Interfacial Magnetism and Exchange Bias in Metal Oxide Thin Films

Abstract

This thesis focuses on the investigations of magnetic thin film structure of cubic and perovskite transition metal oxide magnets, particularly when two different magnetic orderings are competing at such interface. The goal is to understand the various effects of an interfaces that exert on the neighboring materials; to understand the role of the layer structure and microstructure on the overall magnetic properties; and to investigate the feasibility to control such interfacial phenomenon by means of strain, ion implantation, and crystal orientation. Four thin film systems are included: MnxOy/Ni80Fe20, La0.7Ca0.3MnO3/CaMnO3, La0.7Ca0.3MnO3 and BiFeO3/SrRuO3. Oxygen ion implantation was performed on MnxOy/Ni80Fe20 bilayers, and the results show an enhancement of exchange bias and coercivity after the implantation. Polarized neutron reflectometry study reveals a significant change in magnetic spin reversal mechanism due to chemical modification. In the La0.7Ca0.3MnO3/CaMnO3 system exchange bias is assisted by the magnetic frustration at the layer interface. Results suggest, the strength of exchange bias is strongly related to the degree of frustration. Controlling the strain state of La0.7Ca0.3MnO3 shows an effective method to alter the frustration property. Further, the magnetic glassy spins in the La0.7Ca0.3MnO3 epitaxial thin film is studied. Combining DC magnetization, AC susceptibility, and polarized neutron reflectometry measurements, the spin-glass nature of the sample is obtained which spins are freeze around 139 K which just below the ferromagnetic transition of the bulk sample. Finally, for the BiFeO3/SrRuO3 system, an enhancement of net magnetization of the canted antiferromagnetic BiFeO3 was probed in (111)-orientated sample. There is possible exchange interaction of Ru4+ and Fe3+ and the strong orbital p-d hybridization of SrRuO3 which could contribute to the enhanced magnetization.