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 $\mathrm{Pt}$-impurity-doping dependence of current-induced out-of-plane spin polarization in $\mathrm{Fe}$ 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 $\mathrm{Fe}$ acts on the magnetic moments of $\mathrm{Fe}$ at the surfaces, generating out-of-plane spin polarization. Notably, current-induced out-of-plane spin polarization is significantly enhanced with increasing $\mathrm{Pt}$ concentration. $\mathrm{Pt}$ impurities with a large spin-orbit interaction induce the extrinsic spin Hall effect of $\mathrm{Fe}$, and consequently, the anomalous spin-orbit torques are enhanced. Our results provide a guiding principle for strengthening current-induced spin polarization in magnets.