Mesofrequency dynamic hysteresis in thin ferromagnetic films

Abstract

The spectrum of timescales for thin film magnetization reversal processes from103 to10−15 s is reviewed as well as appropriate experimental techniques for their investigation. Thepresent review is motivated by the fact that most studies of magnetization dynamics haveused polycrystalline NiFe thin films, whilst the magnetization dynamics of epitaxial Fe thinfilms, 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 reversalprocesses results in a rich variety in the dynamical behaviour. We review the results of ourtime-resolved magneto-optic Kerr effect measurement of the dynamic hysteresisloop in epitaxial Fe thin films. Coercivity as a function of applied field frequencyHc(f) is measured from the dynamic hysteresis loop in the frequency range , corresponding to experimental timescales in the range , where is the applied field sweep rate at the static coercive fieldHc. Not only are two distinct dynamic regimes forHc(f) found to arise in the experimental timescale range10−1–10−6 s, but also the reversal processes of domain nucleation and wall propagation are seen tocompete at the crossover between the two dynamic regimes, so determining the behaviour ofHc(f). This review gives a historical overview of dynamic hysteresis in the mesofrequencyrange, and surveys recent theoretical descriptions and experiments. It isdemonstrated that dynamic hysteresis experiments on thin ferromagnetic filmsare richly informative of the magnetization dynamics in the mesofrequencyrange. Methods for the interpretation of dynamic coercive field measurementsHc(f) are highlighted, including an adaptation of an existing model of magnetization reversaldynamics in ultrathin magnetic layers.