Dependence of the coercive field and magnetic viscosity coefficient in Nd-Fe-B magnets on the magnetic history of the sample

The dependence of the coercive field H_c and magnetic viscosity coefficient S_v on an initial magnetizing field H_init are determined in NdFeB sintered and MQII magnets. In sintered magnets, coercivity progressively develops as H_init is increased. The variation of S_v is attributed to different values of the activation volume for different grains. The variation of the coercive field deduced from the variation of the activation volume corresponds to the experimental one. In MQII magnets, the high field part of the first magnetization curve is shown to be determined by the same irreversible processes which govern the magnetization processes on the hysteresis loop. The field dependence of the magnetization and of the magnetic viscosity coefficient S_v are analyzed assuming that the angular dependence of the coercive field obeys a simple (1/cos theta ) law.