Enhancement of the spin-orbit torque in a Pt/Co system with a naturally oxidized Co layer

We investigated the effect of surface oxidation of a ferromagnetic Co layer on current-induced spin-orbit torque. A Co thin film was deposited on a Pt underlayer, for which the spin current was expected to be generated through the spin Hall effect. Both the damping- and field-like torques quantitatively determined using harmonic Hall voltage measurements were clearly enhanced in the sample with the naturally oxidized Co layer. Even though the oxidized system possessed a larger perpendicular magnetic anisotropy and nearly the same areal magnetization value, i.e., the same effective ferromagnetic thickness, as those of the unoxidized sample, the electrical current required for magnetization switching was greatly reduced. The results clearly show that surface oxidization of a ferromagnetic thin film deposited on a heavy metal layer plays an important role in enlarging spin-orbit torque.