Spin-orbit torque in MgO/CoFeB/Ta/CoFeB/MgO symmetric structure with interlayer antiferromagnetic coupling

Spin current generated by the spin Hall effect in a heavy metal that would diffuse up and down to adjacent ferromagnetic layers and exert torque on their magnetization is called spin-orbit torque. Antiferromagnetically coupled trilayers, namely, the so-called synthetic antiferromagnets usually are employed to serve as the pinned layer of spintronic devices based on spin valves and magnetic tunnel junctions to reduce the stray field and/or increase the pinning field. Here we investigate the spin-orbit torque in a MgO/CoFeB/Ta/CoFeB/MgO perpendicularly magnetized multilayer with interlayer antiferromagnetic coupling. It is found that the magnetization of two CoFeB layers can be switched between two antiparallel states simultaneously. This observation is replicated by the theoretical calculations by solving the Stoner-Wohlfarth model and the Landau-Lifshitz-Gilbert equation. Our findings combine spin-orbit torque and interlayer coupling, which might advance the magnetic memories with a low stray field and low power consumption.

Figure 1

(a) Schematic of a MgO/CoFeB/Ta/CoFeB/MgO multilayer. The expanded area shows the SHE brought on spin polarization and in turn magnetization switching. $R_{\mathrm{H}}$ curves measured when (b) $H_{\mathrm{ext}}$ is fixed in the $+z$ direction and (c) $H_{\mathrm{ext}}$ is on the $yz$ plane and near the $+y$ direction $(\beta =1^{\circ })$.

Figure 2

(a) Magnetization switching characterized by $R_{\mathrm{H}}$ in the presence of positive and negative external fields fixed in the $y$ direction. (b) Current-induced switching under different external magnetic fields applied in the $+y$ direction.

Figure 3

(a) Sketch of a heavy metal layer sandwiched by two ferromagnetic metal layers and corresponding Cartesian coordinates with relevant orientation of magnetic moments ${\mathbf{m}}_1$ and ${\mathbf{m}}_2$. (b) Hysteresis loop calculated by the Stoner-Wohlfarth model. (c) Current-induced switching under positive and negative external fields exhibited by the angular coordinates of ${\mathbf{m}}_1$ and ${\mathbf{m}}_2$ and (d) the $z$ component of total magnetization $M_{\mathrm{z}}$. (e) Critical $\tau$ for magnetization switching vs the external field by solving the torque balance equation.

Figure 4

(a) Orientation of magnetic moments ${\mathbf{m}}_1$ and ${\mathbf{m}}_2$ defined in the macrospin calculation. Magnetization switching trajectories with the same assistant external field $h_y=0.375$, antiferromagnetic coupling $a=2$, and different dimensionless torque coefficients (b) $C_{\left|\right|}=0.75, C_{\perp }=0.4$; (c) $C_{\left|\right|}=1.5, C_{\perp }=0.8$; and (d) $C_{\left|\right|}=3, C_{\perp }=1.6$. The pink solid line and the green solid line represent the initial and final positions of magnetization, respectively. The red curves and the blue curves individually stand for the tracks of magnetic moments ${\mathbf{m}}_1$ and ${\mathbf{m}}_2$. (e) Time evolution of the $z$ component of magnetization extracted from (b)–(d).