No abstract

Starting at the Curie temperature Tc dynamical 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 low-frequency response, ω/2π<50 MHz, is governed by the domain-wall relaxation, the rate Γd of which speeds up rapidly near Tc. This novel feature appears to be the first proper signature of linear walls predicted by Ginzburg and Bulaevskii for strong anisotropy, χ∥(T) ≳ χ⊥. The faster intradomain relaxation, arising from spin–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), χ(ω) displays an oscillatory behavior with resonance and damping frequencies lying in the GHz region. Their decrease near Tc is described by simple powers to the spontaneous magnetization. A comparison made to the well-understood critical dynamics above Tc suggests an association of these phenomena with dipolar-anisotropic magnetization fluctuations.

Approaching the Curie temperature of LiTbF4 ellipsoids of thicknesses between 0.5 and 7 mm up to T c -r -3x 10~47V, the first clear evidence for critical fluctuations and macroscopic size effects on the domain-wall relaxation is presented. An analog to the Landau-Lifshitz model proposed recently and additional optical data on the domain width suggest associating both phenomena with the existence of linear (Bulaevskii-Ginzburg) walls and with domain branching near the surface.