Phase stability of Fe-Cr alloys in the high temperature paramagnetic phase using Atomistic Spin Dynamics-ab initio Molecular Dynamics (ASD-AIMD)

Calculating phase stability in magnetic alloys in the paramagnetic state from first principles is difficult due to the interplay of several microscopic degrees of freedom. An example of such complex interplay is the phase diagram of Fe-Cr alloys, which presents the so-called γ-loop, a region where fcc (γ) is the most stable. At the γ-δ transition at ~1600K, the bcc structure is stabilized with increasing Cr content, whereas at the α-γ transition at ~1150K the fcc structure is stabilized with increasing Cr content up to ~7% Cr, and above this Cr content the bcc structure is increasingly stabilized. The structure is in a paramagnetic phase at these temperatures, with increasing magnetic disorder as temperature rises.

We employ thermodynamic integration to calculate the Gibbs free energy difference between fcc and bcc Fe-Cr alloys for Cr=0-15% based on simulations that include spin-lattice coupling. We use coupled Atomic Spin Dynamics and Ab Initio Molecular Dynamics (ASD-AIMD), which include magnetic degrees of freedom at finite temperatures.

The results show a capture of the fcc stabilization at the α-γ transition and a bcc stabilization at the γ-δ transition. The present work paves the way for calculating phase stability in complex materials with strong spin-lattice coupling.