Magnetic viscosity in ferromagnets

Abstract

The thermal activation theory of magnetic viscosity is reviewed. Equations are derived relating the maximum slope of the magnetization decrement in (time) curve to absolute temperature, irreversible susceptibility, and the rate of change of activation energy with field at constant temperature, (∂∊/∂H)_T The activation energy ∊ is, in general, a function of the applied field H and an intrinsic function of the temperature T. If, however, (∂∊/∂T)_H=0 then (∂∊/∂H)_T≃28·5k/(dH/dT) where H is the field at which the viscosity curve slope is a maximum. Deviations from this relationship indicate an intrinsic temperature variation of the activation energy ∊ and thus enable thermal activation and intrinsic temperature effects to be separated. Experimental data on magnetic viscosity are re-examined from this point of view.