Efficient charge-spin conversion and magnetization switching through the Rashba effect at topological-insulator/Ag interfaces

We report the observation of efficient charge-to-spin conversion in the three-dimensional topological insulator (TI) Bi₂Se₃ and Ag bilayer by the spin-torque ferromagnetic resonance technique. The spin-orbit-torque ratio in the Bi₂Se₃/Ag/CoFeB heterostructure shows a significant enhancement as the Ag thickness increases to {~}2 nm and reaches a value of 0.5 for 5 nm Ag, which is {~}3 times higher than that of Bi₂Se₃/CoFeB at room temperature. The observation reveals the interfacial effect of Bi₂Se₃/Ag exceeds that of the topological surface states (TSSs) in the Bi₂Se₃ layer and plays a dominant role in the charge-to-spin conversion in the Bi₂Se₃/Ag/CoFeB system. Based on first-principles calculations, we attribute our observation to the large Rashba splitting bands which wrap the TSS band and have the same net spin polarization direction as the TSS of Bi₂Se₃. Subsequently, we demonstrate Rashba-induced magnetization switching in Bi₂Se₃/Ag/Py with a low current density of 5.8×10⁵0.16em0exA/cm².