On the nature of the field induced magnetization reorientation in Dzialoshinsky-Moriya type weak ferromagnets

Abstract

The character of the spin-flop to weak ferromagnetic phase transition at 0°K in a Dzialoshinsky-Moriya type weak ferromagnet is studied as a function of the parameters of the system, using a classical two sublattice model and molecular field theory. The method employed is to expand the system free energy in a two variable Taylor series about the spin-flop phase equilibrium position. including all terms up to forth order. A relationship between the variables is established by requiring that the path followed by the magnetization at the onset of the phase transition be such as to minimize the free energy. In accordance with the Landau theory, conditions are then imposed on the higher order terms of the expansion, thereby determining the critical value of the antisymmetric exchange field at which the character of the phase transition changes from first to second order. For a given isotropic exchange field, this critical field is found to be proportional to the three-halfs power of the uniaxial anisotropy field, when the latter is much smaller than the exchange field. Some applications of these results to the rare earth perovskites are presented.