Effect of chemical disorder on the magnetic anisotropy in L$1_0$ FeNi from first principles calculations

TL;DR

This study uses first principles calculations to explore how chemical disorder influences magnetic anisotropy in L$1_0$ FeNi, revealing that slight Fe enrichment can enhance MAE beyond the perfectly ordered structure.

Contribution

It identifies local chemical environments that increase MAE and demonstrates that chemical disorder and Fe enrichment can improve magnetic anisotropy in L$1_0$ FeNi.

Findings

Partial correlation between local environment and MAE exists only with small deviations from order.

Increased Fe content up to 62.5% enhances MAE.

Perfectly ordered structure does not yield the highest MAE.

Abstract

We use first principles calculations to investigate how deviations from perfect chemical order affect the magneto-crystalline anisotropy energy (MAE) in L$1_0$ FeNi. We first analyze the local chemical environment of the Fe atoms in various partially ordered configurations, using the orbital magnetic moment anisotropy (OMA) as proxy for a local contribution to the MAE. We are able to identify a specific nearest neighbor configuration and use this "favorable environment" to successfully design various structures with MAE higher than the perfectly ordered system. However, a systematic analysis of the correlation between local environment and OMA using smooth overlap of atomic positions (SOAP), indicates only a partial correlation, which exists only if the deviation from full chemical order is not too large, whereas in general no such correlation can be identified even using up to third nearest neighbors. Guided by the observation that the identified "favorable environment" implies an Fe-rich composition, we investigate the effect of randomly inserting additional Fe into the nominal Ni planes of the perfectly ordered structure. We find that the MAE increases with Fe content, at least up to 62.5% Fe. Thus, our study shows that the perfectly ordered case is not the one with highest MAE and that an increased MAE can be obtained for slightly Fe-rich compositions.