Micromagnetics of the Domain Wall Mobility in Permalloy Nanowires

The domain wall mobility in long permalloy nanowires with thicknesses of 2-20 nm and widths of 50-200 nm has been simulated. The domain wall is driven into motion by an external magnetic field and the average wall mobility is calculated after the wall has traveled 2.5 mum along the wire. The results were obtained using the three-dimensional dynamic Landau-Lifshitz equation. We find that the domain wall mobility decreases linearly up to the critical field called the Walker field. The decreasing wall mobility is related to the decrease in the dynamic domain wall length as the applied field is increased. The value of the critical field is dependent on the thickness and width of the wire. At the critical field the mobility decreases by an order of magnitude. Above the Walker field the average mobility remains relatively constant for all driving fields, while the instantaneous mobility shows regions of high mobility with long periods of almost no mobility. For large applied fields the domain wall velocity can be large even though the average mobility is low.