  We study phenomenologically on the basis of two bilinearly coupled Heisen-
berg models the phase diagram of some ferrimagnetic substances. Calculations
are performed with the help of Landau 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
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 temperature dependence of magnetization on the strength of in-
tersublattice interaction and the difference between the effective exchanges of
two ferromagnetically ordered sublattices.
