Domain wall velocity in ultra-thin magnetic films with perpendicular anisotropy

Monte Carlo simulations were performed to study the effect of local variations of the domain wall coercivity on the rate of domain growth in ultra-thin magnetic films with strong perpendicular anisotropy. The analytic models used in previous studies are refined, compared with the Monte Carlo data and shown to be inadequate to describe realistically the domain growth process. When the field is increased a depinning transition occurs where the domain expands by a sequence of avalanches. If the avalanche size is small, the field dependence of the wall velocity is exponential, as in a thermo-activated process, so the full extent of the depinning transition may not be evident in experimental nu(H) curves. The depinning field H-0, when the avalanche size diverges, is weakly dependent on the width of the coercivity distribution. When H > H-0, the phenomenological relation nu = nu(0) + mu'(H - H-0) that is observed experimentally in Au/Co(8 Angstrom)/Au films is shown to result from the temporary obstruction of the domain boundary by relatively hard pinning centres and mu' is found to be larger than the wall mobility.