Highly efficient spin-orbit torque in a perpendicular synthetic ferrimagnet

Perpendicular synthetic ferrimagnet (pSFi) due to the low stray field and net magnetization is expected to replace single ferromagnet as a free layer to optimize the memory devices for high storage density and low-energy consumption. Here, we investigate the dependence of exchange-coupling strength on the thickness of the spacer Ru and current-induced magnetization reversal due to spin-orbit torque in $\mathrm{Ta}/\mathrm{Pt}/{\left[\mathrm{Pt}/\mathrm{Co}\right]}_2/\mathrm{Ru}/{\left[\mathrm{Co}/\mathrm{Pt}\right]}_4/\mathrm{Pt}$ structure with a perpendicular magnetic anisotropy. An oscillating interlayer exchange coupling as a function of Ru spacer layer thickness with a period of 1.16 nm is determined by combining the anomalous Hall effect and the polar magnetic-optical Kerr effect. Furthermore, current-controllable magnetization reversal experiments reveal that the magnetizations of top and bottom layers with antiferromagnetic coupling switch simultaneously due to the combination of spin-orbit torque generated from the two adjacent heavy-metal layers and the interlayer exchange torque. The SOT efficiency $\chi \sim 57.52\mathrm{Oe}/\left({10}^6\mathrm{A}/\mathrm{c}{\mathrm{m}}^2\right)$, corresponding to an effective spin Hall angle ${\xi }_{DL}\sim 0.68$, is estimated by analyzing current-dependent anomalous Hall resistance with our proposed quasistatic balance equation of magnetic moments. In addition to the advantage of minimizing stray field acting on the storage layer, the observed low switching current density and high spin-orbit torque efficiency suggest that the pSFi structure with a high thermal stability factor has great potential to realize the high-density and low-power consumption of nonvolatile magnetic memory devices.

Figure 1

(a) Schematic of a $\mathrm{Ta}/\mathrm{Pt}/{\left[\mathrm{Pt}/\mathrm{Co}\right]}_2/\mathrm{Ru}/{\left[\mathrm{Co}/\mathrm{Pt}\right]}_4/\mathrm{Pt}$ multilayer. (b) The optical microscope image of the Hall bar and the measurement configuration of anomalous Hall resistance.

Figure 2

(a), (b) Out-of-plane PMOKE hysteresis loops of the samples with FM (a) and AFM (b) interlayer exchange coupling. The top inset PMOKE images show magnetic domain structures during the field-driven magnetization switching process. The thickness of the Ru spacer is labeled. (c), (d) Normalized out-of-plane AHE hysteresis loop for samples with 1.5-nm- (c) and 1.0-nm- (d) thick Ru spacer. The blue and orange arrows depict the bottom and the upper magnetic moments. (e) Dependence of interlayer exchange-coupling field on thickness $t_{\mathrm{Ru}}$ of Ru spacer. The crosses represent the FM coupling samples. (f) The interlayer exchange-coupling strength $J_{\mathrm{IEC}}$ vs $t_{\mathrm{Ru}}$. The solid curve is the RKKY formula fitting result.

Figure 3

(a), (d) Anomalous Hall resistance $R_{\mathrm{H}}$ hysteresis loops of $\mathrm{Ta}/\mathrm{Pt}/{\left[\mathrm{Pt}/\mathrm{Co}\right]}_2/\mathrm{Ru}$ control sample with only bottom magnetic layer (a) and $t_{\mathrm{Ru}}=1.0\text{−}\mathrm{nm}$ SFi sample (d) measured with a small DC current $I=0.5\mathrm{mA}$ under out-of-plane (black curve) and in-plane (red curve) magnetic fields. The arrows in (d) illustrate the magnetic moments’ orientations of bottom and upper FM layers under an in-plane field. (b), (e) Current-induced switching of $\mathrm{Ta}/\mathrm{Pt}/{\left[\mathrm{Pt}/\mathrm{Co}\right]}_2/\mathrm{Ru}$ (b) and $t_{\mathrm{Ru}}=1.0\text{−}\mathrm{nm}$ SFi sample (e) under the labeled in-plane external fields $H_{\mathrm{out}}$ in the direction of the current. (c), (f) The critical switching current density $J_{\mathrm{c}}$ vs $H_{\mathrm{in}}$ for $\mathrm{Ta}/\mathrm{Pt}/{\left[\mathrm{Pt}/\mathrm{Co}\right]}_2/\mathrm{Ru}$ (c) and $t_{\mathrm{Ru}}=1.0\text{−}\mathrm{nm}$ SFi sample (f), respectively.

Figure 4

Current-induced magnetization switching of the AFM coupling sample $t_{\mathrm{Ru}}=2\mathrm{nm}$ under an in-plane magnetic field. (a) Field-dependent $R_{\mathrm{H}}$ hysteresis loops measured with an out-of-plane magnetic field and a small DC current $I=0.5\mathrm{mA}$. (b) DC current-dependent $R_{\mathrm{H}}$ hysteresis loops measured with different in-plane magnetic field $H_{\mathrm{in}}$ along the direction of applied current. (c) The critical switching current $J_{\mathrm{c}}$ vs in-plane $H_{\mathrm{in}}$.

Figure 5

Quantitative analysis of the current-induced SOT effective field of the AFM coupling sample $t_{\mathrm{Ru}}=2\mathrm{nm}$. (a) The raw current-dependent anomalous Hall resistance $R_{\mathrm{H}}$ under a series of out-of-plane magnetic fields. The raw $R_{\mathrm{H}}$($I$) includes a significant background noise consisting of the longitudinal resistance due to asymmetrically manufactured lateral electrodes, the Joule heating effect, and other systematic noise. (b) The difference of $R_{\mathrm{H}}$ between the parallel state at high fields and the antiparallel state at the low field, $H_{\mathrm{out}}=0.1\mathrm{kOe}$. (c) Experimental data (symbol) and numerical fitting curves (solid lines) under $H_{\mathrm{out}}=3\mathrm{kOe}$.