UC Davis

AbstractGlobal computational power demands are sharply increasing and existing computing architectures are reaching the limits imposed by fundamental physics. Neuromorphic computing is a philosophy of computer design based on the human brain, which is much more energyefficient than manufactured computers. However, current materials are not optimized for emerging neuromorphic devices that require the ability to manipulate multiple electronic or magnetic states. Complex oxide materials exhibit a rich variety of interesting phenomena such as high temperature superconductivity, metal-to-insulator phase transitions, and colossal magnetoresistance. Perovskites are one of the members of the complex oxide family that have been studied for nearly a century and have shown promise in solving materials science problems in applications including data storage and computing, photovoltaic cells, solid oxide fuel cells, and light emitting diodes. Modern fabrication techniques enable synthesis of a wide variety of nanomaterials such as thin films, heterostructures namely bilayers or multilayers, and nanostructures such as quantum dots. Furthermore, there are many elements that can stably form the perovskite structure. Nanostructuring and the presence of interfaces in these systems allows for the functional properties to be tailored for a specific need and sometimes give rise to emergent phenomena, thus opening the door to many possible avenues of research.

Past studies have shown that perovskite oxide thin films readily undergo topotactic phasetransformations where either anion or cation vacancies form and rearrange into an ordered sublattice forming a new structure with distinct properties. Moreover, alternating the stacking order of heterostructures is a technique that has been proven to elicit differing outcomes in systems containing the same constituent layers. This thesis explores annealing-induced topotactic phase transformations of perovskite thin films and bilayers which cause structural and magnetic changes in the materials. There is limited research on the topotactic phase transformations in La0.7Sr0.3MnO3 thin films, and to date, no study has looked specifically at the effect of stacking order of LaCoO3-La0.7Sr0.3CoO3 bilayers regarding the phases produced by topotaxy. In this thesis, manganite thin films and cobaltite bilayers with alternating stacking order were synthesized and annealed under different conditions. The annealing process initiated topotactic phase transformations which were studied using X-ray diffraction, X-ray photoelectron spectroscopy and soft X-ray magnetic spectroscopy. Beginning from the perovskite phase, manganite thin films and cobaltite bilayers underwent structural and magnetic changes into perovskite-related phases including oxygen-deficient perovskite, Grenier, brownmillerite, and Ruddlesden-Popper phases. In the bilayer systems, the stacking order of the constituent layers influenced which phases were formed under different annealing conditions. The bilayers also behaved differently from their corresponding single layer thin films that were annealed under the same conditions, giving rise to phases that were not present in the single layer configurations. Along with the structural phase transformation, the magnetic properties of the material were also modified. Control of magnetic properties is important for application in spintronics—a growing field of study that makes use of the modulation and detection of spin states in a material for data storage and processing. Furthermore, access and control of numerous unique phases in the same material can pave the path for the next-generation computing devices.