Magnetic viscosity in different Nd-Fe-B magnets

Measurements of magnetic viscosity in Nd-Fe-B magnets of different kinds (sintered magnets, MQII magnets prepared from ribbons, and LDC magnets prepared by liquid dynamic compaction) are reported. In all magnets, the viscosity S and the irreversible susceptibility χirr are found to be proportional. The temperature dependence of the coefficient Sv =S/χirr exhibits a sharp maximum at about 100 K. The temperature dependence of the activation energy E0 which governs magnetic aftereffects is deduced from these data. In sintered and LDC magnets, but not in MQII ones, E0 is found to be approximately proportional to the domain wall energy. It is concluded that the mechanism of coercivity is different in sintered and LDC magnets than in the MQII ones.