Ferromagnetism and crystal lattice stability of bcc and fcc iron

The Stoner theory of itinerant ferromagnetism was used in calculating the magnetic properties of iron within the self-consistent linear muffin-tin orbital approach. The Stoner exchange parameter, I, was found from the linear-response theory as a function of volume and then adjusted by a constant enhancement factor determined by fitting the equilibrium atomic volume of the ferromagnetic (FM) bcc phase. No other adjustments of any quantities were performed. The calculations revealed a somewhat unusual behavior of phases. The nonmagnetic (NM) bcc phase is unstable with respect to spontaneous magnetization. Moreover, the enthalpy of the NM phase exhibits a monotonic decrease with c/a along the so-called Bain deformation path, the bcc modification being unstable with respect to the shear deformation. Ferromagnetism stabilizes the bcc phase. However, the FM fcc phase is unstable with respect to the shear deformation. The enthalpy curve along the deformation path has a cusp corresponding to a first-order phase transition between FM and NM states. The FM contributions to the energy versus atomic volume for different c/a valves are tabulated.