The Neel point for spin-transition systems: toward a two-step transition Original Research Article

The spin-transition (ST) phenomenon may be described by an Ising Hamiltonian with a temperature dependent tranvserse 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.