Magnetic Viscosity in Perpendicular Media

The remanent magnetization of fine particle media of perpendicular anisotropy is known to exhibit a time decay of pseudologarithmic form. The viscous properties are to a large extent determined by the magnetostatic particle interaction and the particle size. A meanfield model of a perfectly aligned ensemble of Stoner-Wohlfarth particles is presented that gives a qualitative description of the hysteretic and time-dependent properties of particulate perpendicular media. The time decay of the remanent magnetization of Alumite media after initial magnetic saturation was measured. The decay was found to be logarithmic within the measuring period, and a coefficient of magnetic viscosity was obtained. Although the reversal mechanism for the particles in Alumite media is known to be incoherent, a good qualitative agreement between the theoretical model and the measurements was found by introducing a reduced effective volume acting as a scaling factor that accounts for the discrepancy in reversal mechanisms.