Spin–orbit torque generated by spin–orbit precession effect in Py/Pt/Co tri-layer structure

In this report, current-induced spin–orbit torques (SOTs) in a permalloy (Py)/Pt/Co tri-layer system are investigated. In addition to the conventional SOTs generated from the spin Hall effect in the Pt layer, we observed a distinct existence of unconventional SOT acting on the in-plane magnetized Py layer in which the polarity of SOT reverses with the magnetization of perpendicularly magnetized Co. From its symmetry and experimental verifications, we conclude that this unconventional SOT, especially the damping-like torque term, originates from the additional spin current generated by the spin–orbit precession effect at the Pt/Co interface. Moreover, we reveal that the polarity of this SOT strongly depends on the interface structure of the ferromagnetic material/non-magnetic metal (NM), which can be controlled by the choice of NM material. The interface structure dependence shows a consistent result with first principle calculations of the spin–orbit precession effect that were previously reported. Our findings not only broaden the origin of the current-induced SOT but also pave the way for the design of high-efficient SOT-based devices.