Nonuniversal dynamic magnetization reversal in the Barkhausen-dominated and mesofrequency regimes

Dynamic magnetization reversal in the mesofrequency range is studied by ac magneto-optical Kerr effect (ac-MOKE) and ac anisotropic magnetoresistance (ac-AMR) magnetometry in a series of epitaxial and polycrystalline thin magnetic films. The dynamic coercive field was found to scale as a power law with scaling exponents [< or =]1/2 depending on the ferromagnetic material. In addition, there is a low sweep rate regime in which the dynamic coercivity reaches a minimum. These findings are explained in the context of reversal proceeding by motion of a few domain walls (~ 1). At dc and low field sweep rates the reversal proceeds between local pinning sites via Barkhausen avalanches and the overall reversal speed is strongly dependent on the field sweep rate. At higher field sweep rates a continuous motion regime is entered in which the reversal velocity depends linearly on the applied field sweep rate and only an average pinning force is experienced by the wall. The fit of the dynamic coercivity vs applied field sweep rate allows the determination of the average nonlocal pinning field. The nonuniversal scaling exponent can be explained using recently developed models and introducing a field rate-dependent number of active domain walls.