Detection of topological surface states by spin pumping at room temperature

Spin pumping on heterostructures made of ferrimagnetic YIG film and topological insulator Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ films has been performed at room temperature. In the presence of topological interface states, spin pumping induced non-equilibrium spin density caused significant resonance field shifts (H$_{\mathrm{res}}$ shifts) of YIG/Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ with respect to bare YIG. The uncommon H$_{\mathrm{res}}$ shifts correspond to clearly resolved changes of gyromagnetic ratio of YIG. As the Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ thickness varied from 4 nm to 20 nm, increasing H$_{\mathrm{res}}$ shifts were observed, while the enhancement of damping constant saturated at the spin diffusion length of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, suggesting the two parameters were of different origins. Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ thickness dependence of spin pumping revealed that Rashba-split 2DEG has comparable effects on the magnetization dynamics. From the change of gyromagnetic ratio, we calculated the imaginary part of spin mixing conductance to be one order of magnitude larger than the real part. Our results showed that with clean and well-defined interface, spin pumping may serve as an effective way to detect spin-polarized surface states.