Statistical Mechanics of Antiferromagnetic Layer Crystal, FeCl2

For the spin system in FeCl 2 we developed a statistical theory with Bethe-Peierls method and performed the numerical calculations to compare the results with experimental data of antiferro-ferro transition. Our results are also compared with those obtained by the molecular field approximation. The short range correlation gives rise to the following effects: the decrease of the Néel temperature, more rapid increase of the sublattice magnetization with decreasing temperature, and larger critical magnetic field of antiferro-ferro transition. From the experimental data of the critical magnetic field of antiferro-ferro transition, the values of two exchange coupling constants are estimated. For these values, it is concluded from theoretical considerations that the antiferro-ferro transition is of the first kind in almost whole the temperature region below the Néel point.