Growth and Characterizations of Complex Oxide Thin Film Heterostructures via Pulsed Laser Deposition

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Growth and Characterizations of Complex Oxide Thin Film Heterostructures via Pulsed Laser Deposition
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english
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Kim, Kyeong-Won
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Doctorate ( Ph.D.)
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University of Florida
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Materials Science and Engineering
Committee Chair:
Norton, David P
Committee Members:
Pearton, Stephen J
Gila, Brent P
Craciun, Valentin
Ren, Fan

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Subjects / Keywords:
films -- magnetism -- oxides -- transport -- zno
Materials Science and Engineering -- Dissertations, Academic -- UF
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Materials Science and Engineering thesis, Ph.D.
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Abstract:
Artificially synthesized oxide thin films and hetero-structureshave shown the novel properties that can be utilized in the next generationelectronics. This dissertation investigates magneto-electric properties of half-metallicdouble perovskite Ba2FeMoO6 thin films and hetero-structuresand p-type doping mechanism in ZnO thin films for the potential application innovel memory device and electronics. Double perovskite Ba2FeMoO6 thinfilms and hetero-structures with Ba1-xSrxTiO3have been prepared by pulsed laser deposition and the structural, magnetic andmagneto-transport properties were investigated. Phase pure Ba2FeMoO6thin films were epitaxially grown on SrTiO3 substrates attemperatures ranging from 700-900°Cunder high vacuum. The magnetic properties of samples improved with high growthtemperatures. Ba2FeMoO6 thin films showed anomalous Halleffect with hole-like behavior under low magnetic field and showed an ordinaryelectron-like transport property under high magnetic field. When thin filmswere grown under low oxygen partial pressure, the magnetic properties weredegraded but the Curie temperature increased. Also unusual positivemagneto-resistance observed in films grown under high vacuum disappeared, whichsuggests that it is associated with the oxygen related defects. The Ba2FeMoO6/Ba1-xSrxTiO3superlattices were synthesized and their structures were confirmed by X-raydiffraction analysis. The superlattices showed the strain induced enhancementof magnetization and the Curie temperature of superlattices were loweredsubstantially with increasing strain. Phosphorus doped ZnOthin films were deposited on ZnO buffer grown under different growth conditionsand the properties of thin films were examined, focusing on the role of bufferlayers. Phosphorus doped ZnO thin films on buffer layer grown at lowtemperature were less conductive as grown state and showed a drastic change inthe transport properties upon the rapid thermal annealing. This suggests thatpoor crystalline quality of the buffer layer grown at low temperature lead tothe more incorporation of phosphorus dopants in following films. A highresolution X-ray diffraction study confirmed the poor crystalline quality ofphosphorus doped thin films on low temperature deposited buffer layer bybroader peaks of omega rocking curves. Also it was observed that themicrostructures of the phosphorus doped ZnO thin film significantly depended onthe buffer layer.
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by Kyeong-Won Kim.
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Thesis (Ph.D.)--University of Florida, 2012.
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Adviser: Norton, David P.
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1 GROWTH AND CHARACTERIZATION S OF COMPLEX OXIDE THIN FILM HETEROSTRUCTURES VIA PULSED LASER DEPOSITION By KYEONG WON KIM A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2012

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2 2012 Kyeong Won Kim

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3 To my parents

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4 ACKNOWLEDGMENTS I could not have achieved this accomplishment without help guidance and love from people around me an d I would like to acknowledge their support First and foremost I thank my advisor Dr. David Norton I am amazed by his range of knowledge and hard work I just cannot thank him enough for his help and attention. I would like to thank my committee members Dr. Stephen Pearton, Dr. Brent Gila, Dr. Valentin Craciun, and Dr. Fan Ren, who have helped me with invaluable advice throughout the doctoral research. Also, I would like to thank Dr. Arthur Hebard and his students, Siddhatha Ghosh and Sanal Buvaev for their helps with the collaboration. I thank my fellow group members specifically Dr. Mat Ivill, Dr. Patrick Sadik, Dr. Fernando Lugo, Dr. Joe Cianfrone, Dr. Seonhoo Kim for their help s in and out of the lab. I thank my friends Dr. Inkook Jun, Dr. Myoung hwan Oh, and Dr. Sanghyun Eom Without their support and presence, it should have been much tougher for me to get though all the processes during my doctoral studies. I also thank my colleagues in the department, specifically Sungwook Mhin, Jinhyung Lee, a nd Hyuksu Han for their help and support. I thank friends in UF Korean Buddhist g roup whom I can share the way to live and think. Also I would like to thank the members in UF Korean baseball league whom I have played with. Finally I express the most sinc ere appreciat ion to my parent s and my brothe r for giving their infinite love and support

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5 TABLE OF CONTENTS P age ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 ABSTRACT ................................ ................................ ................................ ................... 10 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 12 Double Perovskite Ba 2 FeMoO 6 and Superlattices ................................ .................. 12 ZnO for Optoelectronics ................................ ................................ .......................... 13 2 LI TERATURE RE V I EW ................................ ................................ .......................... 15 A 2 FeMoO 6 Double Perovskites ................................ ................................ ............... 15 Ferroelectric Ba 1 x Sr x TiO 3 ................................ ................................ ....................... 17 Multiferroics and Superlattice Approach ................................ ................................ 17 p type Doping in ZnO ................................ ................................ .............................. 19 3 EXPERIMENTAL DETAILS AND CHARACTERIZATION S ................................ .... 27 Target Synthesis ................................ ................................ ................................ ..... 27 Ba 2 FeMoO 6 ................................ ................................ ................................ ...... 27 Ba 1 x Sr x TiO 3 (x=0, 0.5) ................................ ................................ ...................... 28 Phosphorus Doped ZnO ................................ ................................ ................... 28 Thin Film Deposition ................................ ................................ ............................... 28 Substrate Prepara tion ................................ ................................ ...................... 29 Ba 2 FeMoO 6 Ba 1 x Sr x TiO 3 (x=0, 0.5) Thin Films and Hetero Structures ........... 30 Phosphorus Doped ZnO Thin Films ................................ ................................ 30 Characterization Techniques ................................ ................................ .................. 30 X ray Diffraction (XRD) ................................ ................................ ..................... 30 Atomic Force Microsc opy (AFM) and Scanning Electron Microscopy (SEM) ... 31 Auger Electron Spectroscopy (AES) ................................ ................................ 31 Hall Effect and Magnetoresistance ................................ ................................ ... 32 Superconducting Quantum Interference Device (SQUID) ................................ 33 4 MAGNETIC AND MAGNETO TRANSPORT PROPERTIES OF BA 2 FEMOO 6 PULSED LASER DEPOSI TED THIN FILMS ................................ .......................... 41 Introductory Remarks ................................ ................................ .............................. 41 Experimental ................................ ................................ ................................ ........... 42

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6 Result s and Discussion ................................ ................................ ........................... 43 Summary and Conclusions ................................ ................................ ..................... 48 5 THE EFFECTS OF OXYGE N PRESSURE ON THE PR OPERTIES OF BA 2 FEMOO 6 THIN FILMS GROWN VIA PULSED LASER DEP OSITION ............. 57 Introductory Remarks ................................ ................................ .............................. 57 Experimental ................................ ................................ ................................ ........... 58 Results and Discussion ................................ ................................ ........................... 59 Summary and Conclusions ................................ ................................ ..................... 63 6 STRAIN INDUCED ENHAN CEMENT OF MAGNETIZAT ION IN BA 2 FEMOO 6 BASED HET EROSTRUCTURES WITH ( BA,SR)TIO 3 ................................ ............ 72 Introductory Remarks ................................ ................................ .............................. 72 Experimental ................................ ................................ ................................ ........... 73 Results and Discussion ................................ ................................ ........................... 74 Summary and Conclusions ................................ ................................ ..................... 77 7 THE EFFECT OF STRUCT UE ON THE PROPERTIES OF BA 2 FEMOO 6 /BA 0.5 SR 0. 5 TIO 3 SUPERLATTICES ................................ .................... 85 Introductory Remarks ................................ ................................ .............................. 85 Experimental ................................ ................................ ................................ ........... 86 R esults and Discussion ................................ ................................ ........................... 86 Summary and Conclusions ................................ ................................ ..................... 90 8 THE EFFECTS OF ZNO B UFFER LAYERS ON THE PROPERTIES OF PHOSPHORUS DOPED ZNO THIN FILMS GROWN ON SAPPHIRE BY PULSED LASER DEPOSIT ION ................................ ................................ .............. 99 Introductory Remarks ................................ ................................ .............................. 99 Experimental ................................ ................................ ................................ ......... 100 Results and Discussion ................................ ................................ ......................... 101 Summary and Conclusions ................................ ................................ ................... 103 9 CONCLUSIONS ................................ ................................ ................................ ... 110 Ba 2 FeMoO 6 Thin Film and Hetero structures ................................ ........................ 110 p type Doping in ZnO Thin Films ................................ ................................ .......... 111 LIST OF REFERENCES ................................ ................................ ............................. 113 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 121

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7 LIST OF TABLES Table P age 2 1 Saturatio n magnetization ( M S ) at 5K and 295K, Curie temperature ( T C ), and room temperature crystallographic structure as determined from x ray diffraction of the double perovskite AA FeMoO 6 = Ba 2 BaSr, Sr 2 Ca 2 ) ...... 23 5 1 Compositions of Ba 2 FeMoO 6 thin films grown at 700 C under 0.1 and 10 mTorr by Auger Electron Spectroscopy. ................................ ............................. 65 6 1 Periodicities of the superlattices calculated from pos itions of the satellite peaks around (002) peak of Ba 2 FeMoO 6 in Figure 6 1E ................................ .... 79 7 1 Periodicities of the superlattices calculated from positions of the satellite peaks around (002) peak of Ba 2 F eMoO 6 in Figure 7 1B ................................ .... 92 8 1 Hall transport properties of as grown and rapid thermal annealed ZnO:P 0.001 thin films ................................ ................................ ................................ ........... 105

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8 LIST OF FIGURES Figure P age 2 1 The crystal structure of YMnO 3 in the paraelectric and ferroelectric phases and schematic of a MnO 5 polyhedron with Y layers above and below. .............. 24 2 2 Multiferroic properties induced by strain from substrate ................................ ..... 25 2 3 Stick and ball represent ation of ZnO crystal structures ................................ ...... 26 3 1 XRD patterns of Ba 2 FeMoO 6 powders calcined and sintered ............................. 35 3 2 Rietveld results of the diffraction data of Ba 2 FeMoO 6 powders from synchrotron source ................................ ................................ ............................. 36 3 3 Crystal structure of Ba 2 FeMoO 6 from (100) direction generated by CrystalMaker using the refinement data ................................ ............................. 37 3 4 Schematic descripion of pulsed laser deposition (PLD) deposition .................... 38 3 5 Bragg diffraction ................................ ................................ ................................ 39 3 6 The geometry for a high resolution X ray diffraction (HRXRD) ........................... 39 3 7 Schematic description of transport property measurement ................................ 40 4 1 XRD patterns of Ba 2 FeMoO 6 thin films ................................ ............................... 50 4 2 Magnetization curves of Ba 2 FeMoO 6 thin films grown at 700 900 C .................. 52 4 3 The temperature dependence of the magnetization of Ba 2 FeMoO 6 thin films grown at temperatures indicated ................................ ................................ ........ 53 4 4 Magneto resistance of Ba 2 FeMoO 6 thin films ................................ ..................... 54 4 5 Transport properties of Ba 2 FeMoO 6 thin films ................................ .................... 55 4 6 SEM images of the samples grown at 700 to ................................ ........... 56 5 1 XRD patterns of Ba 2 FeMoO 6 thin films grown at 700 C under 0.1 to 10 mTorr O 2 /Ar mixture gas ................................ ................................ ............................... 66 5 2 Magnetization c urves of Ba 2 FeMoO 6 thin films ................................ ................... 68 5 3 The temperature dependence of the magnetization of Ba 2 FeMoO 6 thin films .... 69 5 4 Magneto resi stance of Ba 2 FeMoO 6 thin films grown at the oxygen pressures indicated and measured at 10K ................................ ................................ .......... 70

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9 5 5 Transport properties of Ba 2 FeMoO 6 thin films grown at 700 C under varying oxygen pressure indi cated and measured at 300K ................................ ............. 71 6 1 XRD patterns of Ba 2 FeMoO 6 thin film and heterostructures ............................... 80 6 2 Magnetization curves of Ba 2 FeM oO 6 thin films and hetero structures measured at temperatures indicated ................................ ................................ .. 82 6 3 The temperature dependence of the magnetization Ba 2 FeMoO 6 thin film and heterostructures ................................ ................................ ................................ .. 83 6 4 Magneto resistance of Ba 2 FeMoO 6 thin film and superlattices ........................... 84 7 1 XRD pattern of Ba 2 FeMoO 6 /Ba 0.5 Sr 0.5 TiO 3 (5xN) superlattices .......................... 93 7 2 Lattice parameters and periodicities of Ba 2 FeMoO 6 /Ba 0.5 Sr 0.5 TiO 3 (5xN) superlattices ................................ ................................ ................................ ....... 94 7 3 Magnetization curves of Ba 2 FeMoO 6 thin film and superlattices measured at temperatures indicated ................................ ................................ ....................... 95 7 4 Temperature dependence of the magnetization of Ba 2 FeMoO 6 thin film and superlattices ................................ ................................ ................................ ....... 96 7 5 Magnetoresistance of Ba 2 FeMoO 6 thin film and superlattices measured at 10K ................................ ................................ ................................ ..................... 97 7 6 Transport properties of Ba 2 FeMoO 6 /Ba 0.5 Sr 0.5 TiO 3 (5xN) superlattices .............. 98 8 1 Rocking curves of (002) peaks of ZnO:P 0.005 thin films grown on the buffer layers under oxygen pressures indicated ................................ ......................... 106 8 2 AFM images of ZnO: P 0.005 thin films grown on the buffer layers under oxygen pressures indicated ................................ ................................ .......................... 107 8 3 SEM images of ZnO:P 0.005 films grown on the buffer layers under oxygen pressures indicated ................................ ................................ .......................... 108 8 4 Cross section images of ZnO:P 0.001 thin films and buffer layers grown at 700 C under 10 mTorr O 2 ................................ ................................ ................. 109

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10 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy GROWTH AND CHARACTERIZATIONS OF COMPLEX OXIDE THIN FILM HETEROSTRUCTURES VIA PULSED LASER DEPOSITION By Kyeong Won Kim August 2012 C hair: David P. Norton Major: Materials Science and Engineering Artificially synthesized o xide thin films and hetero structures have shown the novel properties that can be utilized in the next generation electronics This dissertation investigates magneto electric properties of half metallic double perovskite Ba 2 FeMoO 6 thin films and hetero structures and p type doping mechanism in ZnO thin films for the potential application in novel memory device and electronics Double perovskite Ba 2 FeMoO 6 thin films an d hetero structures with Ba 1 x Sr x TiO 3 have been prepared by pulsed laser deposition and the structural, magnetic and magneto transport properties were investigated. Phase pure Ba 2 FeMoO 6 thin films were epitaxially grown on SrTiO 3 substrates at temperatures ranging from 700 900 C under high vacuum The magnetic properties of samples improved with high growth temperatures. Ba 2 FeMoO 6 thin films showed anomalous Hall effect with hole like behavior under low magnetic field and showed an ordinary electron like tr ansport property under high magnetic field. When thin films were grown under low oxygen partial pressure, the magnetic properties were degraded but the Curie temperature increased. Also unusual positive magneto resistance observed in films grown under high vacuum disappeared, which suggests that it is associated with the oxygen related

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11 defects. The Ba 2 FeMoO 6 /Ba 1 x Sr x TiO 3 superlattices were synthesized and their structures were confirmed by X ray diffraction analysis The superlattices showed the strain indu ced enhancement of magnetization and the Curie temperature of superlattices were lowered substantially with increasing strain. Phosphorus doped ZnO thin films were deposited on ZnO buffer grown under different growth conditions and the properties of thin f ilms were examined, focusing on the role of buffer layers. Phosphorus doped ZnO thin films on buffer layer grown at low temperature were less conductive as grown state and showed a drastic change in the transport properties upon the rapid thermal annealing This suggests that poor crystalline quality of the buffer layer grown at low temperature lead to the more incorporation of phosphorus dopants in following films. A high resolution X ray diffraction study confirmed the poor crystalline quality of phosphor us doped thin films on low temperature deposited buffer layer by broader peaks of omega rocking curves. Also it was observed that the microstructures of the phosphorus doped ZnO thin film significantly depended on the buffer layer.

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12 CHAPTER 1 INTRODUCTION Development s and advances in the electronic industry ha ve been fueled by Si related semiconductor research demands for miniaturization and circuit speed Als o technolog ies for display photovoltaic and energy require material properties that cannot be a chieved by the Si. These circumstances drive research in oxide materials. With the advances in thin film growth techniques like pulsed laser deposition ( PLD ) 1 and molecular beam epitaxy ( MBE ) 2 high quality o xide thin films can be deposited with an atomic control 3 4 C omplex oxide hetero structures with abrupt interfaces can often provide for properties that are not ac hieva ble in original parent materials such as extremely high mobility 5 conducting interface between insulating materials 6 8 and novel magnetic propert ies 9 Also, the strain induced by substrates or in superlattice structures has been a remarkable tool that can modulate material properties in unprecedented ways 10 11 These new phenomena and proposed devices based on the materials will be able to provide a path to new physics an d applications for future gene ration electronics D ouble P erovskite Ba 2 FeM o O 6 and S uperlattice s The huge magneto resistance observed in perovskite manganites, so called colossal magnet o resistance (CMR) has attracted significant interest for its potential applications for new type s of memory devices. As early theoretical model s of double completely explain observed properties there have been various theoretical and experimental efforts to elucidate the phenomenon. Among them, a half meta llic ferromagnetic model which was developed based on spin polarized band structure calculations has been widely used to explain related phenomena. M any non

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13 manganite oxide materials have been found to be half m etallic and show large magneto resistance. A 2 FeMoO 6 (A=Ca, Sr, Ba) d ouble p erovskite is one of the materials that show s the half m e tal like property with a huge ma gn eto resistance Since Sr 2 FeMoO 6 was reported to have a high spin polarized state above room temperature, significant research has follow ed to study various aspects of the material 12 Ba 2 FeMoO 6 has been less studied as Sr 2 FeMoO 6 has attracted most of the interest largely due to the highe r Curie temperature of 420K Even with limited research there have been p romising reports of interesting magnetic and magneto electric properties in Ba 2 FeMoO 6 However, there lack s significant studies in thin film Ba 2 FeMoO 6 system. This dissertation investigates the synthesis of Ba 2 FeMoO 6 thin film s by pulsed laser deposition ( PLD) method and corresponding structural, magnetic and magneto transport properties of the thin films. Also, complex hetero structures with Ba 1 x Sr x TiO 3 are synthesized and examined specifically focusing on the effect of strain on the magnetic and magneto electric properties ZnO for O ptoelectronics ZnO has been intensely studied due to its potential for wide bandg ap semiconductor s electronics and optoelectronics. In particular, ZnO is considered a good candidate to replace GaN based III V semiconductors du e to some advantageous material properties such as large exciton binding energy, availability o f single crystal substrate, and easy device processing However, the absence of a reliable high quality p type ZnO has been the main bottleneck for fully reali zi ng th is potential. Thus, a better underst anding of doping mechanisms is required to take advantages of the excellent properties of ZnO. In Chapter 8, the transport properties of phosphorus doped ZnO thin

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14 films are investigated, specifically focusing on the effects of buffer layer synthesi s that provide s some insight into the doping behavior

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15 C HAPTER 2 LITERATURE REVI EW A 2 FeMoO 6 Double Perovskite s Double perovskites (DPs) with the general formula A 2 BB O 6 comprise two interpenetrating simple perovskite (ABO 3 ) structures with a long range ordering of B and B cations in B sites. DPs are recognized as a particular type of half metallic oxide with a fully polarized conduction band, which with a Curie temperature ( T C ) well above room temperature are of interest f or applications that take advantage of the unusual magneto transport and magnetic properties that are associated with the wide variety of available perovskite materials 13 DPs are remarkable materials with localized and itinerant ferromagnetism existing separately on the B and B sublat tices respectively 14 With the absence of both Jahn Teller effects and antiferromagnetic superexchange due to increased separation of B site cations DPs also have higher T C c ompared to manganites 14 Since the half metallic property of Sr 2 FeMoO 6 was predicted and demonstrated A 2 FeMoO 6 ( A =Ca, Sr, Ba) double perovskite s have attracted s ignificant interest for spin tra ns port related research. 12 Unlike c olossal magneto resistance (CMR) typically observed in manganite s oxides materials 15 18 a huge magneto resistance (MR) in the DP s remains at room temperature u nder low magnetic field 12 14 19 24 The crystal struc ture and Curie temperature of A 2 FeMoO 6 DPs depend on the A stie cation and are summarized in T able 2 1 21 Sr 2 FeMoO 6 is the most studied material due to its high Curie temperature relative to other DPs 12 24 35 The stoichiom etry and ordering of B and B cations (Fe and Mo) are known to be important for achieving desired properties The ratio of Fe:Mo can widely

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16 change various properties such as resist i vity, Curie temperature, and saturation magnetization ( M S ) 36 Sarma et al. reported the enhanced magneto resistance at low temperature in an ordered sample 25 However, o rdering is not easy to be evaluated particularly in thin films and thus inversely the saturation magnet ization and magneto resistance are commonly used to estimate ordering state in B site cations. In con trast to reports realizing expected magnetic and magneto electric properties in polycrystalline ce ramic materials it still remains as a challenge to fully reach the expected values e specially for M S and MR in thin film materials. The saturation magnetiz ation values have been changing significantly with processing conditions in the reports 24 29 30 34 35 37 T hin film studies also revealed interesti ng features in transport properties such as a change in conduct ion mechanism, anomalous Hall effect, and unexpected positive magneto resistance which were believed to be associated with t he spin glass phase induced by the disorder 38 40 X ray photoelectron spectroscopy (XPS) studies have shown that the existence of Mo rich phase at the film surface might con tribute to the degradation of magnetic property 34 35 Even with a limited number of reports, there have been some interesting results reported in Ba 2 FeMoO 6 22 41 43 Lee et al reported the effects of Fe/Mo disorder that lead s to degradation of magnetization and magneto resis tance in a similar way as that found in Sr 2 FeMoO 6 43 Grain size and crystalline quality were also reported to induce a large change in the magneto resi s tance of the material 22 44 T hin film stud ies o f this material have not been reported A 2 FeMoO 6 has a different crystal struc ture s in bulk but can be grown in a pseudo cubic structure on properly chosen substrates such as SrTiO 3 LaAlO 3 and MgO 28 30 34

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17 Ferroelectric Ba 1 x Sr x TiO 3 BaTiO 3 is probably the most well known and stu died ferroelectric material. Its C urie temperature is 393 K in which it goes through a phase transition fr om tetragonal to cubic crystal structure s and becomes paraelectric due to its centrosymmetry of a high temperature phase When alloyed with SrTiO 3 the C urie temperature and crystal structure of Ba 1 x Sr x TiO 3 (0
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18 b asically determined by the atomic element The first type of multiferroics is found in perovskite oxide materials which have lone pair electrons in their center atom BiFeO 3 falls into th is category and is the most studied single phase multiferroic material 50 52 When exposed to an electric field, the lone pair electrons of Bi ions at the center can move by applied field and thus induce the ferroelectric ordering T he magnetic ordering comes from Fe ions at the corner. BiMnO 3 and PbVO 3 also belong to this group 53 54 Second, geometrically driven ferroelectricity enables multiferroic properties of materials such as hexagonal manganites R MnO 3 ( R = Ho Lu, Y). In this group of materials, the symmetry is lowered by the tilting of MnO 5 bipyramids, which thus induces an electric dipole moment (shown in Figure 2 1). 55 Third, an electric polarization can be induced by a non symmetric charge ordering in some magnetic insulator such as Fe 3 O 4 56 This material undergoes a metal insulator transition at 125K accompanied by charge ordering of iron ions. Pr 1 x Ca x MnO 3 is also in this category 57 Fourth f erroelectricity can be driven by magnetic ordering, an effect that has been observed in orthorhombic rare earth manganites such as Tb(Dy)MnO 3 and Tb(Dy)Mn 2 O 5 58 60 Last multiferroic properties can be induced by strain. EuTiO 3 is intrinsically antiferromagnetic and paraelectric and turns into a ferroelectric ferr omagnet under a spe cific strain state (shown in Figure 2 2). 11 S ingle phase multiferroic materials are r are due to the mutually exclusive conditions driving ferromagnetism and ferroelectricity, partially fil l ed or empty transition metal orbitals, respectively 61 Due to the limited number of sin gle phase multiferroic materials and a generally weak correlation in those materials, a lternative approaches have been proposed by combining one m agnetic material and one ferroelectric material

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19 into horizontal and vertical composite s 62 68 The horizontal hetero structure called superlattice can be deposited as a few layers of alternating magnetic and ferroelectric materials b y means of the atomic control of thi n film deposi tion techniques 62 63 67 It is even possible to engineer magnetic ordering through superexchange interactions and compositional ordering that breaks an inversion symmetry to increase polarizations 64 69 70 Despite improved properties and interesting phenomena arising from the interface there still remain challenges as to find ing a good combination of those materials because two other materials should have comparable processing conditions p type Doping in ZnO ZnO generally crystallizes in the hexagonal wur t zite structure with lattice parameters of a=3.25 c=5.21 The ionic radii are 0.60 for Zn 2 + cation and 1.38 for O 2 anion, respectively. The crystal structures shared by ZnO are hexagonal wur t zite, cubic zinc blende, and rock salt as shown in Figure 2 3 At ambient conditions the thermodyn amically stable structure is wur t zite. The zinc blende can be stabilized only by growth o n cubic substrates and the rock salt structure may be obtained under high pressures 71 ZnO is an intrinsically n type direct bandgap semiconductor with the bandgap of 3 .2 eV. There are various native defects in ZnO : vacancies (V O and V Zn ), interstitials (Zn i and O i ), and antisite defects (O Zn and Zn O ). The formation energy of these defects which determin es their concentration strongly depends on the growth condition. I t is possible to get a high electron concentration of 10 21 /cm 3 even without extrinsic doping 72 The intrinsic n type behavior of ZnO is known to be due to Zn interstitials (Zn i ) and oxygen vacancies (V O ), which are known as hole killer defects for intentional p type

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20 acceptor dop ants Whether Zn i or V O is dominant is s till controversial. Look et al. suggested that Zn i is dominant shallow donor with the ionization energy of 30 50 meV. 73 However, the first principle study su ggested that none of those native defects shows high concentration sh allow donor characteristics 74 In addition to the intrinsic defects, hydrogen is thought to be another n type defect that should be taken into consideration Hy drogen has a very small ionic radius and can be easily incorporated into the films during the syn thesis. There have been several reports in which hydrogen acts as a shallow donor with the ionization energy of 30 meV 74 75 It is difficult to prevent the incorporation of hydrogen during growth, but hydrogen can be extracted at a high temperature 76 N type doping is relatively easy compared to p type doping. Group III elements such as Al, Ga, and In and group VII elements such as F and Cl can be used to replace Zn and O sites respectively. Myong et al. 77 synthesized Al doped ZnO films by photo assisted metal organic chemical vapor deposition ( MOCV D ) method and obtained highly conductive films with a minimum resistivity of 6.2 x 10 4 successfully used in various applications as n type layers in light emitting diodes as well as transparent Ohmic contacts. As mentioned already, p type doping is the most challenging issue for ZnO based junction devices. By far, group V anions such as N, P, As, and Sb are the most studied elements, motivated by the obvious opportunity to hole dope via substituting anions for the 2 oxygen sites. Nitrogen is thought to be an ideal acceptor dopant due to the similar ionic size to oxygen ( 1.46 for N 3 anion ) 78 There have been successful reports of p type doping with nitrogen by using various growth techniques including MBE and

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21 PLD 79 80 However, a high hole concentrat ion is difficult to achieve by N doping due to the difficulty of achieving high concentration incorporation into the films. Successful phosphor us doping has been reported by several groups but the mechanism of p type conduction b y phosphor us doping is not clearly understood 81 82 P has much lar ger ionic radius than that of O and thus it is not favorable energetically to substitute O site due to the large mismatch and strain energy from it ( 2.12 for P 3 anion ). Theoretical studies predict that phosphor us forms defect complex P Zn 2V Zn which re sult s in a shallow acceptor state 83 84 It should be noted that p type conduction with phosph or us doping is achieved by thermal annealing following film deposition. Morhain et al. 85 reported p type doping with As by PLD without post deposition annealing. The unipolar doping behavior of ZnO is commonly observed in many w ide bandgap semiconductors 86 87 For example, GaN, ZnO, ZnS and ZnSe are easily doped to n type while p type doping is difficult. By contrast, ZnTe is easy to get p type doping and difficult to get n type doping. This topic is well summarize d by Zunger 88 He explains preferential doping behavior in terms of band theory and chemical potential and suggests basic rules for doping based on them. Due to the dif ficulty of p type doping, ZnO has been used as an n layer for hetero structural LEDs in which other materials such as NiO and GaN were employed as a p layer 89 90 Recently, there have been several promising reports on ZnO homojunction LEDs. Until now most successful ZnO based LEDs were fabricated by using group V elements as a p type dopant. Tsukazaki et al. 91 proposed a repeated temperature modulation (RTM) technique as a reliable and reproducible way to fabricate p type ZnO. The study us ed two different temperatures for p layer growth; a low temperature for high

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22 concentration of nitrogen incorporation and a high temperature for high crystalline quality and hydrogen extraction at the same time Also, the results showed diodes with good rec tification and better signal to noise ratio of the electroluminescence spectrum tha n previous reports. Lim et al. 92 repor ted ultraviolet electroluminescence by phosphor us doping. Post deposition rapid thermal annealing was used to activate acceptor dopants in the study. The p type films show a high hole concentration of 10 19 /cm 3 T he intensity of near band edge emission was further increased and the deep level emission was greatly suppressed by using Mg 0.1 Zn 0.9 O layers as energy barrier layers to confine the carrie rs to the high quality n layer.

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23 Table 2 1. Saturation magnetization ( M S ) at 5 K and 295K, Curie temperature ( T C ) and room temperature crystallographic structure as determined from x ray diffraction of the double perovskite AA FeMoO 6 (AA = Ba 2 BaSr, Sr 2 Ca 2 ). (Adapted from 21 ) Nominal composition M S (5K) B fu 1 ) M S (295K) B fu 1 ) T C (K) Cell parameters Ba 2 FeMoO 6 3.53 1.33 308 Cubic Fm3m a = 8.0697(2) BaSrFeMoO 6 3.4 1.89 340 Cubic Fm3m a = 7.9798(2) Sr 2 FeMoO 6 2.95 1.78 385 Tetragonal P4 2 /m a = 5.5724(2) c = 7. 9006 (2) Ca 2 FeMoO 6 3.51 2.22 365 Monoclinic P2 1 /n a = 5.4131(2) b = 5.5209(2) c = 7.7065(2) = 89.95(5)

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24 Figure 2 1 The crystal structure of YM nO 3 in the paraelectric and ferroelectric phases and s chematic of a MnO 5 polyhedron with Y layers above and below The trigonal bipyramids depict MnO 5 polyhedra and the spheres represent Y ions. A ) The stacking of two consecutive MnO 5 layers and the sandw iched Y layer, looking down the c axis in the paraelectric phase B ) A view of the ferroelectric phase from perpendicular to the c axis, showing the layered nature of YMnO 3 The calculated atomic positions of the C) centrosymmetric and D) ferroelectric str uctures The numbers give the bond lengths in The arrows indicate atomic displacements with respect to the centrosymmetric structure (Adapted from 55 ) A B C D

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25 Figure 2 2 Multiferroic proper ties induced by strain from subs trate. A ) First principles epitaxial phase diagram of EuTiO 3 +2% (biaxial tension), calculated in 0.1% steps. Regions with paraelectric (PE), ferroelectric (FE), antiferromagnetic (AFM) and ferromagnetic (FM) behavio r are shown. B ) Schematic of unstrained bulk EuTiO 3 C ) E p itaxially strained thin film EuTiO 3 on the DyScO 3 substrate showing the in plane expansion due to biaxial tension. (Adapted from 11 ) A B C

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26 Figure 2 3 Stick and ball representation of ZnO crystal structures: A ) C ubic rock salt ( B 1) B ) C ubic zinc blende ( B 3) C ) H exagonal wurtzite ( B 4) The shaded gray and black spheres denote Zn and O atoms, respectively. (Adapted from 71 ) C B A

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27 CHAPTER 3 EXPERIMENTAL DETAILS AND CHARACTE RIZATIONS Target Synthesis Ba 2 FeMoO 6 The target was synthesized by a conventional solid state reaction. The starting materials were high purity (99.99% or higher) powde rs of BaCO 3 Fe 2 O 3 and MoO 3 First, the powders were mixed for 24 hours by dry ball milling and the mixed powder was calcined at 900C under reducing atmosphere of 5% H 2 buffered with Ar gas for 6 hours. The calcined powder was ground by mortar and pestle. It was pressed into a one inch diameter die by hand press and then further pressed under 150 M P a for three minutes by cold isostatic pressure. It was sintered at 1150C under the same reducing atmosphere for four hours. The calcined and sintered powders w ere examined by X ray diffraction and synchrotron diffraction. The XRD patterns of calcined and sintered powders in air and reducing atmosphere were show n in Figure 3 1. The final powders heat treated under reducing atmosphere showed only peaks from Ba 2 FeM oO 6 marked with stars in contrast to the powders processed in air containing impurity phases The diffraction data obtained from synchrotron source were analyzed with Rietveld refinement using the GSAS program. The refined crystal structure of Ba 2 FeMoO 6 is cubic (Fm 3m) with the lattice parameter of 8.0752 and the obtained composition after refinement is Ba 2.0374 Fe 1.0935 Mo 1.0065 O 6.4476 summarized in Figure 3 2 The crystal structure was generated with the data using the Crystal Maker program shown in Figu re 3 3

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28 Ba 1 x Sr x TiO 3 (x=0, 0.5 ) The target s w ere synthesized by a conventional solid state reaction. The starting materials for a target were high purity (99.99% or higher) powders of Ba Ti O 3 SrTi O 3 First, the powders were mixed to desired compositions us ing mortar and pestle for an hour It was pressed into a one inch diameter die by hand press It was then sintered at 1 400 C in air for 10 hours. Phosphorus Doped ZnO The target s of undoped and 0.1& 0.5 atomic % doped ZnO w ere synthesized by a conventional solid state reaction. The starting materials for a target were high purity (99.99% or higher) powders of ZnO and P 2 O 5 First, the powders were mixed to stoichiometric compositions using mortar and pestle for an hour It was pressed into a one inch diamete r die by a hand press It was then sintered at 1 200 C in air for four hours. Thin Film Deposition All the thin films and hetero structures were deposited by pulsed laser deposition (PLD) in this research. In this technique, a high power pulsed laser is use d to ablate a target. P ulsed ablations melt, evaporate, and ionize the surface of a target and lead to formation of a pla sma plume in which atoms and molecules are transferred onto a sub s trate. In Figure 3 4 a sketch is shown for schematic of PLD system w ith the image of a plasma plume. PLD has been widely used to deposit high quality thin film materials. T his technique has significant advantages over other film deposition techniques. It allows the deposition from almost any materials with the limitation o f bandgap and absorption of the laser energy I t can provide the stoichiometric material transfer from target to film of

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29 any desired composition (under optimized growth conditions), the creation of solid solution, multi cation epitaxial films, which are of ten difficult to make the targets of desired compositions. Also, it can give a path to multil ayers of different materials by using multiple targets. Despite these advantages, PLD is still confined to research purposes due to three main reasons. First, the deposited area is relatively small. Second, a plasma plume created during ablation is highly directional and therefore induces non uniform thicknesses over a film. Last, the extended period of ablation of a target sometimes results in f ormations of particu late in the plume and thus incorporation of them in film materials. Substrate Preparation (100) oriented SrTiO 3 substrates are used to deposit perovskite thin films and hetero structu r es. As received SrTiO 3 substrates a re etched and then annealed to acquir e TiO 2 terminated surfaces. The detailed procedure is as follows. First, as received SrTiO 3 substrates were rinsed in deionize d water for 5 minutes and blown dry with nitrogen gas. Second they were etched in buffered oxide etchant for 15 20 seconds, rinsed in deionized water for 5 minutes, and blown dry with nitrogen gas. Last, the substrates were annealed at 900 C for three hours in flowing oxygen. All substrates were rinsed with trichloroethylene (TCE), acetone, and methanol for 10 minutes each and th en b lown dry with nitrogen gas. ZnO and phosphorus doped ZnO thin films were grown on c plane (0001) sapphire substrates. The substrates were rinsed with trichloroethylene (TCE), acetone, and methanol for 10 minutes each and then blown dry with nitrogen gas be fore deposition

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30 Ba 2 FeMoO 6 Ba 1 x Sr x TiO 3 (x=0, 0.5 ) Thin Films and Hetero Structures Before the deposition, t he chamber was pumped down to the base pressure of ~ 10 9 Torr The samples were deposited at temperatures ranging from 600 900C. The atmosphere an d pressure used were ~ 10 7 Torr (without any external gas feed in), 1 10 mTorr for pure O 2 and 0.1 100 mTorr O 2 /Ar (0.2% oxygen ) mixture gas The growth condition for each sample was described in detail in the experimental part of chapter 4 7 The laser e nergy and the repetition rate were 1.5 J/cm 2 and 1 5 Hz, respectively. Phosphorus Doped ZnO Thin Films The chamber was pumped down to the base pressure of ~ 10 8 Torr ZnO buffer layers were grown under two conditions: low and high temperature buffer layers were grown at 400 C under 20 mTorr O 2 and 700 C and 1 mTorr O 2 Phosphorus doped ZnO were grown at 700 C under 10 & 150 mTorr O 2 The laser energy and the repetition rate were 1.5 2 J/cm 2 and 1 Hz, respectively. After the deposition, samples were rapid t hermal annealed at 900 C for 3 5 minutes under oxygen atmosphere to convert the carrier type in thin films. Characterization Techniques X ray D iffraction (XRD) X ray diffraction is a widely used non destructive technique to investigate the phase, crystallo graphic structure, lattice spacing of various forms of materials such as powder, single crystal and thin films. The basic principle of x ray diffraction is Bragg equation for diffraction, n = 2d sin where n is the order of diffraction, is the wave length of an incident x ray beam, d is

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31 incident beam and the atomic plane. Figure 3 5 shows the basic interpretation of Bragg diffraction condition The Philips APD 3720 was u s ed to examine the crystalline phases and the periodicity of superlattice samples at the Major Analytical Instrumentation Center (MAIC) at University of Florida In addition, used to examine the epitaxy and lattice parameters of samples. The detailed geometry information of this high resolution XRD was depicted in Figure 3 6 Atomic Force Microscopy (AFM) and S canning E lectron M icroscopy (SEM) Atomic force microscopy (AFM) or scanning probe microscopy (SPM) is the most commonly used technique to examine the surface structure and roughness of a sample. For high quality device fabrication, smooth film surfaces are req uired. An AFM is a mechanical imaging instrument that measures the three dimensional topography as well as physical properties of a surface with a sharpened probe. The sharpened probe is positioned close enough to the surface such that it can interact with the force fields associated with the surface. An image of the surface is reconstructed by monitoring the precise motion of the probe as it is scanned over the surface A Digital Instruments Dimension 3100 was used to observe the surface morphology of thin films at the MAIC facility. Scanning electron microscopy (SEM) i s a basic tool to observe the microstructure of a sample. At the MAIC facility, a JEOL JSM 6335F field emission gun SEM was used to observe the microstructure of sample surfaces as well as vertical structures Auger Electron Spectroscopy (AES) Auger elec tron spec troscopy is a quantitative analysis technique to investigate the chemical properties of materials. T his technique is based on Auger Effect which is an electronic process resulting from inter and intrastate transitions of electrons in an

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32 excited atom. When a sample is probed by electron beams, an inner shell electron can be removed leaving behind a hole. As this state is unstable, the hole can be filled by an outer shell electron which loses energy when it drops to the inner shell. If this transit ion energy is greater than the orbital binding energy, it induces an emission of electron in the outer shell. Since orbital energies are unique to an atom of a specific element, the ejected electrons can provide the information about the chemical compositi on of a surface. Due to the nature of Auger electron s this technique provides a high sensitivity to surface. A spot line, and area analysis can be carried out with help of an electron microscope. Perkin Elmer PHI 660 AES at the MAIC facility was used t o examine the chemical properties of thin films. Hall Effect and Magnetoresistance Hall effect is the generation of potential difference over an electrical conductor perpendicular t o both an electric current flowing along the conductor and an external mag netic field when it is put in the magnetic field applied at right angles to the current upon application of the magnetic field The basic principle is schematically depicted with the v an de r Pauw sample geometry used in this research in Figure 3 7 As def lecting direction that a charge carrier experiences depends on the type of a carrier, it can be used to investigate the type, concentration, mobility of charge carrier s The magneto resistance effect is a change in the electrical resistance R of a substanc e when a magnetic field is applied on it The value is defined as R / R 0 where R 0 is the resistance under no magnetic field. The value of magneto re s istance is very small for most materials, but is relatively large (a few percent) for strongly magnetic materials. Very large values of magneto re s istance have been observed in certain special materials and thin film arrays. The resistance of thin film structures composed of

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33 alternating ferromagnetic and nonmagnetic materials shows a substantial difference (~10%) in resistivi ty depending on whether the magn etizations in the m agnetic layers are parallel or antiparallel. This gives what is known as Giant Magneto resistance ( GMR ) which has found an application in the read heads for a hard disk drive. And certain perovskite oxides containing manganese and rare earths show an even larger magneto resistance (100% or more), leading them to be called Colossal Magneto resistive materials (CMR). They are also candidates for magnetic field detectors in computer drives. More recently, double perovskite oxide Sr 2 FeMoO 6 shows large magneto re s istance (~30%) even at a room temperature. In this research, Lakeshore 7507 Hall Effect Electronic measurement system and Quantum Design Physical Properties Measurement System (PPMS) were utilized to investigate the transport properties of samples, spec ifically focusing on the behavior under applied magnetic field. Superconducting Quantum Interference Device (SQUID) Superconducting quantum interference device (SQUID) is a highly sensitive magnetometer that can measure generally about 10 7 emu. This devic e is based on the tunneling of superconducting electrons across a very narrow gap, called a Josephson though each of two Josephson junctions when a superconducting current flo ws through the ring. A change in magnetic flux through the ring by an applied magnetic field current adds to the measuring current in one junction, and subtracts in the other. Because of the wave nature of the superconducting current, the result is a periodic appearance of resistance in the superconducting circuit, and the appearance of a

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34 voltage. Each voltage step corresponds to the passage of a single flux quantum acro ss the boundary of the ring. The existence of the flux quantum was demonstrated in somewhat similar experiments on superconducting rings; its value is h /2e = 2.07 x 10 7 weber or Tm 2 This sensitivity is rarely nee ded to measure a magnetic field and in pra ctice the device is most commonly linked to a coil to measu re the flux from a small sample and thus the sample magnetization. In this form, it is called a SQUID magnetometer. Since a superconducting Josephson junction device requires low temperature operat ion, it is usually used in conjunction with a superconducting solenoid. In this research, Quantum Design Magnetic Properties Measurement System (MPMS) sample magnetometer was used to investigate the Ba 2 FeMoO 6 thin film and superlattice samples.

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35 Figur e 3 1. XRD patterns of Ba 2 FeMoO 6 pow d ers calcined and sintered Images give for A ) air and B ) reducing atmosphere The peaks marked with star represent the phases from Ba 2 FeMoO 6 A B

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36 Chi square = 6.72 % Structure : C ubic F m 3m a = b = c = 8.072 a lpha = b eta = gamma = 90 Composition: Ba 2.0374 Fe 1.0935 Mo 1.0065 O 6.4476 Figure 3 2. Rietveld results of the diffraction data of Ba 2 FeMoO 6 powder s from synchrotron source.

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37 Figure 3 3. Crystal structure of B a 2 F e M o O 6 from (100) direction generated by CrystalMak er using the refinement data

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38 Figure 3 4. Schematic descripion of pulsed laser deposition (PLD) deposition. Images given for A) for system and B ) a plasma plume. A B

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39 Figure 3 5. Bragg diffraction Figure 3 6 The geome try for a high resolution X ray diffraction (HRXRD)

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40 Figure 3 7 Schematic description of transport property measurement. Images given for A) Hall effect and B) van d er Pauw geometry. A B

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41 CHAPTER 4 MAGNETIC AND MAGNETO TRANSPORT PROPERTIES OF B a 2 FeMoO 6 PULSED LASER DEPOSITED THIN FILMS Introductory Remarks Half metallic oxides have attracted a significant interest in the field of spin polarized transport applications. How ever, in most material systems, the high spin polarization disappears at room temperature due to a low Curie temperature ( T C ). Since Kobayashi et al. 12 reported that half metallic double perovskite Sr 2 FeMoO 6 remains as high spin polarized state at room temperature due to its high T C up to 420K, this material has been of interest for its spin polarized transport properties and considered a good candidate for device application 19 20 38 93 94 A simulation study suggested that the defect free Sr 2 FeMoO 6 has a high T C of 450K and a saturation magnetization of 4 B per formula unit (f.u.) at a low temperature with a long range antiferromagnetic ordering among Fe 3+ ions and Mo 5+ ions 20 However, the experimental values of a saturation magnetization have been reported to be lower than 4 B /f.u. in most studies; achieving this value seems more challenging in thin film materials. Ba 2 FeMoO 6 is a member of the double pero v skite A 2 FeMoO 6 (A=Ba, Ca, Sr) family and reported to have T C as high as 367K 19 Even though Ba 2 FeMoO 6 has attracted less interest mainly due to the lower T C than Sr 2 FeMoO 6 there have been multiple reports of over 30% negative magneto resistance for polycrystalline ceramic samples. 22 44 However, there lack studies on thin film Ba 2 FeMoO 6 In this study, we have prepared epitaxial Ba 2 FeMoO 6 thin films on SrTiO 3 substrate and investigated their magnetic and ma gneto transport properties. Samples grown at high temperatures showed higher values in saturation magnetization and magneto resistance. Interestingly, a change from positive to negative magneto

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42 resistance was observed under increasing magnetic field. Also, an anomalous Hall effect was observed accompanied by conduction mechanism changes for different temperature ranges. Experimental The Ba 2 FeMoO 6 thin films were deposited on SrTiO 3 substrates by pulsed laser deposition with a stoichiometric target. The targ et was synthesized by a conventional solid state reaction. The starting materials for a target were high purity (99.99% or higher) powders of BaCO 3 Fe 2 O 3 and MoO 3 First, the powders were mixed for 24 hours by dry ball milling and the mixed powder was cal cined at 900C under reducing atmosphere of 5% H 2 buffered with Ar gas for 6 hours. The calcined powder was ground by mortar and pestle. It was pressed into a one inch diameter die by hand press and then further pressed under 150 M P a for three minutes by c old isostatic pressure. It was sintered at 1150C under the same reducing atmosphere for four hours. The calcined and sintered powders were examined by X ray diffraction and synchrotron diffraction. The diffraction data obtained from synchrotron source wer e analyzed with Rietveld refinement using the GSAS program. The refined crystal structure is cubic with the lattice parameter of 8.0752 and the obtained composition after refinement is Ba 2.00 Fe 1.07 Mo 0.99 O 6.33 The thin films were deposited under vacuum a t temperatures ranging from 600 to 900C for two hours The chamber was pumpe d down to the base pressure of ~ 10 9 Torr and the pressure during deposition was in the order of 10 7 Torr. The 248 nm wavelength KrF laser was employed to ablate the ceramic targ et. The laser energy and the repetition rate were 1.5 J/cm 2 and 4 Hz, respectively. The growth rate is around 0.1 per laser pulse.

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43 The crystal structure and phase of the thin films were examined by X ray diffraction (Phillips APD3720 and Phillips MRD pert ). Magnetization, Hall effect and magneto transport were respectively measured using Quantum Design Magnetic Properties Measurement System (MPMS) and Physics Properties Measurement Syst em (PPMS). In addition, a field emission scanning electron microsco pe (JEOL 6335F) was u sed to observe the microstructure of samples. Results and Discussion Figure 4 1 A shows the XRD pattern s of the Ba 2 FeMoO 6 under ~ 10 7 Torr without external gas feed in and 1x 10 3 Torr oxygen ambient. An i mpurity BaMoO 4 phase is observed in the sample grown under oxygen ambient in contrast to the phase pure sample grown under ~ 10 7 Torr. Figure 4 1 B shows the XRD patterns of the Ba 2 FeMoO 6 thin films grown at 600 900 ~ 10 7 Torr. All films are phase pure and oriented along the c directions in which the ( 00 l ) peaks of films are observed next to the ( 00 m ) peaks of SrTiO 3 substrate. And the epitaxi al growth is confirmed by in plane phi scan in Figure 4 1 C and the reciprocal space map shows that the film is slightly relaxed as shown in Figure 4 1 D As the growth temperature increases, the ( 004 ) peaks of the films show a continuous shift to high angle which indicates a decrease of the c lattice parameter. This might be due to compositional change of Fe:Mo and the different lattice sizes of BaFeO 3 (4.02 ) and BaMoO 3 (4.04 ) 95 though a systematic change was not observed in Fe:Mo ratio from SEM/EDS analysis, of which the resolution might not be good enough for a slight change in composition. The Fe:Mo composition can vary with the growth temperature due to the different volatility and sticking coefficients during the deposition. Also, the oxygen content is another possibility that can affect the lattice parameter change. It is known

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44 that the lattice parameter can be affected by oxygen content in the sublattice s of BaFeO 3 and BaMoO 3 as well as the double perovskite SrFeMoO 6 38 96 Magnetization loops with magnetic fields oriented perpendicular to the c axis are plotted in Figrure 4 2. At 10K, Figure 4 2 A shows typical ferromagnetic behavior in which the saturation magnetization with values near 0.4, 0.8, B /f.u. for the 700, Also as shown in the lower panels of Figure 4 2 B plotted with magnetic field on an expanded scale, the coercive fields increase for the higher gr owth temperature samples, although the values are small. It is clear from the above discussion that both the magnitude of the magnetization and the coercive field improve with higher growth temperature, presumably because of better crystalline quality. But the low values of M S B B /f.u. can be explained by the compositional variance, i.e., B:B' site order disorder and oxygen content. It is well known that this double perovskite material and the substructural BaFeO 3 and BaMoO 3 are very sensitive to the oxygen content 94 96 98 Sarma et al reported that disordering of Fe/Mo sites in Sr 2 FeMoO 6 has a significant negative effect on magneto transport properties of the material. 25 The coercive field increases with increasing growth temperature, though the values are still small below 1 kOe. At 300K, the samples still show the saturation of magnetization bu t the coercive field is strongly diminished In Figure 4 3 A C the temperature dependence of magnetization i s shown in both field cooled (FC) and zero field cooled (ZFC) protocol s. In the zero field cooled method, a sample is cooled from room temperature without any applied magnetic field and then a small magnetic field of 100 Oe is applied at 10K. The magnetization is measured as a

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45 function of temperature during warm up under a constant field. In field cooled case, the sample is cooled from room temperatu re to 10K in an applied magnetic field and magnetization is measured during warm up T he sample grown at T C higher than 300K. It is consistent to the fact that the saturation magnetization remains almost the same at 10 K Also the FC data in Figure 4 3 A show an increase of magnetization during warm up, which is unusual as in FC mode magnetization should always decrease with increasing temperature (increased thermal energy destroys spin alignment and decreases magnetization). It is interesting to note that the saturation magnetization depends on growth temperature while b oth the Curie temperature (near where M deviates from a constant value) and the irreversibility temperature (marked by the deviation between FC and ZFC curves) remains roughly T C and M can be explained using antisite (AS) disorder, where Fe & Mo exchange their site positions without changing carrier density n It has been shown both theor etically 14 and experimentally 99 that AS disorder systematically reduces the saturation magnetization ( M S ) without affecting T C in Sr 2 FeMoO 6 T he curves show that the Curie temperatures ( T C ) are around 250 K rep orted for Ba 2 FeMoO 6 ceramics 19 The FC data in Fig ure 4 3 B shows a decrease of magnetization during cooling followed by an increase at lower temperatures, which may be due to the presence of both ferromagnetic and antiferromagnetic couplings in these non optimized films Fig ure 4 4 A shows the magneto resistance of the samples. All samples show typical negative magneto resistance at a high magnetic field. However, the sample

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46 resistance below 2 T There is a report that Sr 2 FeMoO 6 thin films showed the positive or negative magneto resistance when grown at different temperatures 38 but in that report the samples showed only one characteristic depending on growth condition throughout the whole magnetic field range whil e in the magneto resistance reflects a positive contribution at low fields and a stronger negative contribution at high fields. The positive magneto resistance is not fully understood. A similar type of field dependence of magneto resistance w as reported in dilute magnetic semiconductor of (Ga,Mn)As 100 in which the positive magneto resistance was believed to be induced by the rotation of spins and it disappeared when the magnetic field was applied parallel to t he sample plane. We resistance of 3% at a temperature of 10K and a magnetic field of 7 T The small value of magneto re s istance is believed to be mainly due to disorder in the films. The degradation of magneto resist ance by disorder has been reported in double perovskite ceramics 25 32 44 The resistance value which is to some extent due to cancellation by the positive region observed at small magnetic fields In Figure 4 4 B the magneto resistance decreases with increasing temperature and it almost disappears at room temperature which is expected from the thermal dependence of magnetization results. The resistivity and Hall resistivity are plotted in Figure 4 5 A and B respectively. The resistivity decreases with increasing growth temperat ure for all samples and a large dependence on temperature is observed from the sample grown at 7

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47 different temperature ranges. The resistivity increases logarithmically with decreasing temperature below 20 K, which is observed in Kondo system. Even though Kondo system originally describes the transport property of a metallic system with magnetic impurities 101 there have been reports of Kondo like behavior in ferromagnetic materials including ferromagnetic perov skite oxide materials 102 103 It is believed that this behavior is correlated with the spin glass phase induced by disorder 37 It is also another indication of the disorder in the B cation arrangements of the A 2 B B O 6 double perovskite. Segregation into the clusters of BaFeO 3 or BaMoO 3 may induce a significant change in magnetic and magneto e lectric properties of double perovskite since it is well known that those two materials have quite different properties. Above 50K, on the other hand, the temperature dependence can be described by R exp[(T 0 /T) 0.25 ]. This change in a conduction mechanism i s very similar to the report on epitaxial double perovskite Sr 2 FeMoO 6 thin films, though there are small discrepancies in temperature ranges for each dependency 39 In ferromagnetic materials the transverse resistivity is given by xy = R H B + R A 0 M, with the magnetization M and the ordinary and anomalous Hall coefficients R H and R A respectively 104 The Hall resistivity is measured at constant temperatur es of 10, 100 and 300K up to 7 T and shown in Figure 4 5 B In low magnetic fields, the Hall resistivity is dominated by the anomalous Hall contribution which behaves hole like as indicated by the positive slope. This positive sign arises because the single itinerant electron associated with the t 2g band of the Mo has a spin antiferromagnetically coupled to the localized S=5/2 core spins on the Fe sites. In high fields the ordinary Hall effect is

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48 dominant and the data show a linear negative slope which indicates an electron like behavior. Assuming nave single band model negative slopes at magnetic field from 4 to 7 T gives a carrier density of 0.4, 0.6 and 4 electrons per f.u. for 10, 100 and 300K, respectively. This anomal ous Hall behavior is also seen in Sr 2 FeMoO 6 systems, where the origin of this anomal ous Hall effect is believed to be induced by skew scattering 39 In Figure 4 6, the SEM images of samples show grain like structure in which the size of the features increases with increasing growth temperature. For films grown at 800 and 900 smaller particles with the size of tens of nanometer are observed though there is no explicit second phase observed from the XRD results. They tend to homogeneously all over the area for 800 The temperature dependence of resistivity might be induced by this microstructural change in part. It is consistent with the report that resistivity and magnetoresistance decreased with increasing grain size in Ba 2 FeMoO 6 ceramics study 22 The coercive field increases with increasing growth temperature. However, Poddar et al. reported that the coercive field decreased with increasing grain size in polycrystalline Sr 2 FeMoO 6 37 This discrepancy su ggests that the strain induced by the substrate can play a role in the properties of thin film double perovskite materials. Summary and Conclusions In summary, structural, magnetic and magneto transport properties were investigated in Ba 2 FeMoO 6 thin films grown by pulsed laser deposition. The grain size, the conductivity, the saturation magnetization and the coercive field all increase with increasing growth temperature, thus indicating that substrate temperatures on the order of 900

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49 T C of these films remains constant while magnetization and conductivity improve is consistent with previous experimental results 99 and recent theoretical notions 14 that antisite disorder (i.e., exchange of Fe and Mo atoms) is responsible for diminished magneti c properties. Clearly, the attainment of higher quality double perovskite films will require precise stoichiometry, improved ordering on the B and B sites and a comprehensive understanding of the role of substrate induced strain.

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50 Figure 4 1 XRD pa tterns of Ba 2 FeMoO 6 thin films Images given for A) sample grown at 900C under 10 7 Torr (without external gas feed in) and 10 3 Torr O 2 and B) samples grown at 600 900 C under 10 7 Torr The peaks marked with asterisk (*) are from the XRD sample holder ( which is not covered by 5x5 mm2 samples). C) In plane phi scan D) R eciprocal space map of the sample grown at 700 C A B

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51 Figure 4 1 C ontinued C D Film Substrate

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52 Figure 4 2 Magnetization curves of Ba 2 FeMoO 6 thin films grown at 700 900 C Images given for samples measured at A B) 10K and C D) 300 K A B C D

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53 Figure 4 3 The temperature dependence of the magnetization of Ba 2 FeMoO 6 thin films grown at temperatures indicated

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54 Figure 4 4 Magneto resistance of Ba 2 FeMoO 6 thin films. Images given for A) sample s grown at 700 K and B measured at temperatures indicated. A B

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55 Figure 4 5 Transport properties of Ba 2 FeMoO 6 thin films. Images given for A ) r esistivity of samples grown at 700 B ) Hall resistivity of a sample grown at 900 A B

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56 Figure 4 6 SEM images of the samples grown at 700 to

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57 C HAPTER 5 THE EFFECTS OF OXYGEN PR ESSURE ON THE PROPERTIES OF Ba 2 F e M o O 6 THIN FILMS GR OW N VIA PULSED LASER D EPOSITION Introduct ory Remarks Double perovskites (DPs ) with the general formula A 2 BB O 6 comprise two interpenetrating simple perovskite (ABO 3 ) structures with a simple three dimensional ordering of the B and B cations. Double perovskite s are known as a particular type of half metallic oxide with a fully pol arized conduction band, which with Curie temperatures higher than room temperature are good candidates for applications that take advantage of the unusual magneto transport and magnetic properties that are associated with various perovskite materials avail able for chemical modification 13 Double perovskites are remarkable materials with localized and itinerant ferromagnetism existing separately on the B and B sublattices respectively 14 With the absence of both Jahn Teller effects and antiferromagnetic superexchange due to increased distance between B site cations, double perovskites also have higher Curie temperatu r es comp ared to manganites 14 Among the different types of double perovskites, Sr 2 FeMoO 6 has been most studied since Kobayashi et al. reported that this half metallic Sr 2 FeMoO 6 remains as high spin polarized state above room temperature 12 Clearly such materials are good candidates for device applications 19 20 38 93 94 A simulation study suggested that the defect free Sr 2 FeMoO 6 has a high T C of 450K and a saturation magnetization of 4 B per formula unit (f.u.) at a low temperature with a long range antiferromagnetic ordering between the localized core spins on the Fe 3+ sites and the itinerant spins on the Mo 5+ sites 20 However, the experimental values of a saturation magnetization have been

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58 lower than 4 B /f.u. in most studies; achieving this value seems more challenging in thin film materials. Ba 2 FeMoO 6 is also a member of the double perovskite A 2 FeMoO 6 (A=Ba, Ca, Sr) family and is reported to have a T C as high as 367K 19 Even though Ba 2 FeMoO 6 has a ttracted much less interest largely due to a lower T C than Sr 2 FeMoO 6 there have been multiple reports of over 30% negative magneto resistance for polycrystalline ceramic samples 22 44 However, t o our knowledge there is no significant study on thin film Ba 2 FeMoO 6 In the previous chapter, we discussed prepar ing epitax ial thin films under vacuum and examined structural, magnetic, and magneto transport properties. In this chapter, we have deposited epitaxial Ba 2 FeMoO 6 thin films under oxygen atmosphere and investigated the effect of oxygen pressure on the properties of t hin films We have found that the films prepared under a low oxygen pressure show ed a high saturation magnetization value while the magneto resistance was enhanced with higher oxygen pressure during deposition. Interestingly, a n unusual positive magneto re sistance observed in the sample grown at a high temperature under vacuum disappeared by by the addition of oxygen Experimental We have prepared epitaxial Ba 2 FeMoO 6 thin films on SrTiO 3 substrates by pulsed laser deposition with a stoichiometric target. Th e target was synthesized by a conventional solid state reaction under reducing atmosphere of H 2 /Ar (5% Hydrogen mixture gas The detailed procedures and properties of the target synthesized were described in the c hapter 3

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59 The thin films were deposited at temperatures of 700 and 800C for an hour The base pressure of chamber was ~ 10 9 Torr and the pressure during deposition was ranging from 0.1 to 10 mTorr of O 2 /Ar (0.2% ox y gen) mixture gas, corresponding to 2 .0 x 10 7 to 2 .0 x 10 5 Torr of oxygen partial pre ssure. The 248 nm wavelength KrF laser was utilized to ablate the ceramic target. The laser energy and the repetition rate were 1.5 J/cm 2 and 5 Hz, respectively. The growth rate is 0.2 per laser pulse. The structural properties of the thin films were exa mined by powder and high resolution X ray diffraction using Phillips AP facility Magnetization, Hall effect and magneto transport were respectively measured using a Quantum Design Magnetic Properties Measurement Sys tem (MPMS) and Physics Properties Measurement System (PPMS). Also A uger E lectron S pectroscopy (AES) was used to investigate the chemical properties R esults and Discussion Figure 5 1 A shows the XRD pattern s of the Ba 2 FeMoO 6 thin films grown at 700 under 0.1 to 10 m Torr of O 2 /Ar mixture gas All films were phase pure and oriented along the c directions in which the ( 00 l ) peaks of films we re observed next to the ( 00 m ) peaks of SrTiO 3 substrate. An un identified peak around 32 degree is overlapped wi th one of the peaks from the sample holder for XRD measurement due to small sample size of 5x5 mm 2 As it is not matched with any peak positions of possible impurities such as BaMoO 4 or Fe 3 O 4 which are most likely to form under a high oxygen pressure, it is believed to be originated from the substrate holder with unusually high intensity for the sample grown at 10 mTorr In Figure 5 1 B the ( 004 ) peak of films were observed to shift toward a high angle, i.e. small lattice spacing with increasing oxygen pr essure. For the sample grown under

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60 10 mTorr, the peak showed broadening and spitting which might be due to phase segregation of Fe and Mo rich phases in the film and in turn can lead to degradation of magnetic properties. Fig ure 5 1 C shows the ( 0 0 4 ) and ( 642) peak s of the samples gro w n at 700 under 0.1 and 10 mTorr. The a and c lattice parameters obtained from ( 004 ) and ( 6 42 ) peak positions are 8.120 and 8.176 and 8.066 and 8.082 for 0.1 and 10 mTorr samples respectively. Both samples are elongated along c direction by strain induced b y substrate of smaller lattice spacing (3.905 ). This change in lattice parameter is believed to be induced by compositional variations under different oxygen partial pressure during deposition. Sr 2 FeMoO 6 thin film has shown a high sensitivity to oxygen p ressure as low as 10 6 to 10 4 Torr 94 T he lattice parameter have been reported to be affected by oxygen content in the sub structural BaFeO 3 and BaMoO 3 as well as the double perovskite SrFeMoO 6 38 96 98 The other possible chemical modulation is a change in ratio of cations (Ba:Fe:Mo) especially in B site cations. Due to the different lattice sizes of BaFeO 3 (4.02 ) and BaMoO 3 (4.04 ), antisite defects i.e., exchange of Fe and Mo atoms and phase segregations can lead to the lattice parameter change Auger electron spectroscopy ( AES ) was used to measure the chemical compositions for samples grown at 700 T he composition s obtained by AES were summarized in T able 5 1. I t should be noted that the s urface state of a sample can affect significantly the intensity of the peak for each ele ment and t he sensitivity factor of each element can change the results However, the relative amounts of cations with varying growth condition can provide important aspects of how the oxygen pressure influences the film property. The Fe:Mo ratio is larger for the 10 mTorr sample than the

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61 0.1 mTorr sample and this increased off stoichiomet r y is believed to be responsible for degraded magnetic properties of the samples under high oxygen pressure. The values of Fe:Mo ratio are larger than the ideal ratio of o ne, which might be the crucial factor for the low saturation magnetization 36 43 Due to the limitation of the m easurement mentioned above, the comparative quantitative characterization is further required to confirm the chemical composition. The magnetization curves measured at 10 and 300K wa re plotted in Figure 5 2 At both growth temperatures, the saturation mag netization ( M S ) decreased with increasing oxygen pressure during deposition and they are about 0.6, 0.4, and 0.1 B /f.u. for 0.1, 1, 10 mTorr samples, respectively. But there is no significant change in M S with the measurement temperature, which is consist ent with the result of temperature dependent magnetizations shown in Figure 5 3 It s hows that all films grown at 700 have a Curie temperature higher than 300K. In contrast, the films grown at 800 show increased Tc in the one grown under higher oxygen pressure. At 10K, t he samples grown under 1 mTorr has smaller coercive field than the sample s grown under 0.1 and 10 mTorr which have almost the same values The coercive filed was diminished at 300K for all samples. This effect, namely the smaller coerciv e value at a specific pressure and increasing with oxygen pressure is also observed in the samples grown at 800 The film grown under 0.5 mTorr has a smaller coercive field than the film s grown under vac uum and 5 mTorr Figure 5 4 shows the magneto resistance of the samples grown at 700 and 800 All samples show typical negative magneto resistance at 7 00 The m agneto resistance increased with increasing oxygen pressure which interestingly showed the

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62 opposite tendency to the saturation magnetization. The saturation magnetization and magneto resistance are the two characteristics used mo st commonly to evaluate this double perovskite oxide material M ost studies have shown that those two properties s how the same tendency with the processing conditions which are believed to affect stoichiometry and disordering. Thus, the changes in magnetization and magneto resist ance observed above suggest that there is another factor other than stoichiome try and disordering, which is dependent on the oxygen pressure during deposition. The increase of magneto resistance with increasing oxygen pressure was observed in the samples g rown at 800 Interestingly, the positive portion observed in the sample grown under vacuum disappeared in the samples grown under oxygen atmosphere. This positive portion under small field which was reported in dilute magnetic semiconductor of (Ga,Mn)As is not clea rly understood 100 The partial pressure of oxygen is as low as 1x10 6 Torr for 0.5 mTorr of O 2 /Ar mixture gas and it clearly shows the high sensitivity of this material to oxygen. T his sensitivi ty suggests that the positive magneto resistance might be associated with oxygen related defects. A closer look at the graph in the small field range gives more d etailed changes. First, when a magnetic field is applied, the samples show a sharp decrease ( up to 1 kOe) and a small plate au appear s (1 to 2 kOe) for all samples. The differences emerge after this point. Samples grown under oxygen showed an ordinary negative MR under increased applied field. In contrast, the sample grown under vacuum has extended plateau up to 4 kOe and start ed to increase until reac hing a maximum around 15 kOe. The resistivity and Hall resistivity are plotted in Figure 5 5. The resistivity in creases with increasing oxygen pressure and a large dependence on temperature is observed

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63 from the sample grown under 1 0 mTorr. The temperature dependence of resistivity is similar to that of films grown under vacuum, in which a conduction mechanism change is observed with temperature Interestingly, it reflects correlation between resistivity and magneto resistance. The s ample grown under 10 mTorr shows a high er resistivity and magneto resistance than the samples grown under 0.1 and 1 mTorr. In ferromagnetic materials, the transverse resistivity is given by xy = R H B + R A 0 M, with the magnetization M and the ordinary and anomalous Hall coefficients R H and R A respectively 104 The Hall resistivity is measured at 300K under an applied magnetic field up to 7 T and shown in Figure 5 5 B An amomalous Hall effect is observed in all samples. In low magnetic fields, the Hall resistivity is dominated by the anomalous Hall contribution which behaves hole like as indicated by the positive slope. This positive sign arises because the single itinerant electron associated with the t 2g band of the Mo has a spin antiferromagnetically coupled to the localized S=5/2 core spins on the Fe sites. In high fields, the ordinary Hall effect is dominant and the data show a linear negative slope which indicates an electron like behavior. The anomalous portion is suppressed in the film grown under high oxygen pressure due to low magnetization (in the second term of equation, R A 0 M ). This anomal ous Hall behavior is also seen in Sr 2 FeMoO 6 systems, where the origin of this anomal ous Hall effect is believed to be induced by skew scattering 39 S ummary and Conclusions In summary, the structure and magnetic properties were investigated in Ba 2 FeMoO 6 thin films grown by pulsed laser deposition focusing on the effect s of oxygen pressure during deposition The s aturation magnetization de creases with

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64 increasing oxygen pressure while magneto resistance was enhanced by higher oxygen pressure. This opposite tendency of magnetization and magneto resistance is unusual in this double perovskite. As it cannot be explained by the stoichiometry and disordering, oxygen related defects are believed to be responsible for th e odd behavior s of the samples Disappearance of positive magneto resistance even with very low oxygen partial pressure during deposition reflects the high sensitive of this double perovskite to oxygen which is another hint of existence of oxygen related defects.

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65 Table 5 1. Compositions of Ba 2 FeMoO 6 thin films grown at 700 C under 0.1 and 10 mTorr by Auger Electron Spectroscopy Sample Ba Fe Mo O Fe/Mo 0.1 mTorr A s grown 36.97 13.60 5.38 44.05 2.53 sputtered 30 sec 39.28 16.35 6.30 38.07 2.60 10 mTorr As grown 35.65 18.52 6.13 39.69 3.02 sputtered 30 sec 39.92 24.39 7.19 28.50 3.39

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66 Figure 5 1. XRD patterns of Ba 2 FeMoO 6 thin films grown at 700 C under 0 .1 to 10 mTorr O 2 /Ar mixture gas Images given for A) scanned from 20 to 80 degree and B) (004) peaks of samples. C) Omega rocking curves of (004) peaks.D) (642) peaks in theta 2theta scan A B

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67 F igure 5 1. Continued C D

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68 F igure 5 2 Magnetization curv es of Ba 2 FeMoO 6 thin films Images given for samples grown at A B) 700 C and C D) 800 C measured at temperatures indicated A at 10K B at 300K C at 10K

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69 F igure 5 3 The temperature dependence of the magnetization of Ba 2 FeMoO 6 thin films Images given for samples grown at A B) 700 C and C D) 800 C (a) A B To be replaced with a new graph A B C D

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70 F igure 5 4 Magneto resistance of Ba 2 FeMoO 6 thin films grown at the oxygen pressures indicated and measured at 10K. Images given for samples grown at A ) 700 C and B C) 800 C A C B

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71 Figure 5 5 Transport properties of Ba 2 FeMoO 6 thin films grown at 700 C under varying oxygen pressure indicated and measured at 300 K. Images given for A) r esi stivity and B) Hall resistivity A B

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72 C HAPTER 6 STRAIN INDUCED ENHAN CEMENT OF MAGNETIZATION IN B a 2 F e MoO 6 BASED HET EROSTRUCTU R ES WITH (Ba,Sr )TiO 3 Introduct ory Remarks Multiferroics, especially ferroelectric ferromagnets, have attracted significant interest due to novel phenomena and the potential for various applications 49 50 59 68 70 105 109 As a limited number of single phase multiferroic materials exists due to the mutually exclusive nature of orbital structures required for ferroelectricity and ferromagnetism 61 horizontal or vertical hetero structu r es have been explored as an alternative way to achieve the multiferroic propert y 70 Since Kobayashi et al reported that the half metallic double perovskite Sr 2 FeMoO 6 remains as high spin polarized sta te at a room temperature due to its high T C up to 420K, this material has been of interest for its spin polarized transport properties and considered as a good candidate for spintronics application 12 19 20 38 93 94 A simulation study suggested that the defect free Sr 2 FeMoO 6 can have the high T C of 450K and the saturation magnetization of 4 B per formula unit (f.u.) at a low temperature with antiferromagnetic ordering among Fe 3+ ions and Mo 5+ ions 20 Ba 2 FeMoO 6 is a member of the double pero v skite A 2 FeMoO 6 (A=Ba, Ca, Sr) family and reported to have T C as high as 367K 19 Even though Ba 2 FeMoO 6 has attracted mu ch less interest mainly due to lower T C than Sr 2 FeMoO 6 there have been multiple reports of over 30% negative magneto r esistance for polycrystalline ceramic samples 22 44 However, the thin film studies of this material ha ve not been done much. In previous chapters, we have prepared epitaxial thin film Ba 2 FeMoO 6 under v arious growth conditions and investigated the magnetic and magneto transport properties of the thin films. In this chapter, we prepared hetero epitaxial multilayers of Ba 2 FeMoO 6

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73 with ferroelectric (Ba,Sr)TiO 3 o n SrTiO 3 substrate by pulsed laser deposition and investigated their magnetic and magneto electric properties focusing on the effect of strain induced by different coupling layers and in different hetero structures E xperimental Ba 2 FeMoO 6 thin film and hetero structu r es with Ba 1 X Sr X TiO 3 (x=0 and 0.5 ) were synthesized on SrTiO 3 substrates by multiple target pulsed laser deposition. The targets were synthesized by a conventional solid state reaction with high purity (99.99% or higher) powders of BaCO 3 Fe 2 O 3 MoO 3 BaT i O 3 and SrTiO 3 The detailed proce dure for target fabrication and analysis on the powders were explained in chapter 3. All samples were deposited at 700C under 0.5 mTorr O 2 /Ar (0.2% oxygen) mixture gas. In case of the Ba 2 FeMoO 6 / Ba 1 X Sr X TiO 3 superlttices (SLs) f ive layers of double perovs kite Ba 2 FeMoO 6 were coupled with BaTiO 3 Ba 0.5 Sr 0.5 TiO 3 of 20 layers and 25 pairs of the structure were repeated Also, bilayer (BL) structu r es of BaTiO 3 on Ba 2 FeMoO 6 and Ba 0.5 Sr 0.5 TiO 3 on Ba 2 FeMoO 6 were deposited to compare the effect of strain induced i n the different multilayer structu r es. The volume of Ba 2 FeMoO 6 layer was kept the same for all structures to exclude the dependency of magnetization on volume The 248 nm wavelength KrF laser was utilized to ablate the ceramic target. The laser energy and t he repetition rate were 1.5 J/cm 2 and 2 Hz, respectively. The growth rate is around 0.2 for Ba 2 FeMoO 6 and for BaTiO 3 and Ba 0.5 Sr 0.5 TiO 3 The crystal structure phase of the thin film, superlattices and bil a y ers were examined by X ray diffraction (Phillip s APD3720). Also the periodicities of superlattices were obtained from the X ray diffraction. Magnetization and magneto transport were respectively measured using a Quantum Design Magnetic Properties Measurement System (MPMS) and Physics Proper ties Measure ment System (PPMS).

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74 R esults and Discussion Figure 6 1 A shows the XRD pattern s of Ba 2 FeMoO 6 thin film, Ba 0.5 Sr 0.5 TiO 3 on Ba 2 FeMoO 6 bilayer, and Ba 2 FeMoO 6 /Ba 0.5 Sr 0.5 TiO 3 superlattice S ate l lite peaks of a superlattice next to ( 00 l ) peaks of the SrTiO 3 s ubstr ate clearly showed the superlattice structure. A close look at the area around ( 00 2 ) peak of the substrate revealed t he structural differences among three different structures. Ba 0.5 Sr 0.5 TiO 3 on Ba 2 FeMoO 6 bilayer showed two separate peaks for Ba 2 FeMoO 6 ( 00 4 ) and Ba 0.5 Sr 0.5 TiO 3 ( 002 ) while only Ba 2 FeMoO 6 ( 004 ) peak was observed for the thin film In addition, t he ( 004 ) peak s that shifted to high er angle from thin film to bilayer to superlattice indicate the strained state of Ba 2 FeMoO 6 layer in hetero structu r es, more in the superlattice, compared to the bilayer Fig ure 6 1 C and D show the ( 004 ) peaks of bilayers and superlattices. In case of bil a yers, only one peak was observed for BaTiO 3 on Ba 2 FeMoO 6 bilayer due to very similar lattice parameter s of BaTiO 3 (4.002 ) on Ba 2 FeMoO 6 ( 8.0 6 ) while the two separate peaks were observed for Ba 0.5 Sr 0.5 TiO 3 ( 002 ) and Ba 2 FeMoO 6 ( 004 ) Also the higher peak intensity of BaTiO 3 on Ba 2 FeMoO 6 bilayer suggests the overlapped peaks of lattice matching layers the log scale) However, ( 004 ) peak of Ba 2 FeMoO 6 layer in the two bilayers have the same position, which suggests that strain dependence on the top layer. In contras t, ( 004 ) peak of Ba 2 FeMoO 6 /Ba 0.5 Sr 0.5 TiO 3 superlattice clearly shifted more to a high an gl e than that of Ba 2 FeMoO 6 /BaTiO 3 superlattice which suggests more strained state of Ba 2 FeMoO 6 layer in the superlattce coupled with the layer with a smaller lattice p arameter of Ba 0.5 Sr 0.5 TiO 3 (3.954 ) Even with very similar lattice size of the two materials, Ba 2 FeMoO 6 /BaTiO 3 superlattice has

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75 more shifted ( 004 ) peak for Ba 2 FeMoO 6 layer than the bilayers. It clearly shows that superlattice structure can give more stai n and big ger dependence on the latti ce parameter of coupled layers. The periodicity, of the superlattice s was calculat ed using an equation (L i L j ) / 2 ( sin i j ), where, L i and L j are satellite peak indices i j are corresponding Bragg a ngles and is the wavelength of an incident X ray 110 And the results are shown in table 6 1. The periodicities obtained are 113 and 115 on average for Ba 2 FeMoO 6 /BaTiO 3 and Ba 2 FeMoO 6 /Ba 0.5 Sr 0.5 TiO 3 superlattice, respectively. Figure 6 2 shows the magnetization curves of thin film, bilayers, and superlattices. In Figure 6 2 A at 10 K, the saturation magnetization ( M S ) increased substantially for both superlattices and esp ecially B a 2 FeMoO 6 /Ba 0.5 Sr 0.5 TiO 3 superlattice showed a huge increase over 300% in M S compared to the thin film. In contrast, BaTiO 3 on Ba 2 FeMoO 6 bilyer remained similar to the thin film and Ba 0.5 Sr 0.5 TiO 3 on Ba 2 FeMoO 6 bilayer showed decreased magnetization This change of magnetization might be due to the strain induced in the hetero structu r es. It is know n that strain from substrate or in superlattice can enhance the magnetization 111 113 E xcept the Ba 0.5 Sr 0.5 TiO 3 on Ba 2 FeMoO 6 bilayer, the enhancement in magnetization has the same tendency with the amount of strain indicated by the peak shift in in X ray diffraction of hetero structures. As the strain from subs trate S r T i O 3 is common to both bilayer samples, the interface at the top of Ba 2 FeMoO 6 l ayer is believed to be responsible for the difference between the two bilayer samples. In the bilayer structure s the strain is limited only to layers at the interface a nd there i s no difference in ( 004 ) peak positions of Ba 2 FeMoO 6

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76 l ay er in the two bilayers. T he small difference of 0.7 % in lattice parameters of Ba 2 FeMoO 6 and Ba T i O 3 is not expected to affect the magnetization much which is consistent to the result that th e magnetization curves for the bilayer and thin film are very similar throughout all the magnetic field. I t is not clearly understood what suppressed the magnetization of Ba 2 FeMoO 6 layer in the bilayer with Ba 0.5 S r 0.5 T i O 3 although the lattice mismatch, 1. 9% is larger between two materials compared to the bilayer with Ba T i O 3 A coercive field decreased in superlattices while it remained almost the same for Ba T i O 3 on Ba 2 FeMoO 6 bilayer. Figure 6 2 C ) show s the magnetization curves measured at 300K. Under sma ll magnetic field, superlattces showed larger saturation magnetization (indicated by a steep slope) than thin film and BL samples However, the data point got scattered as the magnetic field increased and so it i s difficult to evaluate the magnetization ex actly The coercive field was diminished for all samples at room temperature. In Figure 6 3, the temperature dependence of magnetization was shown in both field cooled (FC) and zero field cooled (ZFC) methods. In the zero field cooled method, a sample is c ooled from room temperature without any applied magnetic field and then a small magnetic field of 100 Oe is applied at 10K. The magnetization is measured as a function of temperature during warm up under a constant field. In field cooled case, the sample i s cooled from room temperature to 10K in an applied magnetic field and magnetization is measured during warm up. The curves show that Ba 2 FeMoO 6 thin film rop sharply up to a room temperature. In contrast, all superlattice s and bilayers started to decrease from low temperatures (as low as) around 30K. Those graphs look similar in shape, but

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77 interesting difference s were observed in the irreversibility temperature (marked by the deviation between FC and ZFC c urves) The temperature was lowest around 30K for Ba 2 FeMoO 6 / Ba 0.5 Sr 0.5 TiO 3 superlattice and got higher for Ba 2 FeMoO 6 / BaTiO 3 superlattice and BaTiO 3 on Ba 2 FeMoO 6 bilayer This tendency in this specific temperature is inversely proportional to the amount o f strain in Ba 2 FeMoO 6 layer induced by the coupled layer in SL and BL samples. Therefore, it is thought that the strain is responsible for the lowered Curie temperature s and different temperature dependency of magnetization in hetero structures. Figure 6 4 shows the magneto resistance of the thin film and superlattice samples. The magnetore s istance increased slightly in Ba 2 FeMoO 6 /BaTiO 3 superlattice and remained almost same in Ba 2 FeMoO 6 /Ba 0.5 Sr 0.5 TiO 3 superlattice Clearly, the magneto resistance has not sh own the same dependence on strain induced by coupling layers in superlattice s to the M S and T C Singh et al reported the magneto electric coupl ing depends on the layer and ,in the study, only ferroelectric BaTiO 3 or Ba 0.5 Sr 0.5 TiO 3 enhanced the magneto resis tance in SLs with ( L a, C a) M n O 3 while paraelectric SrTiO 3 enhance the magneto resistance 114 Magneto capacitance measurement is useful to check the coupling between magnetic and ferroelectric materials. However, it was difficult to investigat e further because Ba 2 FeMoO 6 is conductive as half metal and even Ba 1 X Sr X TiO 3 layer is not very insulating because of the growth condition of low oxygen partial pressure. S ummary and Conclusi ons In summary, the structural, magnetic and magnetotransport pr operties were investigated in Ba 2 FeMoO 6 thin film and heterostructures coupled with Ba 1 X Sr X TiO 3 synthesized by pulsed laser deposition. The superlattice structure was confirmed by

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78 XRD analysis of satellite peaks and the relative amount of strain in Ba 2 Fe MoO 6 layer was observed by the peak shift. The strain induced enhance ment of magnetization was observed in superlattice samples in contrast to the bilayer samples which showed no enhancement or even suppression. The Curie temperatures decreases significant ly in all the heterostructues and the decrease showed the same tendency with the strain i n Ba 2 FeMoO 6 l ayer induced by coupled layers.

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79 Table 6 1. Periodicities of the superlattices calculated from positions of the satellite peaks around ( 002 ) peak of Ba 2 F eMoO 6 in Figure 6 1 E Satellite peak index BFMO/BTO SL BFMO/BSTO SL peak position (degree) Periodicity () peak position (degree) Periodicity () 2 20.49 20.61 1 21.27 115 .1 21.41 112 .2 0 22.09 109.6 22.19 115 .2 +1 22.91 109.7 22.97 115 .4 +2 23 .67 118.6 23.75 11 5.5

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80 Figure 6 1 XRD patterns of Ba 2 FeMoO 6 thin film and heterostructures Images given for A) 2 scan B ) ( 004 ) peaks of A ), C) ( 004 ) peaks of bilayers D) superlattices and E) s atellite peaks around ( 002 ) peak of Ba 2 FeMoO 6 in the superlattices. A B

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81 Figure 6 1 C ontinued D E

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82 Figure 6 2 Magnetization curves of Ba 2 FeMoO 6 thin f ilms and hetero structu r es measured at temperatures indicated A C B A B C

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83 Figure 6 3 The temperature dependence of the magnetization Ba 2 FeMoO 6 thin film and heterostructures. Images given for A) Ba 2 FeMoO 6 thin film B) BaTiO 3 on Ba 2 FeMoO 6 BL, C) Ba 2 FeMoO 6 / B aTiO 3 SL and D) Ba 2 FeMoO 6 /Ba 0.5 Sr 0.5 TiO 3 SL. A B C D

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84 Figure 6 4 Magneto resistance of Ba 2 FeMoO 6 thin film and superlattices

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85 CHAPTER 7 THE EFFECT OF STRUCTUE ON THE PROPERTIES OF B a 2 F e M o O 6 /B a 0.5 S r 0.5 T i O 3 SUPERLATTICES Introduct ory Remarks A 2 FeMoO 6 (A=Ca, S r, Ba) is a double perovskite family which is a half metallic ferrimagnet with a high Curie tem perature ( T C ) over room temperature. Sr 2 FeMoO 6 has been most studied due to a high T C among them since Kobayashi et al. demonstrated the high saturation magnetiz ation ( M S ) close to the calculated value and a huge magneto resistance in which the significant portion of it remained up to room temperature under a low magnetic field. 12 This gives substantial advantages in terms of device application over the manganite materials which also show a huge magneto resistance call ed colossal magneto resistance (CMR) but with a high magnetic field and relatively low temperature required. This ma kes Sr 2 FeMoO 6 a prime candidate for spintronic rela ted device application s 19 20 38 93 94 A simulation study showed that a long range antiferromagnetic ordering of Fe 3+ ions and Mo 5+ ions is a key aspect for achieving the high quality sample E xperimental works followed showing degradation of magnetic property by disordering, lowered M S to less than 4 B per formula unit and magneto resistance 20 25 26 36 Ba 2 FeMoO 6 is a member of the A 2 FeMoO 6 (A=Ba, Ca, Sr) family and is reported to have T C as high as 367K 19 Ba 2 FeMoO 6 has shown a huge magneto resistance of over negative 30% for polycrystalline ceramic samples in multiple r eports 22 44 With T C well over room temperature, Ba 2 FeMoO 6 can be one of the material selections for spintronic applications. However, this material has not been studied for a high quality film synthesis and its properties.

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86 In previous chapter s Ba 2 FeMoO 6 hetero structures with (Ba,Sr)TiO 3 were deposited and the properties of them were examined and compared with th ose of the thin film s In this chapter we prepared hete ro epitaxial Ba 2 FeMoO 6 /(Ba,Sr)TiO 3 superlattices on SrTiO 3 substrate by multiple target pulsed laser deposition and investigated their magnetic properties specifically focusing on the dependence on t he structure of the superlattice Experimental The Ba 2 FeMoO 6 / Ba 0.5 Sr 0.5 TiO 3 superlattices were synthesized on SrTiO 3 substrates by multiple target pulsed laser deposition. The targets were synthesized by a conventional solid state reaction with high purity (99.99% or higher) powders of BaCO 3 Fe 2 O 3 MoO 3 BaTiO 3 and SrTiO 3 The detailed procedure for target fabrication is explained with diffraction studies of the target synthesized in the chapter 3. [Ba 2 FeMoO 6 ] 5 /[ Ba 0.5 Sr 0.5 TiO 3 ] N sup erlattices were deposited at 700 C under 0.5 mTorr O 2 /Ar (0.2% oxygen) mixture gas. Five layers of double perovskite Ba 2 FeMoO 6 were coupled with Ba 0.5 Sr 0.5 TiO 3 of N layers. The 248 nm wavelength KrF laser was utilized to ablate the ceramic targets. The la ser energy and the repetition rate were 1.5 J/cm 2 and 2 Hz, respectively. The structural properties of the superlattices were examined by X ray diffraction (Phillips APD3720). Magnetization and magneto transport were respectively measured using a Quantum D esign Magnetic Properties Measurement System (MPMS) and Physics Proper ties Measurement System (PPMS). Results and Discussion Figure 7 1 A shows the XRD pattern of Ba 2 FeMoO 6 / Ba 0.5 Sr 0.5 TiO 3 5x5 superlattice Sate l lite peaks were observed along with ( 00 l ) pe aks of SrTiO 3 substrate.

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87 A close look at the ( 002 ) peak area shown in Figure 7 1 B clearly show s the decreased average lattice spacing of [Ba 2 FeMoO 6 ] 5 /[Ba 0.5 Sr 0.5 TiO 3 ] N superlattice s with increasing layers of Ba 0.5 Sr 0.5 TiO 3 indicated by peak s shift ing to a high angle In Fig 7 2 A t he average lattice spacing, d 001 of the superlattices were obtained using the zero order superlattice peaks and plotted with the calculated ones using the half of the double perosvkite Ba 2 FeMoO 6 ( 4.03 ) and Ba 0.5 Sr 0.5 TiO 3 ( 3.954 ) The larger lattice spacing of superlattices than the calculated values suggests that the layers comprising the superlattices are elongated along the c direction which is partially attributed to in plane compressive strain from the substrate SrTiO 3 (3.9 05 ). It is known that the layers comprising superlattices suffer compressive or tensile strain depending on their relative lattice parameters of the parent materials and the strain affects the magnetic and ferroelectric properties 115 117 T he difference between experimental and calculated values decreases with increasing layers of Ba 0.5 Sr 0.5 TiO 3 which suggests that the effect of strain decrease with N. Ba 2 FeM oO 6 layer in a superlattice suffers more strain with the thicker Ba 0.5 Sr 0.5 TiO 3 layer. Thus the smaller difference indicates a relaxed Ba 0.5 Sr 0.5 TiO 3 layer in thicker superlattices. In Figure 7 1B t he distances between adjacent satellite peaks decreased w ith increasing Ba 0.5 Sr 0.5 TiO 3 layer thicknesses The periodicity, of a superlattice can be calculated using an equation i L j ) / 2(sin i j ), where, L i and L j are satellite peak indices, i j are corresponding Bragg angles, and is the wavelength of X ray 110 From obtained positions of satel lite peaks around ( 001 ) of SrTiO 3 substrate, the periodicities of superlattices were calculated and

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88 summarized in T able 7 1 and plotted with the Ba 0.5 Sr 0.5 TiO 3 layer thickness, N, i n Figure 7 2 B The chemical modulation matched well with the designed structure for 5x5 and 5x10 superlattices and started to deviate from the SL 5x15. This discrepancy for thicker samples might be associated with the change of target surface state or fluc tuation of laser energy difference in periodicity with the extended ablation. Figure 7 3 shows the magnetization curves of the superlattices. In Figure 7 3 A, at 10 K, the superlattices showed the enhance d saturation magnetization ( M S ) compared to thin film s The increased M S are believed to be attributed to the strain in the superlattices. There have been reports on the strain induced enhancement of magnetization 11 1 113 5x15 and 5x20 superlattices in which Ba 2 FeMoO 6 layers experience larger strain show the larger increase in M S than 5x10 superlattice. paramagnetic b ehavior, which is not cle arly understood. All superlattices showed a smaller coercive field than the thin film and the values are almost the same. In Figure 7 3 C at 300 K, the superlattices except the 5x5 sample show the enhancement of M S similar to the behavior at 10K although t he differences among samples are less clear due to the scattered data point s around 0.3 T. I t is interesting that the 5x5 super l attice showed the saturation in magnetization and the value is slightly lowered than the thin film. One possibility for t his odd behavior of the sample is correlation between Ba 2 FeMoO 6 and Ba 0.5 Sr 0.5 TiO 3 layers. The Ba 0.5 Sr 0.5 TiO 3 layers in the 5x5 superlattice experience the largest strain by the Ba 2 FeMoO 6 layers, which might modify the property of Ba 0.5 Sr 0.5 TiO 3 layer. One would need to investigate the change with temperature in detail to understand the origin of paramagnetic to ferromagnetic transition.

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89 In Figure 7 4 the temperature dependence of magnetization was shown in both field cooled (FC) and zero field cooled (ZFC) meth ods. In the zero field cooled method, a sample is cooled from room temperature without any applied magnetic field and then a small magnetic field is applied at 10K. The magnetization is measured as a function of temperature during warm up under a constant field. In the field cooled case, the sample is cooled from room temperature to 10K in an applied magnetic field and magnetization is measured during warm up. The thin film sample has a Curie temperature over 300K as shown in Figure 7 4 A In case of the 5x5 SL sample, during warm up the magnetization increased up to around 80 K and then decreased slowly in both modes disappear up to a room temperature. As for the rest of superlattices, in contrast, the magnetization started to decrease from lo wer temperatures. And the temperatures at which the magnetization reached at max imum and started to drop in the ZFC mode decreased with increasing Ba 0.5 Sr 0.5 TiO 3 layer thickness. These results suggest that the lowered Curie temperature of the Ba 2 FeMoO 6 lay er in superlattices is attributed to the strain induced by the Ba 0.5 Sr 0.5 TiO 3 layer. It should be noted that even with the decreased magnetization at elevated temperatures, superlattices with thicker Ba 0.5 Sr 0.5 TiO 3 layer s showed larger saturation magnetiza tions at 300K A significant reduction in magnetization and T C was reported in a manganite based superlattice, in which the charge transfer at interface was believed to induce the change 118 Magneto resistances of the thin film and superlattices are shown in Figure 7 5. A l arge increase is observed in 5x5 and 5x1 0 superlattices T he enhancement diminish in 5x20 superlattices. The dependence on structure of magnetization and magneto resistance was reported in La 0. 7 Ca 0. 3 MnO 3 /BaTiO 3 superlattices, in which the strain

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90 induced in the hetero structures is maximized at a specific thickness of BaTiO 3 layer and the strains play a particular role in coupling of the two layers. 116 It is interesting that the magnetization and magneto resistance showed the same tendency with the thickness of coupled layers in that report, while magnetization and magneto resistance does not show a cl ear correlation in this study. Figure 7 6 shows the transport properties of the thin film and superlattices. In Figure 7 6A t he resistivity decreases with increasing Ba 0.5 Sr 0.5 TiO 3 layer thickness which suggests that Ba 0.5 Sr 0.5 TiO 3 l ayer is not insulating due to the low oxy gen pressure during deposition. Also, the charge transfer at the interface might increase during an extended growth for thick Ba 0.5 Sr 0.5 TiO 3 l ayers. In figure 7 6B, all superlattices show anomalous Hall effect behaving hole like indicate d by the positive slope unde r a low magnetic field and show ordinary Hall effect behaving electron like under a large magnetic field. The anomalous portion increase s with increas ing thickness of Ba 0.5 Sr 0.5 TiO 3 layer that indicates the enhanced magnetizatio n although the difference is not clearly seen in the magnetization curves measured at 300K in Figure 7 3C. Summary and Conclusions In summary, the structural and magnetic properties were investigated in Ba 2 FeMoO 6 thin film and [Ba 2 FeMoO 6 ] 5 /[Ba 0.5 Sr 0.5 TiO 3 ] N (5xN) superlattices prepared by pulsed laser deposition. The structural changes of superlattice s were confirmed by XRD analysis with changes in the average lattice spacing The magnetic properties of Ba 2 FeMoO 6 layer significantly depend on the structure of the superlattices. At both 10 K and 300 K, superlattice s with relatively thick Ba 0.5 Sr 0.5 TiO 3 showed strain induced enhance ment of magnetization. A significant reduction in the Curie temperatures was observed and the magnitude of reduction increased with the strain. Interestingly,

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91 magneto resistance showed difference dependency on the st ructure from the magnetization, enhancement in superlattices with thin Ba 0.5 Sr 0.5 TiO 3 layers. The decreased resistivity of superlattices with thick Ba 0.5 Sr 0.5 TiO 3 layer su ggests the leaky characteristic of the ferroelectric layer under the growth condition.

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92 Table 7 1. Periodicities of the superlattices calculated from positions of the satellite peaks around ( 002 ) peak of Ba 2 FeMoO 6 in Figure 7 1B Sample BFMO/BSTO 5x5 BFMO /BSTO 5x10 BFMO/BSTO 5x15 BFMO/BSTO 5x20 Superlattice peak index Periodicity () Periodicity () Periodicity () Periodicity () SL 2 SL 3 79.9 109.3 SL 1 SL 2 60.5 80.1 96.5 108.1 SL 0 SL 1 60.7 80.2 95.5 108.3 SL +1 SL 0 60 80.4 95.8 108.4 SL +2 SL +1 59.4 80.5 95.9 108.6 SL +3 SL +2 80.7 96.1 107.5

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93 Figure 7 1 XRD pattern of Ba 2 FeMoO 6 /Ba 0.5 Sr 0.5 TiO 3 (5xN ) s uperlattice s. Images given for A) 5x5 superlattice and B ) satellite peaks around ( 001 ) peak of substrate SrTiO 3 STO (002) STO (003) STO (001) A STO (001) SL 0 SL 1 SL +1 SL +2 SL 2 B

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94 Figure 7 2 Lattice parameters and periodicities of Ba 2 FeMoO 6 /Ba 0.5 Sr 0.5 TiO 3 (5xN) superlattices. Images given for A ) d 001 lattice spacing and difference between experimental and calculated ones. d cal and d exp are calculated value from referenc es of bulk polycrystalline materials and experimental vales from XRD measurement, respectively. B ) P eriodicities of s uperlattices. A B

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95 Figure 7 3. Magnetization curves of Ba 2 FeMoO 6 thin film and s uperlattices measured at temperatures indicated A B C

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96 Figure 7 4. Temperature dependence of the magnetization of Ba 2 FeMoO 6 thin film and s uperlattices A B C D E

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97 Figure 7 5. Magnetoresistance of Ba 2 FeMoO 6 thin film and s uperlattices measured at 10K

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98 Figure 7 6 Transport properties of Ba 2 FeMoO 6 /Ba 0.5 Sr 0.5 TiO 3 (5xN) superlattices. Images given for A) t emperature dependent r esistivity and B) Hall resistivity of measured at 300K A B

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99 CHAPTER 8 THE EFFECTS OF Z n O BUFFER LAYERS ON THE PROPERTIES OF PHOSPHORUS DOPED Z n O THIN FILMS GROWN O N SAPPHIRE BY PULSED LASER DEPOS ITION Introduct ory Remarks Zinc oxide has attracted significant interests largely due to its potential utility as a wide gap semiconductor for opto electronics 71 119 121 Research on ZnO thin film transistors and transparent conducting oxides have been driven by increasing demands for transparent electronics and photovoltaics with other competing semiconducting tr ansparent oxide materials 122 125 Significant investment of effort has been made to realize highly reproducible high quality p type ZnO thin film which is cr ucial to expanding the applications of ZnO based electronics 79 91 126 128 Also, there have been recent reports of p type doping in nanowires and nanoparticles 129 133 In spite of some successful research related to p type doping and light emitting devices 91 134 135 there remain significant challenges to understanding and con trolling on the mechanisms for acceptor doping and the roles of related defects in ZnO 131 136 The majority of studies investigating acceptor doping in ZnO have employed sapphire as the substrate. The large lattice mismatch between substrate and film induces formation of defects that degrade the crystalli ne quality may inhibit the realization of p type doping. There is some debate on the roles of the different defects on doping 136 137 The native defects induced by the lattice mismatch during the deposition can compensate the acceptor dopants. There have been some studies on the use of undoped ZnO buffe r layers in the growth of for p type ZnO thin films 91 Previous studies suggest that the buffer layer has significant effects on the properties of doped layer subsequent. In this study, phosphorus doped ZnO thin films were deposited

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100 on ZnO buffer layers that were grown under different conditions. Their structure and transport properties were analyzed in order to elucidate the role o f the buffer layers. E xperimental Undoped and phosphorus doped ZnO thin films were synthesized using pulsed laser deposition with ceramic targets synthesized with high purity powders. Phosphorus was chosen as a dopant for p type doping. The targets were m ade by the conventional solid state reaction and the starting materials were ZnO and P 2 O 5 The powders were mixed with stoichiometric compositions of 0.1 and 0.5 mole percent of phosphorus and were pressed into one inch diameter pellets. The pellets were sintered for four hours at 1200C in air. C plane sapphire was used as the substrate. A 248 nm KrF laser was utilized. The laser energy density and the repetition rate are about 1.5 J/cm 2 and 1 Hz, respectively. The chamber was pumped down to the base pres sure of low 10 7 Torr. Two types of buffer layers were considered. For one set, a low temperature buffer layer was deposited. In particular, the growth conditions for this undoped ZnO buffer layer were 400C under 20 mTorr O 2 For a second set, a high tem perature buffer layer was selected. In this case, the undoped ZnO buffer layer was deposited at 700C under 1 mTorr O 2 T he thickness of buffer layer wa s 50 nm for both temperatures. And the thick buffer layers were additionally grown to observe the verti cal microstructures. Phosphorus doped ZnO thin films were then deposited on th e buffer layers at 700 C under oxygen atmosphere with the pressure range between 1 0 150 mTorr. The thicknesses of phosphorus doped ZnO layers were at least 600 nm to minimize the contribution of buffer layers. After growth, samples were cooled down under the same oxygen atmosphere to room temperature. After the deposition, the films were rapid thermally annealed at 900C for three minutes under oxygen atmosphere.

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101 Field emission s canning electron microscopy (JEOL JSM 6335F) and atomic force microscopy (Veeco Dimension 3100) were utilized to observe the microstructure and surface of the samples. X ray diffraction ( ) was utilized to analyze the c rystalline qualities of thin films. Hall effect measurement ( Lakeshore 7507 Hall Effect) was carried out to investigate the transport properties of phosphorus doped ZnO thin films including carrier type, resistivity, carrier concentration, and mobility. R e sults and Discussion Table 8 1 shows the Hall transport properties of ZnO:P 0.001 thin films. As for the films grown on high temperature buffer layer, as deposited ZnO:P 0.001 thin films showed n type conductivity which is consistent with previous efforts on ZnO:P 0.005 thin films showing that phosphorus can induce either donor or acceptor states in ZnO 81 126 After the rapid thermal annealing, the films grown on the high temperature buffer layer showed reduced electron carrier concentrations and increased resistivity but remained n type. The relatively small change of mobil ity compared to resistivity and carrier concentration suggests that the rapid thermal anneal affected the defect structure associated with phosphorus rather than the overall crystallinity of the film. Note that this change by annealing does not appear to b e associated with defects solely related to Zn and O. Previous reports showed that the transport properties of undoped ZnO thin films change little with this type of rapid thermal annealing treatment 81 In contrast, the as grown phosphorus doped ZnO films deposited on low temperature buffer layers show n type conductivity but have much larger resistivity than films on high temperature buffer layer. After annealing, the film grown under high oxygen pressure of 150 mTorr showed p type conduct ion while the sample grown under

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102 10 mTorr remained n type. It should be noted that the transport properties of annealed samples are difficult to quantify with Hall effect measurements due to contributions from the semi insulating phosphorus doped ZnO and t he low temperature undoped ZnO buffer layer. Despite the difficulty in quantifying carrier concentration, it is noted that the increases of resistivity are much larger in the films on the low temperature buffer layer than for films on high temperature buff er layers. Considering that the rapid thermal annealing primarily affects defects associated with phosphorus, the larger change in resistivity suggests that more dopants are incorporated during deposition and converted by the rapid thermal annealing in the films on low temperature buffer layer. It suggests that the poorer crystalline quality of the low temperature buffer layer induces a higher defect density in the subsequent phosphorus doped film, acting as favorable sites for defect incorporation related to the phosphorus dopants. Also, it is clear that a low electron carrier concentration of as grown state is important in converting the carrier type by the rapid thermal annealing. Note that after annealing, the mobility slightly increased in the films gro wn on high temperature buffer layers in contrast to the decreased mobility in the films on low temperature buffer layer. It is not entirely clear what yields the different tendencies. Figure 8 1 shows the omega rocking curves of ZnO ( 002) peaks for ZnO:P 0. 005 thin films grown on different types of buffer layers. The diffraction results clearly show broader peaks rocking curves, and hence higher defect density, for phosphorus doped ZnO thin films grown on the low temperature buffer layer as compared to the h igh temperature buffer layer. It is assumed that this reflects a higher concentration of phosphorus incorporation in the poorer crystalline quality films. Also, the oxygen

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103 pressure during deposition has a significant effect on the crystalline quality of th e films for those grown on the low temperature buffer layer. Films deposited on the high The surface microstructure was also examined using atomic force microscopy and scanning electron microscopy. Again, the use of a high temperature appears to induce a lower density of defects. From Figure 8 2 and 8 3, it is shown that the films grown on high temperature buffer layer have smoother film surfaces and less particulates. I n addition, the low oxygen pressure during film deposition leads to the smooth surface. It is noted that oxygen deficient growth condition may lead to more donor defects including oxygen vacancies that can act a donor defect in ZnO thin films. This can be seen from the table which shows that phosphorus doped ZnO thin films grown under low oxygen pressure are more conductive when grown on the same buffer layer. Another interesting feature is the microstructure of the ZnO:P 0.00 1 thin films. Figure 8 4 shows the cross section images of ZnO:P 0.001 thin films. The structures of the films are very different on each buffer layer. After comparing with the microstructures of the buffer layers, it is found that the microstructures of ZnO:P 0.001 thin films are mainly determined by the structures of buffer layers. Summary and C onclusions In conclusion, we have investigated the structure and transport property of phosphorus doped ZnO thin films by PLD grown on different buffer layers. The films grown on low temperature buffer layer are more resistive with a low carrier concentration and show significant changes in electronic properties by the rapid thermal annealing. It suggests that more dopants are incorporated in the films on low crystalline quality buffer layer grown at low temperature during deposition. In spite that a high

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104 temperature buffer is not appropriate for p type doping, it can be used to improve the crystalline quality and surface morphology of doped ZnO layer.

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105 Table 8 1. Hall transport properties of as grown and rapid thermal annealed ZnO:P 0.001 thin films As grown Annealed As grown Annealed A) 1 50 mTorr O 2 Carrier Concentration (cm 3 ) 1.29E+19 3.04E+17 1.83E+18 3.31E+15 Mobility (cm 2 /Vs) 3.24E+01 3.87E+01 1.07E+00 5.31E 01 cm) 1.50E 02 5.30E 01 3.19E+00 3.56E+03 Carrier Type n n n p Buffer layer on HTB on LTB 1 0 mTorr O 2 Carrier Concentration (cm 3 ) 3.51E+19 6.11E+17 5.18E+18 3.44E+14 Mobility (cm 2 /Vs) 4.54E+01 5.61E+01 7.67E+00 1.76E+00 Resistivit 3.91E 03 1.82E 01 1.57E 01 1.03E+04 Carrier Type n n n n Buffer layer on HTB on LTB

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106 Figure 8 1 Rocking curves of (002) peaks of ZnO:P 0.005 thin films grown on the buffer layers under oxygen pressures indicated A B

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107 Figure 8 2. AFM images of ZnO:P 0.005 thin films grown on the buffer layers under oxygen pressures indicated. RMS roughness values are 13.3, 11.4, 10.2, and 4.84 nm from A ) to D ). A C B D

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108 Figure 8 3 SEM images of ZnO:P 0.005 films grown on the buffer layers under oxygen p ressures indicated A C B D

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109 Figure 8 4 Cross section images of ZnO:P 0.001 thin films and buffer layers grown at 700 C under 10 mTorr O 2 Images given for A) ZnO:P 0.001 thin films grown on low temperature buffer layer B ) ZnO:P 0.001 thin films grown on hig h temperature buffer layer C ) low temperature buffer and D) high temperature buffer A C B D

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110 CHAPTER 9 CONCLUSIONS Ba 2 FeMoO 6 Thin Film and Hetero structures Double perovskite Ba 2 FeMoO 6 thin films were grown under various growth conditions by pulsed laser deposit ion and the structural, magnetic and magneto transport properties were investigated, focusing on the effects of growth temperature and oxygen partial pressure. Phase pure Ba 2 FeMoO 6 thin films were epitaxially grown on SrTiO 3 substrates at temperatures rang ing from 700 900C under high vacuum while an impurity BaMoO 4 phase coexisted when the films were deposited with pure oxygen gas. The thin films grown at high temperature showed a better saturation magnetization ( M S ) and magneto resistance but its Curie te mperature ( T C ) is low, around 250K compared to the previous bulk studies. The low M S (1.1 B /f.u. for 900 C sample) and T C (around 250K) of thin films were believed to due to the antisite defects, i.e., disordering of Fe and Mo atoms in B sties. Also Ba 2 FeMoO 6 thin films showed interesting transport properties; a change in conduction mechanism s with temperature and anomalous Hall effect. The s amples showed an anomalous Hall effect with hole like behavior under low magnetic field and showed an ordinary electron like transport property under high magnetic field. The portion of anomalous Hall effec t decreased due to increasing electron carriers at elevated temperatures. When thin films were grown under low oxygen partial pressure with O 2 /Ar mixture gas, the magnetic properties were degraded but the Curie temperature increased. Auger electron spectro scopy (AES) study revealed that the ratio of Fe:Mo got further from one the ideal stoichiometry wi th increasing oxygen pressure during deposition which is consistent with XRD results that showed the smaller lattice spacing of the thin

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111 films grown under higher oxygen pressure. This increased off stoichiometry was responsible for the degradation of magnetization of the films grown under high oxygen partial pressure. Also unusual positive magneto resistance observed in films grown under high vacuum disappea red, which suggests that it is associated w ith the oxygen related defects. The Ba 2 FeMoO 6 based hetero structures with Ba 1 x Sr x TiO 3 were synthesized and their structures and magnetic properties were examined. The superlattice structures were confirmed by sa tellite peaks and the periodicities were calculated. The Ba 2 FeMoO 6 / Ba 1 x Sr x TiO 3 superlattices showed the strain induced enhancement of magnetization compared to the thin film and the enhancement was dependent on the strain induced by coupled layer. The Cur ie temperatures of superlattices were lowered substantially. The effects of superlattice structure were investigated with the samples having varying Ba 1 x Sr x TiO 3 layer thicknesses. The saturation magnetization was enhanced in superlattices with thick Ba 1 x Sr x TiO 3 layers at 10K and 300K. The Curie temperatures of superlattices were suppressed significantly by strain induced in the superlattice structures Interestingly, the enhancement of magneto resistance was observed in the superlattices with the thin Ba 0 .5 Sr 0.5 TiO 3 layer unlike the magnetization. The decreased resistivity of superlattices with thick Ba 0.5 Sr 0.5 TiO 3 layer indicates the ferroelectric layer is undesirably leaky under the growth condition. The change in the portion of anomalous Hall effect is consistent with the change in magnetization induced by the strains in superlattices. p type Doping in ZnO Thin Films Phosphorus doped ZnO thin films were deposited on two different types of ZnO buffer layers; high temperature buffer layer (HTB) and low tem perature buffer layer

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112 (LTB) were deposited at 700C under 1mTorr O 2 and 400C under 20 mTorr O 2 respectively. The properties of phosphorus doped ZnO thin films were investigated focusing on the the effects of f buffer layers. Phosphorus doped ZnO thin fil ms on LTB were less conductive as grown state and showed a drastic change in the transport properties upon the rapid thermal annealing. This suggests that poor crystalline quality of the LTB lead to the more incorporation of phosphorus dopants in following films. The p type conduction was observed only in the phosphorus doped thin film s grown on LTB. All the phosphorus doped ZnO thin films grown on HTB showed n type conduction regardless of the growth condition. A high resolution X ray diffraction study rev ealed that phosphorus doped ZnO thin films grown on LTB had the poor crystalline quality indicated by much broader peaks of omega rocking curves. Also it was observed that the microstructures of the phosphorus doped ZnO thin film significantly depended on the buffer layer. HTB might be useful to improve the surface morphology and crystalline quality, although it is not proper for the incorporation of dopants.

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121 BIOGRAPHICAL SKETCH Af ter graduating from Susung High School in 1996, Kyeong Won Kim attended Seoul National University He received his B achelor of S cience and M aster of S cience in m aterials s cience and e ngineering in 2000 and 2002 respectively During the master studies, h e conducted research on sintering and microstructure of lead zirconate titanate ceramics under the supervision of Dr. Doh Yeon Kim And then he joined Republic of Korea Air Force in 2003 After finishing three year service as a maintenance officer in 2006 he worked for LG Electronics Institute of Technology where he conducted research on thin film solar cells He chose to pursue his doctoral degree He began his graduate studies at University of Florida in 2007 and also joined Dr. David Norton group He received his Ph.D. from the University of Florida in the summer of 2012. His research interests include the synthesis of oxide thin films and heterostructures and doping in ZnO thin films for novel applications in electronics and photonics. When not in the lab, he enjoy s reading, running, and playing and watching ball games