Ferrimagnetism in a two-sublatttice bilinearly coupled Heisenberg model

We study phenomenologically on the basis of two bilinearly coupled Heisenberg models the phase diagram of some ferrimagnetic substances. Calculations are performed with the help of Landau free energy obtained through applying the Hubbard-Stratonovich transformation to the initial microscopic Heisenberg Hamiltonian. The phase transitions within the model are of second order with the emergence of a compensation point at lower temperatures for some values of the parameters of the system. The main phase is a two-sublattice collinear ferrimagnet but also a metastable non-collinear phase is present within the exchange approximation presented here. The numerical results give a detailed description of the temperature dependence of magnetization on the strength of intersublattice interaction and the difference between the effective exchanges of two ferromagnetically ordered sublattices.