Lattice relaxations in disordered Fe-based materials in the paramagnetic state from first principles

TL;DR

This paper introduces a first-principles method for lattice relaxation in disordered Fe-based magnetic materials in the paramagnetic state, highlighting the importance of magnetic disorder considerations for accurate structural and energetic predictions.

Contribution

The authors develop a straightforward approach to perform structural relaxations in the paramagnetic regime within the DLM framework, applicable with standard ab initio codes.

Findings

Vacancy relaxations are twice as large in the DLM state compared to ferromagnetic state.

Formation energies of vacancies and interstitials are significantly affected by magnetic state relaxations.

Considering the DLM state reduces the mixing enthalpy in Fe-Cr alloys by about 10%.

Abstract

In this work we propose a method for the structural relaxation of magnetic materials in the paramagnetic regime, in an adiabatic fast-magnetism approximation within the disordered local moment (DLM) picture in the framework of density functional theory (DFT). The method is straight forward to implement using any $ab$ $initio$ code that allows for structural relaxations. We illustrate the importance of considering the disordered magnetic state during lattice relaxations by calculating formation energies and geometries for an Fe vacancy and C insterstitial atom in bcc Fe as well as bcc Fe$_{1-x}$Cr$_x$ random alloys in the paramagnetic state. In the vacancy case, the nearest neighbors to the vacancy relax towards the vacancy of 0.16 {\AA} (-5% of the ideal bcc nearest neighbor distance), which is twice as large as the relaxation in the ferromagnetic case. The vacancy formation energy calculated in the DLM state on these positions is 1.60 eV, which corresponds to a reduction of about 0.1 eV compared to the formation energy calculated using DLM but on ferromagnetic-relaxed positions. The carbon interstitial formation energy is found to be 0.41 eV when the DLM relaxed positions are used, as compared to 0.59 eV when the FM-relaxed positions are employed. For bcc Fe$_{0.5}$Cr$_{0.5}$ alloys, the mixing enthalpy is reduced by 5 meV/atom, or about 10%, when the DLM state relaxation is considered, as compared to positions relaxed in the ferromagnetic state.