Ferromagnetism and spin transition: an attempt at a unifying approach

Under certain conditions both ferromagnetic (FM) and spin transition (ST) compounds may be described by an Ising Hamiltonian. The goal of this Letter is to explore this analogy. For FM compounds the transition is induced by a crossing of the spin-up and spin-down local states through the application of a magnetic field. For ST compounds the transition corresponds to an inversion of population between low-spin and high-spin local states due to a change of temperature, the driving force being the difference in degeneracy between these states. The analogies and differences between the phase diagrams of the two phenomena are also discussed.