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

By measuring the linear ac susceptibility of single crystals of SrFe₁₂{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 ${T}*0.99{T}C$. There the kinetic coefficient of the wall relaxation ${L}w$ 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 Lw in almost quantitative agreement with predictions of the Landau-Lifshitz-Bloch approach for the mean-field region away from T*. Effects of fluctuations in the critical region of the disordered phase remain unexplained.