Ferromagnetic resonance study of interlayer exchange coupling in topological insulator/ferrimagnetic insulator heterostructures

Introducing magnetic order in topological insulators via magnetic proximity coupling when in contact with a ferrimagnet is a promising way to realize novel topological physics such as quantum anomalous Hall effect. We have performed ferromagnetic resonance (FMR) of heterostructures made of yttrium iron garnet (YIG) films of varying thickness (15-30 nm) and Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ films (25 QL) from 300 K down to 5 K. Frequency- and angle-dependent FMR show sizable Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$-induced effective field in bilayer samples at room temperature, which is manifested as enhanced in-plane magnetic anisotropy over that of single layer YIG. The origin of such magnetic anisotropy occurring at interface is clarified by the YIG thickness dependence study. As the temperature decreases, the exchange effective field builds up pronouncedly and the FMR remains detectable. Specifically, for the bilayer sample of thin YIG (15 nm), the exchange effective field can induce FMR at microwave frequency of at least 3.5 GHz in absence of applied field below 50 K. Our study reveals the role of strong interlayer exchange coupling between Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and YIG in magnetization dynamics, leading to potential field-free spintronics application.