UC Davis

Perovskite oxides, with the ABO3 structure where the A cations are typically alkaline earth or rare earth elements and B cations are transition metals, have attracted significant attention due to their potential applications in spintronics, magnetic devices, and neuromorphic computing. These materials exhibit a wide range of functional properties, including ferromagnetism, metal to insulator transitions, and controllable phase transformations, which arise from the intricateinterplay of spin, charge, lattice, and orbital degrees of freedom. Importantly, perovskite oxides demonstrate a remarkable sensitivity to external stimuli such as external magnetic fields, lattice strain, and ion migrations, allowing for precise control over their emerging magnetic and electronic properties. This characteristic distinguishes them from metallic systems, offering unique path for the design and manipulation of functional materials with tailored properties. As a result, perovskite oxides hold great promise for the development of advanced electronic and magnetic devices with enhanced performance and functionality.

Controlling interfacial magnetic phenomena between hard and soft ferromagnetic (FM) layers in heterostructures plays a key role in next generation spintronic and magnetic memory devices. In this dissertation, the tuning of interfacial properties through control of the thin film strain and layer thickness in La2/3Sr1/3CoO3 (LSCO)/La2/3Sr1/3MnO3 (LSMO) bilayers are discussed. Soft x-ray magnetic spectroscopy was performed to study the valence states and bonding configurations of the transition metal ions as well as the magnetic exchange coupling of the bilayers. Magnetocrystalline anisotropy and exchange bias were further explored LSCO/LSMO bilayers grown on NdGaO3 substrates, where soft x-ray linear dichroism spectra highlighted the differing electron occupancy along the two in plane directions of the LSMO layer with increasing LSCO layer thickness.

Investigating the potential of ion migration induced alteration on functional properties has become a prominent research focus in the rapidly evolving field of neuromorphic computing. Perovskite oxides, including cobaltites (LSCO), manganites (LSMO), and ferrites (La0.7Sr0.3FeO3, LSFO), have emerged as prime candidates for their remarkable characteristics, such as high oxygen vacancy conductivity, relatively low oxygen vacancy formation energy, and the strong interplay between magnetic and electronic properties with oxygen stoichiometry. This dissertation delves into the evolution of physical properties exhibited by LSCO, LSMO, and LSFO thin films when subjected to highly reducing environments, specifically H2 and vacuum conditions. In the cobaltite systems, the introduction of H2 leads to the formation of oxygendeficient phases at lower temperatures compared to vacuum annealing but remain in the perovskite phase without phase transitions at higher hydrogenation temperatures. The manganites, on the other hand, require more reduced conditions under vacuum annealing to achieve a mixed phase of brownmillerite and Ruddlesden Popper phases. Notably, the ferrite films maintain their oxygen deficient perovskite phase even when subjected to the most reducing conditions but shows significant differences in their electronic properties. The strong correlation between crystal structure and the magnetic/electronic properties underscores the potential of leveraging ion migration as a foundation for emerging applications such as neuromorphic computing.