Antiferromagnetic-ferromagnetic transition in FeRh

First-principles band-structure calculations based on the augmented-spherical-wave method and the fixed-spin-moment procedure are used to determine the volume dependence of the total energy and the local moments in ordered FeRh. The calculations reveal the coexistence of antiferromagnetic (AF) and ferromagnetic (FM) solutions over a wide range of volume. The zero-pressure equilibrium state is found to be AF with ∼±3.0${\mathrm{\mu }}_{\mathit{B}}$ iron local moments and precisely zero rhodium local moments, in agreement with experiment, and the calculated lattice constant is within 0.14% of the experimental value. A metastable ferromagnetic state with iron and rhodium local moments of ∼3.1${\mathrm{\mu }}_{\mathit{B}}$ and 1.0${\mathrm{\mu }}_{\mathit{B}}$ lies just above the AF state and has a minimum energy at a lattice constant ∼0.5% larger than the AF state, implying that when the system undergoes the AF-FM transition at finite temperatures, the transition is accompanied by an enhanced thermal expansion. At expanded volumes, the FM state becomes energetically more favorable than the AF state. Calculations also show that type-II AF is more stable than type-I AF in the CsCl structure, in agreement with experiment.