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 Fe20Ni80 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.

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Direct observation of current-induced propagation of purely transverse magnetic domain walls with spin-polarized scanning electron microscopy is reported in Fe30Ni70 nanowires. After propagation, the domain walls keep their transverse nature but switch polarity in some cases. For uniform Ni70Fe30 wires, the effect is random and illustrates domain-wall propagation above the Walker threshold. In the case of Ni{70}Fe_{30}/Fe wires, the transverse magnetization component in the wall is entirely determined by the polarity of the current pulse, an effect that is not reconciled by present theories even when taking into account the nonuniform Oersted field generated by the current.

Magnetic domain walls have been studied in micrometer-sized Fe20Ni80 elements containing geometrical constrictions by spin-polarized scanning electron microscopy and numerical simulations. By controlling the constriction dimensions, the wall width can be tailored and the wall type modified. In particular, the width of a 180• Néel wall can be strongly reduced or increased by the constriction geometry compared with the wall in unconstrained systems.PACS numbers: 75.60. Ch, 75.75.+a, 75.70.Kw For almost a century, the width of magnetic domain walls (DWs) has been believed to be determined by material properties only. However, recent investigations on DWs in nanometer-scale systems have revealed new physical properties due to the geometrical confinement of the magnetization. A reduction of the Bloch wall width has been predicted 1 and observed 2 in nanometer-sized constrictions. This effect is thought to be the origin of the large magneto-resistance measured in nanocontacts, and explained by ballistic transport through a narrow DW pinned within the contact.3,4,5 Furthermore, domain walls are now being investigated as tiny individual magnetic objects that can be manipulated in view of their potential for application in novel magnetic logic or memory devices.6 Of interest in this field are the possibilities of pinning DWs at constrictions and of displacing them using a magnetic field 7 or an applied current. 8,9,10,11,12 For all these phenomena, the key parameter is the magnetic structure of the domain walls. For a basic understanding as well as for potential applications, it is important to gain quantitative insight into how DW properties can be modified via the geometry.The prediction of DW narrowing in a constriction was based on a ferromagnetic model system containing a planar Bloch wall.1 Because dipolar contributions in the constriction were neglected, the problem was onedimensional and could be solved analytically. The vast majority of small elements, however, exhibits DWs of Néel type, with a nonvanishing magnetization component perpendicular to the wall. In these walls, the dipolar energy determines the wall profile to a large extent, and hence the problem is more intricate.In this paper, we investigate Néel-type walls in elements containing constrictions of controlled dimensions. The experimental results obtained by scanning electron microscopy with spin analysis (spin-SEM 13 or SEMPA 14 ) are compared with micromagnetic simulations. We demonstrate how the wall properties can be tuned both by the element size and the constriction dimensions. Constraining a DW in a micrometer-sized element strongly reduces the wall width compared with the width in an infinite film. By appropriately tuning the constriction dimensions, the Néel wall width can further be decreased, or alternatively, increased until the wall splits into two separate walls.The constrictions were fabricated in thin, micrometersized rectangular elements by using electron-beam lithography and Ar dry etching of Fe 20 Ni 80 thin films. These films w...