BLOCH-WALL PHASE-TRANSITION IN THE SPHERICAL MODEL

The temperature-induced second-order phase transition from Bloch to li near (Ising-like) domain walls in uniaxial ferromagnets is investigate d for the model of D-component classical spin vectors in the limit D - -> infinity. This exactly solvable model is equivalent to the standard spherical model in the homogeneous case, but deviates from it and is free from unphysical behaviour in a general inhomogeneous situation. I t is shown that the thermal fluctuations of the transverse magnetizati on in the wall (the Bloch-wall order parameter) result in the diminish ing of the wall transition temperature T-B in comparison to its mean-f ield value, thus favouring the existence of linear walls. For finite v alues of T-B an additional anisotropy in the basis plane x, y is requi red; 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 one.