Magnetization and spin dynamics in complex oxide heterostructures

Information processing via pure spin currents requires new materials for efficient generation and transport of pure spin currents. One promising class of materials for this application is the complex oxides, which offer a wide variety of properties including ferromagnetic insulating behavior, superconductivity, and antiferromagnetism. I will explore each of these properties for spintronic applications in this thesis. I have developed a series of low damping ferromagnetic insulators in thin film form as a medium for low loss spin wave propagation. I optimized the thickness and composition of MgAl(2-x)FexO4 (MAFO) thin films for low Gilbert damping measured via ferromagnetic resonance (FMR). In an effort to understand spin current transport across interfaces and quantify spin transport parameters, I then interfaced MAFO thin films with Cu and Pt capping layers and compared this system to another spinel ferrite used for spin current generation, Ni0.65Zn0.35Al0.8Fe1.2O4 (NZAFO). I found the interfaces in these heterostructures played a dominant role in the FMR properties, so much so that quantifying spin current transport parameters became difficult. To eliminate the polycrystalline interface and improve transport across the interface, I then interfaced MAFO with another spinel oxide, CrCo2O4. Even in this isostructural bilayer, I found the interface dominated the FMR properties and led to damping increases on the order of that observed in MAFO/Pt bilayers. These results highlighted the importance of considering the interface in spin pumping heterostructures when analyzing spin transport parameters. In the next part of the thesis, I investigate the interaction between superconductivity and spin current transport in isostructural La0.67Sr0.33MnO3 (LSMO)/ YBa2Cu3O7 (YBCO) bilayers. Superconductors have the potential to have extremely long spin diffusion lengths, but these parameters have yet to be measured in anisotropic d-wave superconductors such as YBCO. Using a custom FMR insert for our cryostat, I probed spin current transport across the superconducting transition. However, I again found the interface between the LSMO and YBCO layers played an important role. The diffuse layer between the two led to complex behavior across the superconducting transition that was due to other sources of damping increase than spin pumping, such as two-magnon scattering (TMS) processes. To alleviate this issue, I also studied YBCO / Ni20Fe80 bilayers, which were known to have a lower contribution from TMS. While I did find some evidence for the coexistence of spin pumping and superconductivity in these bilayers, the polycrystalline interface appeared to play an important role in the spin dynamics. As before these results underscored the importance of the spin source/sink interface when analyzing spin pumping heterostructures. In the final part of the thesis, I developed antiferromagnetic materials for high-frequency spintronic applications. Antiferromagnetic materials have much higher characteristic frequencies, pushing the timescale for dynamics into the THz regime. I began with fabricating free-standing NiO membranes for ultrafast electron diffraction (UED) experiments. I used a new technique to fabricate oxide membranes, and maintained high crystalline quality and orientation while transferring thin films onto Si TEM grids for the UED experiment. I will then present analysis of the Debye-Waller effect in (001) NiO membranes. I then characterized the structure and spin-Hall magnetoresistance in Cr2O3 thin films on various orientations of Al2O3 substrates. Cr2O3 has been shown to undergo resonance at high frequencies in bulk, but there has yet to be an analogous study for thin films. I measured a crossover between ferromagnetic behavior dominated by uncompensated moments at the surface and bulk antiferromagnetic behavior in the magneotransport. These results combined demonstrated the viability of the NiO and Cr2O3 systems for future experiments for high-frequency spintronic applications

Keywords: Applied Physics.
Advisor: Suzuki Yuri (Applied physicist)
Feldman Ben (Benjamin Ezekiel)
Hwang Harold Yoonsung