Activation volume and coercivity in aluminum-substituted Pb-M hexaferrites

PbFe12-xAlxO19 samples were obtained by the chemical coprecipitation method with compositions x=0.0, 3.0, 4.0, 6.0. As was expected, significant differences in magnetization reversal mechanisms were found at room temperature in these specimens. A magnetic viscosity study was carried out in the temperature range of 75-300 K in order to obtain information related to the mechanism of magnetization behavior. A logarithmic time dependence of the magnetization was observed during a period of 20-1200 s. The magnetic viscosity Sv and the activation volume Va increase as temperature is increased; the samples x=3.0 and 4.0 show the most rapid increase of the activation volume with temperature. Except for the samplesx=3.0 and 4.0, the experimental temperature dependencies of the coercive field are in agreement with a model due to Givord and co-workers in which the magnetization reversal occurs in a volume equal to the activation volume