In-plane field-driven crossover in the spin-torque mechanism acting on magnetic domain walls in Co/Ni

We investigate the in-plane field dependence of current-induced magnetic domain wall motion in asymmetric Co/Ni nanowires. The application of an in-plane field changed the direction of domain wall motion. In a systematic study, by varying the magnetic layer thickness, we show that the in-plane field induces a crossover from spin Hall torque to adiabatic spin-transfer torque for the magnetic domain wall motion. Furthermore, the dependence of the threshold current density on the in-plane longitudinal field provides a way for determining the strength of the Dzyaloshinskii-Moriya interaction.

Figure 1

Schematic drawing of the device structure and measurement setup. Directions of perpendicular and in-plane longitudinal fields are defined as $+H_Z$ and $+H_L.+J$ shows a direction of current flowing through a magnetic layer.

Figure 2

DW motion probability ($P_{\mathrm{DW}}$) as a function of current densities (J) for $t_{\mathrm{Co}/\mathrm{Ni}}=3.0\mathrm{nm}$ under an in-plane longitudinal field ($H_{\mathrm{L}}$) for (a) a down-up DW and (b) an up-down DW. $P_{\mathrm{DW}}$ are estimated by ten times measurements for each J.

Figure 3

(a) Schematic illustration of the DW structure in a Co/Ni nanowire that is contacted with a Pt layer. DW structures for a down-up and an up-down DW are expected to be in the intermediate state between the Bloch DW and Néel DW due to the reduced effect of the DMI in the intermediate thickness. (b) A Néel DW structure is expected when $H_{\mathrm{L}}$ is applied parallel to $H_{\mathrm{DMI}}$. (c) A Bloch-like DW structure is expected when $H_{\mathrm{L}}$ is applied in the antiparallel direction to $H_{\mathrm{DMI}}$. The purple arrow shows the magnetic moment inside DW.

Figure 4

Threshold current density ($J_{\mathrm{th}}$) as a function of in-plane longitudinal field $\left(\right|H_{\mathrm{L}}\left|\right)$ for several magnetic layer thicknesses: (a) $t_{\mathrm{Co}/\mathrm{Ni}}=3.0\mathrm{nm}$, (b) $t_{\mathrm{Co}/\mathrm{Ni}}=3.9\mathrm{nm}$, and (c) $t_{\mathrm{Co}/\mathrm{Ni}}=4.8\mathrm{nm}$. Red and blue triangles show a down-up DW and an up-down DW, respectively. $J_{\mathrm{th}}$ is defined as J, where $P_{\mathrm{DW}}$ reaches to 0.5. Error bars show 20/80% probabilities. $|H_{\mathrm{L}}|$ indicates an absolute value of $H_{\mathrm{L}}$. Schematic diagrams in (c) show a transition of the DW structure with $H_{\mathrm{L}}$.