Anomalous spin-orbit torque switching in synthetic antiferromagnets

We report that synthetic antiferromagnets (SAFs) can be efficiently switched by spin-orbit torques (SOTs) and the switching scheme does not obey the usual SOT switching rule. We show that both the positive and negative spin Hall angle (SHA)-like switching can be observed in Pt/SAF structures with only positive SHA, depending on the strength of applied in-plane fields. A switching mechanism directly arising from the asymmetric domain expansion is proposed to explain the anomalous switching behaviors. Contrary to the macrospin-based switching model where the SOT switching direction is determined by the sign of SHA, this switching mechanism suggests that the SOT switching direction is dominated by the field-modulated domain wall motion and can be reversed even with the same sign of SHA. The presence of this switching mechanism is further confirmed by the domain wall motion measurements. The anomalous switching behaviors provide important insights for understanding SOT switching mechanisms and also offer novel features for applications.

Figure 1

The SOT switching with a (a) positive or (b) negative SHA. $J$ is the injected current density, and m is the unit magnetization vector of adjacent magnets. m switches to up or down for the positive or negative SHA, respectively. (c) SOT-induced anomalous switching observed in this work, in which m can be switched to both up and down states under the SOT with the same sign. (d) Top view of a Hall bar structure showing the configurations of electrical measurements and coordinate system. (e) Magnetic properties of the Pt/SAF structure characterized by AHE (black) and VSM (blue). The red arrows show the switching sequence of magnetization. The insets show the magnetization configuration at each field stage.

Figure 2

The current-driven magnetization switching under (a) ±1 kOe and (b) ±5 kOe in-plane magnetic fields. (c) The magnetization switching induced by ±21-mA current pulses as a function of in-plane field. The red and black arrows indicate the corresponding switching sequence of red and black curves, respectively. (d) $M_R/M_S$ ratio (black) and $|H_t|$ (blue) as a function of $t_{\mathrm{Ru}}.\left|H_t\right|=0$ indicates no AMS observed.

Figure 3

$t_{\mathrm{Ru}}$-dependent current-induced magnetization switching with (a) $t_{\mathrm{Ru}}=0.44\mathrm{nm}$ and (b) 0.88 nm. (c) The current-induced magnetization switching in a control sample with a thicker BML. The arrows indicate the switching sequence. Inset of (c) shows the relative thickness of BML and TML. (d–f) Illustrations of SOT switching based on DW motion. (d) All DWs move with the same velocity, and domains keep the same shape during the current-driven DW motion. (e, f) The different velocities of $\uparrow {\downarrow }_{\mathrm{BML}}$ and $\downarrow {\uparrow }_{\mathrm{BML}}$ DWs induce the expansion or contraction of opposite domains during the current-driven DW motion. Yellow arrows represent DW velocities.

Figure 4

The current-driven DW motion as a function of current density under (a) positive and (b) negative in-plane fields. Red and blue represent ±1 kOe and ±4.5 kOe external fields, respectively. (c, d) The schematic of in-plane field-modulated DW motion for (c) a single magnetic layer and (d) AFM coupled bilayer. (e) $V_D$ as a function of in-plane field driving by the pulses with a current density of $3.97\times {10}^7\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$. The inserted illustrations illustrate the domain expansion/contraction at each in-plane field region, consistent with the four SOT switching regions shown in Fig. 2. For all figures, diamonds and circles represent $\uparrow {\downarrow }_{\mathrm{BML}}$ and $\downarrow {\uparrow }_{\mathrm{BML}}$ DWs, respectively. For clarification, the error bars are omitted (see Supplemental Material [33] for the determination of error).