Direct Imaging of Stochastic Domain-Wall Motion Driven by Nanosecond Current Pulses

Magnetic transmission x-ray microscopy is used to directly visualize the influence of a spin-polarized current on domain walls in curved permalloy wires. Pulses of nanosecond duration and of high current density up to $1.0\times {10}^{12}\mathrm{A}/{\mathrm{m}}^2$ are used to move and to deform the domain wall. The current pulse drives the wall either undisturbed, i.e., as composite particle through the wire, or causes structural changes of the magnetization. Repetitive pulse measurements reveal the stochastic nature of current-induced domain-wall motion.

Figure 1

(a) Optical micrograph of a curved permalloy wire on a ${\mathrm{Si}}_3{\mathrm{N}}_4$ membrane with four gold contacts. A slit on the left side avoids dc current flow through this part. (b) MTXM image and (c) scheme of a vortex wall in the section between the inner contacts. (d) Measured AMR. The up-sweep (down-sweep) is plotted in blue (red). The arrows indicate the magnetization states. Shown are schemes of the two saturated states, the two global-vortex states, and the two onion states. At remanence the onion states exhibit either a tail-to-tail (blue) or a head-to-head (red) DW in the region of interest for the pulse experiments.

Figure 2

(a) MTXM image of a double-vortex wall before and (b) after sending ten current pulses of 1.0 ns duration and 4.0 ns periodicity through the wire. (c) Sketch of the wall positions in (a) and (b) illustrating the motion of the double-vortex wall as composite particle. (d) Another pulse sequence generates a vortex-antivortex pair. (e)–(h) Sequence of MTXM images of a double-vortex wall taken after single 1.0 ns long current pulses with a current density of $1.0\times {10}^{12}\mathrm{A}/{\mathrm{m}}^2$. The lower vortex moves successively to the left edge of the wire.

Figure 3

Efficiency of DW displacement for 49 pulses of 1.0 ns duration. The current density of the pulses was increased from $7.5\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2$ to $1.0\times {10}^{12}\mathrm{A}/{\mathrm{m}}^2$ after pulse No. 25.

Figure 4

Micromagnetic simulation of the influence of a spin-polarized current with a current density of $1.0\times {10}^{12}\mathrm{A}/{\mathrm{m}}^2$ on a double-vortex wall in a curved permalloy wire of $1\mu \mathrm{m}$ width and 80 nm thickness. The time difference between (a) and (b) is 1 ns. The right edge of the wall has moved 110 nm.