Antiferromagnetism of Ferrous Chloride Tetrahydrate FeCl2·4H2O

The antiferromagnetism of the monoclinic crystal Fe${\mathrm{Cl}}_2$·4${\mathrm{H}}_2$O, especially its ordered spin arrangement, is considered. By replacing the spin operators in the appropriate spin Hamiltonian by the classical spin vectors, we look for the lowest energy state of the spin arrangement. A model similar to one proposed by Spence et al. is obtained as the lowest energy arrangement of the spins, although it differs in detail. In doing so, we assume the four-sublattice model and isotropic exchange interactions between magnetic ions up to the fourth nearest neighbors. The susceptibilities in the directions of the magnetic principal axes are calculated at 0°K in a classical way. Comparison with the experimental results permits us to estimate the exchange integrals in reasonable order of magnitude.