Single-ion weak antiferromagnetism and spin-flop transition in a two-sublattice ferromagnet

The ground state of a Heisenberg ferromagnet with noncollinear single-ion anisotropy axes of two sublattices is investigated in the external magnetic field applied in the anisotropy axes plane. The noncollinearity of local anisotropy axes leads to new effect - the first order spin reorientation phase transition of the spin-flop type. The transition field depends on the value of anisotropy and orientation of the sublattice axes. Stability analysis of magnetic states shown that the field induced transition is accompanied by the magnetization hysteresis. The dependencies of the spin-flop transition field, the magnetization jump value and the susceptibility on the anisotropy parameters are determined. The obtained results are used for explanation of the magnetization field dependence in the ferromagnetic crystal PbMnBO$_4$.