ZFC ( ), parallel FC ( ), perpendicular ZFC ( ) and perpendicular FC ( ). Here connecting lines of orange and blue are guides to the eye. 0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000 2T2g 4T2g 2T1g 4T1gNiII(NC)6 (iii) (iv) (ii)Energy (cm-1)(i)FeIII(CN)60 2 4 6 8 10 12 2Eg 2T2g 2T1g 2T1g 2Eg 2A1g 2T1g 2A2g 2T2g 2Eg 2T2g 4A2g 2A1g 4T1g 4Eg 2Eg 4T2g 4A1g 2A1g 2T2g 2T1g 2Eg 2T2g 2T1g 2A2g 6A1g 4T2g 4T1g 1A1g 1T2g 1Eg 1T1g 3T1g 1A1g 1T2g 3T1g 1Eg 3T2g3A2g 2T2g Energy (eV) 0 10 20 30 0.0 0.5 1.0 1.5 2.0 M (10-4 emuG/cm2)T (K)

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206 Figure 6 39. Ligand field energy levels and rotational magnetism of CoFe. (a) A multiplet calculation can be performed to show the el ectronic energy levels for the Co2+/Co3+ and Fe3+/Fe2+ ions in the Prussian blue network On the left, Co3+Fe2+ energy levels are shown for (i ) no spinorbit coupling and ( ii) using reported values of spin orbit coupling for the free ions [ 5 ] On the right, Co2+Fe3+ energy levels are sh own for ( iii) no spinorbit coupling and (iv ) using reported values of spin orbit coupling for the free ions [ 5 ] (b) Rotating the Co Fe powder after photoirradiation at 5 K and 100 G has little effect on the magnetism. Inset: Detail showing the robustness of the magnetization with respect to rotation. (c) Rotating the CoFe film after photoirradiation at 5 K and 100 G reduces the magnetism. For (b) and (c), orientation and photoirradiation is shown in the timeline above the graph.

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207 Figure 6 4 0 Ligand field energies of CuFe. A multiplet calculation can be performed to show the el ectronic energy levels for the Cu2+ and Fe3+ ions in the Prussian blue network On the left, iron energy levels are shown for (i ) no spinorbit coupling and ( ii) u sing reported values of spin orbit coupling for the free ion [ 5 ] On the right, copper energy levels are shown for ( iii) no spinorbit coupling and (iv ) using reported values of spinorbit coupling for the free ion [ 5 ] Figure 6 41. Magnetic susceptib ility of Cu Fe thin films. The temperature dependences of the low field, 100 G, magnetizations are shown for (a) 200 cycle Rb0.5Cu4.0[Fe(CN)6]2.7 nH2O thin film s and (b) 20 cycle Rb0.5Cu4.0[Fe(CN)6]2.7 nH2O thin film s, parallel ZFC ( ), parallel FC ( ), perpendicular ZFC ( ) and perpendicular FC ( ). Here connecting lines of orange and blue are guides to the eye. 0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000 2T2g 4T2g 2T1g 4T1gCuII(NC)6 (iii) (iv) (ii)Energy (cm-1)(i)FeIII(CN)62T2g2E2g0 2 4 6 8 10 12 2Eg 2T2g 2T1g 2T1g 2Eg 2A1g 2T1g 2A2g 2T2g 2Eg 2T2g 4A2g 2A1g 4T1g 4Eg 2Eg 4T2g 4A1g 2A1g 2T2g 2T1g 2Eg 2T2g 2T1g 2A2g 6A1g 4T2g 4T1g2T2g Energy (eV) 0 10 20 30 0.00 0.05 0.10 0.15 0.20 M (10-4 emuG/cm2)T (K) 0 10 20 30 -0.01 0.00 0.01 0.02 0.03 0.04 0.05 M (10-4 emuG/cm2)T (K)

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208 -2 -1 0 1 2 -4 -2 0 B[0] B[x] ( 0M0)distance from center of film (cm) Figure 6 42. Demagnetizing fields in films uniformly magnetized perpendicular to the surface. (a) The change in the B field in and outside the 2 m high film with respect to the value at the center of the film, B[0] = 9.0031631 x 105 0M0. (b) The B field is small c ompared to 0M0, and continuous across the film boundary. (c) Inside the film, the H field opposes the magnetization with maximum demagnetization, which is equivalent to N = 1 in Equation 6.2. -20 -10 0 10 20 0.0 0.2 0.4 0.6 0.8 1.0 B ( 0M0)distance from center of film (cm) -4 -2 0 2 4 0.0 0.2 0.4 0.6 0.8 1.0 H (M0)distance from center of film (cm) Figure 6 43. Demagnetizing fields in films uniformly magnetized parallel to the surface. (a) The B field in and outside the 1 cm long film. (b) Inside the film, there is no demagnetizing H field. This lack of demagnetization is equivalent to N = 0 in Equation 6.2. -10 -5 0 5 10 0 2 4 6 8 10 B ( 0M0)distance from center of film (cm) -2 -1 0 1 2 -1.0 -0.5 0.0 H (M0)distance from center of film ( m) (a) (b) (c) (a) (b)

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209 Figure 644. Fitting magnetization of Ni Cr thin films in low field. (a) The low field volume susceptibility used for modeling magnetization with Equation 6.7. (b) The magnetization at H = 100 G of a thin film of Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O in parallel ( ) and perpendicular ( ) orientations is modeled with meanfield theory ( ), an anisotropic superexchange constant ( ), mean field theory with a demagnetizing factor ( ), and the raw parallel data adjusted with a demagnetizing factor to approximate the perpendicular orientation ( ), where N = 0.07 and N = 0.84. 0 50 100 150 0 2 4 6 8 10 lowT (K) 0 20 40 60 80 100 0.0 0.5 1.0 1.5 2.0 2.5 3.0 M (10-2 emuG/cm2)T (K) (a) (b)

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210 Figure 6 45. Fitting magnetization of Ni Cr thin films in high field. (a) The high field volume susceptibility (raw from fitting experimental data without any additional factor of 5) and (b) the high field constant magnetization used for modeling magnetization with Equation 6.14. (c) The magnet ization at H = 40 kG of a thin film of Rb0.7Ni4.0[Cr(CN)6]2.9 nH2O in parallel ( ) and perpendicular ( ) orientations is modeled with meanfield theory ( ), an anisotropic superexchange constant ( ), mean field theory with a demagnetizing factor ( ), a nd the raw parallel data adjusted with a demagnetizing factor to approximate the perpendicular orientation ( ), where N = 0 and N = 1. Figure 6 46. Thickness dependence of thin films. (a) The thickness dependence of the anisotropy measured in 100 G at 2 K for Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O thin films. (b) An AFM measurement showing powder like features growing on the surface of a 30 cycle, thin film (c) An SEM measurement showing powder like features growing on the surface of a 200 cycle, thick film. 0 50 100 150 0.0 0.5 1.0 1.5 0 5 10 M0 (105 A m)T (K) high (10-3) 0 20 40 60 80 100 120 140 0 1 2 3 4 5 M (10-2 emuG/cm2)T (K) 0 20 40 60 80 100 120 140 0.0 0.2 0.4 0.6 0.8 1.0 40 cycle H|| 40 cycle H 200 cycle H|| 200 cycle H 400 cycle H|| 400 cycle H spin cast H|| spin cast H M / M PAR (T = 2 K)T (K) (a) (b) (c) (a) (b) (c)

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211 Figure 6 47. EMR lines of NiCr thin films and powder. (a) A superposition of 40 cycle Rb0. 7Ni4.0[Cr(CN)6]2. 9 nH2O thin film EMR lines in parallel (orange) and perpendicular (blue) orientations, and a bulk powder line. All data was taken at 10 K and f ~ 116 GHz. (b) Observed peak positions for powder ( ), 400 cycle film parallel to the applied field ( ), and 400 cyc le film perpendicular to the applied field ( ). Peak positions using the demagnetizing formalism for the 400 cycle film parallel to the applied field ( ), and 400 cycle film perpendicular to the applied field ( ). 30 35 40 45 50 -2.0 -1.5 -1.0 -0.5 0.0 0.5 Transmission (arb. units)H (kG) 0 50 100 150 39.0 39.5 40.0 40.5 41.0 41.5 42.0 Peak position (kG)Temperature (K) (a) (b)

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212 CHAPTER 7 HETEROSTRUCTURES OF PRUSSIAN BLUE ANALOGUES 7 One of the most compelling features of molecular magnets is their ability to be rationally designed, giving both increased understanding of fundamental physical phenomenon as well as new or enhanced physical effects. Prussian blue analogues (PBAs) are the class of molecular magnets in which metals are bridged by cyanide in order to generate simple cubic structures. The RbaCob[Fe(CN)6]c nH2O (Co Fe) PBA has been of particular interest because it shows long lived persistent photoinduced magnetism at temperatures below nominally 100 K, with long range magnetic order apearing below ~ 20 K [ 1 ] Unfortunately, from a practical standpoint, 20 K is stil l undesirable because of the need for expensive cryogenics to reach such cold temperatures. In attempt to address this problem, CoFe was incorporated into heterostructures including other Prussian blue analogues. First, studies of an atomically mixed ternary Prussian blue analogues containing CoFe will be presented, and then thin film heterostructures containing layers of Co Fe will be presented. The results demonstrate a novel method to increase the photoinduced ordering temperature and to tune the si gn of the photoinduced magnetization. 7 .1 Solid Solutions of Cobalt Hexacyanoferrate 7.1.1 Introduction The ability to purposefully tune magnetic properties of synthetic materials has motivated progress in the area of molecule based magnets. A new class of magnetic coordination compounds was opened when long range magnetic order was discovered in Prussian blue [ 61 ] [ 62 ] [ 63 ] [ 64] and when its atomic and magnetic structures were

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213 elucidated [ 65] [ 55 ] leading to the notion that properties could be controll ed by changing transition metal ions within the parent cubic framework. Binary metal Prussian blue analogues (PBAs), AM [M(CN)6] nH2O (where A is an alkali metal ion, M and M are transition metal ions, and the values of and n depend upon the stoichiometry) and similar materials, have been the subject of extensive research due to their diverse and exciting range of magnetic properties [ 59 ] [ 60 ] [ 114 ] Room temperature magnetic order [ 54 ] [ 11 5 117 ], [ 115] [ 116 ] [ 117 ] photoinduced magnetization [ 1 ] [ 57 ] [ 68 ] [ 81] [ 82] [ 87 ] [ 118 ] [ 119 ] [ 120] [ 121] [ 122] [ 123 ] [ 124 ] [ 90] [ 11 8 124] thermal charge transfer induced spin transitions (CTIST) [ 83] [ 84 ] [ 125] photoinduced tuning of magnetic coupling [ 126 ] anisotropic photoinduced magnetism in thin films [ 10 ] [ 10010 3 ], [ 100] [ 101] [ 102] [ 103 ]and linkage isomerism [ 12 7 13 2 ], [ 127] [ 128] [ 129 ] [ 130 ] [ 131 ] [ 132 ]are among the phenomena observed in this class of compounds. The photoinduced magnetism, discovered by Hashimoto and coworkers in K0.2Co1.4[Fe(CN)6] 6.9H2O, has proven to be one of the more fascinating features of PBAs [ 1 ] Briefly, at low enough temperatures, incident light can cause an electron to transfer from Fe2+ (LS, S = 0) to Co3+ (LS, S = 0), yielding long lived metastable Fe3+ (LS, S = 1/2) CN Co2+ (HS, S = 3/2) pairs that couple antiferromagnetically and give rise to an observed increase in magnetization. An impressive body of work has elucidated the details of the thermal and optical CTIST effects in this series of compounds, ACo [Fe(CN)6] nH2O (A = Na, K, Rb, Cs) [ 1 ] [ 57] [ 68 ] [ 81] [ 82 ] [ 87 ] [ 118] [ 119] [ 120 ] [ 121 ] [ 122 ] [ 123 ] [ 124] [ 90] [ 11 8 124 ] There has been previous interest in the socalled ternary metal PBAs of the form AM ''1 xM 'x[M(CN)6] nH2O (where M '' and M occupy similar lattice sites as determined by x) stemming from the additional effects that are sometimes observed, such as

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214 photoinduced magnetic pole inversion [ 133 ] [ 134] dilution of spin crossover [ 135 ] and magnets having different types of Ne l order [ 136 13 8 ]. [ 136] [ 137] [ 138]This ability to substitute different transition metals in the compound is due to the similar lattice parameters of the cubic binary PBAs. In addition to novel function, new insight into the underlying physical properties of the compounds can be obtained through a study of these mixed PBAs. In this section, the ternary Prussian blue analogue of the form NaNi1 xCox[Fe(CN)6] nH2O will be discussed The proposed structure is one in which a cubic iron sublattice interpenetrates a cubic sublattice containing a statistical mixture of cobalt and nickel ions Figure 71 The use of the sodium cation allows for clear thermal hysteresis [ 84 ] and the presence of Ni2+ gives rise to ferromagnetic superexchange pathways between Ni2+ and Fe3+ with an exchange constant that is similar in magnitude to the Co2+ NC Fe3+ exchange [ 139 ] The role of the ferromagnetic species in the photodecrease can be illustrated by consideri ng the Ni rich ( x 0 ) and Ni poor ( x 1 ) substitution regimes, Figure 7 1 (a) and (b) Although a similar mix of materials yielding Co0.75Ni0.75[Fe(CN)6] 6.8 H2O has already been reported elsewere [ 140 ] the thermally and optically induced bistabilities of the spin states are not present due to the stoichiometry. The studies reported in this section show NaNi1 xCox[Fe(CN)6] nH2O bulk powder displays photoinduced magnetism that can be either positive or negative depending upon the cobalt fraction, x = [C o]/([Co] + [Ni]) the applied magnetic field, and the temperature. These observations are only the second report of a photoinduced decrease in magnetization in this class of photoswitchable coordination compounds.

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215 For the first time, the sign of the phot oinduced change in magnetization can be controlled by tuning chemical composition. Additionally, an x dependence is found for the ordering temperature, the coercive field, the amount of photoactive material, the magnitude of the thermal CTIST, and the width of the thermal CTIST hysteresis loop. Qualitative understand ing of these results can be obtained through the use of s imple molecular field theories. It is important to note that these materials are analyzed with the assumption of antiferromagnetic interactions in the binary NaCo[Fe(CN)6] nH2O material, as is precedented by the majority of the literature. However, additional modeling of the binary materials by the author suggests that the current assignment of ferrimagnetism is ambiguous. As a result, the microscopic origin of the JCoFe may be due to other interactions, such as singleion effects on the nickel. Alternatively the current understandi ng of the material may be correct, and the JCoFe energy may indeed be due to superexchange. This situation is one in which more experimental data is required, and upcoming neutron scattering experiments are scheduled to address these issues. This work wa s published, in part, in the Journal of the American Chemical Society [ 141 ] Those sections contained within the JACS article are copyright of the ACS (copyright release form in Appendix C ). 7.1.2 Synthesis and Chemical Composition Prussian blue analogues NaNi1 xCox[Fe(CN)6] nH2O were synthesized by Dr. Justin E. Gardner by varying the relative cobalt fraction, [Co2+ (aq)]/([Co2+ (aq) + [Ni2+ (aq)]), present during synthesis from 0.0 to 1.0 in steps of 0.2 [ 93 ] [ 141 ] Energy dispersive x ray spectroscopy (EDS) was performed on a JOEL 2010F instrument to establish

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216 transition metal composition. Samples were deposited as methanol suspensions onto 400 mesh copper grids with holey carbon support films, purchased from Ted Pella, Inc. Combustion analysis to determine carbon, hydrogen, and nitrogen (CHN) percentages was performed by the University of Florida Spect roscopic Services laboratory. The resultant chemical formulas given in Table 7 1 were determined from EDS, FT IR, and CHN analyses. The Co, Ni, and Fe ratios were explicitly taken from the EDS results, because the signals for these ions are clean and reproducible. The percentages of C, H, and N were taken directly from combustion analyses. By using the combustion results for the hydrogen content, the amount of oxygen was calculated by assuming all hydrogen and oxygen are in H2O molecules. 7.1.3 T ransmission E lectron M icroscopy T ransmission electron microscopy (TEM) was performed on a JOEL 2010F instrument to establish particle size. Samples were deposited as methanol suspensions onto 400 mesh copper grids with holey carbon support films, purchased from Ted Pella, Inc. Particle sizes were determined from the TEM images by measuring the edge length of more than 50 particles for each composition through the use of ImageJ imaging software [ 73] For identical synthesis protocols, excepting the ratio of Co2+ to Ni2+, the equilibrium size of the particles evolves continuousl y, with particles becoming smaller as more Ni2+ ions are introduced into the lattice, Figure 72 Finally, some control over particle size for a given x value is possible by varying the concentration and the amount of time that the particles are in soluti on before isolation. However, there are no observable changes in the magnetization, for given values of x, as a function of size within the regime studied [ 8 ] [ 72] [ 142 ]

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217 7.1.4 Infrared Spectroscopy A Thermo Scientific Nicolet 6700 spectrometer was used to record Fourier transform infrared (FT IR) spectra, using KBr pellets or powder samples spr ead between NaCl plates. The relative ratios of Fe2+ and Fe3+ were estimated by fitting and subsequently integrating the cyanide stretching peaks in the FT IR spe ctra associated with each species. Extinction coefficients of the cyanide stretching bands of the ternary Prussian blue analogue compounds were estimated from those measured for Ni2+Fe3+ and Ni2+Fe2+ Prussian blue analogue species [ 93] [ 141 ] The FT IR s pectrum of the pure cobalt hexacyanoferrate displays peaks at 2163, 2120, 2090, and 2040 cm1, corresponding to the cyanide stretches of the Co2+Fe3+ (HS), Co3+Fe2+ (LS), Co2+Fe2+, and linkage isomerized Co2+Fe2+ phases, respectively [ 75] The FT IR spectrum of the pure nickel hexacyanoferrate displays peaks at 2160 and 2125 cm1 corresponding to the bridged and terminal cyanide of Ni2+Fe3+ Prussian blue analog, as well as, peaks at 2079 and 2043 cm1 corresponding to the same assignments for the reduced Ni2+Fe2+ sites [ 143 ] As the concentration of Ni2+ in the lattice is increased at the expense of Co2+ ions, the intensities of the three peaks at 2120, 2090, and 2040 cm1 decrease, while that of the peak at 2163 cm1 remains relatively unchanged and a peak at 2125 cm1 emerges. These intensity changes indicate both the reduction in the number of cobalt iron pairs and the subsequent formation of nickel iron pairs. FT IR spectra and the peak fitting results can be found in Figure 73 7.1.5 X Ray Diffraction To investigate the lattice constants and crystal structure, a Philips APD 3720 powder diffractometer, housed in the Major Analytical Instrument Center at the Unive r sity of Florida, was used to perform room temperature x ray diffraction (XRD)

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218 us ing a Cu K source. Between 1020 mg of the same samples used for all other characterizations including magnetometry, except for the x = 1.00 sample that only had 0.6 mg left by the time XRD was performed, were mounted on glass slides and pressed onto squ ares of double sided cellophane tape of ~ 2.3 cm2. The resulting diffractogram s, Figure 7 4 and Figure 75 were used to model the structure by a Rietveld refinement using the EXPGUI [ 53 ] interface for GSAS [ 52] In order to approximate the complicated Prussian blue analogue structure, a singlephase model with m Fm 3 (No. 225) space group symmetry was used. Specifically, the cobalt and nickel atoms were forced to occupy the same site. Atomic occupancies were set by the experimentally determined chemical formulas, excepting the oxygen atoms of the interstitial waters that were allowed to vary as the samples may have dehydrated or hydrated between synthesis and diffraction. The same site symmetries as in Prussian bl ue were used, where the iron vacancies were replaced by the six coordinated oxygen atoms of the ligand water molecules [ 65 ] Placement of the oxygen atoms of the interstitial water molecules at the 32f Wyckoff position [ 68 ] and a relatively small percentage at the 192l position was found to yield a robust local minima during refinement procedure. As clearly displayed by the (4, 0, 0) reflection the unit cell constants change continuously when changing from x = 0 to x = 1, Figure 76 7.1.6 Mean Field C alculations Using simple mean field approximations, investigat ions of the possible effects to be observed in the magnetization were performed first and subsequently refined the model after completing a series of experiments. In order to model the magnetic interactions, an approximation in which superexchange energies act as effective

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219 magnetic fields, socalled Weiss fields, was employed in a manner akin to previous works in similar materials [ 13 2 13 8 ]. [ 133] [ 134 ] [ 136 ] [ 137] [ 138]In order to model the cooperative thermally active CTIST event, a Bethe Peierls Weiss approximation to a phenomenological spincrossover Hamiltonian was implemented [ 97] These numerical studies extend the previous work of others by allowing the highspin fraction, nHS, which is experimentally controlled by irradiation and temperature, to vary along with the relative metal concentration, x, which is dictated by the synthesis. Details of the calculations, including how x and nHS are utilized to provide numerical results that are directly comparable to the experimental data. 7.1.6.1 Low t emperature m agnetization in the m ean f ield Calculations were performed to investigate effects on the low temperature magnetic susceptibility due to the substitution of Ni atoms for Co in NaNi1 xCox[Fe(CN)6] nH2O. For simplicity, consider only superexchange of spins associated with nearest transition metal neighbors, designated as n.n. under the influence of an applied magnetic field, and thus the Hamiltonian has the form = 2 JijSi Sj + g BH Si ,i i j = n n 7 1 where J is an exchange constant, g is the Land factor, B is the Bohr magneton, S is the electronic spin, and H is the applied field. One method of investigating Equation 7.1 is the usual mean field expansion of the spin operator. For the system in question, three magnetic ions must be considered: (a) Fe3+, which can superexchange with either Ni2+ or Co2+, (b) Ni2+, which only superexchanges with Fe3+, and (c) Co2+, which also only superexchanges with Fe3+. Two parameters, x

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220 and nHS, are introduced to keep track of the magnetic species present when calculating the temperature dependence of the magnetization, where x is the molar fraction of Co, x = [Co]/([Co] + [Ni]), and nHS tracks the amount of material that has undergone CTIST. It is important to note that the highspin fraction, nHS, is representative of the amount of material that is actually magnetic. Expres sions for the average spin polarization values of the constituent ions can be derived by minimizing the free energy with respect to simultaneous variation of the spin polarizations of the different sublattices, yielding, SNi = SNi BS g B S Ni H extkBT + 2 Z NiFe J NiFekBT SNi SFe 7 2 SCo = SCo BS g B S Co H extkBT + 2 Z CoFe J CoFekBT SCo SFe and 7 3 SFe = SFe BS g B S Fe H extkBT + 2 Z FeCo J CoFekBT SFe SCo + 2 Z FeNi J NiFekBT SFe SNi 7 4 where BS is the Brillouin function, kB is the Boltzmann constant, and ... denotes an average. Total spin numbers of SNi = 1 for Ni2+, SFe = 1/2 for Fe3+, and SCo = 3/2 for Co2+ were used, with other species being diamagnetic. There is an explicit dependence of Z upon both x and nHS that can be resolved by considering statistical mixing and using the chemical formula, that is to say, ZNiFe = 4.0(1 x) + 3.3x nHS ZCoFe = 4.0(1 x) + 3.3x nHS ZFeCo = 2.7x + 3.3x nHS and ZFeNi = 6.0(1 x). Subsequent to simultaneous solution of Equations 7.3 7.5 the magnetization can be calculated from M Ni = 4 00 ( 1 x ) Ng B S Ni 7 5 M Co = ( 1 83 x + 2 17 x n HS ) Ng B S Co 7 6 MFe = 3 26 x nHS + 4 00 ( 1 x ) NgB SFe 7 7

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221 and M total = D ( M Ni + M Co + M Fe ) 7 8 where an additional parameter D accounts for the presence of magnetic domains after the onset of longrange magnetic order. In practice, D is chosen to be 100, as estimated by fitting the pure materials. At low temperature, if most of the orbital momentum is quenched values of g ~ 2 are reasonable for all species. 7.1.6.2 Mean field m agnetic s usceptibility and s pinc rossover To understand how the magnitude and hysteresis of the thermally induced CTIST are affected by the introduction of Ni atoms to the lattice, an analytical model was employed [ 75 ] This solution exploits the mapping between the extensively studied Ising Hamiltonian and the phenomenological spincrossover Hamiltonian to be considered, = J sisj < i j > T 2 ln g +g si i 7 9 where g+ and g are the degeneracies of the highspin (HS) and low spin (LS) states, JSCO is the intermolecular interaction energy associated with the different spin states, is the octahedral splitting energy, s is a pseudospin keeping track of the spin state, T is temperature, kB is the Boltzmann constant, and refers to a summation over nearest neighbors. In a manner similar to that employed by Hoo et al [ 97 ] it is straightforward to calculate the HS fraction. For these calculations, it is assume d that the entropy content of the different states, the intermolecular interaction, and the energy difference between the HS and LS states are unchanged as x is changed. A small effect on the onset of the transition is expected to arise from the slightly varying Na content in the samples, even while the width of the hysteresis loop is unchanged [ 84] but this effect is not

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222 considered. The number of nearest neighbors to be considered in the spincrossover event for the different samples is taken from the chemical formulas by taking the minimum occupation necessary for the charge transfer. Specifically, zSCO(x = 1.00) = 4.32, zSCO(x = 0.87) = 3.8, and zSCO(x = 0.66) = 4.0, each determined from the switchable Fe content, zSCO(x = 0.45) = 2.7, and zSCO(x = 0.22) = 1.3, each determined from the amount of Co, and zSCO(x = 0.00) = 0 because there are no Co ions in this sample. Values of JSCO = 150 K, ln(g+/g) = 250, and = 550 K are used for all calculations and are determined by fitting the x = 1.00 data. Simultaneous fitting of the data above 250 K for all samples gives gCo = 2.7, gFe = 2.2, and gNi = 2.3. These g > 2 values arise from the incomplete quenching of orbital moments on the ions at high temperatures [ 5 ] Mean fi eld fits for each sample can then be performed in order to calculate nHS above and below the spin transition. 7.1.7 Magnetic Measurements Magnetic measurements were performed using a Quantum Design MPMS XL superconducting quantum interference device (SQUID) magnetometer. A room temperature halogen light source (~12 mW) was used to introduce light into the sample OD (Ocean Optics Model 200), for photomagnetic measurements. Powders were mounted on pieces of cellophane tape around plastic drinking straws to increase the optical crosssection for the photomagnetic studies. High temperature data (T > 100 K) were taken using gelcaps as the sample holders to accommodate additional sample mass. Backgrounds were subtracted from the data by using the measured mass susceptibility of similar sample holders. The same demagnetizing protocol, during which the magnet

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223 field is oscillated to zero by successive ramps starting at 20 kG, was used for all low field measurements in ~10 G. Additionally, the magnet was allowed to relax for more than two hours subsequent to demagnetization and prior to data taking. By using a commercial Hall sensor, an inhouse calibrated Toshiba THS118E, it is likely th at there may be differences of up to ~1 G in the external fields applied to different samples, but for each specimen, the field was not changed between light and dark states for the temperature sweeps, ensuring any resulting effects are not a result of sli ght perturbations of the external field. 7.1.7.1 Low t emperature DC s usceptibility The time dependences of the DC magnetic susceptibilities, = M / H during photoirradiation of the samples are shown in Figure 77 (a). The temperature dependences of the DC magnetic susceptibilities, (T), in ~10 G between 2 K and 30 K for various x values, both before and after photoirradiation, are shown in Figure 77 (b). A clear bifurcation of the fieldcooled (FC) and zerofield cooled (ZFC) curves, with a peak in the ZFC versus T plots, is observed for all samples. The results of the Weiss mean field calculations, as described in Section 7.1.4, are shown in Figure 77 (c). For the mean field calculated susceptibilities of the dark states, the value of the highspin fraction, nHS is dictated by the amount of material measured to undergo spincrossover, whereas the calculated susceptibilities of the photoirradiated states use nHS = 1, where all available material is in the highspin state by definition. All samp les with x > 0 show a change in magnetization due to applied light. Strikingly, at 5 K and in 10 G, the x = 0.66 sample shows a clear decrease in magnetization with photoirradiation in both the calculations and the experimental results.

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224 To a weaker extent, a photoinduced decrease in magnetiz ation is also observed in the x = 0.45 sample. Expanded photoirradiation versus time plots for the x = 0.66 and x = 0.45 samples are shown in Figure 78 7.1.7.2 DC m agnetization Magnetization as a function of the applied magnetic field was measured for all compounds at T = 2 K and up to 70 kG, Figure 79 A scaling of the coercive fields with the mixing fraction, x, was seen. In addition, at the high fields, all samples where seen to have photoind uced increases in magnetization. A check for symmetry of the hysteresis loops as a potential contributing factor to the change in the coercive field was made, Figure 710 7.1.7.3 High t emperature DC s usceptibility The temperature dependences of the DC ma gnetic susceptibility temperature product, T, in 5 kG and between 100 K and 300 K for various x values are shown in Figure 711 (a). To help ensure equilibrium during the spincrossover, a sweep rate of less than 0.5 K/min was employed. The combined Bet hePeierls Weiss spin crossover and Weiss mean field magnetization calculations, as described in Section 7.1.4, are shown in Figure 711 (b) For clarity, the calculated temperature dependence of the high spin fraction, nHS is also shown, Figure 711 (c). All samples with x > 0 appear to show thermally induced CTIST, as evidenced by the abrupt reduction of the magnetic susceptibility upon cooling below ~170 K. These CTIST events can be cycled with temperature and exhibit hysteresis that is character istic of the cooperativity of the transition. Additionally, an evolution of the ferromagnetic slope in T, characteristic of the NaNi[Fe(CN)6] nH2O compound, can

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225 be seen as more Ni is introduced to the lattice in both the numerical and experimental stud ies. Furthermore, the samples show a decrease of the width of the thermal hysteresis, as the amount of Co decreases, and a drastic decrease in the amount of material that undergoes CTIST, when Ni is introduced into the lattice. 7.1.7.4 Physically m ixed x = 0.66 c ompound To make sure that the observed behavior is not due to a physical mixture of the parent compounds on a macroscopic level, a manually mixed sample of separately synthesized nickel hexacyanoferrate and cobalt hexacyanoferrate powders was prepared and studied, Figure 712 and Figure 7 13. For this type of synthesis, the TEM data reveal a bimodal distribution clearly associated with the two distinct sizes of the NaNi[Fe(CN)6] nH2O and NaCo[Fe(CN)6] nH2O powders. The magnetic orderings of both binary species are clearly Vis ible in (T), and the magnetization only increases with irradiation, even though the chemical composition is the same as the x = 0.66 sample showing a photodecrease. 7.1.8 Discussion In the following subsections the three main results of the experimental and numerical work performed on the ternary transition metal Prussian blue analog, NaNi1 xCox[Fe(CN)6] nH2O, are discussed. Highlighted are, first, the observation of a photoinduced decrease in magnetization, second, the scaling of magnetic properties as a function of x, and third, the dependence of the observed CTIST effect upon dilution of the parent cobalt hexacyanoferrate material. Finally, aspects and potential future extens ions of the mean field calculations are examined in light of the experimental results.

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226 7.1.8.1 Photoinduced d ecrease in m agnetization The results of the mean field calculations predict a decrease in magnetization within the ordered state with increasing highspin fraction for NaNi1 xCox[Fe(CN)6] nH2O powders with enough ferromagnetic Ni2+ constituent ions. These predictions are compared to low temperature magneti c susceptibility experiments as a function of x All samples show an increase in magnetization at high field, Figure 79, even those showing the photodecrease at low field, Figure 77 This increase in magnetization at high field, regardless of x, proves that additional spins are being generated, rather than destroyed, during photoirradiation. These results also indicate that the mechanism for the photoinduced magnetization, present in all samples having x > 0, is the same CTIST leading to the persistent long lived metastable states seen in the pure RbCo[Fe(CN)6] nH2O material. The photoeffect can be reproduced and reversed with thermal cycling above approximately 150 K. The abil ity of the meanfield calculations to predict whether a material will have a photoincrease or decrease based upon its composition, Figure 77 substantiates the claim that the observed photoinduced decreases in magnetization can be understood as an interpl ay between ferromagnetic and antiferromagnetic superexchange interactions, as was hypothesized in Figure 71 These points can be further elucidated by focusing on the x = 0.66 sample. This sample has enough ferromagnetically interacting nearest neighbors to begin driving the Fe sublattice parallel to the applied field, while it simultaneously possesses enough Co NC Fe switchable pairs to still show an appreciable CTIST effect. In the low field limit, newly photoexcited Co NC Fe pairs align antiparallel t o the applied field due to the

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227 antiferromagnetic superexchange between Co and Fe ions. Since the Fe ions are already parallel to the field due to the presence of Ni, a net photodecrease in magnetization is observed, Figure 714. If a sufficient external magnetic field is applied, the energy reduction gained by aligning Co NC Fe pairs with the applied field is larger than the superexchange, so a net photoincrease in magnetization is measured, Figure 714. The temperature dependence of both experimental and numerical magnetizations in the low field limit show a decrease in the measured susceptibility below approximately 12 K, above which an increase is observed since the thermal energy is now able to populate the excited states having Co spins parallel to t he applied field. In addition, t here is a time dependence of the photoinduced magnetic effect on the scale of weeks. For example, when the samples were measured again after one month in a freezer at T ~ 248 K, the photodecrease was found to be slightly st ronger by a few percent. This evolution of the magnetic properties may be due to an increase of atomic mixing of the samples arising from solid state diffusion or to the stabilization of the positions of interstitial counterions to regions more prone to i nduce bistabilities in the spin states. As a final point, the expectation of a photoeffect having the opposite sign compared to the pure Prussian blue analogue material due to a mixing of ferromagnetic and antiferromagnetic superexchange interactions is si milar to the photoinduced magnetic pole inversion reported for Fe1 xMnx[Cr(CN)6] nH2O Prussian blue analogue [ 134 ] A fundamental difference, however, is that in the Fe1 xMnx[Cr(CN)6] nH2O system, the applied light is destroying exchange pathways,

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228 whereas additional moments are being generated in the NaNi1 xCox[Fe(CN)6] nH2O system reported here. 7.1.8.2 Scaling of m agnetic p roperties All NaNi1 xCox[Fe(CN)6] nH2O samples studied show spin glass like long range magnetic order, as evidenced by the bifurcation of the FC and ZFC curves, Figure 77 (b). The peak in the ZFC susceptibility is a fingerprint of the spin glass like nature of the order in both parent compounds [ 78 ] [ 14 6 1 50 ] [ 144] [ 145 ] [ 146 ] [ 147 ] [ 148]the presence of which hints at the complicated nature of the magnetism in the samples investigated. It is noteworthy that local minima are present near x ~ 0.8 in the scaling of the magnetic ordering temperature, the coercive field, and the absolute value of the CurieWeiss temperature as a function of x, Figure 715. The observed scaling of magnetic properties in NaNi1 xCox[Fe(CN)6] nH2O can be compared with previous work on ternary transition metal Prussian blue analogues [ 13 6 13 8 ][ 14 9 15 1 ]. [ 136] [ 137 ] [ 138 ] [ 149] [ 150] [ 151 ] [ 152 ]In ternary materials of Cu[CoxFe1 x(CN)6], Ni[CoxFe1 x(CN)6] and Fe[CoxFe1 x(CN)6], a clear monotonic scaling of the transition temperature with x was observed, and these results are dominated by the changing number of magnetic nearest neighbors, since Co3+ on the M site is LS and therefore diamagnetic [ 149 ] Similar monotonic scaling was observed in Ni[CrxFe1 x(CN)6] and Fe[CrxFe1 x(CN)6] [ 150] where the substitution of Cr3+ ( S = 3/2) for Fe3+ ( S = 1/2) provides a threefold increase of the number of superexchange pathways, as the number of unpaired electrons on the M site changes from (t2g)1 to (t2g)3. Finally, the most cogent example is NixMn1 x[Cr(CN)6] nH2O, which displays a clear dip in the ordering temperature and a peak in the coercive field on the background of a linear dependence

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229 interpolating between the values of the parent compounds as x was changed [ 13 6 13 8 ]. A few similarities are obvious when comparing NaNi1 xCox[Fe(CN)6] nH2O and NixMn1 x[Cr(CN)6] nH2O, particularly in the context of simple empirical rules for superexchange [ 38 ] First, both contain a Ni2+ ion that has a ferromagnetic superexchange pathway (eg to t2g). Second, the interaction between Mn2+ and Cr3+ is analogous to the Co2+ interaction with Fe3+ in NaNi1 xCox[Fe(CN)6] nH2O, as there is a competition between ferromagnetic and antiferromagnetic interactions, Figure 716 (a). It is plausible that when two superexchang e energies of opposite sign compete, the net magnetic interaction is particularly susceptible to perturbation when the inter ion distance is changed. The nonmonotonicities observed in the ordering temperatures, as a function of metal substitution, may th erefore be due to a net superexchange that depends strongly on these small distance changes that are introduced with the substitution. In order to reproduce the experimental data, it was necessary to introduce a distance dependence to the Co2+(HS) NC Fe3+(LS) superexchange interaction, Figure 716 (b). The need to introduce distance dependence to the superexchange interaction in order to reproduce the data may seem drastic, but other methods to reproduce the scaling of the magnetic properties were unsucces sful. Two remarkable features are present in the data: the dip in the ordering temperature near x = 0.8 and the unexpectedly large ferromagnetic character of the mixed samples. This increase in ferromagnetic character manifests itself in the lack of a co mpensation point for the mixed ferroferrimagnetic system and in the high temperature slope of T for the mixed samples. Specifically, from 250 K to 300 K, T for the x = 0.66 sample is increasing as

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230 temperature decreases, where a model using the binary magnetic interactions predicts a clear decrease as temperature decreases. For powder samples containing ions in similar environments to ours, the introduction of singleion anisotropy and spinorbit co upling terms can only give rise to a decrease in T as temperature decreases [ 38 ] [ 153 ] [ 154 ] Finally, there are precedents in the literature for such a modification of the superexchange energy. In CsCo[Cr(CN)6] nH2O ferromagnetic compounds having com peting ferromagnetic and antiferromagnetic pathways, a dependence of the superexchange energy on the lattice constant was found [ 155 ] Additionally, an ACo[Fe(CN)6] nH2O material was reported in which ferromagnetic coupling, as opposed to the usual anti ferromagnetic coupling leading to a ferrimagnet, was inferred based upon the hightemperature inverse susceptibility [ 139 ] 7.1.8.3 Spinc rossover d ilution The width of the thermal hysteresis, as represented by TupTdow n, decreases when the cobalt hexacyanoferrate material is diluted, and this trend is correlated with the number of active CTIST nearest neighbors, zSCO, Figure 717. As described in Section 7.1.4.2, zSCO can be calculated from the chemical formula, and the observed narrowing of the hysteresis is an expected result when zSCO decreases. It is worth noting that while the changing number of nearest neighbors is a dominant effect, additional perturbations due to the changing of the local environments of the act ive species are also present. Experimentally, the dilution of spincrossover species has been investigated intensively after being first realized in [FexZn1 x(2 pic)3]Cl2EtOH [ 156 ] [ 157 ] where a gradual reduction in the width of the hysteresis loop was attributed to the many body elastic interactions innate to these

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231 transitions. With respect to CTIST in Prussian blue analogues the field is not as mature and studies are still ongoing. Recently, a CTIST diluted Rb0.70Cu0.22Mn0.78[Fe(CN)6]0.862.05H2O sa mple was compared to its undiluted parent compound Rb0.81Mn[Fe(CN)6]0.951.24H2O, and no appreciable change in the width of the hysteresis loop was observed [ 158 ] Furthermore, there is a striking reduction in the amount of CTIST active material once nick el is introduced into the lattice, Figure 7 18 (a). More specifically, the x = 1.00 material transitions 83% of the amount expected from the chemical formula when sweeping from 300 K to 100 K whereas the x = 0.87 material transitions 16%, and less than 10% transitions in the remainder of the samples with lower x values. The percent of CTIST active material can be established by considering the chemical formula, the room temperature FT IR, and the change in T as the samples are cooled. Although a detailed investigation of the microscopic origins of the observed reduction in spin crossover active material is warranted, the author conjectures that the reduction is related to a Ni induced stabilization of Co NC Fe HS pairs arising from subtle variatio ns of the unit cell parameters, Figure 718 (b). The lattice constants are observed to scale with x in a monotonic fashion that is consistent with changes seen in other ternary metal Prussian blue analogues [ 136 13 8 ][ 14 9 151 ]. [ 136] [ 137 ] [ 138 ] [ 149 ] [ 150] [ 151] [ 152]However, the nonlinear nature of the scaling implies an actual changing of the bond energies in the system as the different systems are mixed. The FT IR data also provide evidence supporting the stabilization of the coordination bond with the incorporation of Ni2+. Using the cyanide stretch associated with the divalent metal to iron bond, plots of the stretching frequency and an effective spring constant as a function of x can be made,

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232 Figure 7 19, implying a stabilization of the bond and an increased rigidity of the lattice with the introduction of nickel ions. Therefore, it may no longer be energetically favorable for Co NC Fe pairs in NaNi1 xCox[Fe(CN)6] nH2O to undergo CTIST due to the added strain that would result for the Ni NC Fe bonds in the system. More specifically, the LS phase of NaCo[Fe(CN)6] nH2O has a lattice constant of 9.9721 whereas the HS phase has a lattice constant of 10.3033 [ 68] which is comparable to the x = 1.00 sample that has a lattice constant of 10.30(7) In contrast, the x = 0.00 nickel hexacyanoferrate species has a lattice constant of 10.23(9) which is comparable to the previously reported value of 10.229 for Ni3[Fe(CN)6]2 [ 139 ] Comparisons of this observation of the reduction of CTIST active material to a recent work studying the dilution of cobalt hexacyanoferrate by diamagnetic Zn2+ at the divalent metal site or by diamagnetic Co3+ at the cyanometallate site [ 135 ] are useful. In particular, a similar sensitivity of the CTIST effect with the substitution of metals is seen by Cafun et al ., [ 135 ] and the reduction in the magnitude of the effect in their samples is also larger than expected by a simple reduction in the spincrossover active species on a molecule by molecule level. It was previously shown by Ksenofontov et al that by application o f hydrostatic pressure to ACo[Fe(CN)6] nH2O powders, a stabilization of the LS phase could be induced in the samples [ 2 ] This pressure sensitivity leads to the obvious contention that the stabilization of the highspin phase for NaNi1 xCox[Fe(CN)6] n H2O may simply be due to an effective negative pressure. Cafun et al [ 135 ] argue that the observed stabilization with metal substitution cannot be due to such an effect because their starting material has a cell size of ~10.32 while the 100% Zn2+ doped material should have a negative pressure due to its cell size of

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233 ~10.40 and the 100% Co3+(CN)6 doped material should have a positive pressure owing to lattice constant of ~10.23 However, these room temperature values all deal with the highspi n lattice constants, and with respect to the low spin lattice constants, the alien species are still larger than the LS Co NC Fe state, and in fact closer in size to the HS Co NC Fe state than the LS Co NC Fe state, suggesting that chemical pressure may st ill be a valid argument for the effect. Finally, more subtle effects on the energy of the cobalt iron charge transfer arising from the presence of neighboring nickel ions may also be present. 7.1.8.4 Mean f ield p redictions versus o bservations The mean fie ld calculations were able to predict whether the magnetization of the chosen samples would increase or decrease with photoirradiation. Upon completion of the experiments, the lack of a compensation point and subsequent negative magnetization, as well as t he general scaling of magnetic properties, was surprising. However, a better agreement between calculations and experiment was found if a distance dependence was introduced for the superexchange constant. In addition, discrepancies between predictions and experiment may stem from the need to choose the simplest Hamiltonian that could capture the spirit of the problem, having only Zeeman and superexchange terms, in order to make the number of free parameters tractable. Future studies with more parameters may be possible once neutron spectroscopy is performed to fix the parameter values to experimental data. Using the machinery described in the previous subsections magnetization as a function of temperature can be calculated. For the first attempt, the superexchange constants for all materials were taken from the nickel to iron and cobalt to iron interactions of the x = 0.00 and x = 1.00 samples, respectively. This method is

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234 appealing because it is predictive and limits the number of model parameters. However, some discrepancies between model and experiment were found, Figure 720 A few additional parameters were tried, with a distance dependent superexchange energy being the most successful at capturing the features observed in the magnetization. The calculations then took on a flavor of fitting the data, with the predictive role already having been fulfilled. First, the susceptibility data from 250 K to 300 K were fit using the meanfield solutions described previously, yielding values for < JNiFe> and < JCoFe>. The nickel to iron superexchange was fixed, and the cobalt to iron sup erexchange was allowed to vary as a function of lattice constant due to the presence of both antiferromagnetic and ferromagnetic superexchange pathways. The distribution of superexchange energies was binned into two populations, CTIST active and CTIST ina ctive, the fractions of both being determined from high temperature susceptibility data ( Figure 721). The CTIST active superexchange value was associated with the population having the closest lattice constant to the x = 1.00 material. 7.1.9 Conclusions It has been demonstrated that the ternary transition metal Prussian blue analogue NaNi1 xCox[Fe(CN)6] nH2O shows a photoinduced decrease in magnetization for certain values of x, temperature, and applied magnetic field. Furthermore, the NaNi1 xCox[Fe(CN)6] nH2O system is the first example of a compound in which superexchange energies control whether incident light increases or decreases CTIST magnetization. As a result, the sign of the photoeffect can be changed by stoichiometry. Although a phot oinduced decrease in magnetization while increasing the number of spins has also been seen in ACo[Fe(CN)6] nH2O thin

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235 films [ 10 ] the microscopic origins are different. In addition, the width of the thermal hysteresis of the CTIST is reduced upon dilution of the active spincrossover species in the ternary mixture. The origins of the experimental observations are nicely explained using mean field calculations. 7.2 Heterostructured Films Containing Cobalt Hexacyanoferrate 7.2.1 Introduction Recently, there has been interest not only in threedimensional systems, but also in two dimensional and quasi two dimensional structures, some of which have been shown to display phenomenon different than that seen in the analogous threedimensional materials [ 10] [ 29 ] [ 76] [93 ] [ 94] [ 9 8 11 2 ] Work on thin films of [ 98] [ 99 ] [ 100 ] [ 101 ] [ 102 ] [ 103] [ 104] [ 105 ] [ 106 ] [ 107 ] [ 108 ] [ 109] [ 110] [ 111] [ 112 ] Co Fe was originally motivated by the functional need to increase the light cross section of the photoactive material, but it is noteworthy that anisotropy in the photoinduced magnetization is seen in the quasi two dimensional geometry [ 10] Originally, the previous student in Professor Meisels lab, Dr. JuHyun Park, had the idea that, by layering the lower ordering temperature CoFe photomagnet between layers of a higher ordering magnet that is not photoactive, a transition between two dimensional and t hreedimensional behavior in the heterostructure might be seen after photoirradiation [ 76] This transition was hypothesized to occur as CoFe spins become magnetic and contribute additional exchange pathways between the higher ordering temperature layers Upon surveying the different possible highTC magnets, nickel hexacyanochromate (Ni Cr) was chosen because of its robustness and similar lattice constant to CoFe [ 54 ] [ 56] Heterostructured films may be generated with a synthesis technique similar to that used for pure phase films, sequential adsorption (Figure 7 22).

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236 Exploration of CoFe/Ni Cr heterostructures in various geometries eventually led to the realization of the desired effect, although in a different manner than originally hypothesized. Heterostructures with sufficiently thick regions of unadulterated highTC magnets and photoactive CoFe, so as to retain bulk like features but sufficiently thin layers so as t o have strong inter layer interactions, show photoinduced modification of long range magnet order up to the limit of the photoinduced structural transition of the Co Fe molecule, ~ 100 K. These highTC photoeffects are due to a propagation of photoinduced structural changes in the CoFe layer propogating to the Ni Cr layers. In the following subsections, these discoveries will be discussed. First, the evolution of studies of CoFe/Ni Cr thin film heterostructures will be presented, leading up to the stru ctures having the largest photoresponse. Second, the optimized heterostructure will be dissected in detail, and the current understanding of the phenomenon will presented. Finally, heterostructures with different chromate Prussian blue analogues will be presented. Elements of the chapter have recently been published in JACS communications [ 159 ] and elsewhere. 7.2.2 Synthesis Synthesis of the desired heterostructure of Ni Cr and CoFe is possible by a sequential deposition method (SD), in which PBA films are generated by sequentially dipping a solidsupport into aqueous solutions containing the desired constituent metals. One such dipping process will often be referred to as one cycle in the following sections. The attractiveness of the SD approach c omes from its combination of simplicity with fine thickness control for generation of homogeneous films of arbitrary PBAs with tunable chemical compositions on a variety of substrates. The synthesis of a heterostructured film is made possible by alternati ng protocols for the SD films of the

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237 sought after component PBAs, the details of which are discussed in Dr. Franz Fryes thesis [ 94] The shorthand notation used to refer to samples first lists the repeat cycle, then the total number of repeats, and final ly the constituent magnets. For example, a film with every other cycle of CoFe and Ni Cr repeated 40 times will be referred to as 1/1 x 40 CoFe/NiCr, and a film with 40 cycles of Ni Cr followed by 40 cycles of CoFe followed by 40 cycles of Ni Cr will be referred to as 40/40/40 NiCr/Co Fe. 7.2.3 Magnetization of Nickel Hexacyanochromate and Cobalt Hexacyanoferrate Heterostructures Due to similar lattice constants and the high ordering temperature of the Ni Cr material, most of the heterostructured fil ms studied were of Ni Cr and CoFe layers. A brief summary of all films to be presented can be found in Table 7 2 7.2.3.1 Slow d eposition m ultilayer f ilms A series of films having the same total number of cycles, but with different numbers of cycles between the alternation of CoFe and Ni Cr depositions, was studied in the SQUID magnetometer. Unlike the rest of the samples to be presented, these were generated using a slow deposition method that included extra washing of the film after deposition [ 94 ] Exemplary samples are a 1/1 x 40 CoFe/NiCr, 5/5 x 20 Co Fe/NiCr, and 40/40 CoFe/NiCr. The temperature dependence of the DC magnetic susceptibilities, = M / H are shown in Figure 7 23 for temperatures between 2 K and 100 K and an external field o f 100 G. The magnetic signals are expressed per cm2. A clear evolution of the magnetic order in the samples can be seen as a progression is made from separate behaviors of the CoFe and Ni Cr magnets to an overall combined magnet that has the addition of superexchange between Co and Cr as well as superexchange between Ni and Fe. The disparity between the changes in

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238 magnetization with photoirradiation can be explained by poor transfer ratios causing less Co Fe content in the film. 7.2.3.2 Stacked f ilms S impler, stacked films of 10/10 CoFe/NiCr and of 10/10 Ni Cr/CoFe were measured to investigate their magnetic and photomagnetic character. The temperature dependence of the DC magnetic susceptibilities, = M / H for temperatures between 2 K and 100 K, an d M versus time irradiated at 5 K and 100 G, are shown in Figure 7 24 (a), Figure 7 25 (a), Figure 7 26 (a), and Figure 7 27 (a). M versus time irradiated is shown in Figure 7 24 (b), Figure 7 25 (b), Figure 7 26 (b), and Figure 7 27 (b). The magnetic signals are all expressed per cm2 of sample. The presence of the Ni Cr component can clearly be seen by the ordering onset at ~70 K and the large magnetic anisotropy exists below this temperature. The increase of magnetization with photoirradiation is clearly coming from the CoFe component. In addition, two ordering temperatures can be seen for both films, with an additional onset at ~10 K. Differences between the stacked films are also apparent, where the anisotropy of the photoeffect is stronger for t he 10/10 Co Fe/NiCr film, which has the Co Fe deposited first, as opposed to the 10/10 Ni Cr/CoFe film, which has the Ni Cr deposited first. A slightly different ordering temperature is also observed for the CoFe component in the films, with the 10/10 C o Fe/Ni Cr film having a lower ordering temperature than the 10/10 Ni Cr/CoFe film. These effects can be explained by low transfer ratios for the first few cycles and by the proximity of the solid support being a key factor in the sign of the photoeffect of the Co Fe.

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239 7.2.3.3 Thin s andwiched f ilms A third set of films, in which a thin layer of CoFe PBA is deposited between two thin layers of Ni Cr PBA, is the so called thin sandwich geometry. Two different heterostructures are presented, 10/5/10 and 10/10/10, where both are NiCr/Co Fe/NiCr, so the additional nomenclature for constituent makeup will be dropped to ease discussion. The temperature dependence of the DC magnetic susceptibilities, = M / H are shown in Figure 7 28 (a), Figure 7 29 (a) and Fi gure 7 30 for temperatures between 2 K and 100 K and an external field of 100 G. For the film 10/5/10 M versus time irradiated is shown in Figure 7 28 (b) for H parallel, and Figure 7 29 (b) for H perpendicular, both in fields of 100 G. M versus time irradiated at 5 K and fields of 100 G and 1 kG is shown in Figure 7 31 and Figure 7 32 for both parallel and perpendicular orientations of the 10/10/10 film with respect to the applied magnetic field. The magnetic signals are expressed per cm2 of sample. The susceptibility data again show the strong presence of Ni Cr magnetism in the samples as well as photoinduced magnetism from the CoFe moeities. However, the most striking feature of the data for the sandwich films is that for the 10/10/10 film, and to a lesser extent for the 10/5/10 film, a clear decrease in magnetization with photoirradiation is seen in an applied field of 100 G in both film orientations. When going to higher external fields of 1 kG, the 10/10/10 film no longer shows a decrease with photoexcitation, but rather an increase for both orientations. 7.2.3.4 Thick s andwiched f ilms At this point, it is worth mentioning that a novel new effect had been seen in the thin sandwiched films, and to help understand the effect, a more easily modeled, solid

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240 solution with a similar set of constituents, NixCo1 x[Fe(CN)6]3/4 nH2O, was studied [ 141 ] Since intimate mixing of foreign species with CoFe strongly damped the photomagnetic bistability, the sandwich geometry was reinvestigated with thic ker layers that were conjectured to possess a larger photoeffect. A larger photoeffect was observed and, in fact, photoinduced changes could be observed as a function of field and temperature in the heterostructures for the first time. Three different heterostructures are presented here, 40/40/40, 20/40/20, and 40/20/40, where all are Ni Cr/Co Fe/Ni Cr, so the additional nomenclature for the constituent makeup will be dropped to ease discussion. For the 40/40/40 film, the temperature dependence of the DC magnetic susceptibilities, = M/ H are shown for parallel and perpendicular orientations in Figure 7 33 (a) and Figure 7 33 (b), respectively, for temperatures between 2 K and 75 K and an external field of 100 G. Field dependence of the magnetization is shown for parallel and perpendicular orientations in Figure 7 33 (c) at 2 K. While kinks in the temperatures sweeps can be associated with ordering of the pure CoFe and Ni Cr phases, the heterostructures show two striking features not observed in the homogeneous phases. First, there is a significant increase in the temperature, from 18 K to 70 K, at which persistent photoind uced changes in the magnetically ordered state are observed. Second, like the thin sandwich heterostructures, the magnetization decreases with light, in contrast to the normal photoinduced increases known for pure Co Fe compounds. The increase in magneti zation at high field is an indication that there is the usual diamagnetic to magnetic transition of the CoFe spins. In addition, aside from an overall scale factor between parallel and perpendicular orientations, the photoinduced effects are the same.

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241 Fo r the 20/40/20 film, the temperature dependence of the DC magnetic susceptibilities, = M/ H are shown for parallel orientation in Figure 7 34 (a), for temperatures between 2 K and 75 K and an external field of 100 G. Field dependence of the magnetization is shown for parallel orientation in Figure 7 34 (b) at 2 K. These films behave similarly to the 40/40/40 films, except that the relative photoinduced decrease is less, so much so that when the CoFe is in the ordered state, an overall increase is obser ved on the background of the decrease, even in 100 G. For the 40/20/40 film, the temperature dependence of the DC magnetic susceptibilities, = M/ H are shown for parallel orientation in Figure 7 35 (a), for temperatures between 2 K and 75 K and an external field of 100 G. Field dependence of the magnetization is shown for parallel orientation in Figure 7 35 (b) at 2 K. Again, these films behave similarly to the 40/40/40 films, but here the photoinduced decrease at low fields and increase at high fields are relatively smaller. 7.2.4 40/40/40 Heterostructure Even better than the thin sandwich films showing a novel photoeffect, the thick sandwich films showed a novel photoeffect clearly resolvable at temperatures much higher than in the pure material, which is particularly clear in the 40/40/40 film, Figure 7 36. Therefore, to glean the underlying nature of the effect observed in all heterostructures, the optimized 40/40/40 film was studied in further detail. These studies include additional measurements in a SQUID magnetometer, Figure 7 37. Transmission electron microscopy was performed to resolve the nanostructure of the sample, Figure 7 38 Nanometer resolved highresolution inelastic x ray scatte ring, EDS, was performed to resolve the chemical makeup as a function of film position,

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242 Figure 7 39. Elastic x ray powder diffraction resolved the lattice constants present in the heterostructure, Figure 7 40. Fourier transform infrared spectroscopy of t he cyanide stretches in the heterostructure, compared to the stretches in the pure constituent materials, also provides evidence for the structure and chemical content of the heterostructures, Figure 7 41 7.2.4.1 40/40/40 film, 10 kG t emperature s weeps To test the nature of the photoinduced effect, temperature sweeps were performed in high fields of 10 kG for both the light and dark states, Figure 7 37 (a). Difference plots between the light and dark states show that 10 kG is sufficient to overcome the photoinduced decrease when the temperature is less than approximately 60 K, Figure 7 37 (b). 7.2.4.2 40/40/40 film, t ransmission e lectron m icroscopy To investigate the structure of the 40/40/40 Ni Cr/Co Fe/NiCr heterostructure, samples were microtomed an d mounted on a transmission electron microscope. Contrast differences in the transmission can be assigned to the different Prussian blue analogue lattices, Figure 7 38 (a) and (b). While discrete regions are clear, interfacial surfaces have roughnesses on the order of 20 nm. It is worth mentioning that additional experiments were done, where the microtome process incited a fracture at the interface between the CoFe and Ni Cr layers, presumably due to the high strain induced by the lattice mismatch, Figu re 7 38 (c). 7.2.4.3 40/40/40 film, e nergy d ispersive x ray s pectroscopy While transmission electron microscopy provides clear evidence for the proposed structure, the chemical composition as a function of the height of the film can provide additional details about the heterogeneous atomic makeup. Energy dispersive x ray

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243 spectroscopy (EDS) were performed on a JOEL 2010F super probe By line scanning an electron beam across the microtome heterostructure, position dependence of the Co Fe fraction can be plotted and directly compared to the model structure, Figure 7 39 Practically, the Co Fe fraction is found by integrating Co and Fe peaks together and integrating Ni and Cr peaks together. The Co Fe fraction is then the total amount of Co and Fe divided by the total amount of Co, Fe, Ni and Cr. 7.2.4.4 40/40/40 film, x ray p owder d iffraction A Philips APD 3720 powder diffractometer was used to perform room temperature x ray diffraction (XRD) using a Cu K source with a primary wavelength of 1.5418 Despite a large background due to the Melinex from the solid support, two clear peaks can be seen, Figure 7 40 Two important conclusions can be drawn from these data. First, the x ray powder diffraction pr ovides additional microscopic evidence of the existence of both Prussian blue analogues in the heterostructure, in proportions consistent with other microscopic measurements. Second, unlike substitutional solids of Co Fe [ 141 ] the heterostructures posses s two distinct lattice constants, showing that, while bonded at the interface, the majority of the constituents remain in structures similar to their pure states. 7.2.4.5 40/40/40 film, i nfrared s pectroscopy Fourier transform spectroscopy was performed on the 40/40/40 heterostructure, as well as the constituent pure materials, Figure 7 41. The heterostructure shows discrete peaks corresponding to the cyanide stretches of the constituents. 7.2.5 Capping Layers of Cobalt and Chromium Hexacyanoc hromates In or der to further explore the novel photoeffect, most prominently present in the 40/40/40 Ni Cr/Co Fe/NiCr heterostructure, different capping layers of

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244 RbaCob[Cr(CN)6]c nH2O (Co Cr) and RbaCrb[Cr(CN)6]c nH2O (CrCr) were used. The Co Cr analogue is known to be a ferromagnet with an ordering temperature near 30 K [ 155 ] and the Cr Cr is a ferrimagnet with an ordering temperature near 200 K [ 116 ] Hexacyanoferrate based capping layers were not used in order to avoid additional charge transfer between the Fe i n the CoFe layer and Fe in the capping layer. 7.2.5.1 Magnetization of c obalt h exacyanochromate and c obalt h exacyanoferrate s andwich h eterostructures Two Co Cr/Co Fe/Co Cr heterostructures are presented, a 40/40/40 and 40/60/40 layering scheme. The temperature dependence of the DC magnetic susceptibilities, = M/ H are shown for 40/40/40 and 40/60/40 CoCr/Co Fe/Co Cr in Figure 7 42 (a) and Figure 7 42 (b), respectively, for temperatures between 2 K and 50 K and an external field of 100 G oriented parall el to the plane of the films. Time dependence during irradiation is shown in the inset of 7.20 (b), showing clear increase below the ordering temperature of CoFe and decrease above the ordering temperature of CoFe. Sharp increases in the temperature sw eeps can be associated with ordering of the pure Co Fe and CoCr phases, at ~10 K and ~30 K, respectively. Clear photoinduced magnetization can be observed well above the ordering temperature of the CoFe, and up to the ordering temperature of the CoCr. The photoeffect is seen to be negative, excepting when dominated by ordered CoFe magnetization. 7.2.5.2 Magnetization of c hromium h exacyanochromate and c obalt h exacyanoferrate s andwich h eterostructures Two Cr Cr/Co Fe/Cr Cr heterostructures are presented, a 40/40/40 and 60/40/60 layering scheme. The temperature dependence of the DC magnetic susceptibilities, = M/ H are shown for 40/40/40 and 60/40/60 Cr Cr/Co Fe/Cr Cr in Figure 7 43 (a) and

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245 Figure 7 43 (b), respectively, for temperatures between 2 K and 300 K and an external field of 100 G oriented parallel to the plane of the films. The difference between the irradiated and dark sample for the 60/40/60 Cr Cr/Co Fe/Cr Cr film is shown in the inset of F igure 7 43 (b), showing detectable changes in the magnetization at temperatures well above the liquid point of nitrogen. Sharp increases in the temperature sweeps can be associated with ordering of the pure CoFe and Cr Cr phases, at ~10 K and ~230 K, res pectively. The photoeffect is again observed to be negative. 7.2.6 Discussion This chapter describes the characterization of cyanometallate Prussian blue analogue heterostructured films, specifically with photomagnetic CoFe as a constituent. The heteros tructure geometry leads to two striking new behaviors, an increase in the ordering temperature of the photomagnetic effect compared to CoFe and a change in the sign of the photomagnetic effect compared to CoFe. The synthesis technique is elegant in it s simplicity, allowing fine control over thickness and constituents, while using room temperature and pressure wet chemistry. The magnetism data presented suggest a new mechanism for PPIM, whereby photoinduced changes in one lattice alter the magnetic res ponse of the other. Many samples were studied to arrive at the current understanding of the effect. Slow deposition multilayer films formulated with slow deposition techniques showed too much intermixing of the lattices and insufficient material transferr ed to the solid supports. The problems with slow deposition led to fast deposition techniques to be tried, with stacked films showing promise due to a modification of the photoeffect, which remained a small fraction of the total magnetization. A small br eakthrough came with a sandwich geometry, in which clear decreases in susceptibility were seen for films, although the

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246 effect was small on the background of the total magnetization. The big break came with the thick sandwich geometry, where the effect was found to be large enough to be resolved as a function of temperature and field. In fact, thick sandwich structures were the first example showing a clear increase of the photoinduced modification of long range magnetic order, much higher than the pure CoFe material. These geometries were engineered to give the optimal effect, observed in a 40/40/40 Ni Cr/Co Fe/NiCr heterostructure. This sample was studied with additional probes, to explore the nanostructure and atomic character. Finally, different ca pping layers, of CoCr and Cr Cr, were fabricated and studied for their photomagnetic effects. The structural probes clearly display the multi layer character of the 40/40/40 NiCr/Co Fe/NiCr heterostructure. The lattice constants and cyanide stretches o f the heterostructure are consistent with those observed in the homogeneous precursor Ni Cr and CoFe materials, and EDS line scans show an evolution of the chemical formula with film height. Finally, TEM images show difference in contrast that can be ass ociated with the different layers of CoFe and Ni Cr. All studies come back to the novel photoeffect observed in the heterostructures and the search to understand the fundamental origins of the photoeffect. To this end, the well documented photoeffect in Co Fe must first be considered. To begin, the mechanism of PPIM in bulk CoFe PBA involves light induced electron transfer from Fe2+ (LS, S = 0 ) to Co3+ (LS, S = 0 ), yielding long lived metastable Fe3+ (LS, S = 1/2) CN Co2+ (HS, S = 3/2) pairs that couple antiferromagnetically, giving rise to a net increase in magnetization in the ferrimagnetic state below 18 K [ 1 ] [78 ] [ 81 ] The electron transfer and change in spin state also lead to a change in lattice constant,

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247 increasing ~0. 2 upon transitioning from the low spin state to the highspin state, Figure 7 44 [ 160 ] [ 161 ] When Co Fe layers are fabricated to be in intimate contact with another analogue that is not photoactive, these structural distortions propagate through the heterostructure, Figure 7 45. The hexacyanochromate based networks have been shown to have a dependence of the magnetic susceptibility upon pressure, with the divalent nickel analogue have the greatest pressure dependence. For Ni Cr, decreases of the magnetization of ~ 50% can be i nduced by the application of hydrostatic pressure of 0.8 GPa [ 162 ] The photomagnetic response of the heterostructure indicates that the structural change in the CoFe PBA layer couples to the M Cr PBA (where M = Co, Cr, or Ni), leading to the change in magnetization due to distortion of the divalent metal octahedral, Figure 7 46. The interesting aspect exists because the other Prussian blue analogue has a much greater ordering temperature compared to the CoFe. The dependence of the highTC photoeffect in the heterostructures on capping layer can be clearly correlated to the pressure dependence of the capping layer. The Ni Cr has the most pressure dependence, and therefore the photoeffect in the heterostructure is the most dramatic. The end result is a m eta magnet, with long range magnetic order, that exhibits large changes in magnetization with the application of light, at unprecedented temperatures for this class of compounds, due to photoinduced structural distortions in the CoFe layer propagating to the previously nonphotoactive capping layer. Finally, it is tantalizing that, in materials with capping layers containing antiferromagnetic exchange pathways (CoCr and Cr Cr), small modifications of the

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248 ordering temperature can be seen, suggesting a phot oinduced modification of the exchange coupling in the samples. Unfortunately, an ideal candidate to further probe this idea, a 4040 40 MnCr/Co Fe/Mn Cr heterostructure, was unable to be synthesized, presumably due to the large lattice mismatch between t he two materials and different space groups [ 163 ] In the future, an interesting set of experiments could study the Mn NC Fe molecule as opposed to the Co NC Fe, and therefore synthesize a Mn Cr/Mn Fe/Mn Cr heterostructure and look for photoinduced changes in the magnetic coupling of the MnCr layer. 7.2.7 Conclusion In summary, heterostructured films consisting of two different Prussian blue analogues, one with a highTC and the other photoactive, have been fabricated for the first time, and this novel arrangement leads to persistent photoinduced changes in magnetization at elevated temperatures. The new behavior is not seen in either pure phase and requires the unique heterostructure arrangement that generates an interface between them. Simple mixing of ions in a threedimensional lattice does not give the same result, and in fact, serves to greatly suppress the amount of CoFe material that is bistable [ 141 ] [135 ] Heterostructures based on coordination polymers are largely unexplored, and these resul ts provide an example of new phenomena arising from engineered coordination polymer based structures that may motivate the rational design of further systems with new applications.

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249 Figure 7 1 The NaNi1 xCox[Fe(CN)6] nH2O material. (a) Structural model for NaNi1 xCox[Fe(CN)6] nH2O ( b ) A decrease in the magnetization with photoexcitation of a Ni dominated material when there is atomic mixing and spins are in an ordered state dictated by the exchange interactions JNiFe > 0 and JCoFe < 0 ( c) The usual increase in the magnetization with photoexcitation of a sufficiently Co dominated material. Table 7 1 Molecular formulas and unit cell parameters for NaCoxNi1 x[Fe(CN)6] nH2O [23] x Proposed molecular formula Unit cell length 0.0 Na0.27Ni2+ 1.0[Fe3+(CN)6]0.73[Fe2+(CN)6]0.02 5.0 H2O 10.23(9) 0.22 Na0.31Co2+ 0.22Ni2+ 0.78[Fe3+(CN)6]0.74[Fe2+(CN)6]0.03 4.4 H2O 10.24(9) 0.45 Na0.34Co2+ 0.45Ni2+ 0.55[Fe3+(CN)6]0.71[Fe2+(CN)6]0.05 4.9 H2O 10.25(6) 0.66 Na0.33Co2+ 0.66Ni2+ 0.34[Fe3+(CN)6]0.67[Fe2+(CN)6]0.08 4.6 H2O 10.26(8) 0.87 Na0.27Co2+ 0.87Ni2+ 0.13[Fe3+(CN)6]0.63[Fe2+(CN)6]0.10 3.8 H2O 10.28(9) 1.0 Na0.31Co2+ 1.0[Fe3+(CN)6]0.72[Fe2+(CN)6]0.04 4.4 H2O 10.30(7) S pair = 0 S pair = 1 H ext S pair = 0 S pair =+1 (c) Photoinduc ed Increase in Magnetization (b) Photoinduced Decrease in Magnetization Fe iii or Fe ii Ni ii Na + H 2 O N C C N ~ 10 Co ii or Co iii H ext (a) h h

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250 Figure 7 2 Typical TEM micrographs for samples reported in Table 7 1 for different values of x. All scale bars shown are 100 nm. A continuous increase in equilibrium particle edge length is observed when cobalt ions are added to the extended networks. The average particle sizes, in nanometers, from left to right are: 15.6 3.4, 26.5 5.3, 28.7 6.9, 38.7 7.7, 117.2 22.7, and 237.8 40.1. x = 0 .00 x = 0 .22 x = 0 .45 x = 0 .66 x = 0 .87 x = 1.00

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251 Figure 7 3 FT IR spectra and fitting parameters of NaNi1 xCox[Fe(CN)6] nH2O as a function of x. Fits ( green ) were done using four Lorentzian lines ( orange ) Gaussian lineshapes were also tried, but the results with the Lorentzian fits had smaller residuals. A standard Levenberg Marquardt algorithm was employed for simultaneous fitting of the four lines until no observable change in 2 was detected. 0

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252 Space group: (No. 225) Wyckoff position g x y z x = 0.00, Na 8 c 0.27 0.25 0.25 0.25 Ni 4 b 1 0.5 0.5 0.5 Co 4 b 0 0.5 0.5 0.5 Fe 4 a 0.75 0.0 0.0 0.0 C 24 e 0.75 0.196(4) 0.0 0.0 N 24 e 0.75 0.296(5) 0.0 0.0 O1 24e 0.25 0.216(2) 0.0 0.0 OA 32 f 0.23 0.283(7) 0.283(7) 0.283(7) OB 192 l 0.06 0.243(3) 0.076(9) 0.189(6) x =0.22, Na 8 c 0.31 0.25 0.25 0.25 Ni 4 b 0.78 0.5 0.5 0.5 Co 4 b 0.22 0.5 0.5 0.5 Fe 4 a 0.77 0.0 0. 0 0.0 C 24 e 0.77 0.211(7) 0.0 0.0 N 24 e 0.77 0.294(0) 0.0 0.0 O1 24 e 0.23 0.205(0) 0.0 0.0 OA 32f 0.23 0.283(2) 0.283(2) 0.283(2) OB 192 l 0.06 0.206(4) 0.078(1) 0.210(2) x = 0.45, Na 8 c 0.34 0.25 0.25 0.25 Ni 4 b 0.55 0.5 0.5 0.5 Co 4 b 0.45 0.5 0.5 0.5 Fe 4 a 0.76 0.0 0. 0 0.0 C 24 e 0.76 0.204(1) 0.0 0.0 N 24 e 0.76 0.290(6) 0.0 0.0 O1 24 e 0.24 0.207(6) 0.0 0.0 OA 32 f 0.23 0.282(2) 0.282(2) 0.282(2) OB 192l 0.06 0.206(4) 0.078(2) 0.210(2) x = 0.66, Na 8 c 0.33 0.25 0.25 0.25 Ni 4 b 0.34 0.5 0.5 0.5 Co 4 b 0.66 0.5 0.5 0.5 Fe 4 a 0.75 0.0 0. 0 0.0 C 24e 0.75 0.204(5) 0.0 0.0 N 24 e 0.75 0.292(6) 0.0 0.0 O1 24 e 0.25 0.218(0) 0.0 0.0 OA 32 f 0.23 0.282(1) 0.282(1) 0.282(1) OB 192 l 0.06 0.206(4) 0.078(2) 0.210(2) x = 0.87, Na 8 c 0.27 0.25 0.25 0.25 Ni 4 b 0.13 0.5 0.5 0.5 Co 4 b 0.87 0.5 0.5 0.5 Fe 4 a 0.73 0.0 0. 0 0.0 C 24 e 0.73 0.197(0) 0.0 0.0 N 24e 0.73 0.279(0) 0.0 0.0 O1 24 e 0.27 0.212(7) 0.0 0.0 OA 32 f 0.23 0.285(6) 0.285(6) 0.285(6) OB 192 l 0.06 0.178(6) 0.070(2) 0.232(0) x = 1.00, Na 8 c 0.31 0.25 0.25 0.25 Ni 4 b 0.00 0.5 0.5 0.5 Co 4 b 1.00 0.5 0.5 0.5 Fe 4 a 0.76 0.0 0. 0 0.0 C 24 e 0.76 0.197(0) 0.0 0.0 N 24 e 0.76 0.279(0) 0.0 0.0 O1 24e 0.27 0.212(7) 0.0 0.0 OA 32 f 0.23 0.285(6) 0.285(6) 0.285(6) OB 192 l 0.06 0.178(6) 0.070(2) 0.232(0) m Fm 3 10 20 30 40 50 60 x = 0.00 x = 0.22 x = 0.45 Intensity (arb. units)x = 0.66 x = 0.87 x = 1.00 2 (degrees) Figure 7 4 Full XRD diffractograms of NaNi1 xCox[Fe(CN)6] nH2O XRD data ( black), fitting parameters and fits ( orange ), with residuals displayed below the peaks ( dark cyan ), and the fitted background Vis ible as a thin line ( green ). Organic parameters were not of a particular interest and many degenerate solutions consisting of slight perturbations to the quoted solutions are possible.

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253 Figure 7 5 XRD of the x = 1 NaNi1 xCox[Fe(CN)6] nH2O Since only 0.59 mg of the x = 1.00 sample was available for the XR D studies, as opposed to the 1020 mg used for x ray powder diffraction experiments on the other samples, a much weaker signal to noise ratio resulted. In addition, the background curve resulting from the sample holder is seen to be comparable to the signal, Figure 7 4 For these reasons, additional data were acquired for the (2,0,0) reflection near 17.1. While a value of 10.3051(9) was obtained for the original fit, a value of 10.3072(6) was generated by fitting this single reflection, with better statistics, using the parameters for the structure from the original fit, and simply refining the unit cell. The fit peak ( orange ) is displayed, with residuals displayed below the peaks ( dark cyan ), and the fitted background is shown as a thin line ( green ). Figure 7 6 Room temperature XRD reflection with background subtracted and intensity normalized t o show the continuous evolution with x. The peak position shifts as x decreases and the line width broadens, reflecting the smaller particle size of the pure Ni Fe analogue. 16.8 17.0 17.2 17.4 Intensity (arb. units)2 (degrees) x = 1.00 34.0 34.5 35.0 35.5 36.0 Intensity (Arb. Units)2 (degrees)1.000.87 x = 0.66 0.45 0.22 0.00 Wyckoff position g x y z x = 1.00, R wp = 0.0170 Na 8 c 0.31 0.25 0.25 0.25 Ni 4 b 0.00 0.5 0.5 0.5 Co 4 b 1.00 0.5 0.5 0.5 Fe 4 a 0.76 0.0 0. 0 0.0 C 24e 0.76 0.197(0) 0.0 0.0 N 24e 0.76 0.279(0) 0.0 0.0 O1 24e 0.27 0.212(7) 0.0 0.0 OA 32 f 0.23 0.285(6) 0.285(6) 0.285(6) OB 192l 0.06 0.178(6) 0.070(2) 0.232(0)

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254 Figure 7 7 Photoinduced magnetization of NaNi1 xCox[Fe(CN)6] nH2O (a) Molar magnetic susceptibility as a function of time irradiated at 5 K and 10 G, measured in a SQUID. Discontinuities in magnetization when the light is turned on and off are due to a subtle heating effect from the applied light. (b) Molar magnetic susc eptibility as a function of temperature in both the dark FC ( and photoirradiated states ( ), measured in a SQUID at 10 G. (c) Meanfield calculations of molar magnetic susceptibilities at 10 G as a function of temperature in both the dark stat e (solid line), where the highspin fraction, nHS, is determined from fitting high temperature susceptibility, and photoirradiated states (dashed line), where nHS = 1. The magnetic signals are expressed per mole of sample using the chemical formulas in Ta ble 7 1 no effect (a) x = 0.00 126 127 128 x = 0.22 85.0 85.5 86.0 x = 0.45 41.5 42.0 42.5 x = 0.66 (emu/mol) 20 40 60 80 x = 0.87 0 10 20 30 40 100 110 120 130 x = 1.00 time (hours) 0 50 100 x = 0.00 (b) 0 50 100 x = 0.22 0 50 x = 0.45 (emu/mol) 0 20 40 x = 0.66 0 50 100 x = 0.87 0 5 10 15 20 25 30 0 50 100 x = 1.00 T (K) 0 50 100 x = 0.00 (c) 0 50 100 x = 0.22 0 50 100 x = 0.45 (emu/mol) 0 20 40 x = 0.66 0 50 100 x = 0.87 0 5 10 15 20 25 30 0 50 100 x = 1.00 T (K)

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255 Figure 7 8 Molar magnetic susceptibility of NaNi1 xCox[Fe(CN)6] nH2O as a function of time irradiated at 5 K and 10 G, measured in a SQUID. The samples are irradiated continuously for time > 0, whe re the step in at t = 0 is associated with a small heating effect. For the x = 0.66 sample, the magnetization has a small initial increase for 10 minutes followed by the large photoinduced decrease. -30 -20 -10 0 10 20 30 40 50 60 42.0 42.2 x = 0.66 (emu/mol)time (minutes)85.0 85.5 86.0 x = 0.45

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256 Figure 7 9 Magnetization versus field for NaNi1 xCox[Fe(CN )6] nH2O (a) Molar magnetic susceptibility as a function of time irradiated at 5 K and 10 G, measured in a SQUID. Molar magnetization for both (b) high field (c) and low field, as a function of magnetic field in both the dark ( st ates ( ), measured in a SQUID at 2 K. High field magnetization always increases after photoirradiation, even for samples showing a photodecrease at low fields. The smallest field on the field sweeps is 100 G. no effect (a) x = 0.00 125 126 127 128 x = 0.22 84 85 86 87 x = 0.45 41.5 42.0 42.5 x = 0.66 (emu/mol) 20 40 60 80 x = 0.87 0 10 20 30 40 100 110 120 130 x = 1.00 time (hours) -1.0 -0.5 0.0 0.5 1.0 x = 0.00 (b) -1.0 -0.5 0.0 0.5 1.0 1.5 x = 0.22 -1.0 -0.5 0.0 0.5 1.0 x = 0.45 M (104 emu G / mol) -1.0 -0.5 0.0 0.5 1.0 x = 0.66 -0.5 0.0 0.5 1.0 1.5 x = 0.87 -20 0 20 40 60 80 -0.5 0.0 0.5 x = 1.00 H (kG) -0.4 -0.2 0.0 0.2 0.4 x = 0.00 (c) -0.4 -0.2 0.0 0.2 0.4 x = 0.22 -0.4 -0.2 0.0 0.2 0.4 x = 0.45 M (104 emu G / mol) -0.4 -0.2 0.0 0.2 0.4 x = 0.66 -0.4 -0.2 0.0 0.2 0.4 x = 0.87 -4 -2 0 2 4 -0.4 -0.2 0.0 0.2 0.4 x = 1.00 H (kG)

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257 Figure 7 10. Checking for asymmetry in the hysteresis loop as a possible explanation of the reduction in HC for the x = 0.66 sample. Field was swept from 100 G to 70 kG to 70 kG and back to 70 kG, all at 2 K. No asymmetry is observed within experimental uncertainty -1000 -500 0 500 1000 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 x = 0.66 +880 G -890 G M (104 emuG/mol)H (G)

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258 Figure 7 11. Thermal induced changes in magnetization of NaNi1 xCox[Fe(CN)6] nH2O (a) T versus T as measured in a SQUID magnetometer with 5 kG applied field. The results of meanfield calculations are shown for (b) T versus T and (c) the highspin fraction, nHS, versus T. The magnetic signals are expressed per mole of sample using the chemical formulas listed in Table 7 1 1.8 2.0 2.2 2.4 (a) x = 0.00 2.4 2.6 2.8 3.0 x = 0.22 3.0 3.1 3.2 x = 0.45 3.10 3.15 3.20 x = 0.66 T (emu K / mol) 3.0 3.2 3.4 x = 0.87 100 150 200 250 300 1 2 3 4 x = 1.00 T (K) 1.8 2.0 2.2 2.4 (b) x = 0.00 2.4 2.6 2.8 3.0 x = 0.22 3.0 3.1 3.2 3.3 x = 0.45 T (emu K / mol) 3.0 3.1 3.2 x = 0.66 3.0 3.2 3.4 x = 0.87 100 150 200 250 300 1 2 3 4 x = 1.00 T (K) x = 0.00 no effect (c) 0.99 1.00 x = 0.22 0.97 0.98 0.99 1.00 x = 0.45 nHS 0.95 1.00 x = 0.66 0.85 0.90 0.95 1.00 x = 0.87 100 150 200 250 300 0.2 0.4 0.6 0.8 1.0 x = 1.00 T (K)

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259 Figure 7 12. Microscopic versus macroscopic mixing. TEM images of the manually mixed powder with x = 0.60 showing the presence of both (a) Co Fe and (b) NiFe powders in the material with the same sizes reported for the x = 0.00 and x = 1.00 materials, respectively ( Figure 7 2 ). All scale bars shown are 100 nm. Figure 7 13. Magnetization of macroscopically mixed NaNi1 xCox[Fe(CN)6] nH2O Susceptibility measurements of manually mixed powder with x = 0.60 in ~10 G showing (a) no decrease in the magnetization with photoirradiation over time, but rather a clear increase and (b) two well defined ordering temperatures present in the mater ial as well as an overall increase in the magnetization at all temperatures from the dark ( photoirradiated ( ) states. The small change i n the susceptibility above ~ 17 K is due to the slightly different measuring fields arising from th e fact that these data predate the careful procedure described in the text ensuring the same measuring field for photoirradiated and dark states. 0 5 10 15 20 25 30 35 0.19 0.20 0.21 0.22 0.23 0.24 0.25 0.26 0.27 (b) (a) (emu/gram) (emu/gram)t (hours) 0 5 10 15 20 25 30 0.0 0.1 0.2 0.3 0.4 T (K)

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260 Figure 7 14. Field dependence of photoinduced magnetization for NaNi1 xCox[Fe(CN)6] nH2O Photoinduced change in susceptibility for the x = 0.66 sample at T = 5 K measured at low field ( H = 10 G) and high field ( H = 1 kG). Photoirradiation is continuous for time > 0. -1 0 1 2 3 4 5 -0.6 -0.4 -0.2 0.0 0.2 H = 1 kG (emu/mol)time (hours)H = 10 G

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261 Figure 7 15. Scaling of magnetic properties of NaNi1 xCox[Fe(CN)6] nH2O (a) The magnetic critical temperature, TC, (b) the coercive field, HC, and (c) the Curie Weiss temperature, CW, for both low spin ( spin ( ) states, as a function of x. Dashed lines are a meanfield interpolation between the two pure materials Solid lines are from mean field fits of CW that allow for a modification of exchange constants as the M NC M distance changes as a function of x; superexchange constants were empirically scaled to 80 % of their fit value for comparison to the low temperature TC values. Coercive fields were obtained at T = 2 K after sweeping to 70 kG. Curie Weiss temperatures, CW, were obtained by fitting from 250 K to 300 K, where nHS and eff are essentially constant. 10 15 20 25 30 (a) 1 2 3 (b)HC (kG) 0.0 0.2 0.4 0.6 0.8 1.0 -20 -10 0 10 20 30 (c)CW (K) TC (K)x

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262 Figure 7 16. Superexchange in NaNi1 xCox[Fe(CN)6] nH2O (a) Energy levels and diagram for the superexchange interactions considered in the material. The Co2+ (HS) ion notably has both ferromagnetic (JF > 0) and antiferromagnetic (JAF > 0) superexchange interactions with Fe3+ (LS), in contrast with the Ni2+. (b) Average values of the exchange constants, Jave, for the Ni2+ NC Fe3+ (x) and Co2+ NC Fe3+ (+) exchange bonds used in order to reproduce the scaling in Figure 7 15. The line is merely a guide for the eye. Figure 7 17. Charge transfer induced spin transition parameters for NaNi1 xCox[Fe(CN)6] nH2O (a) The width of the thermal hysteresis, TupTdow n, and (b) the number of spincrossover active nearest neighbors, zSCO, as a function of x. Here, TupTdow n is defined by the difference in the temperature at which half of the spincrossover active material is highspin when sweeping up in temperature, and the temperature at which half of the spin crossover active material is high spin when sweeping down in temperature. 10.24 10.26 10.28 10.30 -2 0 2 4 6 x = 1 Jave (cm-1)Lattice Constant (A) x = 0 Co ii NC e g Co t 2g Co Fe iii CN e g Fe t 2g Fe Ni ii NC e g Ni t 2g Ni J J A J (a) (b)

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263 Figure 7 18. Amount of CTIST materials in NaNi1 xCox[Fe(CN)6] nH2O (a) The percentage of CTIST active material, %CTIST active, and (b) the unit cell lattice constant, a, as a function of x. Figure 7 19. FT IR parameters in NaNi1 xCox[Fe(CN)6] nH2O (a) The effective spring constant and (b) FT IR frequency for M2+C N Fe3+ as a function of x.

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264 Figure 7 20. Comparison of ferromagnetic versus antiferromagnetic CoFe components. (a) Comparison of T versus T for SQUID magnetometer data ( meanfield calculation ( ) using JCoFe and JNiFe values from the binary materials, and mean field calculation ( ) fitting < JCoFe> from 250 K to 300 K without modifying JNiFe. (b) Comparison of versus T for SQUID magnetometer data ( = FC photoinduced, = FC dark, = ZFC dark), meanfield calculation ( = FC dark, --= FC photoinduced) using JCoFe and JNiFe values from the binary materials, and mean field calculation ( = FC dark, --= FC photoinduced) fitting < JCoFe> from 250 K to 300 K without modifying JNiFe. 100 150 200 250 300 3.0 3.1 3.2 (a) T (emu K/mol)T (K) x = 0.66 0 5 10 15 20 25 30 -40 -30 -20 -10 0 10 20 30 40 (b) (emu/mol)T (K) x = 0.66

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265 Figure 7 21. Modification of superexchange energy in NaNi1 xCox[Fe(CN)6] nH2O (a) The ternary metal NaNi1 xCox[Fe(CN)6] nH2O compounds were found experimentally to have CTIST active and CTIST inactive populations, as approximated from the high temperature magnetic susceptibility and the room temperature FT IR data. (b) A sketch o f the superexchange energy distribution showing < JCoFe>, which is determined by fitting T versus T from 250 K to 300 K. The CTIST inactive portion, can actually have ferromagnetic character in some samples. For simplicity, it is assumed that the elec tronic structure of the CTIST active portion is most similar to the pure x = 1.00 material and thus the magnetic interactions, JCoFe, are similar. CoFeJ (a) (b) CoFeJ < J CoFe > JCoFe

PAGE 266

266 Figure 722. A scheme showing synthesis of a heterostructured thin film using multiple sequential adsorpt ion cycles. FeCo2+[Fe2+(CN)6]3 -and Rb+ Fe Fe Fe Fe Co Co Co One Co -Fe cycle Co Co Co Co Co Co Fe Fe Co Co Co Fe Fe Fe CrNi2+[Cr3+(CN)6]3 -and Rb+ Cr Cr Cr Cr Ni Ni Ni One Ni -Cr cycle Ni Ni Ni Ni Ni Ni Cr Cr Ni Ni Ni Cr Cr Cr CrNi2+[Cr3 +(CN)6]3 -and Rb+ Cr Cr Cr Cr Ni Ni Ni One Ni -Cr cycle Ni Ni Ni Ni Ni Ni Cr Cr Ni Ni Ni Cr Cr Cr

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267 Table 7 2 Nickel hexacyanochromate and cobalt hexacyanoferrate heterostructures studied. Number of cycles Order of deposition Deposition Speed 1/1 x 40 Ni Cr/Co Fe/Ni Cr/Co Fe/ Slow 5/5 x 20 Ni Cr/Co Fe/Ni Cr/Co Fe/ Slow 40/40 Ni Cr/Co Fe Slow 10/10 Ni Cr/Co Fe Fast 10/10 Co Fe/Ni Cr Fast 10/5/10 Ni Cr/Co Fe/Ni Cr Fast 10/10/10 Ni Cr/Co Fe/Ni Cr Fast 40/40/40 Ni Cr/Co Fe/Ni Cr Fast 20/40/20 Ni Cr/Co Fe/Ni Cr Fast 40/20/40 Ni Cr/Co Fe/Ni Cr Fast 40/40/40 Ni Cr/Co Fe/Ni Cr Fast Figure 7 23. Magnetization of slow deposition multilayer films. (a) Plots of FC and ZFC DC magnetic susceptibilities of 1/1 x 40 CoFe/NiCr ( ), 5/5 x 20 CoFe/NiCr ( ), and 40/40 CoFe/Ni Cr ( ) films. The lines are guides to the eye connecti ng the data points, taken every 10 K. (b) M versus time irradiated, are shown for parallel orientations of films 1/1 x 40 CoFe/NiCr ( ), 5/5 x 20 Co Fe/NiCr ( ), and 40/40 CoFe/NiCr ( ) with applied magnetic field of 100 G and at 5 K. 0 20 40 60 80 100 0.0 0.2 0.4 0.6 0.8 1.0 FC [ T = 2 K ]T (K)ZFC H etc. 0.0 0.5 1.0 0 5 10 15 M 10-3 emuG/cm2)time (hours) (a) (b)

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268 Figure 7 24. Magnetization of 10/10 CoFe/Ni Cr thin film oriented parallel. (a) Plots of FC and ZFC DC magnetic susceptibilities versus temperature for the 10/10 Co Fe/NiCr thin film in H = 100 G parallel to the planes of the films are shown. (b) M versus time irradiated is shown at 5 K and 100 G parallel to the film. Figure 7 25. Magnetization of 10/10 CoFe/Ni Cr thin film oriented perpendicular. (a) Plots of ZFC DC magnetic susceptibility versus temperature for the 10/10 Co Fe/NiCr thin film in H = 100 G perpendicular to the planes of the films are shown. (b) M versus time irradiated is shown at 5 K and 100 G perpendicular to the film. 0 20 40 60 80 100 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 -1 0 1 2 3.90 3.91 3.92 3.93 3.94 3.95 (b) ( 10-6 emu/cm2)T (K)ZFC (a) FC ( 10-6 emu/cm2)time (hours)H Co-Fe/Ni-Cr 0 20 40 60 80 100 0.0 0.5 1.0 1.5 2.0 -1 0 1 2 1.920 1.925 1.930 1.935 (b) ( 10-6 emu/cm2)T (K)ZFC (a) ( 10-6 emu/cm2)time (hours) H Co-Fe/Ni-Cr

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269 Figure 7 26. Magnetization of 10/10 Ni Cr/Co Fe thin film oriented parallel. (a) Plots of FC and ZFC DC magnetic s usceptibilities versus temperature for the 10/10 NiCr/Co Fe thin film in H = 100 G parallel to the planes of the films are shown. (b) M versus time irradiated is shown at 5 K and 100 G parallel to the film. Figure 7 27. Magnetization of 10/10 Ni Cr/ Co Fe thin film oriented perpendicular. (a) Plots of FC and ZFC DC magnetic susceptibilities versus temperature for the Ni Cr/Co Fe thin film in H = 100 G perpendicular to the planes of the films are shown. (b) M versus time irradiated is shown at 5 K a nd 100 G perpendicular to the film. 0 20 40 60 80 100 0 5 10 15 20 25 -1 0 1 2 4 5 6 7 (b) ( 10-7 emu/cm2)T (K)ZFC (a) FC ( 10-7 emu/cm2)time (hours) H Ni-Cr/Co-Fe 0 20 40 60 80 100 -1 0 1 2 3 4 5 6 7 8 -1 0 1 2 1.7 1.8 1.9 2.0 (b) ( 10-7 emu/cm2)T (K)ZFC (a) Co-Fe ordering FC ( 10-7 emu/cm2)time (hours) H Ni-Cr/Co-Fe

PAGE 270

270 Figure 7 28. Magnetization of 10/5/10 Ni Cr/Co Fe/NiCr thin film oriented parallel. (a) Plots of FC and ZFC DC magnetic susceptibilities versus temperature of the sandwich 10/5/10 Ni Cr/Co Fe/NiCr PBA thin film in H = 100 G parallel to the planes of the films are shown. (b) M versus time irradiated is shown at 5 K and 100 G parallel to the film. Figure 7 29. Magnetization of 10/5/10 Ni Cr/Co Fe/NiCr thin film oriented perpendicular. (a) Plots of FC and ZFC DC magnetic susceptibilities versus temperature of the sandwich 10/5/10 Ni Cr/Co Fe/NiCr PBA thin film in H = 100 G perpendicular to the planes of the films are shown. (b) M versus time irrad iated is shown at 5 K and 100 G perpendicular to the film. 0 20 40 60 80 100 0 2 4 6 8 10 -1 0 1 2 9.39 9.40 (b) ( 10-6 emu/cm2)T (K)ZFC (a) FC ( 10-6 emu/cm2)time (hours) H 10/5/10 0 20 40 60 80 100 0 1 2 3 4 -1 0 1 2 3.094 3.096 3.098 3.100 3.102 (b) ( 10-6 emu/cm2)T (K)ZFC (a) FC ( 10-6 emu/cm2)time (hours)H 10/5/10

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271 Figure 7 30. Magnetization of 10/10/10 sandwich film versus temperature. Plots of FC DC magnetic susceptibility versus temperature for the 10/10/10 sandwich NiCr/Co Fe/NiCr PBA thin film in H = 100 G are shown for parallel ( ) and perpendicular ( ) orientations of the plane of the film with respect to the applied field. 0 20 40 60 80 100 0 2 4 6 8 10 12 and ( 10 -6 emu/cm2)T (K) Co-Fe ordering H 10/10/10

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272 Figure 7 31. Photoinduced magnetization of 10/10/10 film oriented perpendicular. M versus time irradiated is shown for perpendicular orientations of the 10/10/10 sandwich Ni Cr/Co Fe/NiCr PBA thin film with respect to the applied magnetic field of (a) 100 G and (b) 1 kG, at 5 K. Figure 7 32. Photoinduced magnetization of 10/10/10 film oriented perpendicul ar. M versus time irradiated is shown for parallel orientations of the 10/10/10 sandwich Ni Cr/Co Fe/NiCr PBA thin film with respect to the applied magnetic field of (a) 100 G and (b) 1 kG, at 5 K. -1 0 1 2 4.14 4.16 4.18 -1 0 1 2 1.369 1.370 1.371 (b) ( 10-6 emu/cm2)time (hours)(a) ( 10-6 emu/cm2)time (hours) 1000 G 100 G H 10/10/10 -1 0 1 2 12.66 12.68 12.70 12.72 12.74 12.76 12.78 -1 0 1 2 1.680 1.682 1.684 1.686 (b) ( 10-6 emu/cm2)time (hours)(a) 1000 G 100 G ( 10-6 emu/cm2)time (hours) H 10/10/10

PAGE 273

273 Figure 7 33. Magnetization of 40/40/40 NiCr/Co Fe/NiCr heterostructure. The (T) data, normalized to the area of the 40/40/40 sandwich Ni Cr/Co Fe/NiCr PBA film, are plotted when the externally applied field of 100 G is oriented (a) parallel (black) and (b) perpendicular (grey) to the surface of the film. The closed symbols represent the data prior to irradiation (i.e. dark state), and the open symbols designate the data acquired after 5 hrs of irradiation with white light, but with the light subsequently off, (i.e. PPIM state). (c) The magnetizati on, M, versus magnetic field, H, loops at 2 K are shown when H || film (black) and H film (grey). The closed symbols are before irradiation and the open symbols are after photoexcitation but with the light off. The insets show an expanded region at low magnetic fields, and the coercive fields, HC, are 85 G for H || film and 140 G for H film. 0 5 10 15 0 10 20 30 40 50 60 70 0 5 (10-5 emu / cm2)h (a)h (b) Temperature (K) -60 -40 -20 0 20 40 60 -40 -20 0 20 40 M (10-3 emuG / cm2)H (kG)(c)-400 -200 0 200 400 -20 -10 0 10 20 H (G) -400 -200 0 200 400 -20 -10 0 10 20 H (G)

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274 Figure 7 34. Magnetization of 20/40/20 Ni Cr/Co Fe/NiCr heterostructure. (a) The (T) data, normalized to the area of the 20/40/20 sandwich Ni Cr/Co Fe/NiCr PBA film are plotted when the externally applied field of 100 G is oriented parallel to the surface of the film. The closed symbols represent the data prior to irradiation (i.e. dark state), and the open symbols designate the data acquired after 5 hrs of irradiation with white light, but with the light subsequently off, (i.e. PPIM state). (b) The magnetization, M, versus magnetic field, H, loops at 2 K are shown when H || film. The closed symbols are before irradiation and the open symbols are after photoexcitation but with the light off. 0 10 20 30 40 50 60 70 0.0 0.5 1.0 1.5 2.0 (10-5 emuG/cm2)T (K) (a) -60 -40 -20 0 20 40 60 -4 -2 0 2 4 M (10-3 emuG/cm2)H (kG) (b)-400 -200 0 200 400 -3 -2 -1 0 1 2 3 H (G)

PAGE 275

275 Figure 7 35. Magnetization of 40/20/40 Ni Cr/Co Fe/NiCr heterostructure. (a) The (T) data, normalized to the area of the 40/20/40 sandwich Ni Cr/Co Fe/NiCr PBA film are plotted when the externally applied field of 100 G is oriented parallel to the surface of the film. The closed symbols represent the data prior to irradiation (i.e. dark state), and the open symbols designate the data acquired after 5 hrs of irradia tion with white light, but with the light subsequently off, (i.e. PPIM state). (b) The magnetization, M, versus magnetic field, H, loops at 2 K are shown when H || film. The closed symbols are before irradiation and the open symbols are after photoexcitation but with the light off. 0 10 20 30 40 50 60 70 0 2 4 6 8 10 12 (10-5 emu/cm2)T (K) (a) -60 -40 -20 0 20 40 60 -20 -10 0 10 20 M (10-3 emuG/cm2)H (kG)

PAGE 276

276 Figure 7 36. Magnetization data of the 40/40/40 sandwich Ni Cr/Co Fe/NiCr PBA film (a) The fieldcooled magnetic susceptibility (T) in 100 G oriented parallel to the surface of the film, where (T) is normalized to the area of the film. The closed symbols represent the data prior to irradiation (i.e. dark state), and the open symbols designate the data acquired after 5 hrs of irradiation with white light, but with the light subsequently off, (i.e. PPIM state). (b) The absolute value of the photoinduced changes of = (dark) (light), normalized to the maximum value. The data for the heterostructure is from the left panel, whereas the data for the single phase CoFe PBA thin film is taken from Ref. [ 103 ] (c ) An expanded view of the temporal evolution of the magnetic response is shown during irradiation at 5 K, 45 K, and 60 K, with H || film and H = 100 G. The i rradiation begins at time = 0. 0 20 40 60 80 0 5 10 15 (a) Light state (10-5 emu/cm2)Temperature (K)Dark state 6 8 10 12 14 16 0 1 2 3 (c) 5 K 45 K 60 K (10-5 emu/cm2)time (hrs) 0 20 40 60 80 0.0 0.2 0.4 0.6 0.8 1.0 Single phaseTemperature (K)MAXHeterostructure

PAGE 277

277 Figure 7 37. High magnetic field,10 kG temperature sweeps of 40/40/40 NiCr/Co Fe/NiCr. (a) The (T) data of a 40/40/40 NiCr/Co Fe/NiCr film are plotted when the externally applied field of 10 kG is oriented parallel to the surface of the film. The solid line r epresents the data prior to irradiation (i.e. dark state), and the dashed line designates the data acquired after 5 hrs of irradiation with white light, but with the light subsequently off, (i.e. PPIM state). (b) Difference plots of the light state minus the dark state, M, showing a photoinduced increase at high fields 10 kG, and temperatures. Figure 7 38. Transmission electron microscopy of the 40/40/40 Ni Cr/Co Fe/NiCr heterostructure. (a) A schema of the heterostructure using a shading gradient between layers to illustrate regions at the interfaces where there can be mixing of the two phases. (b) A T EM micrograph shows a cross section of a microtomed sample. The scale bar is 100 nm. The Melinex solid support is located at the bottom in the image. (c) A TEM micrograph showing a fracture at the Co Fe/NiCr interface. The scale bar is 100 nm. 0 20 40 60 80 100 0 1 2 3 4 5 M (10-3 emuG/sample)T (K) (a) 0 20 40 60 0 10 20 30 (b) M (10-5 emuG/sample)T (K) Rb0.8Ni4[Cr(CN)6]3 nH2O Rb0.7Co4[Fe(CN)6]3 nH2O Rb 0.8 Ni 4 [Cr(CN) 6 ] 3 nH 2 O ( a) (b ) (c )

PAGE 278

278 F igure 7 39. An energy dispersive x ray line scan of the 40/40/40 heterostructure. ( left) A schema of the heterostructure using a shading gradient between layers to illustrate regions at the interfaces where there can be mixing of the two phases. ( b ) A l inescan from the EDS provided the CoFe fraction across the film, and the apparently high CoFe fraction deep into the bottom Ni Cr layer arises because the films become rougher with increasing thickness. T he solid support of Melinex is located at the top of the image and the free surface of the heterostructure is located at the bottom.

PAGE 279

279 Figure 7 40. X ray powder diffraction of a 40/40/40 heterostructure. To investigate the crystal structure of the heterostructures, reflections of the (4, 0, 0) plane w ere measured. Experimental intensities are shown (), as well as fits to Lorentzian lines for Co Fe () and Ni Cr () lattices to extract the relative intensities and positions. The peak at 34.8 corresponding to a cubic lattice constant of 10.3 is con sistent with the Co Fe analogue in the high spin state [ 68 ] whereas the peak at 34.0 corresponding to a cubic lattice constant of 10.5 can be assigned to the Ni Cr analogue. 32.5 33.0 33.5 34.0 34.5 35.0 35.5 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 Co-Fe A(Ni-Cr) 352 w(Ni-Cr) 0.5 xc(Ni-Cr) 34.0 A(Co-Fe) 200 w(Co-Fe) 0.4 xc(Co-Fe) 34.8 103 photonsdegrees (2 ) Ni-Cr

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280 Figure 7 41. Cyanide stretching energies in the infrared. Spectra and fit s are shown at left and fitting parameters (the center position, xc, the width, w, and the area, A) are shown at right. (a) The FT IR spectrum of pure cobalt hexacyanoferrate is known to display peaks at 2163, 2120, 2090, and 2040 cm corresponding to the cyanide stretches of the Co2+ NC Fe3+(HS), Co3+ NC Fe2+(LS), Co2+ NC Fe2+, and linkageisomerized Co2+ NC Fe2+ phases, respectively [7]. The experimental data can be fit well with two lines, CoFe1, which is assigned to Co2+ NC Fe3+(HS) and CoFe2, which is assigned to Co3+ NC Fe2+(LS). (b) The FT IR spectrum of pure nickel hexacyanochromate displays peaks at 2160 and 2125 cm corresponding to the bridged Ni2+ NC Cr3+ pairs and terminal cyanides. The experimental data can be fit to two lines, NiCr1, whi ch is assigned to Ni2+ NC Cr3+, and NiCr2, which is assigned to terminal cyanides. (c) In the heterostructured thin film, discrete peaks corresponding to each of the constituents can be seen. The experimental data fit well to three lines, CoFe1 ( Co2+ NC Fe3+(HS)), CoFe2 ( Co3+ NC Fe2+(LS)), and NiCr2 ( Ni2+ NC Cr3+). The observation of these peaks is further evidence for the proposed structure of the 40/40/40 film. 0.00 0.05 0.10 0.15 0.00 0.05 0.10 0.15 2200 2180 2160 2140 2120 2100 2080 0.00 0.05 0.10 0.15 xc(NiCr1) 2145.40 2.07 w(NiCr1) 54.87 7.78 A(NiCr1) 3.80 0.97 xc(NiCr2) 2174.04 0.07 w(NiCr2) 21.18 0.50 A(NiCr2) 2.58 0.20Absorbance xc(CoFe1) 2163.31 0.17 w(CoFe1) 18.62 0.33 A(CoFe1) 2.19 0.04 xc(CoFe2) 2118.01 0.23 w(CoFe2) 38.58 0.51 A(CoFe2) 4.85 0.05Absorbance xc(NiCr2) 2176.20 1.89 w(NiCr2) 25.98 1.78 A(NiCr2) 2.03 0.28 xc(CoFe1) 2161.44 2.16 w(CoFe1) 14.71 7.41 A(CoFe1) 0.14 0.25 xc(CoFe2) 2119.69 0.15 w(CoFe2) 45.13 0.31 A(CoFe2) 7.70 0.05Absorbancewavenumber (cm-1) ( a) ( b) ( c)

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281 Figure 7 42. Magnetization data of CoCr/Co Fe/Co Cr heterostructures. (a) For a 40/40/40 Co Cr/Co Fe/Co Cr heterostructure, t he (T) data, normalized to the area of the film, are plotted when the externally applied field of 100 G is oriented parallel to the surface of the film. The closed symbols represent the data prior to irradiati on (i.e. dark state), and the open symbols desig nate the data acquired after 3 hrs of irradiation with white light, but with the light subsequently off, (i.e. PPIM state). (b) Analogous (T) data for a 40/60/40 Co Cr/Co Fe/Co Cr heterostructure is shown. Insets are the time dependences of the magnetization for temperatures above and below the ordering temperature of the CoFe layer. 0 10 20 30 40 50 0.0 0.5 1.0 1.5 2.0 2.5 (10-5 emu/cm2)Temperature (K)Co-Cr Co-Fe Co-CrH (a) 40/40/40 0 10 20 30 40 50 0.0 0.5 1.0 1.5 2.0 2.5 -1 0 1 2 3 4 1.900 1.905 1.910 1.915 -1 0 1 2 3 4 2.30 2.35 2.40 (10-5 emu/cm2)Temperature (K)40/60/40 (b) (10-5 emu/cm2)time (hours) T = 15 K (10-5 emu/cm2)time (hours) T = 5 K

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282 Figure 7 43. Magnetization data of Cr Cr/Co Fe/Cr Cr heterostructures. (a) For a 40/40/40 Cr Cr/Co Fe/Cr Cr heteros tructure, t he (T) data, normalized to the area of the film, are plotted when the externally applied field of 100 G is oriented parallel to the surface of the film. The closed symbols represent the data prior to irradiation (i.e. dark state), and the open symbols desig nate the data acquired after 3 hrs of irradiation with white light, but with the light subsequently off, (i.e. PPIM state). (b) Analogous (T) data for a 60/40/60 Cr Cr/Co Fe/Cr Cr heterostructure is shown. The inset shows the difference b etween the photoinduced and dark states, showing modification of the magnetization at temperatures well above liquid nitrogen. 0 50 100 150 200 250 300 0 2 4 6 8 (10-5 emu/cm2)Temperature (K) Cr-Cr Co-Fe Cr-CrH (a) 40/40/40 0 50 100 150 200 250 300 0 2 4 6 8 0 50 100 150 200 250 300 -0.08 -0.06 -0.04 -0.02 0.00 (10-5 emu/cm2)Temperature (K)60/40/60 (b) (10-5 emu/cm2)Temperature (K)

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283 Figure 7 44. Photoexcitation of CoFe. In these schema of a Co Fe lattice, the ferricyanide molecules are represented as red octagons and the cobalt ions are represented by gray circles. Bonds between the atoms are represented by gray lines. (a) The low spin Co Fe sample has a lattice constant of ~ 10 [ 160 ] [ 161 ] (b) From EXAFS measurements [ 96 ] it was shown that under photoirradiation, the structural change in the CoFe, increasing the unit cell to ~ 10 .3 takes place in the CoN bonds, while the FeC bonds remain rigid. Photoexcited bonds are represented as thicker, longer yellow lines. This has the effect that Co ligand fields are found to be distorted in different states of photoexcitation. Figure 7 45. Distortions in NiCr. In these schema of a Ni Cr lattice, the hexacyanochromate molecules are represented as blue octagons and the nickel ions are represented by black circles. Bonds between the atoms are represented by gray lines. (a) While in the bulk Ni Cr, simple cubicity is the most energetically favorable, (b) in thin films there may be a tetragonal dist ortion of the Ni coordination. This distortion gives rise to the anisotropy seen in the dark state of films and is only of ancillary interest to the discussion of the photoeffect, as it was discussed in detail previously in Chapter 6. (c) In heterostruct ures, where Ni Cr is in intimate contact with CoFe, structural distortions induce by photoirradiation in the CoFe can propogate to the Ni Cr lattice, distorting the Ni octahedra. ( a) (b ) ( a) (b ) (c )

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284 Figure 7 46. Anisotropy in NiCr. The lower figures are analogous to those presented in Figure 7 45. The upper figures are representing the anisotropy axes present and the subsequent preferred orientation of the spin momentum in an applied field of the same energy scale as the anisot ropy. (a) For a sample with correlated anisotropy, black arrows, macroscopic differences in the magnetization, red arrows, can be seen for different orientations. At the furthest left, a perpendicular orientation of the anisotropy axis and the applied fi eld is shown, with the tendency for spin momentum to lie along the anisotropy axis, and not necessarily along the applied field. Just to the right, a parallel orientation of the anisotropy axis and the applied field is shown, showing the tendency of both energies to align the spins along the field axis. (b) For a sample with random anisotropy, black arrows, there is no angular dependence of the magnetization. However, a reduction of the magnetization, red arrows, compared to the case of no anisotropy is expected for sufficiently low fields, as some moments always prefer to point away from the applied field. H ext ( a) (b )

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285 CHAPTER 8 SUMMARY AND CONCLUSI ONS 8 While each chapter of th is thesis has been presented in such a way as to be largely self contained, a more general summary and set of conclusions will be made in t his chapter The motivation of the preceding experimental and theoretical investigations had two, main driving factors, the investigation of previously unknown science and education of the author and all interested parties These goals will be explicitly considered as a final contemplation of the dissertation is made. In Chapter 2, the experimental techniques are outlined for the purpose of understanding the experimental data presented. Although