Defect control of magnetism in complex oxides [electronic resource]

From the development of the first compass needle to the Information Age fueled by hard drive miniaturization to the emerging field of spintronic devices, humans have long attempted to harness the magnetic properties of materials to do useful work. In this dissertation, we will explore how disruptions to crystallographic order in two types of complex oxide films can be used to tune their magnetic functionality. The first part of this dissertation will discuss exchange bias, a concept that is both fundamentally and technologically interesting, having enabled many applications (e.g., reliable read heads) even as physicists seek answers to explain all of the rich phenomena in these systems. It has been long accepted that disorder, such as that induced by film growth at low temperatures, suppresses exchange bias due to roughness between interfaces (e.g., in a ferromagnetic/antiferromagnetic bilayer). In contrast, we have demonstrated that the deliberate introduction of disorder via nanocrystallinity in Mn-Zn ferrite films is actually coincident with an exchange bias field. We used annealing to tune the degree of disorder in the system and consequently modulate the exchange bias field. This is an alternate method to engineering exchange bias that can easily be controlled post-synthesis. The ability to grow spinel ferrites at room-temperature is technologically promising as well, particularly when integration with dissimilar materials limits the thermal budget. Furthermore, we have been able to grow crystallographically textured ferrite thin films with long-range magnetic order directly on silicon, glass, and a variety of oxide substrates. On all substrates, disordered, intrinsically biased Mn-Zn ferrite films exhibit anomalous initial magnetization and zero-field-cooled effects that are indicative of a rugged energy landscape with competing local minima. In the next part of this dissertation, we investigate transition-metal and rare-earth doping of barium tin oxide films in order to simultaneously achieve transparency to visible light, electrical conductivity, and a nonzero magnetic moment. La-doped barium tin oxide is a transparent conductor with high room-temperature electron mobilities compared to other perovskite-structure films, and an excellent candidate system for attempts to induce long-range magnetic order into a conventionally uncorrelated material. We have employed Fe-doping on the Sn-site to achieve a ferromagnetic signal in these films. Through an alternate approach of using Pr, Nd, and Gd dopants on the Ba-site, we have been able to engineer transparent, conducting films with a paramagnetic response. By counterintuitive optimization of pulsed laser deposition growth conditions, we also report high room-temperature electron mobilities compared to other perovskite-structure complex oxide thin films. Microstructural investigations, however, reveal evidence of extensive dislocations and grain boundaries, indicating that there is still significant room for optimization in this system. Together these results serve to correlate structure-processing-properties relationships in complex oxides, and demonstrate how defects may be used constructively to engineer new magnetic states in spinel and perovskite-structure oxides.

Keywords: Materials Science and Engineering.
Advisor: McIntyre Paul Cameron primary advisor. Thesis advisor
Suzuki Yuri (Applied physicist) primary advisor. Thesis advisor
Chueh William
Wang Shan