Control of Magnetic Behavior in LaMnO3 Thin Films through Cationic Vacancies A Thesis Submitted to the Faculty of Drexel University by Cerina Gordon In partial fulfillment of the requirements for the degree of Master of Science in Materials Science and Engineering June 2017 © Copyright 2017 Cerina Gordon. This work is licensed under a Creative Commons Attribution 4.0 International License. ii Acknowledgements I would like to express thanks to all those who helped me and stood by me during the production of this work. In particular I’d like to thank Mark Scafetta and Dr. Steve May for providing the opportunity to study this problem, Mark for growing the films and Dr. May for prompting the study of this problem and providing the resources to study it. Growth of the films by Mark S. was funded by the National Science Foundatin (DMR- 1151649) I’d also like to thank my lab group for providing training on several of the tests performed, assisting in characterization and providing useful discussion and moral support. Similarly, I’d like to thank my committee members, Dr. Steve May, Dr. Goran Karapetrov and Dr. Garritt Tucker for the valuable discussion following my defense. Finally, I’d like to thank my friends and family for providing support and sticking it out with me, and providing an outside perspective to remind me of exactly what a general audience does and does not know. iii Table of Contents List of Tables ..................................................................................................................... v List of Figures ................................................................................................................... vi Abstract ........................................................................................................................... viii 1. Introduction ............................................................................................................... 1 2. Background ................................................................................................................ 3 Magnets and Magnetization ...................................................................................................... 3 Definition of a Magnet ............................................................................................................. 3 Types of Magnetic Behavior in Materials ............................................................................... 4 Ferromagnetic Behavior ........................................................................................................... 6 Antiferromagnetic Behavior .................................................................................................... 8 Perovskites .................................................................................................................................. 9 Charge balancing ..................................................................................................................... 9 Defects ................................................................................................................................... 10 Magnetic and Electronic Behavior in Perovskites ................................................................. 11 Lanthanum Manganite ............................................................................................................ 14 Bulk LaMnO3 ......................................................................................................................... 14 LaMnO3 Thin Films ............................................................................................................... 15 3. Experimental ............................................................................................................ 18 Samples ..................................................................................................................................... 18 Synthesis and Processing ......................................................................................................... 19 Thin Film Deposition via Molecular Beam Epitaxy (MBE) ................................................. 19 Annealing ............................................................................................................................... 21 iv Characterization ...................................................................................................................... 21 Rutherford Backscattering Spectroscopy (RBS) ................................................................... 22 X-ray Diffraction (XRD) ....................................................................................................... 22 X-ray Reflectivity (XRR) ...................................................................................................... 24 Vibrating Sample Magnetometry ........................................................................................... 26 4. Results and Discussion ............................................................................................ 29 Structural Analysis .................................................................................................................. 29 X-Ray Diffraction .................................................................................................................. 29 Effect of Annealing ................................................................................................................ 32 X-Ray Reflectivity ................................................................................................................. 33 Magnetic Behavior ................................................................................................................... 35 Moment vs Applied Field (MvH) .......................................................................................... 35 Moment vs Temperature and the Paramagnetic Transition Temperature .............................. 41 5. Conclusion and Future Work ................................................................................. 46 v List of Tables Table 2.1: Categorization of magnetic materials by alignment of adjacent spins ............. 4 Table 3.1: List of samples by LaMnO3 film cation ratios ................................................ 18 Table 4.1 Summary of x-ray diffraction results ............................................................... 31 Table 4.2 Film thickness as obtained through x-ray reflectivity analysis ........................ 34 Table 4.3 Transition temperatures ................................................................................... 42 vi List of Figures Figure 1.1 Slowdown in hard-drive improvements ............................................................ 1 Figure 1.2 Seagate's 2008 technology roadmap .................................................................. 1 Figure 1.3 IBM hard drive architecture for perpendicular writing ..................................... 2 Figure 2.1 Alignment and growth of magnetic domains in a ferromagnetic material ........ 6 Figure 2.2 Typical magnetic moment (M) vs. applied field (H) behavior for ferromagnetic materials .............................................................................................................................. 7 Figure 2.3 Example spin canting in a G-type antiferromagnetic material .......................... 8 Figure 2.4: Crystal structure of an ideal perovskite ............................................................ 9 Figure 2.5 d-orbitals and corresponding energy levels ..................................................... 12 Figure 2.6: p-orbitals ......................................................................................................... 12 Figure 3.1 Sample XRR data with simulated fit ............................................................... 25 Figure 4.1: X-ray diffraction data for LaMnO3 films on SrTiO3 ...................................... 29 Figure 4.2 X-ray diffraction peaks following ozone annealing ........................................ 32 Figure 4.3 XRD spectra for LaMnO3 near the (001) substrate peak ................................ 33 Figure 4.4 Sample fit of x-ray reflectivity data ................................................................. 34 Figure 4.5 Magnetization vs applied field at 10K for LaMnO3 films ............................... 35 Figure 4.6 Comparison of AF sample to AF behavior reported in [11] ........................... 36 Figure 4.7 Saturation magnetic moment vs thickness for LaMnO3 films on SrTiO3 ....... 37 Figure 4.8 Saturation magnetic moment as a function of composition ............................ 37 Figure 4.9 Saturation moment as a function of manganese valence state ........................ 38 Figure 4.10 Phase diagram for strontium-doped LaMnO3 ................................................ 39 vii Figure 4.11 Change in saturation magnetic moment due to ozone annealing, as a function of manganese average oxidation state ............................................................................... 40 Figure 4.12 Film moment vs temperature under an applied field of 0.1 T ....................... 41 Figure 4.13 Determination of transition temperature via trend line ................................. 42 Figure 4.15 Film moment […] vs transition temperature ................................................. 43 Figure 4.16 Manganese oxidation state vs transition temperature .................................... 44 Figure 4.17 Film thickness vs transition temperature