Peculiarities of the critical dynamics in magnetically uniaxial PbFe$_{12}$O$_{19}$ hexaferrite

The conditions under which the thermodynamic theory of the critical slowing-down of the order parameter relaxation rate describes the behavior of magnetically uniaxial crystals are formulated. Taking into account the formulated conditions, the peculiarities of the dynamic magnetic susceptibility and the critical slowing-down of the magnetization relaxation rate of PbFe$_{12}$O$_{19}$ in the vicinity of the Curie temperature are studied. The obtained experimental results agree well with the droplet model of phase transitions. Based on the experimental data, an estimated value of the correlation length for the magnetization in the temperature area of the critical slowing-down of the relaxation rate is obtained within the droplet model.