Room-temperature high spin-orbit torque due to quantum confinement in sputtered Bi_xSe_(1-x) films

The spin-orbit torque (SOT) that arises from materials with large spin-orbit coupling promises a path for ultralow power and fast magnetic-based storage and computational devices. We investigated the SOT from magnetron-sputtered Bi_xSe_(1-x) thin films in Bi_xSe_(1-x)/Co₂₀Fe₆₀B₂₀ heterostructures by using d.c. planar Hall and spin-torque ferromagnetic resonance (ST-FMR) methods. Remarkably, the spin torque efficiency (θ_S) was determined to be as large as 18.62 ± 0.13 and 8.67 ± 1.08 using the d.c. planar Hall and ST-FMR methods, respectively. Moreover, switching of the perpendicular CoFeB multilayers using the SOT from the Bi_xSe_(1-x) was observed at room temperature with a low critical magnetization switching current density of 4.3 × 10⁵ A cm^-2. Quantum transport simulations using a realistic sp³ tight-binding model suggests that the high SOT in sputtered Bi_xSe_(1-x) is due to the quantum confinement effect with a charge-to-spin conversion efficiency that enhances with reduced size and dimensionality. The demonstrated θ_S, ease of growth of the films on a silicon substrate and successful growth and switching of perpendicular CoFeB multilayers on Bi_xSe_(1-x) films provide an avenue for the use of Bi_xSe_(1-x) as a spin density generator in SOT-based memory and logic devices.