Enhancement of Current-Induced Out-of-Plane Spin Polarization by Heavy-Metal-Impurity Doping in Fe Thin Films

Current-induced spin polarization in magnets has recently attracted much attention because out-of-plane spin polarization is allowed, as opposed to polycrystalline nonmagnetic heavy metals. We investigate the Pt-impurity-doping dependence of current-induced out-of-plane spin polarization in Fe thin films with the use of magnetocircular dichroism. Current-induced out-of-plane spin polarization is clearly observed and attributed to anomalous spin-orbit torque, in which the spin-orbit torque induced by the spin Hall effect in Fe acts on the magnetic moments of Fe at the surfaces, generating out-of-plane spin polarization. Notably, current-induced out-of-plane spin polarization is significantly enhanced with increasing Pt concentration. Pt impurities with a large spin-orbit interaction induce the extrinsic spin Hall effect of Fe, and consequently, the anomalous spin-orbit torques are enhanced. Our results provide a guiding principle for strengthening current-induced spin polarization in magnets.