Interplay of spin current and magnetization in a topological-insulator/magnetic-insulator bilayer structure

The interaction between accumulated spins on the surface of a heavy metal (HM) and the magnetization of an adjacent magnetic material leads to various spin phenomena, such as spin-orbit torque, spin pumping, and spin Hall magnetoresistance (SHMR). However, the exploration of device applications based on these spin phenomena is often limited by the low charge-to-spin conversion efficiency of the HM. Authors of recent studies have suggested that topological insulators (TIs) are promising candidates for device applications due to their potentially higher charge-to-spin conversion efficiency. Here, we report a multifaceted study of a bilayer structure consisting of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}\mathrm{O}$ (YIG) and demonstrate an approach based on angle-dependent magnetoresistance (ADMR) measurements to determine the effective charge-to-spin conversion efficiency in TIs. Our ferromagnetic resonance measurements demonstrate efficient spin pumping from YIG to ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, which is further confirmed by detection of an electromotive force generated in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ via spin-to-charge conversion. Our ADMR measurements show that the interfacial spin diffusion can significantly affect the charge transport in a way like the SHMR effect and provide an estimate of the charge-to-spin conversion efficiency in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ of $\ensuremath{\sim}0.1--0.4$. Neglecting to account for the large out-of-plane magnetoresistance of the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ results in a fivefold overestimate of the charge-to-spin conversion efficiency.