Atomic Structures and Magnetic Properties of Fe-rich Fe$_{1-x}$Co$_x$ Alloys: A Genetic Algorithm Search

Using genetic algorithm with first-principles calculations, we performed a broad global search for low-energy crystal structures of Fe-rich Fe$_{1-x}$Co$_x$ alloys. We found that Fe-rich Fe$_{1-x}$Co$_x$ alloys are highly configurationally degenerate and there are many additional off-stoichiometric stable structures to the well-known stoichiometric FeCo - B$_2$ structure, giving a possibility for atomistic manipulation of the alloys. The Co-Co nearest-neighbor pair is strongly unfavorable in Fe-rich Fe$_{1-x}$Co$_x$ alloys. The magnetic moment of Fe atom is increasing with Co concentration while that of Co atom is almost constant, inducing a Slater-Pauling curve for magnetic moment per atom. The magnetic moment of Fe atom is strongly dependent on the number of Co nearest-neighbors and it increases with this number.