Structure and magnetism in bcc-based iron-cobalt alloys

The ordering and magnetic properties of bcc-based iron-cobalt alloys have been investigated as a function of the atomic composition. The different states of ordering in Fe-Co alloys, from the fully ordered to the disordered state, and their effect on the atomic volume, the magnetic moment, and enthalpy of formation, were described using the recently developed variational cluster expansion method based on first-principles data. The coarse-grained description of the energetics of the system allowed one to identify several new ordered structures, additional to the known FeCo-B2 (CsCl) ground state, after an extensive combinatorial search in the configurational phase space. It is found that the atomic volume and magnetic moment do not follow Vergard's law for either the ordered or disordered Fe-Co alloys. However, the equilibrium values of the atomic volume and the magnetic moment depend linearly on each other. The implications of such results on Fe-Co nanostructured systems are discussed.