First-principles modeling of magnetism and phase equilibria in binary alloys

The magnetic and thermodynamic properties of iron–cobalt alloys have been investigated by means of first-principles density-functional calculations. A cluster expansion (CE) bridged the quantum and the statistical mechanics, providing a set of magnetically implicit effective cluster interactions (ECIs). Our results show that magnetism is crucial to the ordering tendencies in iron–cobalt: A non-spin-polarized calculation predicts Fe–Co as a phase separating system. This is also true in the case of atomic defects for the B2 (CsCl) phase of FeCo, where magnetism stabilizes the system against a spontaneous formation of antistructure atoms. As a by-product, our analysis helps to elucidate the origin of the controversy on the existence of a low-temperature Fe3Co ordered phase.