Mesofrequency dynamic hysteresis in thin ferromagnetic films

The spectrum of timescales for thin film magnetization reversal processes from 10(3) to 10(-15) s is reviewed as well as appropriate experimental techniques for their investigation. The present review is motivated by the fact that most studies of magnetization dynamics have used polycrystalline NiFe thin films, whilst the magnetization dynamics of epitaxial Fe thin films, in contrast, have been little investigated, especially in the mesofrequency range (10(-1)-10(-6) s). Here, the competition between domain nucleation and domain wall propagation reversal processes results in a rich variety in the dynamical behaviour. We review the results of our time-resolved magneto-optic Kerr effect measurement of the dynamic hysteresis loop in epitaxial Fe thin films. Coercivity as a function of applied field frequency H-c(f) is measured from the dynamic hysteresis loop in the frequency range similar to0.03-3000 Hz, corresponding to experimental timescales t(exp) approximate to H-c/H in the range similar to10(-1)-10(-6) S, where H is the applied field sweep rate at the static coercive field Hc. Not only are two distinct dynamic regimes for H-c(f) found to arise in the experimental timescale range 10(-1)-10(-6) s, but also the reversal processes of domain nucleation and wall propagation are seen to compete at the crossover between the two dynamic regimes, so determining the behaviour of H-c(f). This review gives a historical overview of dynamic hysteresis in the mesofrequency range, and surveys recent theoretical descriptions and experiments. It is demonstrated that dynamic hysteresis experiments on thin ferromagnetic films are richly informative of the magnetization dynamics in the mesofrequency range. Methods for the interpretation of dynamic coercive field measurements H-c(f) are highlighted, including an adaptation of an existing model of magnetization reversal dynamics in ultrathin magnetic layers.