Theory of the Metamagnetic Transition in FeCl2

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Abstract
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.

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