Transit-time-limited wall motion in Permalloy films

A simple quantitative formulation of transit-time-limited wall motion is used to show that such an effect can reasonably explain the observed nonlinear wall motion in Permalloy thin films. The ``critical velocity'' is calculated by equating the intrinsic relaxation time Ti to the transit time Tt for the wall past an individual spin position to obtain Vcrit=δ/Ti, where δ is the wall width. The model correctly predicts (i) the magnitude and (ii) variation with film thickness of the observed knee in the velocity-field curves for wall motion in Permalloy films.