Eddy-Current-Limited Domain-Wall Motion in Thin Ferromagnetic Films

The effect of eddy current losses on domain-wall motion in thin ferromagnetic films has been investigated from a theoretical point of view. Earlier calculations neglecting wall shapes and stray demagnetizing fields do not adequately explain existing experimental data. A general expression has been developed for the current distribution about a moving Bloch wall of arbitrary shape in a thin film of infinite extent and arbitrary thickness. Stray demagnetizing field effects have been included in an approximate manner for a simple 180° wall in which the magnetization rotates linearly through a wall whose width is calculated from static energy considerations. The current distribution was evaluated numerically and the eddy-current-limited wall mobility was obtained as a function of film thickness from 400 to 4000 Å, using material parameters typical for Ni–Fe alloy films with 80% Ni. Although the current distributions exhibited a significant dependence on wall width, the mobilities were not significantly different from previous predictions. In order to explain existing mobility data on the basis of eddy current losses, wall widths much larger than calculated static Bloch wall widths must be assumed. Otherwise, some other loss mechanism must dominate.