Spin-polarized currents can drive the motion of magnetic domain walls. Now, imaging studies show how the shape of domain walls evolve as they move along a ferromagnetic nanowire
On a microscopic length scale, the magnetization of a ferromagnet is uniform. However, over macroscopic length scales, uniform magnetization is energetically unfavourable and domain walls — regions where the magnetization rotates — will form.
When the domain-wall width corresponds to one of the spatial dimensions of the ferromagnet itself, the wall shape may be confined, even if it is forced to move by an external magnetic field or current. Stuart Parkin and colleagues1 from the IBM Research Division in the US studied domain-wall formation and propagation in permalloy nanowires with diameters of 200 nm — narrow enough to permit a single domain wall — to better understand how magnetic field and current affect domain-wall shapes and dynamics.
The wires are indented on one edge so that when a domain wall forms and is forced to propagate along the wire length, it becomes pinned at the notch. The type of domain wall — essentially, the precise pattern of how the magnetization flips from one direction to the other — is determined by measuring the resistance along the wire. Importantly, the authors can monitor domain dynamics in real time and find that although currents affect domain-wall velocity, they have little effect on their precessional frequency.
References
Hayashi, M., Thomas, L., Rettner, C., Moriya, R. & Parkin, S. S. P. Direct observation of the coherent precession of magnetic domain walls propagating along permalloy nanowires. Nature Phys. 10.1038/nphys464 (2006).
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Thomas, J. Domain walls. Nature Nanotech (2006). https://doi.org/10.1038/nnano.2006.186
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DOI: https://doi.org/10.1038/nnano.2006.186
