Switching of Perpendicular Magnetization via ac Spin-Orbit Torque

We theoretically and numerically investigate the switching of perpendicular magnetization by ac spin-orbit torque. We find that the threshold switching current for ac spin-orbit torque is much smaller than that for dc spin-orbit torque. Moreover, the ac spin-orbit torque acts like an effective out-of-plane field in the rotating frame and thus allows field-free deterministic switching of perpendicular magnetization. The ac spin-orbit torque scheme will be useful for energy-efficient operation of spin-orbit torque devices.