Nucleation of 360 deg DWs in a wire using a local circular field*

Understanding domain wall (DW) motion in ferromagnetic nanostructures is important to realize proposed magnetic data storage and logic devices. Interest in $360^{o}$ DWs has increased recently with the recognition that their minimal stray field creates only short range interactions, leading to a potentially higher packing density compared to $180^{o}$ DWs. Our simulations demonstrate the feasibility of nucleating a $360^{o}$ DW at a specific location along a wire by applying a local circular field that is centered in close proximity to the wire. We simulate the field strength as if from a current carrying wire, which can be experimentally realized by passing current through the tip of an AFM [$1,2$]. The successful nucleation of a $360^{o}$ DW depends on the dimensions of the Py wire, on the strength of the circular field, and on the distance of the center of the field from the wire. Once a $360^{o}$ DW is nucleated, its position shifts with time. We use a notch to stabilize the location of the $360^{o}$ DW. We investigate the optimal size and spacing of the notches to allow the greatest packing density with control over the nucleation and annihilation of individual domain walls. [$1$] T Yang et al., Appl. Phys. Lett., $98$, $242505$ ($2011$). [$2$] http://math.nist.gov/oommf