Bloch wall phase transition in the anisotropic spherical model (abstract)

The temperature-induced second-order phase transition from Bloch to linear (Ising-like) domain walls in uniaxial ferromagnets was predicted theoretically1 within the mean-field approximation and observed in the dynamic susceptibility experiments on Sr hexaferrite.2 Here the fluctuational effects at this phase transition are investigated for the exactly solvable anisotropic model of D-component classical spin vectors in the limit D→∞. This anisotropic spherical model is equivalent to the common spherical model in the homogeneous case, but deviates from it and is free from nonphysical behavior in a general inhomogeneous situation. It is shown that the thermal fluctuations of the transverse magnetization in the wall (the Bloch wall order parameter) result in the diminishing of the domain wall transition temperature TB in comparison to its mean-field value, thus favoring the existence of linear walls. For ensuring finite values of TB, an additional anisotropy in the basal plane x, y is required: In purely uniaxial ferromagnets a domain wall behaves like a two-dimensional system with a continuous spin symmetry and does not order into the Bloch wall at any nonzero temperature. The theory qualitatively explains the fact that the value TB=0.99Tc measured on the Sr hexaferrite2 is substantially more remote from Tc than the mean-field estimation TB=0.996Tc, as well as some other characteristics of domain wall relaxation in the critical region.