Magnetism of complex oxide thin films, interfaces and heterostructures probed by synchrotron techniques and model devices.

摘要

The nature of magnetism in materials of reduced dimensions and in the proximity to other materials is both a fundamental and technological question that has yet to be fully understood. This dissertation explores the magnetic properties of spinet and perovskite complex oxide thin films, interfaces and heterostructures by a variety of techniques, with particular emphasis on element-specific synchrotron radiation techniques and magnetotransport of magnetic tunnel junction devices.;The first area of investigation explores the unconventional magnetic ground states found in spinel oxide thin films. In single films of NiMn 2O4, which exhibit an anomalous magnetic ground state, we find that the magnetic double exchange interactions are dramatically changed upon modifying the Mn cation properties of this material. This results in a selective quenching of one of the two equilibrium magnetic phases found in the bulk form. For ultrathin NiFe204 thin films, we reproduce the effect of increased magnetization with decreasing film thickness seen previously in the literature, and using element-specific synchrotron techniques, we directly demonstrate a dramatic increase in the magnetization of the Ni and Fe cations with decreasing film thickness while the cation valences and cation inversion remain largely unchanged.;The second area of investigation explores the unconventional magnetic phenomena at the interfaces between two highly spin polarized materials (Fe 3O4 and La0.7Sr0.3MnO3) with two different magnetic insulators (NiMn2O4 and NiFe2O4). At the isostructural spinel-spinel interfaces (NiMn2O4/Fe3O4 and NiFe2 O4/Fe3O4), we find strong magnetic coupling and magnetic properties not found in either constituent film. At the non-isostructural spinel-perovskite interface (NiMn2O 4/La0.7Sr0.3MnO3 and NiFe2O 4/La0.7Sr0.3MnO3), one surprisingly obtains complete magnetic decoupling between the two adjacent ferromagnetic films due to the frustrated magnetic exchange interactions at the ferrimagnet/ferromagnet interface.;The final area of investigation joins these films and interfaces into functional oxide heterostructures. A novel spintronic architecture is demonstrated, called the hybrid spin filter/magnetic tunnel junction, which implements long range magnetism into the barrier layer of a magnetic tunnel junction device. Heterostructures with magnetic barrier layers at low temperatures (Fe 3O4/NiMn2O4/La0.7Sr 0.3MnO3) and room temperature (Fe3O4/NiFe 2O4/La0.7Sr0.3MnO3) show that long-range magnetism directly influences the spin transport and can provide added functionality to these systems.