  A simple effective model for a description of magnetically ordered insulators
is analysed. The tight binding Hamiltonian consists of the effective on-site
interaction (U) and intersite magnetic exchange interactions (Jz, Jxy) between
nearest-neighbours. The phase diagrams of this model have been determined
within the variational approach, which treats the on-site interaction term
exactly and the intersite interactions within the mean-field approximation. We
show that, depending on the values of interaction parameters and the electron
concentration, the system can exhibit not only homogeneous phases:
(anti-)ferromagnetic (F) and nonordered (NO), but also phase separated states
(PS: F-NO).
