Nanometer Scale Observation of High Efficiency Thermally Assisted Current-Driven Domain Wall Depinning

Nanometer scale observation of the depinning of a narrow domain wall (DW) under a spin current is reported. We studied $\sim 12\mathrm{nm}$ wide 1D Bloch DWs created in thin films exhibiting perpendicular magnetic anisotropy. Magnetotransport measurements reveal thermally assisted current-driven DW motion between pinning sites separated by as little as 20 nm. The efficiency of current-driven DW motion assisted by thermal fluctuations is measured to be orders of magnitude higher than has been found for in-plane magnetized films, allowing us to control DW motion on a nanometer scale at low current densities.

Figure 1

(a) Polar Kerr hysteresis loop of our $F1/\mathrm{Cu}/F2$ spin valves with PMA measured in a continuous film. (b) Schematic diagram of a top view of the device. (c) EHE minor hysteresis loop corresponding to the reversal of the free layer in a $200\times 200{\mathrm{nm}}^2$ Hall cross. A small EHE jump corresponding to DW motion over 10 nm is indicated. The EHE values (1) to (4) refer to the positions indicated in Fig. 1d. (d) Bubblelike behavior of an ideal 1D Bloch DW in a magnetic Hall cross. This process has been demonstrated previously by Kerr microscopy [18, 20]. The electron flow direction is indicated by the horizontal arrows.

Figure 2

EHE as a function of the applied dc current for fields (a) $H_{\mathrm{fre}}=-10\mathrm{Oe}$, (b) $H_{\mathrm{fre}}=-40\mathrm{Oe}$, and (c) $H_{\mathrm{fre}}=-60\mathrm{Oe}$. Measurements have been done by increasing the current from $I=0$ to $I=\pm 3\mathrm{mA}$ and then reducing it to $I=0$, with a DW initially pinned at position (2). The negative (closed symbols) and positive (open symbols) current measurements have been realized independently. The states (1) to (4) refer to the positions in Fig. 1d. The small Joule heating contribution has been subtracted.

Figure 3

Critical positive current as a function of $H_{\mathrm{fre}}\le 0$ with a DW initially pinned at position (2) in Fig. 1d.

Figure 4

Depinning time $\tau (I,H_{\mathrm{fre}})$ at position (2) as a function of dc current for $H_{\mathrm{fre}}=-40\mathrm{Oe}$. Inset: typical measurement of $\tau (I,H_{\mathrm{fre}})$ for a DW initially at the cross input. Under $H_{\mathrm{fre}}=-40\mathrm{Oe}$, the DW propagates by discrete jumps of about 10 nm until it remains pinned at position (2) [cf. Fig. 1d]. Then a dc current $I$ is applied (in this case, $+1.5\mathrm{mA}$) and the depinning time, $\tau (I,H_{\mathrm{fre}})$, is measured.