Nonlinear Domain-Wall Velocity Enhancement by Spin-Polarized Electric Current

The interaction between a dc spin-polarized electric current and a magnetic domain wall in a Permalloy nanowire was studied by high-bandwidth scanning Kerr polarimetry. The full functional dependence of wall velocity on electric current and magnetic field is presented. With the pinning potential nulled by a field, current-induced velocity enhancements exceeded $35\mathrm{m}/\mathrm{s}$ at a current density of $\sim 6\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2$. This large enhancement, more than 10 times that found in pinning-dominated experiments, results in part from an interaction that is nonlinear in current and independent of current direction.

Figure 1

(a) Scanning electron micrograph of nanowire and contiguous large-area film regions (light contrast) and milled regions (dark contrast). Directions of domain-wall propagation, positive applied field, and positive current (opposite to direction of $e^{-}$ flow) are indicated by arrows. (b) Inset shows time-resolved magneto-optic Kerr effect measured $12\mu \mathrm{m}$ from the left film-nanowire junction for $H=46\mathrm{Oe}$ and $j=0$. The temporal centers of reversal transients are plotted versus position for $j=0$ and $j=\pm 5.8\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2$ ($t=0$ when field step crosses the injection field).

Figure 2

Mobility curves for the left wall measured at $j=0$ and $j=\pm 5.8\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2$.

Figure 3

Velocities of the left (open symbols) and right (solid symbols) domain walls as a function of current density for several field step amplitudes. At the lowest field, the right wall is pinned and does not propagate for positive currents.

Figure 4

(a), (b), (c) Antisymmetric ($v_{-}$) and (d), (e), (f) symmetric ($v_{+}$) velocity components of the left (open symbols) and right (solid symbols) walls versus current density for several fields. (g) $v_{-}$ and $v_{+}$ (with zero-current velocity offset subtracted) versus field for $|j|=5.8\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2$.