Magneto-structural transformations via a solid-state nudged elastic band method: Application to iron under pressure

TL;DR

This paper extends the solid-state nudged elastic band method to include magnetization, enabling detailed analysis of magneto-structural transformations like iron's pressure-induced phase change, and provides results consistent with experimental observations.

Contribution

The authors develop a formalism that incorporates non-conserved order parameters into the nudged elastic band method, specifically applied to magneto-structural transformations in iron.

Findings

Calculated coexistence pressure of 8.4 GPa for iron transition

Identified a magnetization discontinuity at the transition state

Predicted a pressure jump of 25 GPa consistent with experiments

Abstract

We extend the solid-state nudged elastic band method to handle a non-conserved order parameter - in particular, magnetization, that couples to volume and leads to many observed effects in magnetic systems. We apply this formalism to the well-studied magneto-volume collapse during the pressure-induced transformation in iron - from ferromagnetic body-centered cubic (bcc) austenite to hexagonal close-packed (hcp) martensite. We find a bcc-hcp equilibrium coexistence pressure of 8.4 GPa, with the transition-state enthalpy of 156 meV/Fe at this pressure. A discontinuity in magnetization and coherent stress occurs at the transition state, which has a form of a cusp on the potential-energy surface (yet all the atomic and cell degrees of freedom are continuous); the calculated pressure jump of 25 GPa is related to the observed 25 GPa spread in measured coexistence pressures arising from martensitic and coherency stresses in samples. Our results agree with experiments, but necessarily differ from those arising from drag and restricted parametrization methods having improperly constrained or uncontrolled degrees of freedom.