Influence of the magnetic proximity effect on spin-orbit torque efficiencies in ferromagnet/platinum bilayers

Current-induced spin-orbit torques in ${\mathrm{Co}}_2\mathrm{FeAl}/\mathrm{Pt}$ ultrathin bilayers are studied using a magnetoresistive harmonic response technique, which distinguishes the dampinglike and fieldlike contributions. The presence of a temperature-dependent magnetic proximity effect is observed through the anomalous Hall and anisotropic magnetoresistances, which are enhanced at low temperatures for thin platinum thicknesses. The fieldlike torque efficiency decreases steadily as the temperature is lowered for all Pt thicknesses studied, which we propose is related to the influence of the magnetic proximity effect on the fieldlike torque mechanism.

Figure 1

(a) The bilayer square resistance for all Pt thicknesses. The solid black squares are the 300-K data, and the open red squares are the 20-K data. In the inset, the Pt resistivity is plotted vs the inverse of the Pt thickness. The intercepts of the solid lines correspond to the bulk resistivity of Pt. In (b), the SOT efficiencies ${\xi }_{DL}$ (the circles) and ${\xi }_{FL}$ (the squares) are shown for different Pt thicknesses at 300 K (the black solid symbols) and 20 K (the red open symbols). The lines connect the data points. For all data the CFA thickness is 1.2 nm.

Figure 2

Summary of $R_{\mathrm{AHE}}$ (the squares) and $R_{\mathrm{AMR}}$ (the triangles) vs $R_{xx}$ for the different bilayers, which are labeled by the Pt thickness. Temperature is an implicit variable, and the minima and maxima of $R_{xx}$ correspond to 10 and 300 K, respectively, for all bilayers except the 1-nm Pt bilayer in which $R_{xx}$ shows a small upturn below 20 K. The dashed lines indicate $R_{MR}\propto R_{xx}^2$, which is expected for MR originating from F shunting alone. The inset magnifies the AHE data for the 1- and 2-nm Pt bilayers. See Fig. 5 in the Supplemental Material [15] for an alternative representation in which temperature is indicated explicitly and details on how $R_{\mathrm{AHE}}$ and $R_{\mathrm{AMR}}$ were measured.

Figure 3

The temperature dependence of the SOT efficiencies ${\xi }_{DL}$ (the black squares, left ordinate) and ${\xi }_{FL}$ (the red triangles, right ordinate 1) for the 1-, 2-, 4-, and 6-nm Pt bilayers as indicated in the figure. The ${\xi }_{FL}$ data have been scaled by a factor of $-1$. The error bars represent the standard errors. The Pt resistivity is shown (right ordinate 2) as the blue open circles, and the lines connect the data points.

Figure 4

Illustrations of (a) the Rashba spin accumulation $S_R$ at high temperatures in the absence of the MPE and (b) MPE order at low temperatures which serves to precess and destroy the transverse Rashba spin accumulation. The magnetization $\widehat{\mathit{m}}$ and current ${\mathit{j}}_e$ are indicated with the red and black arrows, respectively. Right-drifting carriers in N, which make up the current, are drawn as the blue arrows (denoting spin accumulation) with the black dashed trajectories implying scattering events. Plots of the exchange interaction strength vs depth coordinate $z$ in the bilayer, which are schematic and not drawn to scale, are included.