Starting at the Curie temperatureTcdynamical susceptibilities have been measured between 1 Hz and 20 GHz on nonmetallic crystals to determine the influence of progressive ordering on the magnetization dynamics. In uniaxial materials (GdCl3, LiTbF4), the lowhyphen;frequency response, ohgr;/2pgr;50 MHz, is governed by the domainhyphen;wall relaxation, the rate Ggr;dof which speeds up rapidly nearTc. This novel feature appears to be the first proper signature of linear walls predicted by Ginzburg and Bulaevskii for strong anisotropy, khgr;par;(T) gsim; khgr;perp;. The faster intradomain relaxation, arising from spinndash;spin interaction, undergoes a thermodynamic slowing down, the kinetic coefficient of which determines the magnitude of the damping in the walls. In the cubic ferromagnets (EuS, EuO), khgr;(ohgr;) displays an oscillatory behavior with resonance and damping frequencies lying in the GHz region. Their decrease nearTcis described by simple powers to the spontaneous magnetization. A comparison made to the wellhyphen;understood critical dynamics aboveTcsuggests an association of these phenomena with dipolarhyphen;anisotropic magnetization fluctuations.
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