# Concentration tuned tetragonal strain in alloys: application to magnetic anisotropy of FeNi$_{1-x}$Co$_x$

## TL;DR

This study demonstrates how tetragonal strain in FeNi-Co alloys can be tuned by composition to significantly enhance magnetic anisotropy, with potential applications in magnetic materials design.

## Contribution

It introduces a method to control tetragonal distortion via alloy composition, leading to a substantial increase in magnetocrystalline anisotropy in FeNi-Co alloys.

## Key findings

- Maximum tetragonal strain at x=0.5 in FeNi$_{1-x}$Co$_x$
- Enhanced MAE by a factor of 4.5 in FeNi$_{0.5}$Co$_{0.5}$
- Electronic structure calculations confirm strain control of magnetic properties.

## Abstract

We explore an opportunity to induce and control tetragonal distortion in materials. The idea involves formation of a binary alloy from parent compounds having body-centered and face-centered symmetries. The concept is illustrated in the case of FeNi$_{1-x}$Co$_x$ magnetic alloy formed by substitutional doping of the L1$_0$ FeNi magnet with Co. Using electronic structure calculations we demonstrate that the tetragonal strain in this system can be controlled by concentration and it reaches maximum for $x=0.5$. This finding is then applied to create a large magnetocrystalline anisotropy (MAE) in FeNi$_{1-x}$Co$_x$ system by considering an interplay of the tetragonal distortion with electronic concentration and chemical anisotropy. In particular, we identify a new ordered FeNi$_{0.5}$Co$_{0.5}$ system with MAE larger by a factor 4.5 from the L1$_0$ FeNi magnet.