Spin-charge conversion at magnetic insulator/topological insulator interfaces

The development of next-generation spintronic devices has driven extensive studies of spin-charge conversion through measurements of the inverse spin Hall effect and/or the inverse Rashba–Edelstein effect in both three-dimensional and two-dimensional material systems. Topological insulators such as the Bi-chalcogenides are naturally relevant in this context due to the expected large spin-orbit coupling strength and the inherent spin-momentum “locking” in their surface states. We report the observation of robust ferromagnetic resonance-driven spin pumping signals in three-dimensional topological insulator thin films -- Bi$_2$Se$_3$ and (Bi, Sb)$_2$Te$_3$ -- deposited by molecular beam epitaxy on the ferrimagnetic insulator Y$_3$Fe$_5$O$_{12}$. By systematically varying the Bi$_2$Se$_3$ film thickness, we show that the spin-charge conversion efficiency, characterized by the inverse Rashba–Edelstein effect length, increases dramatically as the film thickness is increased from two quintuple layers, saturating above six quintuple layers [Phys. Rev. Lett. 117, 076601 (2016)]. For bulk insulating (Bi, Sb)$_2$Te$_3$ thin films, by electrical gating and varying the chemical compositions, we demonstrate that the spin-charge conversion efficiency follows a constant value when the Fermi level lies within the bulk band gap and shows opposite variation trends when Fermi level enters the conduction and valance bands. Our results uncover the spin-charge conversion mechanism in topological insulators and suggest the dominant role played by spin-momentum “locking” and spin-orbit coupling for surface and bulk states respectively. This work was carried out in collaboration with James Kally, Joon Sue Lee, Tao Liu, Houchen Chang, Danielle Reifsnyder Hickey, K. Andre Mkhoyan, Mingzhong Wu, Anthony Richardella, and Nitin Samarth. We acknowledge support from the Center for Spintronic Materials, Interfaces, and Novel Architectures (C-SPIN), a funded center of STARnet, a Semiconductor Research Corporation (SRC) program sponsored by MARCO and DARPA.