Self-consistent local mean-field theory for phase transitions and magnetic properties of FeRh

FeRh has a phase transition from an antiferromagnetic state (low temperature) to a ferromagnetic state (high temperature) at 360 K. Various explanations for this behavior have been proposed over the past 20 years. However, many of the mechanisms are inconsistent with all the data. Early models were Ising-like, but the large anisotropy fields necessary for this are not found in hysteresis curves. Using a four-spin Hamiltonian, we obtain a complete theoretical description of the field and temperature phase diagram and the magnetic properties for FeRh. The theoretical results are in good agreement with experiments. We use our approach to predict changes in behavior as a function of the thickness of an FeRh film. We find the four-spin Hamiltonian is particularly sensitive to the presence of a surface, with thinner films remaining ferromagnetic over a larger temperature range because the four-spin contribution to the energy (which favors the antiferromagnetic state) is smaller.