Searching for high magnetization density in bulk Fe: the new metastable Fe${}_6$ phase

Abstract

We report the discovery of a new allotrope of iron by first principles calculations. This phase has $Pmn{2}_1$ symmetry, a six-atom unit cell (hence the name Fe${}_6$), and the highest magnetization density (M${}_s$) among all known crystalline phases of iron. Obtained from the structural optimizations of the Fe${}_3$C-cementite crystal upon carbon removal, $Pmn{2}_1$ Fe${}_6$ is shown to result from the stabilization of a ferromagnetic FCC phase, further strained along the Bain path. Although metastable from 0 to 50 GPa, the new phase is more stable, at low pressures, than the other well-known HCP and FCC allotropes and smoothly transforms into the FCC phase under compression. If stabilized to room temperature, e.g., by interstitial impurities, Fe${}_6$ could become the basis material for high M${}_s$ rare-earth-free permanent magnets and high-impact applications such as, light-weight electric engine rotors or high-density recording media. The new phase could also be key to explain the enigmatic high M${}_s$ of Fe${}_{16}$N${}_2$, which is currently attracting an intense research activity.

7 pages, 7 figures

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• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• Impurity-Ion pair induced high-temperature ferromagnetism in Co-doped ZnO
• Theoretical investigation into the possibility of very large moments in Fe16N2