Anomalous magnetic viscosity in the bulk-amorphous ferromagnetNd60Fe20Co10Al10

Abstract

Magnetic viscosity in ferromagnetic materials is the time dependence of magnetization, $M(t),$ in a constant field and is a result of the thermal activation of irreversible processes. On the major hysteresis loop and over short times, $M(t)$ can be represented by a simple logarithmic monotonic function. The anomalous magnetic aftereffect is the nonmonotonic time dependence of the magnetization. The magnetic viscosity of bulk-amorphous ${\mathrm{Nd}}_{60}{\mathrm{Fe}}_{20}{\mathrm{Co}}_{10}{\mathrm{Al}}_{10}$ has been studied on the major hysteresis loop and insight into the magnetization processes has been gained from the determination of the magnetic viscosity parameter S, the (rationalized) irreversible magnetic susceptibility ${\ensuremath{\kappa}}_{\mathrm{irr}},$ and fluctuation field ${H}_{f}.$ Further measurements on both the lower and upper branches of minor loops or recoil curves, for periods extending to 30 h, were made to investigate any anomalous behavior. On the lower branch of a recoil curve, for certain magnetic prehistories, nonmonotonic behavior is observed, with the magnetization initially increasing, reaching a peak, before decreasing. The Preisach model is used to interpret this nonmonotonic behavior and the relationship between the time taken for the magnetization to reach a peak and the applied magnetic field.