Enhancement of Current-Induced Out-of-Plane Spin Polarization by Heavy-Metal-Impurity Doping in $\mathrm{Fe}$ 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.

Figure 1

(a) Schematic of anomalous spin-orbit torque. Charge current (orange arrow) generates a pure spin current (green arrow) flowing along the z direction, inducing spin accumulation at the top and bottom surfaces. Accumulated spins pointing in the y direction cause spin-orbit torque (or, equivalently, effective field denoted by red arrows) on magnetic moments of $\mathrm{Fe}$ (purple arrow). (b) The x component of magnetization as a function of magnetic field parallel to the x direction measured by longitudinal MOKE. (c) Out-of-plane magnetic field (H_z) dependence of Kerr ellipticity without applying charge current. Inset represents the experimental setup for this measurement. Black line is a linear fit to data. (d) Schematic diagram of the measurement system for current-induced spin polarization. HWP, BS, and QWP denote half-wave plate, beam splitter, and quarter-wave plate, respectively. (e) Light-polarization dependence of the voltage signal from the photodetector measured by the lock-in amplifier. Here, α is the angle of the λ/4 plate. Symbols $\leftrightarrow$, $\circlearrowright$, and $\circlearrowleft$ represent linearly, right-handed circularly polarized light, and left-handed circularly polarized light, respectively.

Figure 2

(a) Kerr ellipticity (η_K) as a function of magnetic field along the x direction in ${\mathrm{Fe}}_{0.87}{\mathrm{Pt}}_{0.13}$. (b) Position dependence of Δη_K. Here, Δη_K is defined as Δη_K = [η_K(+M) − η_K(−M)]/2, where η_K(+M) and η_K(−M) are the Kerr ellipticity for M parallel and antiparallel to the x direction, respectively. Light blue shading represents the sample area. (c) Position dependence of H-independent offset signal of Kerr ellipticity (η_{K, offset}), which is defined as η_{K, offset}= [η_K(+M) + η_K(−M)]/2. Error bars correspond to standard deviation.

Figure 3

(a) Kerr ellipticity (η_K) as a function of magnetic field along the x direction in ${\mathrm{Fe}}_{0.87}{\mathrm{Pt}}_{0.13}$ at various current densities. Error bars correspond to standard deviation. (b) Current-density dependence of Δη_K in ${\mathrm{Fe}}_{0.87}{\mathrm{Pt}}_{0.13}$. Dashed black line is a guide to the eye. (c) Current-density dependence of the H-independent offset signal of Kerr ellipticity (η_{K, offset}), which is defined as η_{K, offset}= [η_K(+M) + η_K(−M)]/2. Dashed black line is a guide to the eye.

Figure 4

(a)–(d) Kerr ellipticity (η_K) as a function of magnetic field along the x direction at various $\mathrm{Pt}$ concentrations (x) measured with a current density of $j=8.0\times {10}^9\mathrm{A}/{\mathrm{m}}^2$. (e) $\mathrm{Pt}$-concentration dependence of Δη_K at a current density of $j=8.0\times {10}^9\mathrm{A}/{\mathrm{m}}^2$. Thick green curve is a guide to the eye. Error bars correspond to standard deviation.