Magnetization switching through spin-Hall-effect-induced chiral domain wall propagation

The influence of spin-Hall-effect spin torque (SHE-ST) induced by in-plane charge current was studied in microscale Ta/Co${}_{20}$Fe${}_{60}$B${}_{20}$/TaO${}_x$ films with perpendicular magnetization. Simultaneous electrical transport and polar magneto-optical Kerr effect (MOKE) imaging experiments were used to investigate the switching dynamics. A rich set of switching behaviors was observed, which can be well understood by analyzing a switching-phase diagram and polar MOKE images, considering the competition between SHE-ST and the externally applied magnetic field. Furthermore, we found that domain walls with a particular chirality were dominant in our devices, which suggests the presence of the Dzyaloshinskii-Moriya interaction in the present material system.

Figure 1

(a) Top view of the device consisting of a Ta/CoFeB/TaO${}_x$ stack structure (10 μm scale bar). (b) The effective field ($H_{\mathrm{SHE}}$) acting on the magnetic moment, which is tilted away from the $z$ axis in the xz plane by an external magnetic field. The field is in the xz plane, and the $x$ and $z$ components are also shown. Schematic of the spin texture inside a NW is shown as well. Red, blue and white arrows present the magnetization pointing up, pointing down, and in the DW, respectively. Green and red arrows on the side wall show the directions of the spin polarized electrons. (c) Dependence of anomalous Hall resistance on the perpendicular magnetic field $H_z$ measured at a positive current of +1 mA. (d), (e) Current-induced switching at room temperature in the presence of a constant in-plane magnetic field $H_x$, with (d) $H_x$ = 372 Oe and (e) $H_x=-361$ Oe.

Figure 2

Anomalous Hall resistance as a function of $x$-component field $H_x$ at $I_x$ = ±1 mA, ±6 mA, ±8, and ±15 mA. The external magnetic is $\vec{H}=H_x\widehat{x}+H_z\widehat{z}$, at the polar angle of θ ∼ 88°. The left (right) figures show the change of hysteresis loops when increasing current of positive (negative) values. The red (black) curve corresponds to the magnetic field sweeping forward (backward). A–G in (g) labels the magnetic field values, where polar MOKE images are taken for illustrating the multiple switching due to the competition between $H_z$ and SHE (in the $z$-axis direction), as shown in Fig. 4. The red (black) labels correspond to the magnetic field sweeping forward (backward). The competition is schematically shown by blue ($H_z$) and green (SHE) arrows, where the length of the arrows represents the strength of field.

Figure 3

Experimentally determined switching-phase diagram with the externally applied field $\vec{H}$ at angles of θ ∼ 91°, θ ∼ 90°, θ ∼ 89°, and θ ∼ 88°. For each value of applied field, corresponding switching currents ($I_c$) are obtained by sweeping current. For each value of θ, the area is divided into five regions, represented by I–V. The magnetizations in these five regions are $M_z$ < 0, $M_z$ > 0, $M_z$ > 0, $M_z$ < 0, and bistable ($M_z$ > 0 or $M_z$ < 0), respectively.

Figure 4

MOKE images at different $H_x$ points (A→D→G), shown in Fig. 2 with $I_x=-8$ mA. The red (blue) color shows the up (down) magnetization, which is also labeled by white marks (see Fig. 1). The white arrows in the figures indicate the magnetization direction in the DW, which is considered to approximately follow the direction of $H_x$. The white ⊙ and ⊗ present the magnetization direction in the magnetic domains.

Figure 5

Schematic representation of the domain wall (DW) motion induced by SHE-ST for both right- and left-handed chiral DWs. The formula represents the effective field ($H_{\mathrm{SHE}}$) induced by SHE. The black arrows indicate the magnetizations in chiral DW. The direction of magnetization in the center of DW is consistent with the external field ($H_x$). The directions of $H_{\mathrm{SHE}}$ corresponding to different DW structure and $J_x$ directions are shown by the green and red marks. The left (right) side magnetic configuration corresponds to right- (left-)handed chirality, where the DW moves along (against) the current direction.