Thermodynamics of the two-dimensional superexchange model of Mitsui

The exact solution of the two-dimensional superchange Mitsui model obtained by means of the decoration-transformation method is presented. It is shown that in the region of the parameter values $-1\leqslant a\leqslant1$, $0\leqslant\gamma1$ the model has one phase transition. The expressions for thermodynamical functions are obtained. Near the critical point the temperature dependence of the spontaneous polarization (magnetization), internal energy, specific heat and susceptibility is similar to that of the Ising model.