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.