Time-dependent effects in materials with two-stage reversal processes

We have measured the magnetic viscosity in materials which undertake magnetisation reversal by two distinct processes. The materials examined were Pt/Co multilayer systems which have potential as magneto-optic recording materials. In these systems magnetisation reversal proceeds first by the nucleation of a reverse domain followed by the propagation of the domain wall. Thus, in materials where the domain wall pinning is strong two distinct regions of reversal occur. In consequence, we observe two different forms of variation of magnetisation with time and additionally different values for derived parameters such as the fluctuation field.