Coercivity mechanisms in ferrites and rare earth transition metal sintered magnets (SmCo₅, Nd-Fe-B)

Measurements of the temperature dependence of the coercive field, of the magnetic viscosity, and of the angular dependence of the coercive field are reported for ferrite and rare-earth-transition-metal sintered magnets. A satisfactory explanation for the observed properties is obtained in a simple model, considering that magnetization reversal is initiated in a volume equal to the activation volume and is determined by the formation of a domain wall. From magnetic viscosity measurements, the activation volume is found to be proportional to the cube of the domain wall width, delta . The observed angular dependence of the coercive field reveals that, in the activation volume, the anisotropy is much larger than the coercive field and is not strongly reduced with respect to the bulk. If the coercivity is determined by true nucleation in a fully saturated sample, this is unlike the usual assumption that the magnetocrystalline anisotropy is strongly reduced in the volume of the nucleus.