Domain-wall relaxation near the disorder transition of Bloch walls in Sr hexaferrite

By measuring the linear ac susceptibility of single crystals of ${\mathrm{SrFe}}_{12}$${\mathrm{O}}_{19}$ between 10 Hz and 20 MHz, the domain-wall dynamics have been investigated near the continuous phase transition from Bloch to linear walls at ${\mathit{T}}^{\mathrm{*}}$≊0.99${\mathit{T}}_{\mathit{C}}$. There the kinetic coefficient of the wall relaxation ${\mathit{L}}_{\mathit{w}}$ has a deep minimum, the essential features of which indicate the presence of strong, two-dimensional Ising-like fluctuations of the order parameter of the Bloch walls. Magnetic fields, applied transverse to the easy c axis, increase ${\mathit{L}}_{\mathit{w}}$ in almost quantitative agreement with predictions of the Landau-Lifshitz-Bloch approach for the mean-field region away from ${\mathit{T}}^{\mathrm{*}}$. Effects of fluctuations in the critical region of the disordered phase remain unexplained.