Optical detection of spin-orbit torque in Ta/CoFeB and W/CoFeB bilayers

The spin Hall effect in heavy metals generates a spin-orbit torque (SOT) on a ferromagnet. To date, most SOT studies have been conducted using electrical methods, such as harmonic Hall response and spin-torque ferromagnetic resonance. In this study, we investigated SOT in heavy metals, Ta and W, using an optical method of magneto-optical Kerr effect. The optical method has the advantage of spatial resolution that is used for a clear separation between the SOT and the Oersted field. From a quantitative comparison between the SOT and the Oersted field, we determined the lower bounds of the spin Hall angle of 0.12 and 0.21 for Ta and W, respectively. From a thickness dependence of the spin Hall conductivity, we determined the spin diffusion length of 4 nm for both Ta and W.