Magnetic viscosity in some alloys and particles (abstract)

Abstract

Magnetic viscosity describes the time-dependent changes in magnetization produced when a system is thermally activated over the energy barriers separating magnetic states. This behavior is usually modeled by calculating the time constants associated with surmounting the set of energy barriers. Néel suggested an alternative approach1 in which the Boltzmann energy kT was used to define a fluctuating internal field Hf, which assisted the crossing of the energy barriers. This fluctuating field has its origin in localized magnetization oscillations (i.e., spin-wave excitations). Some evidence for this model was recently noted2 in that maximum magnetic viscosity scaled with reduced temperature T/Tc for a variety of particulate recording materials. We have measured magnetic viscosity near Hc at 300 K in a number of permanent magnet alloys and recording media particles. All the samples showed a magnetization decay of the form M=Xirr(H0−H)+XirrSν ln t, where Xirr is the irreversible susceptibility, H0 and Sν are constants, and M, H, and t have their usual meanings. The scaling of the magnetic viscosity coefficient Sν with reduced temperature again suggests that Hf contributes to the magnetization change. However, the data also indicate that other parameters, e. g., the volume associated with a reversal event, are important factors in magnetic viscosity.