Theory of the metamagnetic transition in FeCl₂

A crystal-field calculation of the low-lying levels of a Fe²⁺ ion in FeCl₂ shows that the lowest lying levels, a doublet and a singlet, are well separated from the remaining levels. The relative isolation of these levels suggest taking the effective spin as unity. The Hamiltonian incorporates a Zeeman term, an exchange term between spins S=1, and an anisotropy term. An RPA Green's function theory is derived, and, because of the highly anisotropic exchange, is reduced to an expansion about the molecular-field theory, in powers of the non-Ising exchange factors. It is found that even in MFT the metamagnetic-transition field has temperature dependence which follows well that of the zero-field sublattice magnetization.