Synthesis and characterization of oxide thin films for spintronics
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Abstract
The pursuit of energy-efficient computing platforms requires breakthroughs in spintronics, where information is encoded in electron spin rather than charge. This thesis focuses on novel oxide systems, specifically ruthenium dioxide (RuO2), a conductive metallic candidate for the emerging altermagnetic phase, which promises compensated magnetic order and strong intrinsic spin splitting, alongside yttrium iron garnet (YIG), the benchmark ferrimagnetic insulator renowned for its ultralow damping. The central objective is to achieve a verifiable and quantitative understanding of spin-current generation, propagation, and electrical detection in these oxide heterostructures. A fundamental scientific challenge addressed is the stringent requirement to distinguish potential intrinsic phenomena, such as non-relativistic spin splitting effects stemming from crystal symmetry, from pervasive extrinsic contributions arising from interfaces, defects, or dimensionality. First, foundational structural quality was established through controlled physical vapor deposition, monitored in situ using reflection high-energy electron diffraction, while ex situ x-ray diffraction provided further verification of crystallographic phase purity and structural coherence. Second, quantitative interface-sensitive magnetotransport experiments (ADMR in RuO2/Py bilayers) were performed, confirming the dominance of interface effects. Third, localized magnetic order was probed using depth-resolved low-energy muon spin rotation (LE-µSR), enabling nanometer-scale distinction between surface and bulk properties. The optimized epitaxial RuO2/TiO2 films also served as a platform for collaborative ARPES, XMCD/XPS, and optical experiments. These studies reported signatures interpreted as consistent with altermagnetic order in RuO2. Finally, the YIG/Pt(Ta) magnetotransport experiments successfully verified the sign dependence of the Hall signal, validating the reliability of the applied analysis framework for extracting spin transport parameters. The integrated experimental data yield critical insights. LE-µSR measurements revealed that static, inhomogeneous magnetic order in RuO2 thin films is confined predominantly to the near-surface region, suggesting an extrinsic origin, likely due to dimensionality or defect effects, rather than a bulk altermagnetic phase. Correspondingly, transport analysis demonstrated that interface-generated spin currents, arising from localized spin-orbit scattering at the RuO2/Py interface, predominantly govern the macroscopic magnetoresistance response, effectively masking potential subtle bulk non-relativistic spin splitting signals. These findings clarify that, in the present generation of oxide heterostructures, macroscopic spintronic functionality is chiefly determined by extrinsic interfacial spin dynamics. While RuO2 is confirmed as a robust, metallic platform for spin injection, its utility hinges on meticulous interface and stoichiometry control to either optimize these dominant interface-generated spin currents or successfully stabilize the intrinsic altermagnetic phase. The challenge is underscored by evidence suggesting the lack of magnetic moment in bulk RuO2. The analytical methodologies and conceptual framework developed in this work pave the way for future studies focusing on epitaxial strain engineering and ultrafast dynamics, accelerating the path toward energy-efficient, oxide-based spintronics devices.