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

TLDR

By mapping total‑energy surfaces in moment‑volume space using local‑spin‑density calculations, the authors chart magnetic phases of bcc and fcc Fe, Co, and Ni, providing a method that accesses phases beyond conventional self‑consistent‑field approaches. They find that magnetic moments can change discontinuously with volume, producing coexistence ranges and metamagnetic behavior, with bcc Co always ferromagnetic, fcc Co able to be nonmagnetic or ferromagnetic with coexistence, Fe fcc exhibiting nonmagnetic, low‑spin, and high‑spin phases that coexist over limited volumes while bcc Fe remains ferromagnetic, and Ni fcc ferromagnetic while bcc Ni can be nonmagnetic or ferromagnetic at expanded volumes, with all phase volume ranges clearly identified.

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.