Single-Ion Weak Antiferromagnetism and Spin-Flop Transition in a Two-Sublattice Ferromagnet

The ground state of a Heisenberg ferromagnet with the noncollinear single-ion anisotropy axes of two magnetic sublattices has been investigated in an external magnetic field applied in the anisotropy axes plane. Noncollinearity of the sublattice local anisotropy axes leads to a new effect called the orientational first-order spin-flop phase transition. The transition field depends on the single-ion anisotropy value and sublattice local axes orientation. An analysis of the stability of magnetic states shows that the transition is accompanied by the hysteretic field dependence of the magnetization. The dependences of the spin-flop transition field, magnetization jump, and susceptibility on the single-ion anisotropy value and axes orientation have been determined. The results obtained are used to explain the field dependence of the magnetization in the PbMnBO(4)ferromagnetic crystal.