Direct Observation of Domain-Wall Configurations Transformed by Spin Currents

Direct observations of current-induced domain-wall propagation by spin-polarized scanning electron microscopy are reported. Current pulses move head-to-head as well as tail-to-tail walls in submicrometer ${\mathrm{Fe}}_{20}{\mathrm{Ni}}_{80}$ wires in the direction of the electron flow, and a decay of the wall velocity with the number of injected current pulses is observed. High-resolution images of the domain walls reveal that the wall spin structure is transformed from a vortex to a transverse configuration with subsequent pulse injections. The change in spin structure is directly correlated with the decay of the velocity.

Figure 1

(a) Topographic image of the device structure showing the Au contacts (white) and the four zigzag ${\mathrm{Fe}}_{20}{\mathrm{Ni}}_{80}$ wires (light gray) with square pads at the bottom. (b) Magnetization configuration in a wire after magnetizing with a field pulse along the direction indicated by the arrow. White (black) corresponds to the magnetization pointing up (down) within the plane; a head-to-head wall is formed at the top bend, a tail-to-tail wall at the bottom. (c) After injection of a single $10\mu \mathrm{s}$ long current pulse through this wire, both domain walls have moved in the direction of the electron flow as indicated by the arrow.

Figure 2

Domain-wall velocity as a function of pulse injection number determined from spin-SEM images (wall $\alpha$: blue circles, dotted line; $\beta$: black triangles, solid line; $\gamma$: green squares, dashed line). The magnetic state has been reinitialized by a magnetic field before pulses 1, 11, and 26, as indicated by the arrows. After pulses 26 to 28, high-resolution images of the domain wall have been taken. The labels are related to the images shown in Fig. 3. Statistical uncertainty of the wall velocity is $0.05\mathrm{m}/\mathrm{s}$.

Figure 3

High-resolution experimental images of the spin structure of domain walls (a),(d),(e) $\alpha$, (b),(f)  $\beta$, and (c),(g) $\gamma$. After the first current injection 26, the walls are all of vortex type (a),(b),(c). After injection 27, wall $\gamma$ has stopped moving and undergone a drastic transformation to a very distorted transverse wall type (g), whereas the mobile wall $\alpha$ has a vortex core and a large transverse component (d). After injection 28, walls $\alpha$ and $\beta$ have also stopped and changed to transverse walls (e),(f). The arrow images are constructed from the two orthogonal in-plane magnetization components taken by spin-SEM. Image size: 1600 nm by 500 nm.