Domain wall behaviour at constrictions in ferromagnetic ring structures

Direct non-intrusive imaging and magnetoresistance measurements have been employed to investigate the head-to-head domain walls occurring in micrometre-sized NiFe and Co rings. In wide rings vortex head-to-head domain walls are found to be present. These vortex head-to-head walls are repelled by constrictions in structures with notches, but they are pinned in energetically favourable positions adjacent to the notches. For rings with sufficiently large notches, a transverse head-to-head wall can be stabilized in the constriction. The type of domain wall is shown to be easily identifiable using magnetoresistance measurements.