Spin-transfer-torque-driven domin-wall dynamics in Permalloy nanowires

Pulse-current-driven domain-wall (DW) dynamics in Permalloy nanowires are studied using high-temporal-resolution magneto-optical techniques. The time-resolved measurements reveal prompt displacements proportional to current density above a well-defined threshold and yield values of DW velocity and spin-flip efficiency that are a factor of 10 higher than those determined in prior experiments. A stochastic variation in the wall displacements associated with termination of the current pulse is observed. The experimental results suggest the presence of both adiabatic and nonadiabatic spin-transfer-torque mechanisms, but do not appear to be fully explained by existing theoretical models.

Figure 1

(Color online) Panel (a): schematic of Permalloy microstructure. Ellipse enclosing a DW indicates $1∕e$ intensity profile of focused laser beam. Inset: MFM image of the microstructure showing an injected vortex DW. Panel (b): pulse-current measurement ($100\mathrm{MHz}$ sampling rate) of microstructure resistance $R\left(t\right)$ that establishes the temperature rise due to Ohmic heating.

Figure 2

(Color online) Panels (a) and (b): high temporal resolution ($0.1\mu \mathrm{s}$/sample) current-stimulated DW displacement produced by a $2\text{−}\mu \mathrm{s}$ pulse. Panel (c): average wall displacement as a function of pulse duration and current density.

Figure 3

(Color online) Panel (a): statistical distribution of DW displacements after 500 measurements for $2\text{−}\mu \mathrm{s}$ pulses. Panels (b) and (c): typical signal-averaged time-resolved DW displacements for two current pulse widths selected to illustrate positive and negative displacement relaxation effects at the pulse termination, associated with upper (light, red dots) and lower (dark, blue triangles) regions of the distribution (refer to text).