  Increasing dampinglike spin-orbit torque (SOT) is both of fundamental
importance for enabling new research into spintronics phenomena and also
technologically urgent for advancing low-power spin-torque memory, logic, and
oscillator devices. Here, we demonstrate that enhancing interfacial scattering
by inserting ultra-thin layers within a spin Hall metals with intrinsic or
side-jump mechanisms can significantly enhance the spin Hall ratio. The
dampinglike SOT was enhanced by a factor of 2 via sub-monolayer Hf insertion,
as evidenced by both harmonic response measurements and current-induced
switching of in-plane magnetized magnetic memory devices with the record low
critical switching current of ~73 uA (switching current density of 3.6x10^6
A/cm^2). This work demonstrates a very effective strategy for maximizing
dampinglike SOT for low-power spin-torque devices.
