Single-ion mechanism of weak ferromagnetism and a spin-flop transition in one- and two-position antiferromagnets

A system of equations describing the ground state in an external magnetic field has been obtained for a twoposition antiferromagnet with noncollinear axes of single-ion anisotropy of moments of different sublattices. It has been shown that the antiferromagnetism vector changes sign at a spin-flop transition. This leads to a first-order magnetic phase transition with hysteresis in the field dependence of the magnetization. Explicit relations have been obtained between the parameters of a microscopic Hamiltonian and the experimentally observed spontaneous moment and spin-flop transition field. It has been shown that the field dependence of the total magnetic moment of the antiferromagnet with magnetic ions at two crystallographically nonequivalent positions (two-position antiferromagnet) obtained within the two-sublattice model is nonlinear above the transition field at any orientation of the crystal with respect to the magnetic field.