Approaching the Curie temperature of the uniaxial Ba and Sr hexaferrites from below in zero magnetic field, a rapid decrease of the domain wall relaxation rate Γw is observed which attains a minimum at T*=0.99 Tc. Above T* the increase of the relaxation, Γw∼(Tc−T)−0.8, agrees with that observed previously on low-temperature ferromagnets1 and is assigned to the presence of linear domain walls. Accordingly, the speeding-up of Γw towards Tc is associated with the divergence of the correlation length of a 3-dimensional Ising-magnet determining the width of the linear wall. The temperature T* is found to be significantly larger than T*0 predicted by mean field work of Bulaevskii and Ginzburg2 for the second order transition from linear to Bloch domain walls, which is characterized by a continuously increasing transverse magnetization, mB ∼ (T*− T)βB, in the wall. Adjusting the temperature variation of the relaxation rate below T* to a recent kinetic theory for elliptic walls,3 one finds βB=0.10 to be close to the 2-dimensional Ising value. Along with the suppression of T* this constitutes the first signature of severe fluctuation effects in domain walls.4
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