Direct observation of remanent magnetic states in epitaxial fcc Co small disks

The magnetic nanostructure of epitaxial fcc Co/Cu(001) circular elements $(\sim 1.7\mu \mathrm{m}$ in diameter) has been imaged with scanning electron microscopy with polarization analysis. The disks are obtained by ultrahigh vacuum deposition of the metal films onto a prepatterned Si(001) substrate. The Si structures are 700 nm high, ensuring that the continuous background film and that of the circular structures are not physically connected. A closed flux configuration (a quadrant configuration) is observed for some of the disks, characteristic of systems with cubic anisotropy. The measured width of the $90°$ domain wall varies from $70\pm 25\mathrm{nm}$ close to the vortex core, up to $150\pm 25\mathrm{nm}$ at a normalized distance ${r/r}_d\approx 0.625$ from the vortex core (where $r_d$ is the domain wall length from the vortex core to the disk periphery), i.e., significantly exceeding the bulk domain wall width, and increasing further with increasing distance from the vortex core. Such a wide domain wall is a consequence of the geometrical constraints imposed by the element, thus defining a geometrically constrained domain wall. This view is supported by detailed micromagnetic simulations that also show that the domain wall width increases dramatically with radial position from the disk center.