Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires

We report direct observation of current-driven magnetic domain wall (DW) displacement by using a well-defined single DW in a microfabricated magnetic wire with submicron width. Magnetic force microscopy visualizes that a single DW introduced in a wire is displaced back and forth by positive and negative pulsed current, respectively. The direct observation gives quantitative information on the DW displacement as a function of the intensity and the duration of the pulsed current. The result is discussed in terms of the spin-transfer mechanism.

Figure 1

Schematic illustration of a top view of the sample. One end of the $L$-shaped wire is connected to a diamond-shaped pad which acts as a domain wall (DW) injector, and the other end is sharply pointed to prevent a nucleation of a DW from this end. The wire has four electrodes made of Cu. MFM observations were performed for the hatched area at room temperature.

Figure 2

(a) MFM image after the introduction of a DW. DW is imaged as a bright contrast, which corresponds to the stray field from positive magnetic charge. (b) Schematic illustration of a magnetic domain structure inferred from the MFM image. DW has a head-to-head structure. (c) Result of micromagnetics simulation (vortex DW). (d) Result of micromagnetics simulation (transverse DW). (e) MFM image calculated from the magnetic structure shown in Fig. 2c. (f) MFM image calculated from the magnetic structure shown in Fig. 2d. (g) Magnified MFM image of a DW. (h) MFM image after an application of a pulsed current from left to right. The current density and pulse duration are $1.2\times {10}^{12}\mathrm{A}/{\mathrm{m}}^2$ and $5\mu \mathrm{s}$, respectively. DW is displaced from right to left by the pulsed current. (i) MFM image after an application of a pulsed current from right to left. The current density and pulse duration are $1.2\times {10}^{12}\mathrm{A}/{\mathrm{m}}^2$ and $5\mu \mathrm{s}$, respectively. DW is displaced from left to right by the pulsed current.

Figure 3

(a)–(k) Successive MFM images with one pulse applied between each consecutive image. The current density and the pulse duration were $1.2\times {10}^{12}\mathrm{A}/{\mathrm{m}}^2$ and $0.5\mu \mathrm{s}$, respectively. Note that a tail-to-tail DW is introduced, which is imaged as a dark contrast.

Figure 4

(a) Average DW displacement per one pulse as a function of the pulse duration under a condition of constant current density of $1.2\times {10}^{12}\mathrm{A}/{\mathrm{m}}^2$. (b) Average DW velocity as a function of the current density.