High Domain Wall Velocities due to Spin Currents Perpendicular to the Plane

We consider long and narrow spin valves composed of a first magnetic layer with a single domain wall (DW), a normal metal spacer, and a second magnetic layer that is a planar or a perpendicular polarizer. For these structures, we study numerically DW dynamics taking into account the spin torques due to the perpendicular spin currents. We obtain high DW velocities: $5\mathrm{m}/\mathrm{s}$ for planar polarizer and $80\mathrm{m}/\mathrm{s}$ for perpendicular polarizer for $I=0.01\mathrm{mA}$. These values are much larger than those predicted and observed for DW motion due to the in-plane spin currents. The ratio of the magnitudes of the torques, which generate the DW motion in the respective cases, is responsible for these large differences.

Figure 1

(a) DW displacement for $J=1\times {10}^7\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$ for a CPP spin valve with the planar polarizer. Inset: sketch of the device. (b) Velocity of the steady-state DW motion as a function of the current density $J$. The dashed line shows a threshold for the domain excitation. Inset: shape of the DW during its steady-state motion for $J=1\times {10}^7\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$; the color code (grayscale) represents the $x$-component of magnetization ranging from 0 to $+M_s$. (c) DW displacement taking into account only the ST (top panel) or only the FLT (bottom panel).

Figure 2

(a) Current-induced DW displacement for $J=0.5\times {10}^7\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$ (circles) and for $J=2\times {10}^7\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$ (triangles) for a CPP spin valve with the perpendicular polarizer. Inset: sketch of the device. (b) Velocity of the steady-state DW motion as a function of the current density $J$. Inset: snapshot of the transformed and eventually stopped DW; the color code (grayscale) represents the out-of-plane component of magnetization ranging from 0 to $+M_s$. (c) DW displacement taking into account only the ST (top panel) or only the FLT (bottom panel), for $J=0.5\times {10}^7\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$ and for $J=2\times {10}^7\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$ .