Spin-orbit torque switching from spin-momentum locked surface states of amorphous BiSe*

Given the recent observation of spin-momentum locked surface states in amorphous Bi2Se3, the textbook three dimensional topological insulator in its crystalline form, it is natural to couple these states to a ferromagnetic layer to manipulate the magnetization for use in potential spintronic applications.

Due to the spin-momentum locked surface states, an electrical current flowing posseses a net spin-polarization, which we use to switch an adjacent ferromagnetic layer using spin-orbit torque. We grow heterostructures of amorphous Bi2Se3 on top of a sputtered Ta/Co/Pt magnetic layer stack, which has perpendicular magnetic anisotropy. We investigate the spin-orbit torque efficiency of the Bi2Se3 by using standard harmonic hall and hysteresis loop shift measurements with devices in Hall-cross geometry. Finally, we are exploring the ultrafast magnetization dynamics by patterning the material stack and depositing an Auston switch on a LT-GaAs substrate, that allows the generation of ps-current pulses, while the magnetic response is observed using time resolved MOKE measurements.

For amorphous Bi2Se3-thicknesses <10nm we find a spin Hall angle that is larger compared to those of conventional heavy metals. The spin-orbit torque efficiency is furthermore decreasing for increasing Bi2Se3-thickness, indicating a dominant contribution of the spin-momentum locked surface states.

Our results have important consequences to technological applications since the growth conditions of amorphous Bi2Se3 are much less stringent than its crystalline counterpart.