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Manipulating the Magnetic Domains of Hole-Doped Manganites by Using Electric Field

Permanent Link: http://ufdc.ufl.edu/UFE0021751/00001

Material Information

Title: Manipulating the Magnetic Domains of Hole-Doped Manganites by Using Electric Field
Physical Description: 1 online resource (102 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: anisotropy, cmr, film, fluid, lpcmo, manganites, nanostructure, pld, stm
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: The observation of colossal magnetoresistance (CMR) and phase coexistence in hole-doped rare earth manganese oxides (manganites) have sustained the interest in these materials for over a decade. In recent years giant magneto resistance (GMR) materials have got recognition in the field of nanotechnology such as read/write memory devices. Because of the ever increasing demand of memory devices, researchers and engineers are compelled to look for new materials and reduce the device sizes even further. We investigate on the nanometer-sized phases co-existing in these manganites by using magnetotransport and scanning tunneling microscopy (STM) measurements. As thin films are required for industrial applications, we have grown thin films of manganites by using pulsed laser deposition on various substrates to obtain diverse physical properties. We will present the temperature dependence of the in-plane resistivity and the current to voltage (I-V) characteristics of these materials. While cooling and warming, resistivity of the manganites we grew shows hysteresis which is a signature of first order phase transition and phase coexistence. We will present an unconventional method (magnetic field being conventional one) to tune one phase over another by applying electric field. This method is unique in that it can be applied precisely to the local phases unlike magnetic field which affects the whole region of application. In an attempt to identify the different phases, we were able to draw a simple but unique phase diagram by using resistance vs. magnetic field (R-H) isotherms. The phase diagram contains two clean phases (metallic and insulator) and two mixed phases (static phase and fluid phase). Our results show a visibly distinct effect of the applied electric field in the region of the phase diagram where it is fluid phase. We call this fluid like phase an electric soft matter state. Our data suggest that the applied electric field orients the metallic domains of the material in the direction of the applied field. Using this electric field driven orientation we have been able to suggest a method to manipulate the magnetic nanostructure of manganites. In addition we will also attempt to obtain a local electronic picture of the material by using a technique called scanning tunneling potentiometry.
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.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Biswas, Amlan.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0021751:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021751/00001

Material Information

Title: Manipulating the Magnetic Domains of Hole-Doped Manganites by Using Electric Field
Physical Description: 1 online resource (102 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: anisotropy, cmr, film, fluid, lpcmo, manganites, nanostructure, pld, stm
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: The observation of colossal magnetoresistance (CMR) and phase coexistence in hole-doped rare earth manganese oxides (manganites) have sustained the interest in these materials for over a decade. In recent years giant magneto resistance (GMR) materials have got recognition in the field of nanotechnology such as read/write memory devices. Because of the ever increasing demand of memory devices, researchers and engineers are compelled to look for new materials and reduce the device sizes even further. We investigate on the nanometer-sized phases co-existing in these manganites by using magnetotransport and scanning tunneling microscopy (STM) measurements. As thin films are required for industrial applications, we have grown thin films of manganites by using pulsed laser deposition on various substrates to obtain diverse physical properties. We will present the temperature dependence of the in-plane resistivity and the current to voltage (I-V) characteristics of these materials. While cooling and warming, resistivity of the manganites we grew shows hysteresis which is a signature of first order phase transition and phase coexistence. We will present an unconventional method (magnetic field being conventional one) to tune one phase over another by applying electric field. This method is unique in that it can be applied precisely to the local phases unlike magnetic field which affects the whole region of application. In an attempt to identify the different phases, we were able to draw a simple but unique phase diagram by using resistance vs. magnetic field (R-H) isotherms. The phase diagram contains two clean phases (metallic and insulator) and two mixed phases (static phase and fluid phase). Our results show a visibly distinct effect of the applied electric field in the region of the phase diagram where it is fluid phase. We call this fluid like phase an electric soft matter state. Our data suggest that the applied electric field orients the metallic domains of the material in the direction of the applied field. Using this electric field driven orientation we have been able to suggest a method to manipulate the magnetic nanostructure of manganites. In addition we will also attempt to obtain a local electronic picture of the material by using a technique called scanning tunneling potentiometry.
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.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Biswas, Amlan.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0021751:00001


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1 MANUPULATING THE MAGNETIC DOMAINS OF HOLE-DOPED MANGANITES BY USING ELECTRIC FIELD By TARA P. DHAKAL A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2008

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2 2008 Tara P. Dhakal

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3 To my parents

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4 ACKNOWLEDGMENTS My graduate studies at the Un iversity of Florida have been a g reat learning experience because of so many people and fac ilities available at the univers ity. First and foremost I would like to thank my advisor Prof. Amlan Biswas for providing me a stress free environment to do research and studies. His willingness to help me and my lab mates at any time at any level is very much appreciated. The fact that he was a new f aculty member gave me exposure to a lot of new things like setting up cryogenics, writing data acquisition programs, designing, etc. I had the opportunity to take graduate level courses from very experienced teachers of our physics department for which I will always be thankful. I also would like to thank my supervisory committee members Prof. Arthur He bard, Prof. Yoonseok Lee, Prof. Dmitri Maslov and Prof. Cammy Abernathy for providing va luable suggestions and guidance during my qualifying exam presentation and other times. In pa rticular I would like to thank Prof. Hebard for providing me an opportunity to collaborate in projects like ma gneto-capacitance of capacitance structures in which manganites were used as one of the electrodes. I al so would like to thank Prof. David Tanner for a collaborative opportunity to do optical measurements on some of the manganites. I would like to thank my lab mates for their friendship, company and encouragement. I have to thank Sung Hee for her readiness to di scuss physics problems with enthusiasm and deep insight. I would like to thank J acob and Devesh for helping me with anything including film growth and designing. I have so many friends to be thankful for. I would like to thank Sinan, Naveen, Guneeta, Sef, Rajiv, Ritesh, Pradeep, Ju n, Abhijit, Priyank, Emre, Cem, Susumu and all the others for being my very good friends and also helping me to broaden my research experience. I also would like to thank everybody in machine shop and in nanofabrication lab for making them such a convenient place to get th ings done quickly. I also would like to thank

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5 people in cryogenics for providing liquid helium and liquid nitroge n 24/7. I would also like to thank Bob for making lab teaching a fun experi ence like providing pizza with nominal price during practice sessions. I would lik e to thank the janitors who keep the department neat, clean and hygienic for all of us. My parents have been a source of inspiration fo r me. I would not be able to come to this point without their selfless sacrifice. I would lik e to thank them for providing me care and love and everything possible. I am also grateful to my wife Kasumi who has tirelessly taken care of my two beautiful daughters while providing me free time to come to school. And finally I would like to thank Mira and Kara for their preci ous smiles which would always cheer me up.

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........8 LIST OF FIGURES.........................................................................................................................9 ABSTRACT...................................................................................................................................11 CHAP TER 1 INTRODUCTION..................................................................................................................13 Double Exchange Mechanism................................................................................................ 13 Crystal Structure of the Manganites....................................................................................... 14 Colossal Magnetoresistance, I-M T ransition and Magnetism................................................ 15 Jahn Teller Distortion......................................................................................................... ....18 2 PHASE COEXISTANCE IN MAN GANITES: PR SUBSTI TUTION.................................. 25 3 MATERIALS AND EXPERIMENTAL METHODS............................................................ 33 Pulsed Laser Deposition.........................................................................................................33 Atomic Force Microscopy......................................................................................................34 Transport Measurements........................................................................................................ 36 Magnetization Measurements.................................................................................................36 4 ELECTRIC FIELD EFFECT.................................................................................................47 Motivation...............................................................................................................................47 Experimental Details........................................................................................................... ...49 Results and Observations....................................................................................................... .50 Hysteresis in Resistivity.................................................................................................. 50 Phase Diagram.................................................................................................................50 Nature of Phase Coexistence...........................................................................................51 Nonlinear Current to Voltage Characteristics................................................................. 53 Conclusion..............................................................................................................................55 5 ANISOTROPY IN TRANSPORT PROPERTIES OF MANGANITES............................... 61 Motivation...............................................................................................................................61 Experimental Results and Discussion..................................................................................... 62 Magnetization as a Function of Electric Field................................................................. 62 Nanofabrication of a Cross Structure..............................................................................64 Transverse Resistance by Lock-in Amplifier.................................................................. 65

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7 Conclusion..............................................................................................................................66 6 SCANNING TUNNELING MICROSCOPY......................................................................... 74 Motivation...............................................................................................................................74 Basic Principle of Sca nning tunneling Microscopy ................................................................ 74 Topography of Sample Surface..............................................................................................75 STM Spectroscopy............................................................................................................... ...75 Designing Low Temperature Scan ning Tunneling Microscope ............................................. 76 Approach Mechanism.............................................................................................................77 Current to Voltage Converter................................................................................................. 78 Feedback System................................................................................................................ ....78 Summing Circuit.....................................................................................................................79 Imaging Graphite............................................................................................................... .....80 Scanning Tunneling Potentiometry........................................................................................81 7 CONCLUSION AND FUTURE WORK............................................................................... 95 Conclusion..............................................................................................................................95 Future Work............................................................................................................................95 Scanning Tunneling Potentiometry.................................................................................95 Multiferroics....................................................................................................................96 LIST OF REFERENCES...............................................................................................................97 BIOGRAPHICAL SKETCH.......................................................................................................102

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8 LIST OF TABLES Table page 1-1 Ionic radii of different ions in LPCMO m anganite............................................................ 20 3-1 Lattice mismatch strain of the film LPCMO grown on differ ent substrates...................... 38

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9 LIST OF FIGURES Figure page 1-1 Double exchange of eg electron in two Mn sites via oxygen 2p state................................ 21 1-2 Structure of the manganites with perovskite lattice........................................................... 22 1-3 Reduction of resistivity by a magnetic fi eld as a function of tem perature in the compound Nd0.67Ca0.33MnO3.............................................................................................23 1-4 Lifting of the eg orbital degeneracy of LaMnO3 by Jahn-Teller distortion........................ 24 2-1 Phase diagram of two ma nganites LCMO and PCMO. .....................................................29 2-2 Resistivity as a function of temperat ure in LPCMO thin film of 300 thickness grown on NGO (110) substrate and LCMO grown on LAO............................................. 30 2-3 Temperature dependence of the phase separated domains in La0.33Pr0.34Ca0.33MnO3 (LPCMO) grown on NGO (110) substrate as seen by magnetic force microscopy (MFM)................................................................................................................................31 2-4 Resistivity as a function of temperat ure of the LPCMO grown on LAO substrate. ..........32 3-1 Pulsed laser deposition system........................................................................................... 39 3-2 X-ray diffraction peaks of the film (La0.5Pr0.5)0.67MnO3 grown on NGO substrate........... 40 3-3 Atomic force microscopy image of (La1-yPry)0.67Ca0.33MnO3 with y=0.5......................... 41 3-4 An AFM image of a 1000 thick film of (La0.5Pr0.5)0.67MnO3, grown on STO (001) substrate...................................................................................................................... .......42 3-5 Magnetization as a function of temperature in a thinner (75 ) strained LP CMO film grown on STO substrate.....................................................................................................43 3-6 Two different methods of measuring re sistivity. (a) 4-probe m ethod (b) 2-probe method......................................................................................................................... .......44 3-7 Resistance ( R ) as a function of tem perature ( T ) of La0.67Mn0.33MnO3 for various oxygen pressure.................................................................................................................45 3-8 Magnetization as a function of applied m agnetization in (La0.4Pr0.6)0.67MnO3 grown on NGO substrate...............................................................................................................46 4-1 Resistivity vs. temperature curves for thin films of (La1 yPry)0.67Ca0.33MnO3 ( y =0.4, 05, and 0.6) on NGO substrates......................................................................................... 56 4-2 Phase diagram of LPCMO created by using R vs. H isotherm s........................................ 57

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10 4-3 I V curv es of the (La1 yPry)0.67Ca0.33MnO3 ( y =0.6) thin film in the cooling cycle with the voltage being ramped up.............................................................................................. 58 4-4 T V phase diagram for the (La0.4Pr0.6)0.67Ca0.33MnO3 thin film in the cooling cycle......... 59 4-5 I-V curv es of the (La1 yPry)0.67Ca0.33MnO3 (y=0.6) thin film in the warming cycle with the voltage being ramped up......................................................................................60 5-1 Current as a function of applied voltage in (La0.4Pr0.6)0.67Ca0.33MnO3..............................68 5-2 Magnetization as a function of magnetic fi eld for three different voltages applied on (La0.4Pr0.6)0.67Ca0.33MnO3...................................................................................................69 5-3 Schematic of the manipulation of me tallic dom ains by using electric field......................70 5-4 Resistance and cross bar structure..................................................................................... 71 5-5 Schematic of tran sverse resistance (RT) measurement while the dc voltage is being applied in longitudinal direction........................................................................................ 72 5-6 Longitudinal resistance (RL) and the transverse resistance (RT).......................................73 6-1 Schematic of scanning of STM tip in constant current mode............................................ 83 6-2 Schematic of the STM spectroscopic tunneling................................................................. 84 6-3 Design of different parts of the STM head........................................................................ 85 6-4 Voltage pulses in coarse approach mechanism.................................................................. 86 6-5 Current to voltage c onverter circuit for STM. ................................................................... 87 6-6 Feedback circuit for STM imaging. The pr oportional and Integral circuit are used. ........ 88 6-7 Summing circuit for the input of piezoelectric scanner. .................................................... 89 6-8 A vacuum can and a probe to hold the STM head............................................................. 90 6-9 Vibration isolation system.................................................................................................91 6-10 Scanning tunneling microscopy flow chart and electronics .............................................. 92 6-11 Calibration scan on graphite at room temperature.............................................................93 6-12 Scanning tunneling Potentiom etry circuit diagram............................................................94

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11 Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy MANUPULATING THE MAGNETIC DOMAINS OF HOLE-DOPED MANGANITES BY USING ELECTRIC FIELD By Tara P. Dhakal May 2008 Chair: Amlan Biswas Major: Physics The observation of colossal magnetoresistance (CMR) and phase coexistence in hole-doped rare earth manganese oxides (manganites) have su stained the interest in these materials for over a decade. In recent years giant ma gneto resistance (GMR) materials have got recognition in the field of nanotechnology such as read/write memo ry devices. Because of the ever increasing demand of memory devices, researchers and engineers are compelled to look for new materials and reduce the device sizes even further. We investigate on the nanometer-sized phases coexisting in these manganites by using magnetotransport and scanning tunneling microscopy (STM) measurements. As thin films are required fo r industrial applications, we have grown thin films of manganites by using pul sed laser deposition on various substrates to obtain diverse physical properties. We will present the temperat ure dependence of the in -plane resistivity and the current to voltage ( I-V ) characteristics of these materi als. While cooling and warming, resistivity of the manganites we grew shows hysteresis which is a signature of first order phase transition and phase coexistence. We will present an unconventional method (magnetic field being conventional one) to tune one phase over another by applying electric field. This method is unique in that it can be applied precisely to the local phases un like magnetic field which affects the whole region of application.

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12 In an attempt to identify the diffe rent phases, we were able to draw a simple but unique phase diagram by using resistance vs. magnetic field ( R-H ) isotherms. The phase diagram contains two clean phases (metallic and insulator) and two mixed phases (static phase and fluid phase). Our results show a visibly distinct effect of the a pplied electric field in the region of the phase diagram where it is fluid phase. We call this fluid like phase an electric soft matter state. Our data suggest that the applied el ectric field orients the metallic domains of the material in the direction of the applied field. Usi ng this electric field driven orie ntation we have been able to suggest a method to manipulate the magnetic nanos tructure of manganites. In addition we will also attempt to obtain a local electronic pictur e of the material by using a technique called scanning tunneling potentiometry.

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13 CHAPTER 1 INTRODUCTION The unique properties of hole-doped manganese oxides (manganites) have generated great interest both due to the cha llenges they pose to our understa nding of fundamental magnetic phenomena and because they show prospect of application in new device technologies [1-4]. Manganites show a sharp insulator to metal trans ition (I-M) as a function of several factors like temperature, electric field, ma gnetic field, light, hydrostatic pre ssure, strain etc [5-10]. The transition due to a magnetic field leads to a co lossal magnetoresistance (CMR) in manganites. The I-M transition is also accompanied by a paramagnetic to ferromagnetic transition as the temperature is lowered. In additi on the transition also brings about a structural change in the material. Thus the physics of manganites is dict ated by the interplay between the magnetism, the electronic properties and th e lattice structure. Due to this complexity, manganites exist in various phases important for fundamental physics and technological aspects as well. As giant magnetoresistance (GMR) materials have already made their way to the real life applications [11-13], the CMR materials show a br ight prospect for device technology. Double Exchange Mechanism Manganites have a chem ical formula RE1-xAExMnO3 where RE is a rare earth ion like La, Pr, Nd etc. and AE is a divalent ion like Ca, Sr, and Pb etc. These materials show colossal magnetoresistive (CMR) effect and an insulator to metal ( I-M ) phase transition. In the undoped limit where x =0 (zero) the material is an antiferrom agnetic insulator. In this case the Mn3+ ions have 4 electrons in the 3 d-shell, and they are surrounded by O2-, forming an octahedron. The crystal field splits the d -orbitals into 2 eg and 3 t2 g orbitals. Hunds coupling favors that 3 electrons populate the t2 g orbitals and 1 electron the eg orbitals. The t2 g electrons are localized and eg electrons become itinerant above a cer tain critical doping and use oxygen p orbitals as a

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14 bridge between Mn ions. Furthermore in the undoped case eg degeneracy is split due to JahnTeller (JT) distortion. This splitting localizes the eg electrons thus making the undoped material an insulator. In this case the energy is minimi zed if the spin alignment is antiferromagnetic. On the other hand a ferromagnetic metallic ph ase is formed in the composition range 0.2< x <0.5 [3]. The strong ferromagnetic behavior of these hole-doped manganites is a consequence of the double exchange mechanism (DE) [14-16]. Th e divalent AE ions when doped in place of the trivalent RE ions results in the mixed valency of the transition metal ions, Mn3+ ( t2g 3 eg 1) and Mn4+ (t2g 3 eg 0) For example, if 20% of the RE3+ cations were replaced by AE2+ cations then 20% of manganese ions have to be Mn4+ instead of Mn3+ to match valency proportion. Since there are no electrons in the eg level of Mn4+, the eg electron of an Mn3+ ion will hop to the neighboring Mn4+ ion via double exchange thus giving rise to conductivity in these mate rials. This DE of Mn3+ and Mn4+ occurs only if the spin of the respective d-shells align parallel to each other as shown in figure 1-1. In un-doped case, the supe r exchange interaction gives antiferromagen tism. Crystal Structure of the Manganites The m anganites have the ABO3 (here A=RE1-xAEx and B=Mn for the present manganites) type of perovskite structure. As can be seen fr om the figure 1-2 (a), the smaller manganese ions form a cubic lattice with the larger RE (La, Pr, etc.) ions at the body center. The ionic radii of La, Pr, etc are shown in table 1-1. The oxygen ions are lo cated at the centers of the cube edges. If we look at the bottom right of th e figure 1-2 (c) we see that the Mn ion is surrounded by six equidistant oxygen atoms to form a regular MnO6 octahedron. The MnO6 octahedron and its 3-D network in perovs kite structure when repeated in space is shown in figure 1-2 (c). Among the octahedra or at the corners of the cubes lie the trivalent rare earth (RE) and/or divalent alkaline earth (AE) cations wh ich have relatively larger ionic radii (see table 1-1). The Mn ions which are at the center of each octahedron and the oxygen ion

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15 at the corners are not shown on the figure 1-2 (c ). The properties of th e perovskite structure compounds can be controlled by cation substi tution and/or oxygen stoichiometry. The substitution of cation actu ally may rotate the MnO6 octahedra and therefore the whole structure may not look like ideal cube as shown in the figure 1-2 (b). The ability to tune such distortion helps us to understand the co-relation among magnetic, structural and electronic degrees of freedom in the transition meta l oxides. In the case of CMR oxides like manganites the perovskite structure is in fact strongly distorted by the combined effect of lattice mism atch (tolerance factor1) and electron instability (J ahn-Teller distortion to be explained in a latter section). Colossal Magnetoresistance, I-M Transition and Magnetism The colossal m agnetoresistance (CMR) which is a huge reduction of resistivity of the material by the application of a magnetic fiel d is one of the most stimulating phenomena observed in manganites in the hol e-doping range of 20 to 50% [17] The term colossal is because of the fact that the negative magnetoresistan ce (MR) found in these ma terials are sometimes close to 100 %. The MR in this case is defined such that the maximum value of MR would never exceed negative 100 % as seen below in the following equation. %1000 0 HMR (1-2) 1 The tolerance factor is defined as OMn ORd d 2 (1-1) where dR-O and dMn-O are average RE cation-oxygen a nd manganese-oxygen distances. A tolerance factor 1 characterizes an unstrained cubic perovskite and < 1 or > 1 represents a strain which is relaxed by dist orting the structure away from the cubic form.

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16 where H and 0 are the resistivities at applied a nd zero magnetic fields respectively. In the same composition range as mentioned abov e, a peak is seen in the resistivity at a temperature usually symbolized by Tp. Such an evolution of resistiv ity is a characteristic of an insulator to metal ( I-M ) transition with decreasing temperature. Figure 1-3 shows such a graph showing both I-M transition and CMR effect for a single crystal of Nd0.67Sr0.33MnO3 The peak in resistivity is accompanied by a magnetic transition at the Curie temperature ( Tc). Below Tc the material is a ferromagnetic metal (FMM) and above Tc, it is a paramagnetic insulator (PMI). These type of transitions can be explained by C. Zener's DE [14] mechanism. In referen ce [14], he has established a quantitative relation between the electrical conductivity ( ) and ferromagnetic Curie temperature ( Tc). T Tc ah xe2 (1-3) where x is composition of a divalent cation (AE) to be doped in th e trivalent rare earth cation ( RE), e is the electronic charge, a is the lattice parameter, h is Plancks constant and T is temperature. The relation (1-3) agrees ex cellently with the compound RE1-xAExMnO3 with RE = La, Nd and AE = Ca, Sr, or Ba studied by Jonker and Van Santen [17] and many others in the composition range 0.2 < x < 0.5 for the temperatures well below the curie temperature ( Tc). Because of the creation of a hole on the Mn4+ site an electron from Mn3+ eg band can easily hop to the hole in the Mn4+ site via O2ion and vice versa with strong on site Hund's coupling. Below Tc hopping of the electron and in turn the FM stat e is enhanced if the relative spins of the d-shell electrons are parallel. But when the temperature is up to near or above Tc the spin alignment gets disordered and eventually the hopping interaction is reduced on average. At this point if we

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17 apply the magnetic field to the material the spin disorder is reduced pushing Tp toward higher temperature and therefore a large MR is expected around Tc. This is a naive description attempted to explain CMR effect by DE interacti on. However the DE theory (cf. the relation (13)) can not explain the higher temp erature transport prope rties and the CMR effect quantitatively. At high temperatures since the spins are all oriented in ra ndom directions, the eg electron of Mn3+ ion can not hop to Mn4+ as allowed by double exchange. This eg electron then has to be thermally activated to be able to drag or distort the embedded lattice with it [3, 18]. So the hopping now occurs at the cost of the nei ghboring lattice distortion or in other words the polaron formation. This distortion now lowers the energy of the one eg electron of Mn3+ and localizes the electron within itself. This is why this material is insulating at high temperature. However as the temperature is increased the el ectron is activated to hop carryi ng the polaron[19, 20] with it. The activated electric c onductivity has the form )/exp( 1 )( TkE T TBa (1-4) Here Ea is the small polaron hopping energy which is a pproximately half of the polaron binding energy [3]. In some manganite, it shows a good fit with T instead of T in equation (1-4) [21]. This effect is known as the polaron hopping mo del. Variable range hopping (VRH) type of expression also holds good above Tc but the characteristic temp erature is found to have physically unrealistic values [1]. The ther moelectric power, as in a semiconductor, is proportional to 1/T but the characteristic en ergy is much smaller that Ea [3]. Thus the polaron hopping model described above is the best model so far to describe the transport behavior of the paramagnetic insulator (PMI) region (above Tc) of a manganite of the form RE1-xAExMnO3 with RE = La, Nd and AE = Ca, Sr, or Ba

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18 The DE is not sufficient to explain CMR observation even at low temperatures. At low temperature when the magnetization is near saturation the resistivity doe s not decrease anymore and one can expect, as explained by DE, that MR tends to zero as T tends to zero. However, contrary to this, MR at 4T has b een found around 35% which increases as T tends to zero in the compound La0.75Ca0.25MnO3 [22]. Jahn Teller Distortion The physics of manganites looks very complex. To explain these complexities, we need other mechanisms besides DE. Some of them are based on electron-lattice interaction, antiferromagnetic super excha nge interaction between the t2g local spins, orbital ordering tendency between the eg orbitals etc. The important electron-lattice interaction arises from the Jahn-Teller type [23, 24] latti ce distortion. The distortion stems from an electronic instability inherent to the Mn3+ ion in an octahedral crystal field. Under the influence of the weak crystal field, Mn3+, which has 4 d-electrons adopts a high-spin configuration with 3 electrons in t2g orbitals and 1 electron in the eg orbital. This single electron in the eg orbital is in the orbitally degenerate state. Such an orbita lly degenerate state is unstable to a distortion and thus is relaxed by lifting the degeneracy. Figure 1-4 shows a schematic of how the eg band of LaMnO3 is split by Jahn-Teller (JT) distortion to make the material insulati ng. This Jahn-Teller distortion splits the eg orbitals by around 1.5 eV [25]. LaMnO3 is not only JT distorted but also orbital-ordered [26-29]. Its spin ordering structure is A -type in which the ferromagnetic ab plane is coupled antiferromagnetically along the c -axis. Such a distortion can be caused by th e reduced size of the rare earth ion that occupies the A site of ABO3 (in our case A is (RE, AE) and B is Mn) structure because it gives rise to a rotation of the MnO6 octahedra in order to fill the empty space created. It has been found

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19 that around 20% of hole doping i.e. creating that much of Mn4+ removes the orbital degeneracy and thus the stru ctural distortion2. The other factors that may cause the distortion are the strain caused by the substrate in case of thin films a nd external perturbations like temperature. Other types of materials that exhibit CM R properties are in the composition range, x ~ 0.5. These kinds of manganites show two transi tions PMI-FMM and FMM-AFMI with lowering temperatures. In the PMI-FMM transition a peak in resistivity is observed, at a temperature that coincides with the Curie temperature ( Tc). The FMM-AFMI transition, which takes place at Neel temperature ( TN), corresponds to the existence of char ge ordered insulati ng state (COI) below TCO TN, where TCO is the temperature at which charge ordering takes place. This effect is due to the fact that in these types of manganites, the Mn3+ and Mn4+ species are in equal amounts and tend to order more easily [22, 30, 31]. In addition the manganites above x =0.5 also show charge ordering. The Mn3+ and Mn4+ are found to be ordered as thin stripes in La0.33Ca0.67MnO3 [32]. There are other type of manganites which are phase separated. Due to the comparable free energies of the COI and FMM phases, certain ma nganites display phase separation in which, the micrometer sized FMM and COI regions are observ ed to coexist at certain temperatures (see chapter 2). We are mainly interested in these phase separated manganites. 2 It should be noted that the Mn4+ ion is Jahn-Teller inactive sin ce it has no unpaired electron in eg orbital.

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20 Table 1-1: Ionic radii of diffe rent ions in LPCMO manganite. Ions Radii () La3+ 1.17 Pr3+ 1.13 Ca2+ 1.14 Mn3+ 0.75 Mn4+ 0.67 O21.24

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21 Double exchange (DE) between Mn3+ and Mn4+ through O2ions: eg 3d4 t2g 3d3 O2Mn3+ Mn4+ O2Mn4+ Mn3+ Super exchange between Mn3+ and Mn4+ ions: O2Mn3+ Mn3+ Figure 1-1: Double exchange of eg electron in two Mn sites via oxygen 2p state. The DE is possible only if the moments of the two cations are parallel. The super exchange interaction aligns the moments antiparallel.

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22 Figure 1-2: Structure of the ma nganites with perovskite latti ce. (a) Mn ions can be seen surrounded by six oxygen ions. (b) RE ions shown on the corner of a cube which contains oxygen octahedron with Mn ion at its center. (c) Network of corner-sharing MnO6 octahedra in the perovskite structure. (a) Mn Re or A O (b) (c)

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23 050100150200250300350400450 0 200 400 600 800 1000 Magnetization (emu/mole)Temperature (K) 0 5 10 15 20 Nd0.67Sr0.33MnO3Single Crystal 0T 7T (m-cm) Figure 1-3: Reduction of resistivity by a magnetic field as a f unction of temperature in the compound Nd0.67Ca0.33MnO3. This sample shows an meta l-insulator transition around 265 K for zero field. The magnetoresistance (MR) at 7T is around 90% at peak temperature. The magnetization curve (gre en) shows that the magnetic transition occurs around the I-M transition.

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24 Figure 1-4: Lifting of the eg orbital degeneracy of LaMnO3 by Jahn-Teller dist ortion. The values of exchange field (Eexch, the weak crystal field (Ecf ) and the and EJT are found to be around 3 eV, 2 eV and 1.5 eV respectively. The Fermi level is in the middle of the Jahn-Teller split labels. The bandwidth is around 1.5 to 2 eV. [25, 26]. 3d4 3d4 Eexch e g 3d z 2t2 g EJ T 3d yz 3d z xMn3+ Ec f 3d xy 3d x 2 y 2 Ef Bandwidth

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25 CHAPTER 2 PHASE COEXISTANCE IN MA NGANITES: PR SUBSTI TUTION It has been found that a diverse range of electronic and ma gnetic phases coexist in manganites [33, 34]. The coexistence of such phases is due to the competition between double exchange (DE ) and Jahn-Teller effect [35]. Figure 2-1 shows the phase diagram of two manganites La1-xCaxMnO3 (LCMO) and Pr1-xCaxMnO3 which have different ground state (GS) at the same carrier density. For example in the composition range x =0.33 where La1-xCaxMnO3 is ferromagnetic metallic (FMM), Pr1-xCaxMnO3 is a charge ordered insulator (COI). We want to destabilize the GS of one by mixing with the othe r. So by mixing these two manganites together we expect to observe phase coexistence. The chemical composition of the compound thus produced, which is our mate rial under study, is: (La1-yPry)0.67Ca0.33MnO3 (where y = 0.4, 0.5 and 0.6) (LPCMO). Such phase coexistence in fact has been observed by us and other groups [6, 5]. In the latter chapter our results on electric field effect on LPCMO will be presented. The coexistence of FMM and COI phases has also been found in strained thin films, for example, La0.67Ca0.33MnO3 (LCMO) at low temperatures [9, 36]. In the thin films of this material grown on LaAlO3 (LAO) substrates, a compressive lat tice mismatch strain leads to 3 D island growth and hence a non-uniform distribution of stra in in the film. The high strain regions of the film are converted to the COI state while the low strain regions remain in the FMM state. Such a phase-separated state is confirmed using low-te mperature magnetic force microscopy (MFM) [37, 38] and magneto-transport measurements. This phas e separated state is not observed in the bulk form of this compound and is caused by the stru ctural inhomogeneties due to the non uniform distribution of strain in the film. Thus it was a clear evidence of the strain induced charge ordered state. The strain weakens the low-temper ature ferromagnetic metallic state and a chargeordered insulator is formed at the high strain regions. This conclusion was drawn because the

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26 sample magnetization shows ferromagnetism wher e as the resistivity measurement under same conditions shows insulator like behavior. It is only possible if FMM and COI phases coexist. However, as the films were grown thicker, th e transport properties of LCMO thin films approached that of the bulk. We would like to investigate if the phase coexistence in LPCMO manganites is solely due to the Pr substitution. We have seen that as the Pr content is lowered, the FMM state becomes the dominant phase at low temperatur es. This is analogous to the be havior of LCMO under substrate induced strain. However, the observed hysteresis in the LPCMO vs. T (as shown in figure 2-2 a) is conspicuously absent in the vs. T data of the strained LCMO (figure 2-2 b). This gives a hint about the origin of phase separation for the two cases. When the phase separation is due to substrate induced strain the two phases are lock ed to the strain landscape of the thin film produced due to the 3-D island growth. It is conc eivable that for LPCMO the strain landscape is not predetermined and is dictated by the martensitic transition at low temperatures. To check this hypothesis we grew thin films of LPCMO on NGO substrates to minimize the lattice mismatch strain. The first direct evidence of the formati on and evolution of these phase separated regions in thin films of LPCMO [19] was seen by Zhang et al They used low temperature magnetic force microscopy (LTMFM) to image the magnetic domain structure of these films. As temperature is lowered from high temperature, th e ferromagnetic domains were seen to form and grow as if they were fluid (figure 2-3). The broad hysteresis in resist ivity of LPCMO is attributed to phase coexistence and the sharp transition to the first order nature of th e transition. To explai n what causes the mixed phases in our LPCMO manganite we have to consid er both the substrate induced strain and the effect of Pr-content. As we know the COI phase and FMM phase are structurally different, the

PAGE 27

27 former being pseudo-tetragonal and the latter being pseudo-cubic. In thin films the built in strain due to the lattice mismatch between the substr ate and the material introduces a structural distortion, which in turn suppresses the FMM phase. As has been seen in LCMO by Biswas et al [9, 36], the metallic region observed on LCMO grow n on NGO substrate is turned into insulating state when grown on LaAlO3 (LAO) (001) substrate. Note that LAO unlike NGO has different lattice parameters compared to LCMO, so it in troduces higher strain (about 2% compressive strain). Our sample is grown on NGO (110) so we expect a minima l substrate induced strain effect (0.52%). Now since Pr has smaller cation ra dius than La, it distorts the cubic perovskite structure of the LCMO. This dist ortion actually now creates a high strain region in the material pinned on the Pr-site. This stra in effect appears, at least through experimental results, fundamentally different from the effect of the substrate induced strain. The sharp transition and the hysteresis are signatures of phase coexis tence in LPCMO whereas LCMO grown on LAO does not show such behavior. LPCMO grown on LAO also shows that the transition broadens, moves toward lower temperature and the hystere sis width shrinks significantly as shown in figure 2-4. So in our material Pr has a big ro le to play. In LCMO grown on LAO the strained region is dominant only on the interface between the substrate and the material whereas in LPCMO the strain, although pinned at Pr-s ites, spreads throughout the material. In addition, in thin films the substrate-induc ed strain does not allow the bulk structural transition to occur. However what we saw is that our resistivity drop is very sharp similar to what is observed in the bulk forms of the material The kink on the resistan ce observed on the bulk LPCMO manganite when the material goes from paramagnetic insulator (PMI) state to the COI state [6] is also absent in our thin film sample. We believe that the Pr cations first introduce a built in strain landscape at the high th in film deposition temperatures (820 C). As the sample is

PAGE 28

28 cooled below the charge orderi ng temperature, the COI phase is formed in the high strain regions. The PMI phases formed at low strain region remain as PMI phases even below the charge ordering temperature because the structural transition is prevented in thin films. This removes the resistivity anomaly at the PMI to COI transition temperature. In addition, the strain landscape is modified due to ther mal contraction of the substrate. As the temperature is lowered further, the FMM phase is nucleated at the lo w strain regions. The FMM and COI phases then follow the strain landscape as the temperature is lowered until at the lowest temperatures a major fraction of the film goes to the FMM state, which is reminiscent of martensitic transformation. And because the lattice constants of the NGO substr ates closely match the la ttice constants of the low temperature FMM phase, the low temperature resi stivity of our thin films is lower than that of bulk LPCMO of similar composition. Hence, a large-scale phase separation is observed in thin films of LPCMO, which exhibit transitions simila r to those observed in single crystals. The fact that we have been able to achieve such sharp transitions in thin films of LPCMO is therefore significant for understanding the underlying mechanism of colo ssal magneto-resistance (CMR) and phase separation. In addition this is also an important step towards possible future applications of this phenomenon since thin films would be required for use in devices. The experiments described in the following ch apters are designed to test the hypotheses outlined above. The sample growth, characterization, transport and surface measurements, and the conclusions drawn ar e described in detail.

PAGE 29

29 Figure 2-1: Phase diagram of two manganites LCMO (upper panel) and PCMO (lower panel [24, 39]. FI PMI

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30 Figure 2-2: Resistivity as a function of temperature in LPCMO thin film of 300 thickness grown on NGO (110) substrate and LCMO gr own on LAO. (a) The resistivity of LPCMO shows a broad hysteresis between warming and cooling cycles of temperature shown in the graph by arro ws. The broad hysteresis and the sharp transitions are attributed to first order t ype of transition. (b) The resistivity as a function of temperature for La0.67Ca0.33MnO3 (LCMO) grown on LaAlO3 (LAO) (001) substrate. The thickne ss of the film is 150 (b) LCMO on LAO 050100150200250 10-410-310-210-1100101102103 (.cm)Temperature (K)on NGO(La0.5Pr0.5)0.67Ca0.33MnO3 (a)

PAGE 31

31 Figure 2-3: Temperature dependence of the phase separated domains in La0.33Pr0.34Ca0.33MnO3 (LPCMO) grown on NGO (110) substrate seen by magnetic force microscopy (MFM). The middle panel shows the resistiv ity behavior of the film. The upper panel shows the cooling run and the images are 6 m 6 m and the lower panel shows the warming run and the images are 7.5 m 7.5 m [5].

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32 050100150200250 10-1100101102 (-cm)Temperature (K) (La0.5Pr0.5)0.67Ca0.33MnO3on LAO Figure 2-4: Resistivity as a f unction of temperature of the LP CMO grown on LAO substrate. For comparison, the LPCMO grown on NGO is reproduced from figure 2-2 (a) on the lower panel. The arrows show the direction of the temperature sweep. 050100150200250 10-410-310-210-1100101102103 (.cm)Temperature (K)on NGO(La0.5Pr0.5)0.67Ca0.33MnO3

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33 CHAPTER 3 MATERIALS AND EXPERIMENTAL METHODS Three com positions of (La1-yPry)0.67Ca0.33MnO3 with y =0.4, 0.5, and 0.6 have been used to study the electrical transport and magnetic properties of this materials. As our focus was on the physics associated with the basic properties of these materials as well as the search for the possible technological applic ations, thin films were grown on different substrates. The substrates used were NdGaO3 (NGO) (110), SrTiO3 (STO) (100) and LaAlO3 (LAO) (100) substrates (see table 3-1 for the strains for all the substrates on LPCMO). Pulsed Laser Deposition Thin films of the above materials were gr own by using a technique called pulsed laser deposition (PLD) [40]. This PLD system was set up in our own laboratory. The laser used was 248 nm excimer laser. In the PLD process a pellet (target) of the desired composition is rotated while the laser pulses are made to hit the target at around 45. The evaporated plume of the plasma is then directed toward the substrate whic h is positioned on the heater such that it is just at the center of the path of the evaporated plumes as shown in the figure 3-1. Before optimizing the growth conditions, we had to play with parameters like pulse rate, pulse energy, oxygen pre ssure during the growth and af ter the growth. The sample conditions were optimized for sharper transitio ns and better conductivity. In figure 3-7, three different graphs of resi stance vs. temperature are show n. All the three films were grown with different oxygen pressures duri ng the growth and afte r the growth during cooling. As we can see that the peak temperature in resistance vs. temperature graph is very sensitive to the oxygen pressure. The oxygen pr essures for sample # 1 during the growth and post growth were 420 mTorr and 450 Torr. The pressures for similar situations for sample #2 and sample # 3 were 520 mTorr and 475 Torr and 600 mTorr and 550 Torr

PAGE 34

34 respectively. The substrate used was NdGaO3 (NGO) (110) and the substrate temperature during growth 820C for all thr ee samples. The cooling rate was 20C and the laser pulse repetition rate was 5 Hz. The pre-ablation was do ne for 5 mins with repetition rate of 10 Hz. The energy of the laser pulse wa s kept at 480 mJ. The arrows s hown in the figure are for the peak temperatures for the R vs. T graph. The conditions used are valid only for La0.67Ca0.33MnO3. For the manganite samples with other compositions, different sets of conditions had to be found. The optimized cond itions are given in the following paragraph. The substrate temperature during the growth is kept at 820C. The film s were grown at an oxygen pressure of 450 mTorr. The frequency of the laser pulse was kept at 5 Hz during the growth and 10 Hz during the pre-ablation. Before and after the growth the heating and cooling rates were kept at 20C respectively. The oxygen pressure during post growth cooling was kept at 430 Torr. Standard -2 x-ray diffractometry was used to check whether the samples grown are epitaxial and are of single chemical phase. The intensity of the x-ray peaks as a function of the angle of incidence is given in the figure 3-2 for the sample (La0.5Pr0.5)0.67Ca0.33MnO3. The arrows indicate peaks due to sample holder and S indicates substrate peaks. The film peaks are not clearly visible due to the nearly perfect lattice match with the substrate. The shallow region of graph on the lower panel of graph is due to the f ilm. The tail on both sides of the substrate would look symmetrical if there was no film deposited on it. Additional techniques like magnetization and scanning probe microscopy were used to ve rify the best growth conditions which will be explained in latter sections. Atomic Force Microscopy Atomic force microscopy (AFM) was first discovered by G. Binnig et al. in 1986 [41]. Since then, AFM is widely used in material and biological sciences [42, 43]. After the films are

PAGE 35

35 grown, we have used atomic force microscopy to see the surface roughness of the material. A commercial atomic force microscope from Digital Instruments was used to scan the samples of manganites grown. The commercial AFM is capable of scanning 12 m12 m in one scan. We have also used it for magnetic force microscopy (MFM) of some of the manganites like La1-xSrxMnO3 which are ferromagnetic even at room temperature. An AFM image of a 75 thick film of a mixed phase manganite grown on STO (100) substrate is shown in figure 3-3 a. The LPCMO films grown on the STO substrate are su bjected to a tensile strain of 1.69% (table 3-1). Because of this high strain, manganites are grown as disconnected islands. However in the film grown on NGO, which has a lo w tensile strain of 0.59%, the manganite islands are grown as connected islands (figure 3-3 b). Both of these films were around 75 thick. As is shown in figure 2-2 (a) of chapter 2, a 300 thick film of LPCMO grown on NGO shows a hysteresis in resistance between the cooling and warming cycl es of temperature and the transition from insulator to metal while cooling is very sharp like that in first order transition. However, a 300 thick film grown on STO substrate is insulating all the way to lowest temperature. To see similar hysteretic behavior typical of LPCM O, we had to grow a film thicker until the islands started to overlap each other. In figure 3-4, an AFM image of a 1000 film of LPCMO grown on STO substrate and its resistance as a function of temperature are shown. The AFM image clearly shows the overlapped islands and resistance show s the hysteresis typical of LPCMO manganites. We also wanted to confirm that the islands grown are ferromagnetic below the insulator to metal transition. For this we measured magnetization of a LPCMO film grown on STO and we chose the 75 thick film where the islands are very well separated from each other. As shown in figure 3-5, the magnetization shows ferromagnetic behavior. Magnetizati on as a function of temperature is shown for both field c ooled and zero field cooled case.

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36 Transport Measurements Transport measurements are extremely important for an experimental physicist. In addition to the simplicity of the techniques used, these measurements give a tremendous amount of information on the electronic and magnetic propertie s of the materials studied. For our sample with lower resistivity (y =0, 0.4, and 0.5), a 4-probe method is used as shown in figure 3-6 (a). In this method, current ( I ) is passed through the 2 ou tside leads and voltage ( V ) is read from the remaining 2 internal leads as shown in the figure. The resistance of the sample ( Rs) then would be V/I In this method since current doesnt flow through the voltmeter (provided the impedance of the voltmeter is very high co mpared to the sample resistance) the contact resistance and the lead resistance are automatically excluded from th e resistance measurement. For the sample with higher resistivity ( y =0.6), a 2probe method as shown in figure 3-6 (b) was used for the resistivity measurements. Here the cu rrent through the circuit would be I=VR/R and the voltage across the sample would be Vs=V0-VR. The sample resistance is Rs=Vs/I. By using this method we avoid connecting a voltmeter across the high re sistance sample which is necessary because the input impedance of the voltmeter is compar able to the sample resistance around the peak resistance temperature. Magnetization Measurements For magnetotransport measurements a high fi eld magnet system from American Magnetics was used. A dc magnetic field of 9T could be appl ied in this system and the temperature could be varied from 1.5K to 300K using a Janis Variab le Temperature Insert. For data acquisition, National Instruments Labview program wa s developed and used. For magnetization measurement a SQUID magnetometer was used wh ich is a shared faci lity within the physics department. It is very hard to measure magnetizat ion of the films grown on the substrates with large paramagnetic behavior, for example, NGO. Figure 3-8 shows the magne tization of the film

PAGE 37

37 (La0.4Pr0.6)0.67MnO3 grown on NGO. The paramagnetic signal is clear visible on the upper panel of the figure. The lower panel shows the M-H loop of the film which shows that the sample is around 67% ferromagnetic. The fully ferromagnetic sample of this type would show a magnetization of 3.67 B per Mn site.

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38 Table 3-1 : Lattice mismatch strain of the film LPCMO grown on different substrates. Note that the lattice parameter of LPCMO film is 3.84 Substrate Crystal structure Lattice parameters () Strain on LPCMO NdGaO3 (NGO) (110) Orthorhombic a= 5.426 b= 5.502 c= 7.706 0.52% (tensile) SrTiO3 (STO) (100) Cubic a=3.905 1.69 % (tensile) LaAlO3 (LAO) (001) Pseudo cubic A=3.789 1.32 % (compressive)

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39 Figure 3-1: Pulsed laser deposition system. (a) LEXtra laser system and the growth chamber. (b) The schematic of the deposition technique. (c) The plum of disintegrated manganite ions and the heater glowing can be seen during a typical film growth. (a) (b) (c)

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40 020406080100120 103104105 S S S Intensity (degrees) S 100 104 108 112 103104 Intensity (degrees)Film Figure 3-2: X-ray diffracti on peaks of the film (La0.5Pr0.5)0.67MnO3 grown on NGO substrate. The lower panel is an expanded region of the upper panel graph, which shows a shallow peak due to the film.

PAGE 41

41 Figure 3-3: Atomic force microscopy image of (La1-yPry)0.67Ca0.33MnO3 with y=0.5. (a) The film grown on a substrate of STO (100). Because of the large lattice mismatch, the material is grown as disconnected islands. (b) A film grown on NGO (110) substrate. Small lattice mismatch lets the manganite grow as connected islands. (a) (b)

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42 050100150200250 10-210-1100101102 film on STOResistivity (.cm)Temperature (K)(La0.5Pr0.5)0.67Ca0.33MnO31000 Figure 3-4: An AFM image of a 1000 thick film of (La0.5Pr0.5)0.67MnO3, grown on STO (001) substrate. Lower panel shows the resistivity of the same film.

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43 050100150200250 0.00000 0.00005 0.00010 0.00015 0.00020 0.00025 75 STO substrate(La0.5Pr0.5)0.67Ca0.33MnO3 Magnetization (emu)Temperature (K)FCZFC Figure 3-5: Magnetization as a func tion of temperature in a thinne r (75 ) strained LPCMO film grown on STO substrate.

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44 Figure 3-6: Two different met hods of measuring resistivity (a) 4-probe method. Here Rs is the sample resistance, V is the voltmeter and A is the current source. (b) 2-probe method. The standard resistance is kept at room temperatur e and voltage dropped across it measured by a voltmeter. Voltage source is used to supply a constant voltage (V0). A V I I RSvoltmeter Current source (a) sample Voltage

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45 050100150200250300 103104105 sample # 1 sample # 2 sample # 3 Resistance (ohm)Temperature (K) Figure 3-7: Resistance ( R ) as a function of temperature ( T ) of La0.67Mn0.33MnO3 for various oxygen pressure.

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46 -1500-1000-500050010001500 -0.04 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 0.04 on NGO(La0.4Pr0.6)0.67Ca0.33MnO3 Magnetization (emu)Field (Oe) -1500-1000-500050010001500 0.0 0.5 1.0 1.5 2.0 2.5 3.0 on NGO(La0.4Pr0.6)0.67Ca0.33MnO3 Magnetization(B/Mn )Field (Oe) Figure 3-8: Magnetization as a function of applie d magnetization in (La0.4Pr0.6)0.67MnO3 grown on NGO substrate. The upper panel shows the magnetization shows linear behavior due to the large paramagnetic substrate b ackground. The lower panel is plotted after subtracting the paramagnetic linear background.

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47 CHAPTER 4 ELECTRIC FIELD EFFECT We have studied the effect of substrate-induced s train on the properties of thin films of the hole-doped manganite (La1 yPry)0.67Ca0.33MnO3 ( y =0.4, 0.5, and 0.6) grown on NdGaO3 (NGO) substrates, in order to distinguish between the ro les played by long-range strain interactions and quenched atomic disorder in forming the micromet er-scale phase separated st ate. We show that a fluid phase separated (FPS) state is formed at intermediate temperatures similar to the strainliquid state in bulk compounds, which can be co nverted to a metallic state by applying an external electric field. In contrast to bulk compounds, at low temperatures a strain stabilized ferromagnetic metallic (FMM) state is formed in the y =0.4 and 0.5 samples. However, in the y =0.6 sample a static phase separa ted (SPS) state is formed simila r to the strain-glass phase in bulk compounds. Our results suggest that the substrate-induced strain is a function of temperature. Hence, we show that the temperat ure induced variation of the long-range strain interactions plays a dominant role in determining the properties of thin films of phase-separated manganites. Motivation Multiphase coexistence in hole-doped manganites is a result of the competition between phases of different electronic, ma gnetic and structural orders, whic h leads to properties such as colossal negative magnetoresistance (CMR) [44, 45]. At low temperatures the two competing phases are the ferromagnetic metallic (FMM) an d charge-ordered insulating (COI) phases. In manganites with greater average A -site cation radii ( Ar) and consequently a larger effective one electron bandwidth ( W) the pseudo cubic FMM phase is fa vored at low temperatures [46]. When smaller ions such as Pr are substituted at the A -site, the pseudo tetr agonal (distorted) COI phase then has a comparable free energy to the FMM phase, resulting in micrometer scale phase

PAGE 48

48 separation [6]. It was shown that in the presence of que nched disorder introduced by the ions of different radii, the similarity of the free energies leads to coexistence of the two competing phases [45]. However, the observation of martensitic strain accommodation in manganites [47] and fluid like growth of the FMM phase observe d in magnetic force microscopy (MFM) images of phase separated manganites [5] suggest that the phases are not pinned, which can be explained by an alternative model which shows that phase separation occurs due to the different crystal structures of the FMM and COI phases and the resu ltant long range strain interactions [44]. In fact, due to this behavior the phase separated state in manganites has been described as an electronic soft matter state [45, 48], which are similar to the phases observed in the materials such as liquid crystal [49]. To understand the underlying mechan ism for micrometer scale phase separation in manganites and propose possible technical applications, it is essential to distinguish between the roles played by quenched disord er and long range strain interactions. If long range strain interactions are the principal cause of pha se coexistence, then it should be possible to control the elas tic energy landscape with substrat e induced strain. Strain-induced phase separation has been clearl y observed in thin films of La0.7Sr0.3MnO3 grown using laser molecular beam epitaxy [50] and substrate strain is known to a ffect the nature of structural transitions in ferroelectrics [51]. On the other hand, the effect of quenched disorder can be estimated from the effect of isovalent substitution of La-ions by the smaller Pr-ions. In this chapter we report our results on the separate eff ects of substrate induced strain and isovalent substitution in thin films of the manganite (La1 yPry)0.67Ca0.33MnO3 (LPCMO), and compare our results to bulk LPCMO. The T H phase diagram of bulk LPCMO clearly shows two distinct types of phase separation (PS), a strain-li quid (dynamic PS) and a strain-glass (frozen PS) regions [52]. We show that in thin films of LPCMO, a fluid phase separated (FPS) state is

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49 formed at intermediate temperatures similar to the strain-liquid state in bulk materials. However, a strain stabilized FMM phase is formed at low temperatures leading to a sharper and larger drop in resistivity compared to bulk samples. The stra in stabilized FMM phase transforms to a static phase separated (SPS) state (analogous to the strain-glass state in bulk LPCMO when the Pr content is increased. The FPS a nd SPS states were named based on the fluid like and static behavior of the FMM regions in the temperatur e range of the FPS and SP S states, respectively, as observed in MFM images of LPCMO [5]. Corresponding to our nomen clature of the phase separated states we show that an external electri c field transforms the FPS state to a metallic state whereas there is negligible electric field effect once the sample reaches the SPS state. Experimental Details We have grown thin films of (La1 yPry)0.67Ca0.33MnO3 (LPCMO) (y =0.4, 0.5, and 0.6) using pulsed laser deposition (PLD). The film s were grown in an oxygen atmosphere of 450 mTorr on NdGaO3 (NGO) (110) and SrTiO3 (STO) (100) substrates kept at 820 C. All the films described here are 30 nm thick and were grown at a rate of about 0.05 nm/s. These growth conditions were optimized to obtain an insulato r-to-metal transition temperature while cooling TIM (cooling) close to that observed in bulk compounds of similar composition and the minimum transition width at TIM (cooling). Such an optimization is crucial for mapping the phase diagram of LPCMO, since the properties of thin films of this compound vary markedly depending on the growth conditions. Standard 2 x-ray diffraction data show that the films are epitaxial and of a single chemical phase. Since the resistan ce of the films can be as high as 1 G the resistivity of the films was measured with a two-probe method using a constant voltage source, as shown in the lower inset of figure 4-1, with V0 set at 5 V. We measured the voltage VR across the standard resistor R The current in the circuit then was VR/ R and the voltage across the sample VS= V0 VR. Hence, the sample resistance RS = ( V0 VR)R / VR. As a check, the low temperature resistivity was

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50 also measured using a standard four-probe method. The I V curves to be shown later were measured by varying V0. For the vs. T curves, the temperature was varied at a rate of 2 K/min. Results and Observations Hysteresis in Resistivity The vs. T data for three LPCMO films grown on NGO ( y =0.4, 0.5, and 0.6) and one LPCMO film grown on STO ( y =0.5) are shown in figure 4-1. An expected reduction of TIM (cooling) is observed with increasing Pr concentration due to the reduction of Ar. The width of the hysteresis between warming and cooling cy cles of temperature drops sharply when y is reduced from 0.5 to 0.4 (Fig. 4-1, top inset). A remarkable feature of the resistivity is that, unlike in bulk (La1 yPry)0.67Ca0.33MnO3 (LPCMO) [6], the residual resistivity 0 ( measured at 10 K) does not change from y =0 (La0.67Ca0.33MnO3, LCMO) to y =0.5 and then rises sharply for y =0.6 (Fig. 4-1, top inset). Phase Diagram We mapped the T H phase diagram of the y =0.6 sample by measuring resistance R vs. H curves at different temperatures for the cooling cy cle (Figs. 4-2 right pane l). Since the effect of H is irreversible, the sample wa s reset after every field sweep by heating it to 150 K and then cooling it to the set temperature. The data poi nts were obtained by loca ting the magnetic field corresponding to the steepest change in R at a given temperature, i.e., where dR / dH is maximum (Fig. 4-2 right panel). The squares and triangles represent the melting and freezing fields, respectively (Fig. 4-2, left panel). The melting fi eld line is extended to ze ro field by including the TIM (cooling) at zero field from figure 4-1. The inverted triangles represent the melting field at low temperature ( T 50 K). Since the transition wi dths can be large in thin films (Fig. 4-2, right panel), as shown by the error bars in the left panel of Fig. 4-2, we have used a more direct

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51 method of constructing the T H phase diagram. We plotted the difference in log(R) between up sweep and down sweep of H log(Rup) log(Rdown) in the T H plane as a 2D color plot (Fig. 4-2, left panel). The two methods of plotting the phase diagra m give similar results except that the SPS region can be clearly distinguished only with th e second method. Four di stinct regions can be clearly identified in this phase diagram. Tw o pure phases namely the COI state and the FMM state and two mixed phase states, namely, the fl uid phase separated (FPS) state and the static phase separated (SPS). As mentioned earlier, the nomenclature of the mixed phase states is based on the electric field effect (explained in the later sections) and the previously reported MFM images of LPCMO thin films [5]. Nature of Phase Coexistence To elucidate the nature of phase coexistence in our thin films of LPCMO, we have to understand the combined effect of substrate induced strain and Pr substitution. NGO (110) has an orthorhombic structure with two in-plane distan ces of 3.853 and 3.861 at room temperature, which stabilizes the pseudo cubic structure with negligible strain when manganites such as LCMO are grown on NGO [36, 9]. Hence the subs trate tends to stabilize the pseudo cubic FMM phase at low temperatures in LPCMO samples grown on NGO (110) substrates. Conversely, Pr has a smaller cation radius than La and hence, substitution of La ions by Pr ions favors a distorted crystal structure [53]. The distortion produced by Pr substitution reduces the TIM (cooling) in our LPCMO thin films similar to bulk LPCMO [6]. On the other hand, strain induced by the NGO substrate removes the resi stivity anomaly at the charge-ordering temperature ( TCO) seen in bulk LPCMO [6], which is an effect of the suppression of the bulk structural transition near TCO [54]. Therefore, although the films ar e under negligible substrate strain at room temperature, the structural phase separation of the bulk material as the temperature

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52 is lowered [6] results in a strain build up in the thin film. A similar effect has been shown for ferroelectric thin films where the strain in the film is released by the formation of domains with different structures aided by the defects in the film [55]. A similar structural separation in manganites would lead to phase separation into FMM and COI regions and the observed reduction of TIM (cooling). Furthermore, the thermal contraction of the substrate modifies the strain landscape leading to the fluid nature of the phases as has been observed in MFM images of LPCMO thin films [5]. A detailed temperature dependent stru ctural study of the thin films is required to verify the above hypothesis. However, it is clear from figures 41 and 4-2 that in spite of substrate induced strain, th e phase diagram of the LPCMO ( y =0.6) sample is similar to that of bulk LPCMO [52]. In contrast to the y =0.6 sample, 0 for the y = 0.4 and 0.5 samples drops to a value consistent with the pure FMM phase of the y =0 sample (Fig. 4-1, upper inset). Magnetization measurements also show that the y =0.5 sample has a saturation magnetization ( Msat) consistent with a pure FMM phase. Msat is estimated by measuring the M-H loops for the thin films and then subtracting the paramagnetic background due to the NGO substrate. Hence, these two samples have a pure FMM phase at low temperat ures in contrast to the strain-glass phase observed in bulk LPCMO [52], because the NGO substrate favors the pseudo cubic FMM phase at low temperatures. When the Pr concentration is increased to y =0.6, 0 increases by about an order of magnitude to 8.7 m -cm. The value of Msat for this sample is consistent with 67 % of the material being in the FMM phase at low temp eratures (lower panel of the figure 3-6) As shown in the left panel of Fig. 4-2, a phase sim ilar to the strain glass phase in bulk LPCMO (the SPS state) appears in the phase diagram of thin film LPCMO only when the Pr concentration is increased to y =0.6. It was suggested in Ref. [52] that the strain-liquid to strain-glass transition is

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53 due to the interaction of the long-range strain with the quenched atomic disorder. The observation of the FPS state and the absence of the SPS state in the y =0.5 sample show that under substrate induced strain, l ong-range strain is the drivin g force behind micrometer scale phase separation in manganites. Figure 4-1 also shows the resistivity of a y =0.5 film on an STO (100) substrate (STO has a cubic structure with a lattice constant of 3.905 which translates to a biaxial tensile strain of 1.2% for a thin film of a manganite such as LCMO grown on STO. The film was insulating down to the lowest temperatur e of 5 K (data not shown down to 5 K due to high resistance values) which suggests a large stra in effect. However, atomic force microscope images revealed different growth modes for the films on NGO and STO which makes it impossible to quantify the change in the strain on these two differe nt substrates since it is known that different growth modes lead to different local stra in distributions [36, 9]. Nonlinear Current to Voltage Characteristics To realize potential applications, an accessibl e handle is needed to manipulate the phase separation in these thin films and one candidate is an extern al electric field. Previous measurements of the electric field effect showed a large drop in the resistivity of charge-ordered manganites on the application of an electric field [7]. Electric field effects have also been observed in thin films of LPCMO [56]. Large electric current effects have been observed in LPCMO crystals due to Joule heatin g of the metallic regions [57]. In our samples, the voltage was applied to the LPCMO thin films using two indium contacts 0.75 mm apart. The circuit for measuring the I V curves is the same as the one used fo r measuring resistivity. Figure 4-3 shows the I V curves for the y =0.6 sample for the cooling run. At a threshold voltage Vth, the current across the film rises abruptly. This electric field effect is irrevers ible as shown for the 59 K curve (red) in Fig.4-3. The sample stays in the low resistance state even when the electric field is

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54 removed. To recover the high re sistance state we heat the film to a te mperature above the resistivity hysteresis region and then cool it down to the next desired temperature. We have plotted the quantity log(dI /dV) calculated from these I V curves as a function of temperature and voltage to construct a T V phase diagram as shown in Fig. 4-4 a. The observed electric field effect is not a heating e ffect as reported by Tokunaga et al. [57] since heating should increase the resistance in the temperature range shown in Fig. 4-3. We also observed a similar electric field effect while cooling the y =0.5 sample. Using an extended double exchange Hamiltonian, Gu et al. showed that the application of an electric fi eld favors the FMM phase over the COI phase [58]. The authors also showed that Vth decreases as the number of high resistance elements is decreased from a 100 to 50 % of the sample, wh ich qualitatively agrees with the observed variation of Vth as a function of temperature shown in Fig. 4-4 a (decreasing temperature is analogous to decreasing numb er of high resistances). A schematic picture of the phase coexisten ce in LPCMO for the cooling cycle in the T V plane is shown in Fig. 4-4 b. As the size of the metallic regions (shown in black) increases, the local electric field across the smaller insulating regions (shown in white) is enhanced, which leads to the predicted and observed decrease in Vth with decreasing temperature. Above Vth, percolation of the metallic regions in the film resu lts in the sharp rise in the conductivity of the film. Further rise in the voltage across the film increases the current flowing through the metallic regions resulting in local heating and a decrease in conductivity sim ilar to the results of ref. [57] as seen in Fig. 4-4 a (gray regions in Fig. 44 b). When the sample is cooled down to the SPS state, the metallic regions form a percolating path and increasing the voltage across the sample only results in a larger current a nd Joule heating. In contrast to the cooling run, no sharp increase

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55 in current was observed during the warming run as shown in Fig. 4-5, which suggests that there is no enhancement of the local electric field. This is due to the static FMM regions in the warming run [5]. The I-V behavior during cooling for a similar resistance value is shown for comparison (red curve). Therefore, once the sample is cooled to the SPS region, the FMM and COI phases are locked in space. Since the FMM regions percolate through the sa mple, application of a voltage leads to Joule heating of the FMM regions. On further warming the FMM regions homogeneously transform to a high temperature insulating phase with no local enhancement of the electric field. Conclusion In conclusion, substrate induced strain modifies the mechanism of micrometer-scale phase separation in manganites. At intermediate temperatures [ T ~ TIM (cooling)], long range strain interactions lead to a fluid pha se separated state analogous to the strain-liquid phase in bulk LPCMO [52]. However, below a critical Pr concentration ( y 0.5) the FPS state transforms to a strain stabilized FMM phase at low temperatur es, unlike the strain-liquid to strain-glass transition in bulk LPCMO [52]. A static phase se parated state analogous to the strain-glass phase in bulk LPCMO is observed at low temperatures only when the Pr concentration (and hence the quenched atomic disorder) is incr eased above a critical value ( y 0.6). An external electric field provides an effective means to modify the phase separation in manganites since it lowers the resistance of the FPS state by two orders of magnitude due to a local electric field enhancement. However, an electric field has negligible effect on the SPS state. Further experiments using low temperature MFM are needed to find the microsc opic mechanism of the el ectric field effect. Our model shown in figure 4-4 (b), can verified by using low temperature MFM as it can image the shape of the magnetic domains.

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56 050100150200250 10-1100101102103104105106107 0.00.20.40.6 50 100 150 200 2 4 6 8 TIM (warming) (m-cm)Temperature (K) y = 0.6 y = 0.5 y = 0.4 TIM (cooling) Cooling Warming y TIMB0 (m-cm) Figure 4-1: Resistivity vs. temperature curves for thin films of (La1 yPry)0.67Ca0.33MnO3 ( y =0.4, 05, and 0.6) on NGO substrates. The open black squares show the resistivity behavior of an LPCMO ( y =0.5) film on STO. Cooling and warm ing directions are indicated by arrows. The dotted lines mark the range of temperatures for the I V curves shown in figure 4-2. The upper inset shows the variati on of the transition temperatures and low temperature resistivity 0 with y. The lower inset shows the setup for measuring the two probe resistance us ing a constant voltage source. All the films measured are 300 thick.

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57 Figure 4-2: Phase diagram of LPCMO created by using R vs. H isotherms. The right panel: R vs. H isotherms for the y =0.6 sample in the cooling cycle for various temperature. The left panel: The T H phase diagram for the y =0.6 thin film in the cooling cycle. The squares and triangles represent the melti ng and freezing fields, respectively. These data points were obtained by locating the ma gnetic field corresponding to the steepest change in R at a given temperature, i.e., where dR / dH is maximum (see right panel).

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58 10 100 10-210-1100101102103 Voltage ramping up Voltage ramping down 56 K 55 K Current(A)Voltage (V) 57 K 58 K 59 K 60 K 61 K Vth Figure 4-3: I V curves of the (La1 yPry)0.67Ca0.33MnO3 ( y =0.6) thin film in the cooling cycle with the voltage being ramped up. The red curve is the I V curve at 59 K with the voltage being ramped down. The irreversibility nature of the I-V curve can be seen.

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59 Figure 4-4: T V phase diagram for the (La0.4Pr0.6)0.67Ca0.33MnO3 thin film in the cooling cycle. (a) The dI/dV is plotted as color. The black a nd white circles are the values of threshold voltage ( Vth) at corresponding temperatures. This plot is overlapped on the color plot. (b) Schematic representation of the phase coexistence in the T V plane. The black white and grey regions are FMM, COI and heated FMM respectively.

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60 10 100 10-1100101102103 56K 56.5K 76K 77K 78K 81K 82KCurrent ()Voltage (V)cooling cycle warming cycle 0 501001501021031041051061071081091010 (La0.4Pr0.6)0.67Ca0.33MnO3 Resistance ()Temperature (K) 5 V 50 V-20 0 20 40 60 80 100 -ER% Figure 4-5: I-V curves of the (La1 yPry)0.67Ca0.33MnO3 (y=0.6) thin film in the warming cycle with the voltage being ramped up. I-V curves (red) for the cooling cycle are shown for comparison. The lower panel shows resistance curves for two voltages. It can be seen that the resistance during cooling is larg ely reduced where as the same for warming run remains the same. The negative electric resistance (electric version of CMR) calculated from these two resistances was close to 100% near peak temperatures shown by red curve on the lower panel of the graphs.

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61 CHAPTER 5 ANISOTROPY IN TRANSPORT PROPERTIES OF MANGANITES In the last c hapter we described the effect of an electric field on a phase-separated manganite. Our experimental results which reveal the underlying mechanism of the electric field induced insulator-to-metal transition will be described here. We have studied the dynamics of fluid like phases [52] of ferroma gnetic metal (FMM) and charge ordered insulator (COI) observed in thin films of (La0.4Pr0.6)0.67Ca0.33MnO3 (LPCMO) under the influence of electric field. The electr ic field (set by applying voltage difference across the material) alters the fluid phases in a way to make the material more metallic [59]. To check if the enhanced metallicity is associated with the increase in the size of the FMM domains, we measured magnetization using SQUID magnetome ter with and without electric field. The saturation magnetization remains same in eith er case showing that the FMM domains do not increase in size. This led us to hypothesize that the domains stretch themselv es out along the direction of electric fiel d to give rise to the abrupt percol ation. This assumption was verified by measuring the transverse resistance while a volta ge difference was applied longitudinally across the material. For this a cross bar (each bar 10 m wide and 40m long) of manganite was fabricated by using photolithography and wet etching. At a threshold voltage when one leg of the cross bar went through percolation showing a sharp increase in current, th e resistance across the other leg of the cross bar increased. This increas e in resistance is thus attributed to the FMM domains being stretched in the di rection of the electric field. Motivation The LPCMO manganites have shown hysteresis in resistivity between warming and cooling cycle and the transition from insulator to metal has been very sharp in particular during the cooling cycle. This behavior reveals the phase separation in these materials and the first order

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62 nature of the phase transitions. The phase diagra m of this material as shown in figure 4-2 of chapter 4 shows four distinct regions. Ther e are two pure phases called ferromagnetic metal (FMM) and charge ordered insulator (COI). Th e remaining two phases are called fluid phase separated state (FPS) and static phase separated state (SPS). We have also called the FPS state an electronic soft matter like state. We have seen a strong el ectric field effect when the LPCMO manganites are in FPS state [59]. A voltage differe nce across the sample is applied to see the effect of electric field. Figure 5-1 shows the cu rrent vs. voltage characteristics of LPCMO at a temperature of 50K which is well within the FPS regi me. It is clearly seen that current increases sharply as the voltage applied is above the threshold voltage ( Vth). Once the applied voltage is above the threshold voltage, the current vs. voltage behavior is ohmic for a while. However as the voltage is increased further the current vs. volta ge flattens out because of the Joule heating of the metallic filaments [57]. In addition once the sa mple reaches a low resistance state it continues to remain in the low resistance state even if the voltage is ramped down. Now we wanted to find out what happens to the local phases when th e current through the sample sharply increases above the threshold voltage. Experimental Results and Discussion Magnetization as a Function of Electric Field The increase in current due to the applicati on of electric field can be interpreted as increased metallicity. As we know metallicity in manganites is associated with ferromagnetism. So the obvious question would be, is the increase in current due to the incr ease in the size of the ferromagnetic domains? To answer this question we have measured the magnetization in the material as a function of electric field. For this purpose a 5mm 5mm thin film of LPCMO grown on NGO (110) of thickness 300 was used. Two thin gold wires were soldered along the two opposite sides of the sample for the purpose of applying voltage difference between them.

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63 Magnetization as a function of external magnetic field (M-H loop) was measured for three different conditions using a SQUID magnetomete r (figure 5-2). The first one was measured without the electric field. Th e second one was measured by applying 40 V which is above the threshold voltage (20 V) of this material. However the measurement was done after ramping the voltage down to zero. The last one was done keeping the sample at 40 V for the duration of the measurement. The saturation magnetization in th ese three different cas es did not show any appreciable difference as shown in figure 5-2. Ho wever the coercive field in case of the voltage being applied during the whole tim e of the measurement is decreased. This could have been due to the domain movement caused by the Joule heati ng as the voltage is applied for considerably long time when the sample is predominantly in the metallic state. The M-H isotherms overlapping on top of each other surely rules out the possibility of the ferromagnetic domains size increase by the applica tion of electric field. There could be two possibilitie s that cause the sudden rise in current as the voltage increased above the threshold limit. One maybe that the ferromagnetic and hence metallic domains stretch in the direction of applied electric field. As we know that when a metallic conductor is exposed to an otherwise uniform el ectric field, the positiv e and negative charges will separate to two opposite direc tions [60]. For the sake of simp licity, if we assume the metallic domains coexisting in the matrix of charge ordere d insulator as spherical, the charge distribution would be in two opposite ends as shown in figur e 5-3 a. As a matter of fact there are some calculations done on the basis of effective medi um approximation which shows that the metallic domains are in fact spherical [61]. The electric field in the region between the metallic regions is very high because of the smaller size of the domains. The size of th ese domains is in the order of hundred to thousand nanometers which gives electric field as high as 107 V/m. When such a high

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64 electric field is set up between these metallic do mains, which are fluid like in nature (see the phase diagram in chapter 4), they could elongate in the direction of the fi eld. Another possibility is a metallic filament connecting the ferromagneti c metallic domains. These filaments are created by dielectric breakdown of the insu lating regions in favor of meta llic regions [62]. The formation of such filamentary connected path was previous ly suggested by Garbarino et al [63]and Wu et al [64] in single crystals of La5/8-yPryCa3/8MnO3 (y~0.4). The schematic of this type of filamentary connection is shown in figure 5-3 b. Nanofabrication of a Cross Structure We would like to distinguish between these two possibilities menti oned in the previous subsection. For this purpose we had to make a cross structure of LPCMO as shown in the upper right corner of figure 5-4. The th ickness of the film is 300 and the dimensions of each bar of the cross are 10 m wide and 40 m long. If our first assumption that the metallic domains stretch in the direction of app lied electric field, then the resistance along the sample in this direction should decrease. Because of this elongation along the direction of E-field, the sample gets deprived of the metallic region in the transverse direction which would increase the resistance in this direction a lthough slightly. To be able to me asure this slight increase of resistance in the direction perpendicular to the a pplied electric field, we had to make the cross like structure. This way we could apply voltage across one leg of the cro ss but would not affect the metallic regions of another leg except the ones at the center square of the cross. The cross bar (each bar 10 m wide a nd 40m long) of our LPCMO manganite was fabricated by using photolithography and we t etching. Karl Suss MA6 Mask Aligner was used for patterning the manganite sample with UV light of wavelength 348 nm. After the fabrication, we measured the resistance of each bar to make sure that the transitions are as good as that of the bulk sample. The resistance vs. temperature graph is shown in the figure 5-4. The transitions are

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65 sharp and the resistances between warming and c ooling cycles for both the legs show hysteresis typical of bulk LPCMO. The resistances for bot h the legs match very well throughout the whole temperature except at low temperatures. The diffe rences at low temperatures could be due to different cooling rates as it is harder to stabil ize temperature at low temperatures. We did not reduce the width of the bars below 10 m because we wanted to retain the properties of the bulk of the material. Reducing the width further, th e effect of single domain would dominant [65]. Now we are ready to test whic h of our assumptions is true. Transverse Resistance by Lock-in Amplifier As shown in figure 5-4, we ca ll the resistance along the leg gh longitudinal resistance (RL). While the dc voltage is ramped up between two ends of the leg gh of the cross bar, we would measure the resistance along another leg ef of the cross bar. We call th is the transverse resistance (RT). We expect that if the electric field applied across gh elongates the metallic domains in the direction of the field, then th e transverse resistance should increase. However, our initial measurements showed that voltage drop across ef was affected by the voltage applied across gh. To remove the effect of the voltage across gh on the measurement of RT we used an ac technique. We applied a small sinusoidal voltage of 0.1 V (rms) at 23 Hz across ef and measured RT by using the 2 probe method expl ained in chapter 3. The only di fference in this case is that the voltage source is replaced by an SR830 Lock in amplifier. The small voltage was used to make sure that its not strong enough to cause the electric field effect. The ac voltage was used to remove the effect of the dc voltage (applied across gh) since the Lock-in amplifier measures only the voltage at a particular frequency. The transv erse resistance was measured by using the real part of the voltage ( VAB) since across the test resistor the voltage and current are in phase. The schematic of the measurement circuit is shown in figure 5-5.

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66 As we expected, the transverse resistance increased (albeit by only about 2% while RL drops by more than 2 orders of magnitude) as the longitudinal voltage was increased above the threshold voltage (figure 5-6). Fo r this sample geometry the th reshold voltage is around 20 V. Before reaching the threshold voltage we can se e in figure 5-6 that both the longitudinal and transverse resistances are not changing, whic h is a test of the effectiveness of our ac measurement set up. As the voltage reaches 20 V and beyond, the longitudinal resistance (RL) is decreased. This happens because the metallic domains are brought closer by the elongation of their shape in the direction of applied electric field. RL would decrease until the domains touch each other and equipotentials ar e redistributed. There is no fu rther electric field induced elongation of the domains above Vth. However, Joule heating coul d still change the domain shapes. While RL is decreasing, the resistance in the transverse direction (RT) keeps on increasing. It should be noticed that both the longitudinal and tran sverse legs of the cross bar share the 10 m10 m square at the crossing point. The domains within this shared piece of square are exposed to the dc electric field applied along gh. As the metallic domains elongate in one direction, they open up more insulating region in the transverse dire ction although slightly. This is the reason the transverse resistance (RT) keeps on increasing as the metallic domains in the longitudinal direction stretch more and more. Once the voltage is high enough to completely stretch the metallic domains, there is no further change in both RL and RT. If the decrease in the longitudinal resistance was due to the thin fila mentary connecting path, th ere either wouldnt be a change in the resistance in the tran sverse direction or it would decrease. Conclusion The resistance of the LPCMO manganite show s sharp transition and a wide hysteresis between the warming and cooling cycle of temperat ures. This behavior is a signature of phase separation. The ferromagnetic metallic (FMM) doma ins in this material can be manipulated by

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67 using electric field. The sharp increase in current as electric field is applied is due to these FMM domains being stretched in the direction of the fi eld. In addition our measurements rules out the assumption that the sharp increase in current could be due the filamentary connecting path.

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68 0 1x10-52x10-53x10-54x10-55x10-5020406080100 50K Voltage (V)Current (A) Vth Figure 5-1: Current as a functi on of applied voltage in (La0.4Pr0.6)0.67Ca0.33MnO3. The directions of voltage applied are shown by arrows. A non linear increase in current is seen at a threshold voltage ( Vth). The temperature of measuremen t is 50K which is in FPS state.

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69 -1500-1000-500050010001500 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0V 40V ON then OFF 40V ONMagnetization (B / Mn)Magnetic Field (Oe)50K Figure 5-2: Magnetization as a f unction of magnetic field for thr ee different voltages applied on (La0.4Pr0.6)0.67Ca0.33MnO3. This measurement was done at 50K.

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70 Figure 5-3: Schematic of the ma nipulation of metallic domains by using electric field. The red circles are ferromagnetic metallic domains and the cyan background is the charge ordered insulator. (a) The electric field elongates the me tallic domains in the direction of applied field. The elongated metallic dom ains, which were otherwise spherical, are shown by dashed ovals. (b) The electric field creates thin metallic filaments by dielectric breakdown.

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71 030609012015010610710810910101011 Leg ef Leg gh Resistance (ohm)Temperature (K) Figure 5-4: Resistance and cross ba r structure. The lower panel show s the resistance of 2 legs of the LPCMO cross bar as a function of temp erature. The cross bar (each bar 10 m wide and 40m long) is shown on the upper panel of the figure.

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72 Figure 5-5: Schematic of transverse resistance (RT) measurement while the dc voltage is being applied in longitudinal direction. Th e voltage across the load resistor VAB is the difference between the voltages at two channels A and B.

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73 Figure 5-6: Longitudinal resistance (RL) and the transverse resistance (RT). The black curve is the longitudinal resistance (RL) measured as a function of dc voltage. As the voltage is increased above the thres hold voltage the longitudinal resistance decreases but the transverse resistance (RT) (blue curve) increases.

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74 CHAPTER 6 SCANNING TUNNELING MICROSCOPY Motivation In previous chapter we showed that the LPCM O exists in phase sepa rated states. We would like to obtain conclusive evidence in support of our model for phase separation in manganites and the electric field effect. One of the tools to probe the nature of phase separation of manganites is a scanning tunneling microscope (STM) [66, 67]. We built a low temperature STM which will be used to determine the local phase of a manganite with spatial resolution of the order of 1 nanometer. Basic Principle of Scanning tunneling Microscopy Scanning tunneling microscopy (STM) is a tool which can take an image of a surface with atomic resolution [68-70]. The basic principle of STM is the tunneling of an electron from a tip made of a metal wire to a metallic or semiconduc ting surface. When the tw o metal electrodes (tip and the metallic surface to be scanned) are brought to a close proximity of a few angstroms, a bias voltage applied between them will cause th e electron to tunnel through them. The tunneling current ( I ) through the junction is exp onentially dependent on the junction (barrier) width ( s ) [71] as described by )2exp( s I (6-1) with the decay rate 22 m (6-2) where is the effective barrier height a nd m is the mass of an electron.

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75 This exponential decay of the tunne l current is a key to the imaging of a surface with nanometer scale resolution. When the metal tip is brought closer to the sample surface by a fraction of a nanometer, the tunneling current increases significantly. Topography of Sample Surface The exponential decay of tunnel current as the junction width increases is used to measure the height of the metal tip from the sample surface with high resolution. This height can then be converted to give the topographic image of the sample surface. As shown in figure 6-1, when the tip is scanned across the surface of the sample keeping the tunnel current constant (which is done by using a feedback system to be explained in subsequent section), we can get the surface topography by following the tip movement. The tip is made to raster scan the sample surface in x-y direction by attaching it to a piezoelectric scanner. The pi ezoelectric scanner is also attached to a z-voltage which is controlled by the feedback circuit to keep the sample-tip height constant. STM Spectroscopy The geometrical view of the atomic structure as explained in the previous section can be obtained by the dependence of current on the tip-sample position. No w keeping the tip fixed at a point close to the sample surface and measuring the current-bias voltage dependence as well as differential conductivity ( dI/dV ) gives us information about the local electronic structure of the material. Figure 6-2 shows the schematic of the STM spectroscopy. The tunneling current is found to be approximate ly proportional to the electronic density of states as shown in the following equation: dEEtENENIs eV t)( ~ )()(0 (6-3) where, Nt( E ), Ns( E ) and )( ~Et are tip and sample density of states (DOS) and the barrier penetration factor respectively.

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76 Now the derivative of the tunnel current ( I ) w.r.t. the bias voltage ( V ) is therefore proportional to the local DOS of the sample. )( ~ )( EtEN dV dIs (6-4) Thus the tunneling spectra will give us th e electronic state at a particular point and combining this with the topographic image, we can extract a spatial distribution of the local electronic states. Designing Low Temperature Scanning Tunneling Microscope The most important part for a low temperature STM head is its cylindrical macor body which contains the piezo-electric material, spring a nd the tip as shown in the following fig. 6-3. The STM head has to be compact so that its resonance frequencies are much higher than the frequencies of the usual sources of mechanical vibrations wh ich are on the order of a few hundred Hz and lower. A design for such a compact STM head is described below. The macor cylinder has a diameter of 1.5 inch (38.10 mm) and its height is 2 inch (50.80 mm). A V-shaped groove has been cut along the height of the cylinder so as to place an approach sledge. The approach sledge is also made of macor. The lower half of its body is a triangular prism like and upper half part is cylindrical. Smoo th sapphire plates are attached on the 3 faces of the prism like part of the sledge. On top of the cylindrical part of the sledge, a cylindrical piezoelectric scanner is attached which is meant for swaying the tip in x-y direction and also stretch or retract in z-directi on. Four shear piezo stacks are glued on the inner wall of the Vgroove of the main macor cylinder, 2 on each wall On top of each piezo stacks are attached the sapphire plates of almost the same size as the shear piezo-stacks. These sapphire plates actually provide the contact areas between the shear pi ezo stacks and the sapphire prism faces of the approach sledge. Once the sledge is placed on the V-groove of the macor body on top of the

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77 shear piezo stacks, it is covere d from the top by a macor plate which has remaining two shear piezo stacks attached underneath it. The macor plat e is then pressed from the top by a ruby ball and a phosphor bronze spring plate. The spring plat e is then screwed to the macor body to keep everything taut. The details of the desi gn are described in the figure caption. Approach Mechanism The tip which is on the top of the approach sled ge is first brought as close as quarter of a millimeter by a technique called walker stepping [72]. In this technique a snap voltage is first applied to one of the piezos, stage A as shown in figure 6-4. This snap voltage is applied such that the shear piezo stack twists backward but can't pull the sledge along with it because the sledge is held tight by other five piezos which are kept still and the voltage increase is instant. Then the snap voltages are applied to all other stac ks one after another (B, C, D, E, etc.). When all the piezo stacks suffer a twist, the voltages on all the stacks are slowly decreased as shown in figure 6-4 (b). This will bring th e piezo stacks slowly back to thei r original state and while doing this they also carry the appro ach sledge along with them by fr iction. Thus the sledge, which carries the tip and piezo scanner on its top, moves forward by a sm allest distance. This process continues until the tip is very close to the sample surface. To retract the tip just opposite voltage pulses are applied to the shear piezo stacks. Caution has to be maintained not to crash the tip on the sample surface. The closenes s between the sample and the tip is monitored by watching the tip movement through an optical microscope. Th e maximum of the voltage pulses applied on the shear piezo stacks are +/200 V. This set of voltage pulses can be applied at different frequency. This is used when the tip is already in a close proximity with the sample. By changing the frequency of the pulses, the tip can be brought to a tunneling range in a controlled manner. A special circuit was developed with help of the electronic shop for this purpose.

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78 Current to Voltage Converter The current to voltage converter circuit is s hown in figure 6-5. An STM tunnel junction has a resistance of about 1 G This means that for tip-sample biases of around 1V, tunneling current through the STM junction will be around 1 nA or less (for lower biases). Because of the high impedance of the tunnel j unction and the extremely small currents which have to be measured, this current has to be converted into the voltage signal and then amplified further before it is fed to the analog to digital conve rter (ADC) which reads the voltage signals. The current to voltage conversion and amplification is done by the current amplifier. Our current to voltage converter has two stages. The first stage has a current to voltage conversion factor of 107 V/A. The second stage has a voltage gain of 10 giving a total gain of 108 V/A. The circuit is powered by two 9V batteries which have a co mmon ground with the signal ground. The whole circuit, including the batteries, is housed in an aluminum box. The current amplifier can be connected to either electrode of the tunnel junctio n (tip or sample). The input of our current to voltage converter is connected to the sample by a coaxial cable. The current to voltage converter circuit and the feedback circuit to be explained in the subsequent section were made with the help of the electronics shop of the department. Feedback System A feedback system is very important for imaging in a scanning tunneling microscopy. The circuit diagram used for feedback system is shown in figure 6-6. In our feedback circuit only the proportional (P) and the Integral (I) are used. The response due to a differential circuit is very abrupt, which is not good when the tip sample distance should be controlled in a nanometer scale. Since the tunneling current can be positive or negative depending on the polarity of the bias, a rectifier circ uit is used to feed the m odulus of the current amplif ier output to the feedback circuit. This is used for carrying out bias depe ndent imaging. The rectif ied output of the current

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79 amplifier is first compared with a reference voltage (Vref) and the difference is given to the PI stage, which has two parallel gain stages, proportiona l (P) and integral (I), w ith adjustable gains. The reference voltage changes the distance betw een tip and the sample. The outputs of P and I gain stages are added and the sign of the output is reversed, if necessary, with a unity gain, inverting amplifier. The output of the feedback circuit is then added to the xand ypiezo signals as explained in the summing circuit in the follo wing section. The result ant signals are then applied to the four quadrants for x-y scanning and to keep the tip -sample height constant during topographic scans. The output of the feedback ci rcuit can also be given to the z-piezo through a sample and hold circuit for tunne ling spectroscopy. For such measurements the bias voltage is swept across a certain range while keeping the feedback in the hold mode to keep the tip sample distance constant. The sample a nd hold circuit is controlled by the computer. The op-amps used in the feedback circuit are OP-07, and an LF398 AN was used for the sample and hold circuit. An LT1077 was used for reference circuit. Summing Circuit The summing circuit consists of four operationa l amplifiers (op-amp) as shown in figure 67. The output of the feedback circ uit, which is the z-piezo signal is one of the inputs for all the four op-amps. The other four inputs are X+, X-, Y+, Y-. The X+ and Xare for swaying the piezo scanner in x-direction and Y+, and Yare for doing the same in y-direction. The outputs of the amplifiers X+ + Z, X+ Z, Y+ + Z and Y+ Z are then fed to the four quadrants of the piezo scanner. The X+ + Z and X+ Z are connected to the two di rectly opposite qua drants of the scanner whereas Y+ + Z and Y+ Z are connected to the remaining two opposite quadrants. By applying X or Y voltages this way, the piezo scanne r can sway in xor ydirection respectively. The resultant motion of the piezo gives a raster scan of the sample surface. Higher voltages should be applied for larger scans. The power supplies and the Xs and Ys for our summing

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80 circuit were used from the DI ACCESS MODU LE of our commercial Nanoscope AFM from digital instruments. The inside of the scanner piezo is a non-se gmented cylinder and is grounded. Imaging Graphite A constant current mode is used to get a topographical image of the sample surface. To get a fine atomic resolution image, we need a clean sharp tip. For this purpose a freshly cut Platinum : Rhodium (Pt : Rh ; 87:13 wt%) wire of 0.01 inch (0.25mm) diameter is used. Once the tip is inserted into the tip holder and once the coarse a pproach is done as explained in the previous section, the probe which is holding the STM head is first inserted into the vacuum can (figure 68) and then the vacuum can is put into the Dewar which has a 2.5 inch wide superconducting magnet bore. Such a wide bore magnet bore was specially ordered for our STM system from American Magnetics. The Dewar whic h contains our STM vacuum can is then air lifted from the ground for vibration isolation as shown in the figure 6-9. The MICRO-g vibration isolation system from Technical Manufact uring Corporation (TMC) was us ed. Three isolators used are rated for maximum load at an internal pressure of 80 psi. The tip is th en brought closer to the sample surface by manually applying one set of volta ge pulses at a time. While the tip is brought closer to the sample, a bias voltage is applied between the sample and the tip, until a tunneling current is seen. Once a stable tunnel current is achieved, the f eedback system is turn ed on (see the flow chart in figure 6-10). Then we apply voltages to the cylindrical scanner which has the tip on its head. The scanner piezo is made such that when all X-Y-Z electric configur ations are applied, it can produce a roaster scan along the x-y direction and also can contract and stretch itself in z direction. During the scanning, the tunneling curren t is then fed to the current to voltage (I-V) converter circuit box. This changes the tunnel current into voltage and this voltage is then sent to the feedback circuit which it compares with the reference voltage. If the tip comes closer to a

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81 bump on the surface, the tunnel current would ri se exponentially which is then sensed by the feedback circuit after it gets converted to the voltage by I-V c onverter. Then to keep the tunnel current constant the feedback system would pull the tip away from the sample. On the other hand if the tip comes to a ni che rather than a bump, the tunnel curr ent would decrease. In this case the feedback system will push the tip closer to the surface by applying more z-voltage to the cylindrical scanner. This way the tip would neither crash on a bump nor move flat without noticing the niche, but would just follow the geometry of the surface with nanometer scale resolution. This movement of the tip when repeated in all x-y-z direction will give us the atomic resolution picture of the surface scanned. Figur e 6-11 gives an image of graphite at room temperature. Graphite was used for calibration pur poses as the inter atomic distances between the graphite atoms are already known. The piezoelectr ic scanner has a gain of 2 nm/V, which was found by looking at the tip scan distance at a certain voltage. Scanning Tunneling Potentiometry The atomic imaging and the STM spectroscopy ex plained in the previous sections are done in the material with metallic conductivity. Tunneli ng is extremely difficu lt in a material with high resistivity. In such a case the alternative techni que called scanning t unneling potentiometry (STP) is very significant in characterizing the di stribution of potential ac ross the materials [73, 74]. The circuit diagram for this STP technique is shown in figure 6-12. As shown in figure, it has two feedback system, one for usual topography and another is for poentiometry. The sample and the tip is biased by a spatially uniform and ac voltage and a spatially varying dc voltage is applied between the two ends of the sample. The t unnel current due to the ac voltage is fed to the current to voltage amplifier which converts the cu rrent signal into a voltage signal. The voltage signal is sent to a lock-in amplifier which only lets the voltage of the same frequency as that of ac bias used between the tip and the sample pass through it. The output of the lock-in amplifier is

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82 then fed to the feedback system for topographic scanning. The voltage signal from the current to voltage amplifier is also fed to the integrator of the potentiometry feedback circuit. The output of the integrator is then added with the non varyi ng ac bias and supplied to one end of the sample. The output signal of the integrator added with the non varying ac bias is again added with a varying dc bias and the resultant is supplied from another end of the sample. This way, a constant ac bias is kept between the sample and the tip, but the dc bias can be varied to nullify the local potential between the tip and the sample. The dc voltage required to null the voltage between the tip and a point in the sample is actua lly the potential at that specific point. This dc voltage is then sent to the computer for imagi ng the potential distributi on across the sample. For a material like LPCMO, the potential signal to be de tected could be very small. The sensitivity of this detection depends on the bit size of the an alog to digital converter (ADC) card. As you can see from the circuit diagram, two integrators are used in two feedback system. Care should taken not to use similar time constants for the two inte grators. A separation of two time constants by a factor of 5-7 are reported [73] good enough to avoi d the cross talk between the two integrators. This technique will be very useful to image a potential distribution of our LPCMO manganite which coexist in both metallic and insulating phase.

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83 Figure 6-1: Schematic of scanning of STM tip in constant current mode. The dotted line gives the topography of one scan. I V Sample VZ Feedback x y z Tip Piezoelectric scanner

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84 Figure 6-2: Schematic of the ST M spectroscopic tunneling. The left side of the figure shows the relative Fermi levels when bias voltage V is applied. The wiggling vertical line on the sample side shows the different local density of states. When bias voltage is applied, the electron tunnels from the occupied states of the tip to the uno ccupied states of the sample. The right side of the figure shows the computed spectros copy characteristics.

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85 Figure 6-3: Design of di fferent parts of the STM head. (a ) 3-D AutoCAD drawing of the STM head. (b) Real picture of the STM head. (c) The top view drawing of the macor body with a V-shaped groove. (d) The front view of the macor cylinder. There are altogether 6 shear piezo stacks, 2 on each side of the macor cylinder and 2 beneath the macor plate. It holds the approach sl edge with the ruby ball and spring plate on top of it. Sapphire plates are attached on t op of the piezo stack. The spring plate made of phosphor bronze. All the wires from the STM head are anchored on the copper disk. (a) (b)

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86 Figure 6-4: Voltage pulses in co arse approach mechanism (a) The top view of the triangular macor approach sledge held by shear piezo stacks on its sides. Two more shear piezo stacks on top are not shown. (b) High volta ge amplifier positive output pulses to be applied at the shear piezo stacks for coarse approach. A C B E D E A B C DApproach sledge Shear piezo Time (ms)

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87 Figure 6-5: Current to voltage converter circuit for STM.

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88 Figure 6-6: Feedback circuit for STM imaging. The proportional and Integr al circuit are used.

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89 Figure 6-7: Summing circ uit for the input of pi ezoelectric scanner.

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90 Figure 6-8: A vacuum can and a probe to hold the STM head. Left panel: A probe which holds the STM head on its copper base. Right panel: A vacuum can. Copper base Copper cup

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91 Figure 6-9: Vibration isol ation system. The STM probe is put in to the vibration isolation system once the tip and sample are in tunneling range. The sample transfer is done by a manipulator rod.

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92 Figure 6-10: Scanning tunneling microscopy flow ch art and electronics. (a) Flow chart for STM function. (b) Feedback circuit t ogether with other electronics. z-coordinates for image data Gain of 108 V/amp (a) (b)

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93 Figure 6-11: Calibration scan on gr aphite at room temperature 3.521 mV 1.760 mV 0.0 mV

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94 Figure 6-12: Scanning tunneling Potentiometry ci rcuit diagram as taken from Ref. [73].

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95 CHAPTER 7 CONCLUSION AND FUTURE WORK Conclusion We set up a pulsed laser deposition (PLD) la b for film growth. Thin films of phase separated manganites of the form (La1-yPry)0.67Ca0.33MnO3 (where y = 0.4, 0.5 and 0.6) (LPCMO) were grown. The resistivity anomaly present at PMI-COI transition of the bulk LPCMO is removed in the thin film by substrate induced strain. In case of the LPCMO films grown on NGO (110) substrate, the low temper ature strain stabilized the FMM phase. We created a phase diagram of LPCMO which clearly showed four distinct regi ons namely FMM, COI, FPS and SPS state. A strong electric field effect was observed when this material was in the FPS state. This electric field effect was used to manipul ate the ferromagnetic domains in the FPS state by fabricating a nano-structure in the form of a cross bar. Our results showed that the FMM domains are stretched in the di rection of the electric field applied but their overall volume remained same, which was confirmed by magnetiz ation measurements. In addition, we built a low temperature STM which will be used to imag e the microscopic proces ses in phase separated manganites. Future Work Scanning Tunneling Potentiometry To image a high resistance material like mangan ite using an STM is a difficult job since the sample and tunnel junction resistance may be comp arable. But at low temp erature, some of the manganites have low resistivity of about 1 m -cm and can be considered as reasonably good metals. Some of these phase separated manganites we are working on have also shown the presence of thin insulating domain walls at low temperature when the bulk of the material is a good metal. This conclusion was drawn as t unneling magnetoresistance (TMR) effect was

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96 observed by reducing the dimension of this material to nanometer scale [65] To visually confirm the presence of the insulating domain walls, we will use our low temperature STM for scanning tunneling potentiometric [73, 74] im ages of manganite thin films. Multiferroics As explained throughout this dissertation, manganites show ferromagnetic properties. The ferromagnetic domains in manganites can be manipul ated by using electric field. I would like to be involved in the search of the manganite base d materials which show fe rroelectricity [75] in addition to ferromagnetism. It has been observed in the compounds like RMnO3 (R being Ho to Lu and Y) that magnetic and electric domains are coupled [76-79]. Since it is very rare to have a multiferroicity in one single material, I will also work in techniques to fabricate multiferroics in particular involving ferroelectric and ferromagne tic materials. I will use variable temperature magnetic force microscopy to see how the magnetic components are spread in the ferroelectric matrix. In addition I will also look for the pos sibility of fabricating devices incorporating manganites [80-83].

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102 BIOGRAPHICAL SKETCH Tara Dhakal was a fourth child of his pa rents and was born in a sm all village called Punarbas, in the far western part of Nepal. Af ter graduating from Puna rbas Janata High School, he went to the beautiful tourist city of P okhara, where he joined Prithvi Narayan College affiliated under Tribhuvan University for his undergraduate degree in physics, mathematics, and statistics. He finished his underg raduate degree as valedictorian in 1994. For this he was awarded a gold medal from King Birendra of Nepal. In 1995 he joined the central department of physics, Tribhuvan University, Kathmandu, Nepal, for his ma sters degree in physics. He went to Japan in 1998 to pursue an M.S. in material science from Shimane University under a Japanese government scholarship (Monbusho). There, he st udied the effect of doping on the transport properties of hole-doped manganites. He also prepared and studied samples of high Tc superconductors. He graduated from the materi al science department, Shimane University, Matsue, Japan, with an M.S. degree in physics in May 2001. He then joined the University of Florida in summer 2002. During that summer he worked in Prof. Hillss lab to study organic superconductor by using micr o waves. In 2003 he joined the Biswas lab for his PhD research. He started to study the properties of phase separated manganite by using bulk probes like magneto-transport and local probes like the low temperature scanning tunneling microscope (LTSTM). He was able to find a region of this material where it acts like an electronic soft matter where the magnetic domai ns can be manipulated by using electric field. He also helped to set up a new lab and built a low temperature scanning tunneling microscope (LTSTM). He is planning to use his recently built LTSTM to image the phase coexistence in manganites by using a technique called scanning t unneling potentiometry (STP). In the future, he plans to study multiferroics by using tools like the low temperature scanning tunneling microscopy, the low temperature magnetic forc e microscopy, and transport measurements.