Spin-Transfer Torques in Dual-Gated Bismuth Selenide Topological Insulator Devices

Recent theoretical and experimental work on topological insulator / ferromagnet bilayers suggests that bismuth selenide can act as a source of spin current for applying a spin transfer torque to an adjacent magnetic layer. To help determine the mechanism of the in-plane and out-of-plane spin torques, we fabricate dual-gated bismuth selenide devices with a ferromagnetic permalloy nanowire positioned between the gates to act as an absorber of spin currents. We use the spin-torque ferromagnetic resonance technique to measure current-induced torques acting on the permalloy nanowire. We will attempt to distinguish between surface and bulk mechanisms for the torque by sweeping a uniform voltage applied to both gates to tune the carrier density. We will also study whether the surface spin current can be modified by applying different gate voltages to induce a large gradient in the electron chemical potential near the permalloy wire. Such a modification is expected as a consequence of locking between the orientations of the electron wavevector and spin in topological insulator surface states.