Experimental Study of Spin Current Transport in Heterostructures

Author(s): Xu, Yadong | Advisor(s): Shi, Jing | Abstract: Spin current transport plays an important role in modern solid state physics. Research efforts on this field not only reveal fundamental principles, but also promote engineering applications. We explore spin current transport in various heterostructures. We first give a brief introduction to the recent advancements in spin current transport, including the observation of spin Hall effect (SHE) in materials with strong spin-orbit coupling, the discovery of spin Seebeck effect (SSE) in both conductive and insulating magnetic materials, and the realization of spin current transport without free charge motion in magnetic insulators. Then we present the results related to spin current transport in ferromagnetic metal (FM)/normal metal (NM) heterostructures, driven by spin pumping or heat flow. In the spin pumping experiment, we observe inverse SHE signals at ferromagnetic resonance states and anomalous Nernst effect (ANE) signals due to a vertical temperature gradient. In the longitudinal SSE experiment, we disentangle SSE, ANE and proximity effect contributions and discover the spin current draining effect, i.e., an adjacent NM changes the spin chemical potential and induces an additional spin current in the FM. The third part provides details about the observation of magnon-mediated current drag effect. We grow high quality Pt/yttrium iron garnet (YIG)/Pt(Ta) trilayer heterostructures and realize electronic signal transmission through YIG, a magnetic insulator, by magnon current. A charge current in the bottom Pt layer induces spin accumulation by SHE. The electronic spins convert to magnons in YIG and convert back to electronic spins in the top Pt(Ta) layer. The study of topological SSE in YIG/topological insulator (TI) heterostructures is presented at last. Magnons driven by a vertical temperature gradient are converted to a charge current in TI surface states. We tune the Fermi level by changing TI composition ratio or applying a top gate and demonstrate the dominant role of the topological surface states in the magnon-charge conversion.