The Neel point for spin-transition systems: Toward a two-step transition

The spin-transition (ST) phenomenon may be described by an Ising Hamiltonian with a temperature dependent transverse field. This Hamiltonian is analyzed for ''antiferromagnetic'' or ''heterophilic'' coupling, both in the mean-field approximation (MFA) and with a Monte Carlo simulation (MCS). Both methods show that above a critical value of the coupling parameter, a two-step spin transition is obtained. This two-step behavior corresponds to ''antiferromagnetic'' ordering. Then, a global phase diagram is proposed, valid for Ising magnetic and spin-transition systems, for both ''ferromagnetic'' and ''antiferromagnetic'' couplings.