Magnetism and Magnetotransport in Complex Oxide Thin Film Heterostructures
The nature of magnetism at thin film surfaces and interfaces is not yet fully understood, yet it is quite important for both fundamental studies and technological applications. In this dissertation, I present a study of the magnetism and magnetotransport in single thin film layers as well as at interfaces of Fe3O4/spinel chromite/LSMO and Fe3O4/spinel chromite/Fe3O4 heterostructures. To begin with, investigations of single layer thin films on metallic oxides such as perovskite structure SrRuO3 and spinel structure LiTi2O4 elucidate the dependence of transport properties on parameters such as thickness, film strain state, and crystal orientation. In addition, the magnetism of CoFe2O4 thin films is examined while dynamically altering the strain state via the temperature-dependent lattice parameter of piezoelectric BaTiO3 substrates. Detailed spectroscopy experiments indicate that magnetism at the (110) LSMO and (111) LSMO surfaces are not suppressed compared to (001) LSMO interfaces. In addition, no magnetic coupling was observed between LSMO and spinel chromite layers above 100K. In contrast, the (110) Fe3O4 surface exhibited a significant change in anisotropy accompanied by an enhanced magnetization in the spinel chromite layer to beyond room temperature. At the isostructural interface, there is strong ferromagnetic coupling between Fe and Cr ions in bilayers. Our results on Fe3O4 and LSMO surfaces, combined with measurements on the angular, field and temperature dependence of junctions with LSMO and Fe3O4 electrodes, indicate that spin polarization is not intrinsically suppressed at a surface or interface but that magnetization and spin polarization depends on the crystal surface orientation, strain state and surface or interface reconstruction.
This research was supported by the Office of Naval Research (N00014-97-1-0564) and the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
complex oxide; pulsed laser deposition; manganite; ferrite; thin film; magnetotransport; perovskite; spinel
dissertation or thesis