Anomalous Current-Induced Spin Torques in Ferrimagnets near Compensation

While current-induced spin-orbit torques have been extensively studied in ferromagnets and antiferromagnets, ferrimagnets have been less studied. Here we report the presence of enhanced spin-orbit torques resulting from negative exchange interaction in ferrimagnets. The effective field and switching efficiency increase substantially as CoGd approaches its compensation point, giving rise to 9 times larger spin-orbit torques compared to that of a noncompensated one. The macrospin modeling results also support efficient spin-orbit torques in a ferrimagnet. Our results suggest that ferrimagnets near compensation can be a new route for spin-orbit torque applications due to their high thermal stability and easy current-induced switching assisted by negative exchange interaction.

Figure 1

(a) Schematic of film stack and transport measurement geometry. (b) $R_{\mathrm{AHE}}$ plotted as a function of Gd concentration. The insets show opposite $R_{\mathrm{AHE}}$ hysteresis loops for the Co and Gd dominated samples. (c) Coercivity of deposited films using MOKE and that of the fabricated devices using AHE measurements. The left (right) inset shows a hysteresis loop for Co (Gd) rich sample using MOKE. Note the reversal of hysteresis loops across compensation. (d) Saturation magnetization versus Gd concentration measured by VSM.

Figure 2

(a),(b) 1st and 2nd harmonic Hall voltage measurements with an in-plane field applied parallel ($\parallel$) and perpendicular ($\perp$) to the current direction. Colored lines represent the quadratic and linear fitting for extracting the $H_L$ and $H_T$. Note that the net magnetization ($M$) is in the upwards direction during measurements for both the cases. (c),(d) Current-induced switching loops for the Co and Gd rich samples. An offset due to Hall bar misalignment has been removed.

Figure 3

(a) Normalized longitudinal effective field ($H_L$) and switching efficiency ($\chi$) with a normalized value of $1/M_s$. $H_L$, $\chi$, and $1/M_s$ for various samples are normalized to their respective values of the ${\mathrm{Co}}_{80}{\mathrm{Gd}}_{20}$ sample. The inset in (b) shows the normalized macrospin switching efficiency ($\eta$) for various compositions.

Figure 4

(a) Schematic representation of various fields acting on the two sublattices with (i) negative and (ii) positive exchange interaction. (b) Macrospin simulation results. Simulations were carried out in the presence of anisotropy and external fields. Plot shows normalized $H_L$ of ferrimagnet compared with normalized $1/M_s$ for different $B$ concentrations. The scaling trend of normalized $H_L$ of an equivalent ferromagnet (similar $M_s$) is also shown. Values are normalized with respect to the values for $A_{99}{B}_1$.