HALF METALLIC Fe3O4: AN EXPERIMENTAL STUDY ON IMPURITY DOPING AND THE GIANT MAGNETORESISTANCE EFFECT DEBASHISH TRIPATHY NATIONAL UNIVERSITY OF SINGAPORE 2007 To my father… HALF METALLIC Fe3O4: AN EXPERIMENTAL STUDY ON IMPURITY DOPING AND THE GIANT MAGNETORESISTANCE EFFECT DEBASHISH TRIPATHY (B. Eng.(Hons.), BITS Pilani, India) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2007 It is the possibility of having a dream come true that makes life interesting. – “The Alchemist” by Paulo Coelho Acknowledgements I feel deeply indebted to several people who have contributed in different ways towards the work accomplished in this thesis. First and foremost, I would like to express my sincerest gratitude towards my supervisor, Assoc. Prof. Adekunle Adeyeye for giving me the opportunity to work on this exciting topic. His constant motivation, support, guidance and encouragement in all aspects varying from research to personal life, have made my candidature a truly enriching experience. I would also like to acknowledge Dr. Christopher Boothroyd and Dr. Santiranjan Shannigrahi from Institute of Materials Research and Engineering (IMRE), Dr. S. N. Piramanayagam from Data Storage Institute (DSI), and Dr. Xingyu Gao from Singapore Synchrotron Light Source (SSLS), for their professional help and enlightening discussions regarding my experimental work. I would also like to express my appreciation towards Ms. Kelly Low and Dr. Jixuan Zhang from Department of Materials Science and Engineering for their invaluable help in letting me use the TEM sample fabrication and imaging facilities. I would like to acknowledge the help of Ms. Van Li Hui from Faculty of Science for SQUID measurements and also like to thank the ISML lab officers, Ms. Loh Fong Leong and Mr. Alaric Wong, and Ms. Ah Lian Kiat from MOS Device Laboratory, for their help and support during the last four years. During the course of my PhD, I have had the privilege of working closely with the students and staff in Assoc. Prof. Adekunle’s group, which has benefited me immensely. I would like to acknowledge Goolaup for always being such a great companion and helping me out with the cryostat on innumerable occasions. I would i Acknowledgements also like to thank Chenchen, Yunsong, Wangjin and Kiam Ming for all the enjoyable moments we have shared in ISML. Outside the group, I would like to mention my friends “CX” Xingzhi, Sreenivasan, Lalit, and Saurabh for always making the lab a fun place to work. I would like to acknowledge the NUS research scholarship, and NUS Research Grant No. R263-000-283-112 for providing financial support to this project. I would like to thank my entire family in India for all their support, faith and advice during my stay in Singapore. I especially owe this thesis to my late father who always believed in me, and who has been the source of inspiration, perseverance, and determination for all my endeavours. Finally, but most importantly, I would like to mention my pillar of strength; my wife Shikha. For proof reading this thesis and for your unwavering help, emotional support and understanding in all matters; in the lab and at home – thank you so much! ii Table of Contents Acknowledgements Table of Contents Summary i iii viii List of Tables x List of Figures xi List of Symbols and Abbreviations xvii Statement of Originality xxi Chapter Introduction 1.1 Background 1.2 Motivation 1.3 Focus of Thesis 1.4 Organization of Thesis References Chapter Theoretical Background 11 2.1 Introduction 11 2.2 Half Metals 11 2.2.1 Spin Polarization and Band Structure of Ferromagnetic Metals 11 2.2.2 Band Structure and Classification of Half Metals 13 2.3 Crystal Structure of Fe3O4 16 2.3.1 Inverse Spinel Structure and the Verwey Transition 16 2.3.2 Magnetic Ordering in Fe3O4 18 2.3.3 Antiphase boundaries in Fe3O4 19 iii Table of Contents 2.3.4 Substitutions in the Spinel Structure 2.4 Spin Dependent Transport Phenomenon 21 22 2.4.1 Anisotropic Magnetoresistance Effect 22 2.4.2 Giant Magnetoresistance Effect 24 2.5 Coupling Mechanism in Multilayer Films 26 2.5.1 Interlayer Exchange Coupling 27 2.5.2 Pin Hole Coupling 28 2.5.3 Néel Coupling 28 2.6 Granular Films 29 2.7 Summary 31 References 32 Chapter Experimental Techniques 37 3.1 Introduction 37 3.2 Fabrication Processes 37 3.2.1 Ultraviolet Photolithography 37 3.2.2 Deposition Techniques 39 3.2.3 Lift-off and Wire Bonding 40 3.3 Structural Characterization Techniques 41 3.3.1 X-ray Diffraction 41 3.3.2 X-ray Photoelectron Spectroscopy 43 3.3.3 Transmission Electron Microscopy 44 3.4 Magnetometry Techniques 47 3.4.1 Vibrating Sample Magnetometer 47 3.4.2 Superconducting Quantum Interference Device Magnetometer 49 iv Table of Contents 3.5 Magnetoresistance and Electrical Measurements 52 3.5.1 Room Temperature Setup 52 3.5.2 Low Temperature Setup 53 3.6 Summary 54 References 56 Chapter Microstructure and Magnetotransport Properties of Impurity 57 Doped Fe3O4 Films 4.1 Introduction 57 4.2 Sample Preparation 58 4.3 Structural Properties of Co doped Fe3O4 Films 58 4.3.1 X-Ray Diffraction Measurements 58 4.3.2 X-Ray Photoelectron Spectroscopy Analysis 60 4.3.3 Transmission Electron Microscopy Analysis 62 4.4 Magnetic Properties of Co doped Fe3O4 Films 63 4.5 Transport Properties of Co Doped Fe3O4 Films 67 4.6 Magnetoresistance Behavior of Co Doped Fe3O4 Films 70 4.6.1 MR of undoped Fe3O4 films 70 4.6.2 MR of 17% Co doped Fe3O4 films 73 4.7 Structural Properties of Cu doped Fe3O4 Films 74 4.7.1 X-Ray Diffraction Measurements 74 4.7.2 Transmission Electron Microscopy Analysis 75 4.8 Magnetic Properties of Cu doped Fe3O4 Films 77 4.9 Transport Properties of Cu Doped Fe3O4 Films 79 4.10 Magnetoresistance Behavior of Cu Doped Fe3O4 Films 80 v Table of Contents 4.11 Summary 82 References 84 Chapter Giant Magnetoresistance in Fe3O4/Cu/Ni80Fe20 Spin Valve 86 Structures 5.1 Introduction 86 5.2 Sample Preparation 87 5.3 Structural Properties 89 5.4 Magnetic Properties 91 5.5 CIP Magnetoresistance 94 5.5.1 Room Temperature Longitudinal and Transverse MR 94 5.5.2 Room Temperature Giant Magnetoresistance 98 5.5.3 Temperature Dependence of GMR 5.6 CPP Magnetoresistance 101 104 5.6.1 Room Temperature CPP GMR 104 5.6.2 Temperature Dependence of CPP GMR 108 5.7 CIP versus CPP Configuration 111 5.8 Summary 112 References 114 Chapter Magnetic and Giant Magnetoresistive Properties of an all 116 oxide Fe3O4-Al2O3 Granular System 6.1 Introduction 116 6.2 Sample Preparation 117 6.3 Structural and Chemical Properties 118 6.3.1 X-ray Diffraction Measurements 118 vi Table of Contents 6.3.2 Transmission Electron Microscopy Analysis 119 6.3.3 X-Ray Photoelectron Spectroscopy Analysis 120 6.4 Magnetic Measurements 122 6.5 Transport Properties 125 6.5.1 Resistance as a function of Temperature 125 6.5.2 Current – Voltage Characteristics 128 6.6 Tunneling Giant Magnetoresistance 130 6.6.1 Room Temperature Magnetoresistance 130 6.6.2 Temperature Dependence of Tunneling GMR 134 6.7 Summary 135 References 136 Chapter Conclusion and Outlook 139 7.1 Overview 139 7.2 Summary of Results 140 7.3 Recommendations for Future Work 142 References 145 Appendix 146 vii VI Magnetic and Giant Magnetoresistive Properties of an all oxide Fe3O4-Al2O3 Granular System insulator granular films such as Fe-Al2O3 and Fe-SiO2, it assumes significance for the Fe3O4-Al2O3 granular films due to the rapid decrease in the spin polarization of Fe3O4 with increasing temperature due to finite-temperature spin disorder, thermally activated spin mixing and magnon and phonon effects [33]. Moreover, the strong decrease of the GMR ratio with increasing temperature may also be due to spin-flip scattering [34], which is caused by magnetic impurities in the tunnel barriers or by the excitation of bulk magnons. As temperature increases, the probability of spin-flip scattering increases and thus the GMR ratio also decreases. 6.7 Summary A systematic study of the structural, magnetic and magnetotransport properties of an all oxide Fe3O4-Al2O3 granular system prepared by cosputtering has been presented in this chapter. XRD and TEM analyses comprehensively show that with increasing Al2O3 content, the Fe3O4 grain size reduces drastically, and the grains distribute in the amorphous Al2O3 matrix to form a well defined granular structure. Magnetic properties of the granular films closely follow the structural changes with coercivity and remanent magnetization of the films decreasing with increasing Al2O3 content and the magnetization curves gradually changing from ferromagnetic to superparamagnetic. Temperature dependent resistivity measurements and highly nonlinear I-V characteristics confirm that the spin dependent tunneling of electrons featured by log R ∝ T−1/2 dominates the transport properties of the films. This tunneling transport causes an isotropic GMR effect in the granular films, the magnitude of which decrease with increasing Al2O3 content due to increase in the tunnel barrier width. The GMR effect also exhibits strong temperature dependence and decreases with increasing temperature for all the granular films. 135 VI Magnetic and Giant Magnetoresistive Properties of an all oxide Fe3O4-Al2O3 Granular System References [1] R. A. de Groot, and K. H. J. Buschow, J. Magn. Magn. Mater. 54-57, 1377 (1986). [2] X. 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B 58, 9212 (1998). 138 Chapter Conclusion and Outlook 7.1 Overview This thesis provides a detailed insight into the effects of impurity doping on the microstructure and magnetotransport properties of half metallic Fe3O4 films, and subsequently investigates the giant magnetoresistance (GMR) phenomenon in Fe3O4 incorporated spin valve multilayers and granular systems. The key issues which have been addressed in this thesis are: • Modification of the structural, magnetic and magnetoresistance (MR) behaviour of Fe3O4 films as a consequence of impurity doping; the modified properties depending on the kind of impurity, and the preferential occupation of octahedral or tetrahedral sites in the inverse spinel structure. • Understanding the magnetization reversal process in Fe3O4 based spin valve structures and a comparison of the GMR effect in such structures for both current-in-plane (CIP) and current-perpendicular-to-plane (CPP) configurations. • Exploring the GMR effect and characterizing the structural and magnetic properties of granular films consisting of highly spin polarized Fe3O4 grains embedded in an insulating Al2O3 matrix. These issues have been addressed by the fabrication of various Fe3O4 films and structures by magnetron sputtering and systematic characterization of the microstructure, electronic and magnetic properties, and temperature dependent MR behaviour. This chapter summarizes the main results which have been presented in the thesis and also provides recommendations for further work in this area. 139 VII Conclusion and Outlook 7.2 Summary of Results Firstly, Co and Cu doped Fe3O4 films were fabricated using the cosputtering technique. Structural investigations of the impurity doped films revealed that the films were polycrystalline in nature with a preferred growth direction, while retaining the cubic spinel structure of Fe3O4. It was also observed that the doped films consist of a large density of grain boundaries. These films were characterized further by studying the temperature dependence of resistance, which shows a smooth behaviour without any discontinuity. It was concluded that the transport properties are dominated by electron tunneling across antiferromagnetically (AF) coupled grain boundaries. The magnetic properties of the doped films were markedly sensitive to the impurity doping concentration with both saturation magnetization and coercivity increasing with increasing doping concentration. The increase in saturation magnetization was attributed to a distribution of Co and Cu ions on both tetrahedral and octahedral interstitial sites in the spinel structure. The MR behaviour for the undoped Fe3O4 films showed linear magnetic field dependence for in-plane measurements and quadratic dependence for out-of-plane measurements, which is in good agreement with the model of spin dependent tunneling across AF coupled grain boundaries. In contrast, the in-plane MR curves for doped Fe3O4 films deviated from the linear field dependence due to the high cubic anisotropy in the doped films. Moreover, the MR ratio of the doped films decreased with increasing doping concentration. The temperature dependence of MR ratio for the doped films showed that the MR ratio decreases with increasing temperature due to reduction in the spin polarization of the films and enhanced probability of spin flipping processes. This was followed by a systematic study of Fe3O4 (60 nm)/Cu(tCu)/Ni80Fe20 (15 nm) spin valve structures, which exhibit marked changes as the thickness of the Cu 140 VII Conclusion and Outlook spacer layer tCu was varied in the range nm≤tCu≤30 nm. It was observed that for tCu = nm, the Ni80Fe20 and Fe3O4 layers are strongly exchange coupled. This results in AMR effect dominating the transport properties of the spin valve structures. For tCu≥5 nm however, the interlayer exchange coupling between the Ni80Fe20 and Fe3O4 layers decays rapidly, thereby causing a positive GMR effect in the spin valve structures. The CIP GMR effect is primarily attributed to spin dependent electron scattering at the Fe3O4/Cu and the Cu/Ni80Fe20 interfaces, while the CPP GMR has additional contributions from bulk scattering of electrons within the ferromagnetic layers. At room temperature, the GMR ratio was found to decrease monotonically with increasing tCu for both CIP and CPP configurations. Moreover, it was also observed that for a fixed tCu, both GMR ratios increase with increasing Fe3O4 layer thickness. The temperature dependence of GMR ratio was also studied and it was observed that for both geometries, the GMR ratio decreases monotonically with increasing temperature due to reduction in the spin polarization of Fe3O4, enhanced spin flip scattering and electron magnon interactions. The CPP GMR results are in good agreement with the Valet-Fert model for the long spin diffusion length limit. Finally, a comparison between CIP and CPP GMR measurements across varying temperatures and tCu showed that the magnitude of GMR is always greater in the CPP configuration. The study of GMR effect in Fe3O4 was then extended to granular films consisting of Fe3O4 grains dispersed in an insulating Al2O3 medium. Detailed structural analysis for the granular films confirmed that the Fe3O4 grain size decreases drastically with increasing Al2O3 content in the films. The magnetic properties of the granular films were directly influenced by these structural changes, with both coercivity and remanent magnetization decreasing with increasing Al2O3 content. Moreover, it was also observed that as the Al2O3 content in the granular films was 141 VII Conclusion and Outlook increased, the magnetization curves gradually changed from ferromagnetic to superparamagnetic like Langevin curves. Temperature dependent resistivity measurements featured by log R ∝ T−1/2, and highly nonlinear current-voltage characteristics represented by Simmon’s expression, confirmed the spin dependent tunneling transport of electrons across insulating tunnel barriers. This tunneling conductance depends on the relative orientation of magnetization between Fe3O4 grains, and thus causes an isotropic tunneling GMR effect in the granular films. It was observed that the GMR ratio decreases with increasing Al2O3 content due to increased tunnel barrier width and interactions with magnetic impurities in the barriers. Moreover, the GMR ratio also exhibits strong temperature dependence and decreases with increasing temperature for all the granular films. 7.3 Recommendations for Future Work Normally, the spacer layer materials for spin valves and tunnel junctions are nonmagnetic metals and insulating oxides respectively. Recently, there has been considerable interest in making magnetoelectronic devices that combine ferromagnetic materials with organic molecules for “organic spintronics” [1-6]. Of particular interest are -conjugated organic semiconductors (OSE), which have extremely weak spin orbit interactions and weak hyperfine interactions, thus making the spin diffusion length extremely long [7]. These properties make them ideal candidates for spin polarized electron injection and transport applications. As an extension of the study on spin valve structures presented in chapter 5, the device schematic illustrated in Fig. 7.1 is extremely attractive for further investigations. The organic spacer layer is chosen to be the small -conjugated molecule 8-hydroxy-quinoline aluminium (Alq3), which is a commonly used material in organic light emitting diodes (OLED) [8]. Alq3 has 142 VII Conclusion and Outlook excellent chemical and thermal stability, and can be easily deposited as thin films under high vacuum by sublimation techniques. A combination of the highly spin polarized properties of Fe3O4 and the organic properties of Alq3, integrated into a single device can initiate a variety of exciting new applications in organic spintronics. V+ V– FM2 Ni80Fe20 Organic Fe3O4 Substrate Alq3 I– I+ FM1 Fig. 7.1 Schematic representation of a spin valve structure that consists of two ferromagnetic electrodes separated by an organic spacer layer. Another possible study involves combining half metallic Fe3O4 and multiferroic materials [9], into heterostructures for spintronic applications. Multiferroics are an emerging class of magnetic materials in which several ferroic orders (magnetic and electric) coexist, with some coupling between them (the magnetoelectric effect) [10]. Most of them are ferroelectric and antiferromagnetic, a notable exception being BiMnO3 which is ferromagnetic [11]. By taking advantage of the magnetoelectric effect occurring in insulating antiferromagnetic multiferroic films, the electric field induced magnetization provides control of the magnetic pinning in exchange biased spin valve structures or tunnel junctions. As shown in Fig. 7.2(a), the multiferroic film is used as a barrier in tunnel junctions. Such tunnel barriers are attractive because of the intrinsic large electric field occurring in these layers. For the 143 VII Conclusion and Outlook exchange biased spin valve structures shown in Fig. 7.2(b), the multiferroic thin film is used as a tunable pinning bottom layer. Such device configurations allow for pure electrical control of the magnetic configurations in spin valves and tunnel junctions. (a) (b) U V FM2 (Ni80Fe20) V FM2 (Fe3O4) Multiferroic Film FM1 (Fe3O4) NM (Cu) FM1 (Fe3O4) Multiferroic Film Fig. 7.2 Schematic illustration of devices involving multiferroic films as (a) junction barrier, and (b) pinning layer. 144 VII Conclusion and Outlook References [1] E. Arisi, L. Bergenti, V. Dediu, M. A. Loi, M. Muccini, M. Murgia, G. Ruani, C. Taliani, and R. Zamboni, J. Appl. Phys. 93, 7682 (2003). [2] A. H. Favis, and k. Bussmann, J. Appl. Phys. 93, 7358 (2003). [3] P. A. Dowben, and R. Skomski, J. Appl. Phys. 95, 7453 (2004). [4] J. R. Petta, S. K. Slater, and D. C. Ralph, Phys. Rev. Lett. 93, 136601 (2004). [5] Z. H. Xiong, D. Wu, Z. V. Vardeny, and J. Shi, Nature 427, 821 (2004). [6] M. S. Meruvia, M. L. Munford, I. A. Hümmelgen, A. S. da Rocha, M. L. Sartorelli, A. A. Pasa, W. Schwarzacher, and M. Bonfim, J. Appl. Phys. 97, 026102 (2005). [7] V. Kirinichnyi, Synth. Met. 108, 173 (2000). [8] S. Forrest, P. Burrows, and M. Thompson, IEEE Spect. 37, 29 (2000). [9] N. A. Hill, J. Phys. Chem. B 104, 6694 (2000). [10] M. Fiebig, J. Phys. D 38, R123 (2005). [11] N. A. Hill, and K. M. Rabe, Phys. Rev. B 59, 8759 (1999). 145 Appendix Relevant Publications A. Journals [A1] “Effect of Cobalt Doping Concentration on the Structural and Magnetic Properties of Half Metallic Fe3O4”, D. Tripathy, A. O. Adeyeye, S. N. Piramanayagam, C. S. Mah, X. Gao, and A. T. S. Wee, Thin Solid Films 505, 45 (2006). [A2] “Magnetic and Transport Properties of Co-Doped Fe3O4 Films”, D. Tripathy, A. O. Adeyeye, C. B. Boothroyd, and S. N. Piramanayagam, Journal of Applied Physics 101, 013904 (2007). [A3] “Microstructure and Magnetotransport Properties of Cu Doped Fe3O4 Films”, D. Tripathy, A. O. Adeyeye, C. B. Boothroyd, and S. Shannigrahi, Journal of Applied Physics (in press). [A4] “Effect of Spacer Layer Thickness on the Magnetic and Magnetotransport Properties of Fe3O4/Cu/Ni80Fe20 Spin Valve Structures”, D. Tripathy, A. O. Adeyeye, and S. Shannigrahi, Physical Review B 75, 012403 (2007). [A5] “Giant Magnetoresistance in Half Metallic Fe3O4 Based Spin Valve Structures”, D. Tripathy and A. O. Adeyeye, Journal of Applied Physics 101, 09J505 (2007). [A6] “Current Perpendicular to Plane GMR in Half Metallic Pseudo Spin Valve Structures”, D. Tripathy and A. O. Adeyeye, Journal of Applied Physics (in press). 146 Appendix [A7] “Magnetic and Tunneling Giant Magnetoresistive Properties of an all oxide Fe3O4-Al2O3 Granular System”, D. Tripathy, A. O. Adeyeye, and S. Shannigrahi, Physical Review B 76, 174429 (2007). B. Conferences [B1] “Effect of Cobalt Doping Concentration on the Magnetic Properties of Half Metallic Fe3O4”, D. Tripathy and A. O. Adeyeye, presented at the 3rd International Conference on Materials for Advanced Technologies, Singapore, July 3-8, 2005. [B2] “Magnetoelectronic Properties of Cobalt Doped Half Metallic Fe3O4”, D. Tripathy and A. O. Adeyeye, presented at the 5th IEEE Conference on Nanotechnology, Nagoya, Japan, July 11-15, 2005. [B3] “Effect of Co Doping on the Structure and Magnetic Properties of Half Metallic Fe3O4 Films”, D. Tripathy, A. O. Adeyeye, X. Gao, C. S. Mah, S. N. Piramanayagam, and A. T. S. Wee, presented at the Spintech III Conference, Awaji Island, Japan, August 1-5, 2005. [B4] “Microstructure and Magnetoelectronic Properties of Co-Doped Half Metallic Fe3O4 Films and Devices”, D. Tripathy, A. O. Adeyeye, and C. B. Boothroyd, presented at the 2nd MRS-S Conference on Advanced Materials, Singapore, January 18-20, 2006. [B5] “Microstructure and Magnetotransport Properties of Cu Doped Fe3O4 Films”, D. Tripathy, A. O. Adeyeye, and C. B. Boothroyd, presented at the 52nd Magnetism and Magnetic Materials Conference, Tampa, Florida, USA, November 5-9, 2007. 147 Appendix [B6] “Giant Magnetoresistance in Half Metallic Fe3O4 Based Spin Valve Structures”, D. Tripathy and A. O. Adeyeye, presented at the 10th Joint Magnetism and Magnetic Materials Conference/International Magnetics Conference, Baltimore, Maryland, USA, January 7-11, 2007. [B7] “Current Perpendicular to Plane GMR in Half Metallic Pseudo Spin Valve Structures”, D. Tripathy and A. O. Adeyeye, presented at the 52nd Magnetism and Magnetic Materials Conference, Tampa, Florida, USA, November 5-9, 2007. Other Publications A. Journals [A1] “Electronic Properties of Field Aligned CrO2 Powders”, D. Tripathy and A. O. Adeyeye, Physica B 368, 131 (2005). [A2] "Fabrication and Magnetoelectronic Properties of Field Aligned CrO2 Powders", D. Tripathy and A. O. Adeyeye, Physica Status Solidi (b) 243, 732 (2006). [A3] “Effect of Fe2O3 on the Transport and Magnetic Properties of Half Metallic Fe3O4”, D. Tripathy, A. O. Adeyeye and C. B. Boothroyd, Journal of Applied Physics 99, 08J105 (2006). [A4] “Planar Hall Effect in Orthogonal Submicrometer Co Wires”, Y. S. Huang, C. C. Wang, A. O. Adeyeye, and D. Tripathy, Journal of Applied Physics 99, 08C508 (2006). [A5] “Temperature Dependence of Magnetotransport Properties of Ni80Fe20 /Fe50Mn50/ Ni80Fe20 Trilayers”, K. M. Chui, D. Tripathy, and A. O. Adeyeye, Journal of Applied Physics 101, 09E512 (2007). 148 Appendix [A6] “Sc modified multiferroic BiFeO3 thin films prepared through a sol-gel process”, S. R. Shannigrahi, A. Huang, N. Chandrasekhar, D. Tripathy, and A. O. Adeyeye, Applied Physics Letters 90, 022901 (2007). B. Conferences [B1] “Spin Dependent Transport in Half Metallic CrO2 Powders”, D. Tripathy and A. O. Adeyeye, presented at the 49th Magnetism and Magnetic Materials Conference, Jacksonville, Florida, USA, November 7-11, 2004. [B2] “Effect of Fe2O3 on the Transport and Magnetic Properties of Half Metallic Fe3O4”, D. Tripathy and A. O. Adeyeye, presented at the 50th Magnetism and Magnetic Materials Conference, San Jose, California, USA, October 30November 3, 2005. [B3] “Interlayer Exchange Coupling in Cu/Co/Cu(tCu)/Ni80Fe20/Cu Nanostructures”, D. Tripathy, A. O. Adeyeye, and N. Singh, presented at the 50th Magnetism and Magnetic Materials Conference, San Jose, California, USA, October 30November 3, 2005. [B4] “Planar Hall Effect in Orthogonal Submicron Co Wires”, Y. S. Huang, C. C. Wang, A. O. Adeyeye, and D. Tripathy, presented at the 50th Magnetism and Magnetic Materials Conference, San Jose, California, USA, October 30November 3, 2005. [B5] “Magnetization reversal in half metallic Fe3O4 based pseudo spin valve nanomagnet arrays”, D. Tripathy, A. O. Adeyeye, and N. Singh, presented at the 2006 International Magnetics Conference, San Diego, California, USA, May 8-12, 2006. 149 Appendix [B6] “Magnetic Properties of Ellipse Shaped Fe3O4 Based Pseudo Spin Valve Nanomagnet Arrays”, D. Tripathy, A. O. Adeyeye, and N. Singh, presented at the 17th International Conference on Magnetism, Kyoto, Japan, August 20-25, 2006. [B7] “Temperature Dependence of Magnetotransport Properties of Ni80Fe20 /Fe50Mn50/ Ni80Fe20 Trilayers”, K. M. Chui, D. Tripathy, and A. O. Adeyeye, presented at the 10th Joint Magnetism and Magnetic Materials Conference/International Magnetics Conference, Baltimore, Maryland, USA, January 7-11, 2007. 150 [...]... dominates the transport properties of the doped films as well The magnetic properties and magnetoresistance (MR) behaviour at various temperatures are markedly sensitive to the doping concentration and the distribution of impurity ions on the tetrahedral and octahedral interstitial sites of the cubic spinel structure of Fe3O4 The second part of this thesis investigates the magnetization reversal processes and. .. investigations presented henceforth in this thesis 1.3 Focus of Thesis The main focus of this thesis is to provide a detailed investigation and understanding of the issues enumerated above in § 1.2 The first part of this thesis addresses the effects of impurity doping on the inherent properties of Fe3O4 films This will provide a knowledge of how doping concentration and distribution of impurity cations across... transport behaviour and the observed magnetic and structural properties 1.4 Organization of Thesis Chapter 1 discusses the background and motivation for the work presented in this thesis Chapter 2 reviews the band diagrams of various categories of half metals followed by an insight into the inverse spinel structure of Fe3O4 and the resulting magnetic and electrical behaviour Spin dependent transport phenomena,... systematic study of the magnetization reversal process and the temperature dependent GMR effect in Fe3O4/ Cu/Ni80Fe20 spin valve structures as a function of spacer layer and Fe3O4 layer thicknesses for both CIP and CPP configurations The strong dependence of microstructure, magnetic and tunneling transport properties of a Fe3O4- Al2O3 granular 5 I Introduction system on the Al2O3 volume fraction, is examined... The basic spintronic devices are based on simple two terminal trilayer elements using either the giant magnetoresistance (GMR) effect [5], or the tunnel magnetoresistance (TMR) effect [6], and three terminal devices such as the Johnson transistor [7,8] So far, the most extensively used applications of TMR and GMR devices are in read heads for hard disk drives [9,10], and magnetic random access memories... fundamental and technological studies in the area of spintronics The first part of this thesis presents a detailed study on the evolution of the structural, magnetic and magnetotransport properties of Fe3O4 films with impurity doping such as Co and Cu in order to find suitable magnetic materials for potential applications The impurity doped films show a preferential growth direction and the microstructure... establishes why Fe3O4 is the focus of several studies involving highly spin polarized materials The crystallographic structure of Fe3O4, the resulting magnetic behaviour and electrical conductivity and the possibility of ion substitutions are presented in § 2.3 Spin dependent transport phenomenon, with emphasis on anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR) effects, are discussed... electron mobilities In either case, the asymmetry which makes spin electrons behave differently from spin ferromagnetic exchange field splits the spin and spin electrons arises because the conduction bands, leaving different band structures at the fermi surface As a result, the number of electrons participating in the conduction process is also different for each spin channel 1.2 Motivation The ferromagnetic... studied and drastic changes are observed as the Al2O3 fraction in the films is varied The Fe3O4 grain size decreases with increasing Al2O3 fraction and the magnetic properties are directly influenced by these structural changes The spin dependent tunneling of electrons across insulating barriers dominates the transport properties, and results in the observation of a tunneling GMR effect in the granular... across the interstitial sites of the Fe3O4 crystal structure results in a drastic modification of the microstructure, magnetic properties, and MR behaviour of Fe3O4 films The conclusions drawn from this study will aid in the fabrication of modified Fe3O4- based materials with tailor-made magnetic and transport properties The second part gives a detailed insight into the effect of spacer layer and Fe3O4 . To my father… HALF METALLIC Fe 3 O 4 : AN EXPERIMENTAL STUDY ON IMPURITY DOPING AND THE GIANT MAGNETORESISTANCE EFFECT DEBASHISH TRIPATHY (B. Eng.(Hons.),. HALF METALLIC Fe 3 O 4 : AN EXPERIMENTAL STUDY ON IMPURITY DOPING AND THE GIANT MAGNETORESISTANCE EFFECT DEBASHISH TRIPATHY NATIONAL UNIVERSITY. concentration and the distribution of impurity ions on the tetrahedral and octahedral interstitial sites of the cubic spinel structure of Fe 3 O 4 . The second part of this thesis investigates the magnetization