MAGNETIC VISCOSITY, FLUCTUATION FIELDS, AND ACTIVATION-ENERGIES (INVITED)

Time dependence of magnetization in ferromagnetic materials was first described towards the end of the 19th century. Subsequently, two types of mechanisms responsible for time dependent behavior were identified and became known as"diffusion" and"fluctuation" after-effect or viscosity. The former depends on thermally induced motion of impurity atoms. The latter is a consequence of thermal activation of irreversible domain processes such as domain-wall motion and the nucleation of domains of reverse magnetization. Fluctuation viscosity affects, to a greater or smaller extent, all magnetic materials subject to hysteresis. In the late 1940s descriptions of magnetic viscosity in terms of fluctuation fields (Néel) and activation energy distributions (Street and Woolley) were developed. The two approaches will be described. An analysis of the time dependent phenomena exhibited by magneto-optical films will be presented as a simple example of the application of activation energy modeling.