UC Riverside

Spintronics has been an active research area for decades, as pure spin current which is free of Joule heating oers a promising possibility to build next-generation electronic devices with revolutionarily high energy efficiency. Spin-related transport phenomena such as anomalous Hall effect (AHE), spin Seebeck effect (SSE) and spin Hall magnetoresistance (SMR), serving as reliable probes, help researchers understand the generation, propagation and conversion of spin current as well as the mechanisms behind in ferromagnetic-materialbased and antiferromagnetic-material-based spintronic systems. In Chapter 1, I will go through a brief introduction to those spin-related transport phenomena especially in ferromagnetic insulators (FMI) and antiferromagnetic insulators (AFMI) related systems.

FMI-based heterostructures with normal metals (NM) that have strong spin-orbit coupling, such as yttrium iron garnet YIG/Pt bilayers, have attracted a great deal of attention. In such systems, electric signal can only be generated in the metal layers adjacent to the FMI layer, which allows for more valid investigations into the pure spin current related phenomena. To better study various spin-related transport phenomena in FMI/NM bilayers, good control over the magnetic anisotropy of FMI is essential. In Chapter 2, I will go through the techniques we use to engineer and manipulate magnetic anisotropy in FMI by choosing proper substrates that provide desired strain, tuning thickness of FMI and varying temperature. These tuning capabilities enable us to discover topological Hall effect (THE) in TmIG/Pt bilayers at room temperature and conduct systematically study to explore its physical origin.

In FMI/NM bilayers where NM has strong spin-orbit coupling, there have been some debates on whether those spin-related transport phenomena originate from the magnetic proximity effect (MPE) that NMs get magnetized by adjacent FMI layer or come from pure spin current contribution via spin Hall and inverse spin Hall effects in the NM layer. In Chapter 3, I will discuss my work on temperature dependence study of AHE in TmIG/Pt and TmIG/Cu/Pt heterostructures. The distinct behaviors in AHE temperature dependence w/o Cu spacer indicate that MPE dominates the AHE signal.

In recent years, great efforts have been devoted to the study of spintronics in antiferromagnetic insulator(AFMI) related heterostructures thanks to its unique properties of ultrafast spin dynamics and robustness against external field. AFMI working either as sources generating terahertz spin current or as a spacer in between FMI and NM that manipulates magnon propagation offer numerous interesting questions to be fully explored. Cr2O3 which is an uniaxial AFMI, has been a popular candidate for these study purposes. In Chapter 4, I will review the progress of recent AFMI related spin transport study. Then discuss in details of my research on the study of YIG/Cr2O3/Pt heterostructures, which begins from the growth and characterization of Cr2O3 thin films on FMI YIG and then the observed unique behaviors in SSE measurements. The results are to be associated with the characteristic properties of Cr2O3 which reveal the existence of AFM magnon contribution in SSE.