Ferromagnetic phases of bcc and fcc Fe, Co, and Ni

Abstract

The different magnetic phases of the bcc and fcc forms of Fe, Co, and Ni are studied by analyzing total-energy surfaces in moment-volume parameter space obtained from energy-band calculations using a local-spin-density approximation. The surfaces, found by calculating total energies while holding both the magnetic moment and the volume fixed, offer a method for studying phases that are inaccessible to traditional self-consistent-field methods. We find that magnetic moments can change discontinuously with volume and that there are ranges of coexistence for different magnetic phases. In the multiphase ranges, these elemental magnetic systems exhibit metamagnetic behavior. Our results show that bcc Co is ferromagnetic for all volumes studied, that fcc Co can exist in either a nonmagnetic or a ferromagnetic phase, and that there is a range of volumes where the two phases can coexist. For Fe, the bcc form exhibits a stable ferromagnetic phase for all volumes considered, but the fcc form can exist in any of three phases---a nonmagnetic, a low-spin, and a high-spin phase---all of which can coexist in limited volume ranges. For Ni, the fcc form exhibits a stable ferromagnetic phase, but the bcc form can exist in both a nonmagnetic and, at expanded volumes, a ferromagnetic phase. The volume ranges for all magnetic phases are clearly identified for the bcc and fcc forms of Fe, Co, and Ni.