Enhanced torque efficiency in ferromagnetic multilayers by introducing naturally oxidized Cu

Spin-orbit torque (SOT) in the heavy elements with a large spin-orbit coupling (SOC) has been frequently used to manipulate the magnetic states in spintronic devices. Recent theoretical works have predicted that the surface oxidized light elements with a negligible SOC can yield a sizable orbit torque (OT), which plays an important role in switching the magnetization. Here, we report anomalous-Hall-resistance and harmonic-Hall-voltage measurements on perpendicularly magnetized Ta/Cu/[Ni/Co]₅/Cu-CuO_x multilayers. Both torque efficiency and spin-Hall angle of these multilayers are largely enhanced by introducing a naturally oxidized Cu-CuO_x layer, where the SOC is negligible. Such an enhancement is mainly due to the collaborative driven of the SOT from the Ta layer and the OT from the Cu/CuO_x interface and can be tuned by controlling the thickness of Cu-CuO_x layer. Compared to the Cu-CuO_x-free multilayers, the maximum torque efficiency and spin-Hall angle were enhanced by a factor of ten, larger than most of the reported values in the other heterostructures.