Magnetism of Complex Oxide Thin Films and Heterostructures by Jodi Mari Iwata A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Engineering – Materials Science and Engineering and the Designated Emphasis in Nanoscale Science and Engineering in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Junqiao Wu, Chair Professor Yuri Suzuki Professor Ramamoorthy Ramesh Professor Constance Chang-Hasnain Fall 2012 Magnetism of Complex Oxide Thin Films and Heterostructures Copyright © 2012 by Jodi Mari Iwata Abstract Magnetism of Complex Oxide Thin Films and Heterostructures by Jodi Mari Iwata Doctor of Philosophy in Materials Science and Engineering Designated Emphasis in Nanoscale Science and Engineering University of California, Berkeley Professor Junqiao Wu, Chair Studies of magnetism at reduced scales have revealed new phenomena that are distinctly different from their bulk counterparts providing insight to the fundamental mechanisms that govern magnetism and other correlated properties. To this end, the use of heteroepitaxy and heterostructures is invaluable for investigating magnetism at reduced dimensions and at surfaces and interfaces. This dissertation is a compilation of investigations examining the magnetic properties of spinel-structure oxide thin films and heterostructures. Of particular interest are non-collinear spin systems as closely competing exchange interactions between magnetic moments give rise to a plethora of ground state degeneracies and phenomena inaccessible in the bulk. The first part of this dissertation highlights the use of heteroepitaxial lattice distortions as a method to tune spin functionality and potentially lift ground state degeneracies more broadly in frustrated magnets. It discusses the first synthesis of heteroepitaxial thin films of the frustrated canted-moment ferrimagnet, CuCr2O4, and demonstrates the modification of exchange interaction strengths which results in greater collinear spin ordering and enhanced magnetization compared to the bulk. The data illustrates the sensitivity of the strong competing exchange interactions suggesting that strain is a promising instrument for perturbing the delicate balance of the exchange interactions in frustrated materials. The second part of this dissertation probes magnetic proximity effects induced by interfaces and unconventional transport properties when CuCr2O4 is incorporated as a barrier layer in magnetic tunnel junctions comprised of ferromagnetic La0.7Sr0.3MnO3 and Fe3O4 electrodes. It is surprising that a heterostructure composed entirely of magnetic materials can achieve distinct magnetic and resistive switching given the complexities present at the two barrier-electrode interfaces. Studies of the CuCr2O4/Fe3O4 interface again illustrate the delicate balance between exchange interactions as proximity effects by Fe3O4 are believed to modulate alignment in the Cr moments. Additionally, the bias dependence of JMR displays a local 1 minimum at zero bias which is believed to be the result of the Fe3O4 band structure and spin filtering properties of the CuCr2O4 barrier. 2 To my grandparents and the men of the 100th Infantry Battalion and 442nd Regimental Combat. May your legacy live on forever. i Table of Contents List of Figures .............................................................................................................................. iv List of Acronyms ......................................................................................................................... ix Acknowledgements ...................................................................................................................... x Chapter One: Introduction to the basics of complex oxides 1.1 Introduction .............................................................................................................................. 1 1.2 The Pauli Exclusion Principle and Hund’s Rules .................................................................... 2 1.3 Crystal Field Theory and Its Extensions .................................................................................. 3 1.4 The Jahn-Teller Theorem ......................................................................................................... 8 1.5 The Spinel Crystal.................................................................................................................... 8 1.6 Ferrimagnetism ........................................................................................................................ 9 1.7 Frustrated Magnetism ............................................................................................................ 12 1.8 Superexchange ....................................................................................................................... 14 1.9 Magnetic Anisotropy ............................................................................................................. 15 1.10 Organization of Dissertation ................................................................................................ 17 Chapter Two: Experimental Methods 2.1 Thin Film Growth Using Pulsed Laser Deposition .............................................................. 19 2.2 Structural and Chemical Characterization Techniques .......................................................... 22 2.2.1 Atomic Force Microscopy ...................................................................................... 22 2.2.2 X-ray Diffraction .................................................................................................... 22 2.2.3 Rutherford Backscattering Spectroscopy ................................................................ 23 2.2.4 X-ray Absorption Spectroscopy .............................................................................. 25 2.2.5 Resonant X-ray Scattering ...................................................................................... 26 2.3 Magnetic Characterization ..................................................................................................... 26 2.3.1 SQUID Magnetometry ............................................................................................ 26 2.3.2 X-ray Magnetic Circular Dichroism ....................................................................... 27 2.3.3 Neutron Reflectivity................................................................................................ 27 2.4 Electrical Transport ................................................................................................................ 28 Chapter Three: Tuning Magnetism in CuCr2O4 Thin Films 3.1 Frustration in Spinels ............................................................................................................. 31 3.2 Experimental Methods ........................................................................................................... 35 3.3 Structural and Chemical Properties ....................................................................................... 35 3.4 Magnetic Properties ............................................................................................................... 41 ii 3.5 Discussion .............................................................................................................................. 45 3.6 Conclusion ............................................................................................................................. 46 Chapter Four: La0.7Sr0.3MnO3/CuCr2O4/Fe3O4 Magnetic Tunnel Junctions 4.1 Introduction to Oxide Magnetic Tunnel Junctions ................................................................ 47 4.2 Experimental Methods ........................................................................................................... 50 4.3 Structural Properties............................................................................................................... 50 4.4 Magnetic Properties ............................................................................................................... 52 4.5 Transport Properties ............................................................................................................... 57 4.6 Discussion .............................................................................................................................. 65 4.7 Conclusion ............................................................................................................................. 66 Conclusion .................................................................................................................................. 67 Appendix ..................................................................................................................................... 68 References ................................................................................................................................... 83 iii List of Figures 1.1 The spinel (MN2O4) oxide crystal structure. ...................................................................... 2 1.2 A comparison of the relative energies due to crystal field splitting for a 3d transition metal cation in octahedral and tetrahedral environments. ................................................. 3 1.3 Schematic depicting the orientation of the d orbitals in an octahedral crystal field. Oxygen ligands are represented as point charges. The x, y, and z axes are orthogonal. .......................................................................................................................