Increased magnetocrystalline anisotropy in epitaxial Fe-Co-C thin films with spontaneous strain

TL;DR

This study demonstrates that adding 2 at% carbon to epitaxial Fe-Co thin films induces spontaneous strain, significantly enhancing magnetocrystalline anisotropy while maintaining high polarization, offering a promising route for rare earth free permanent magnets.

Contribution

The paper shows that carbon addition stabilizes spontaneous strain in Fe-Co films, leading to improved magnetic properties, which aligns well with theoretical predictions.

Findings

Spontaneous strain of 3% tetragonal distortion achieved with 2 at% carbon.

Magnetocrystalline anisotropy exceeds 0.4 MJ/m³ in strained films.

High polarization of 2.1 T maintained in the strained phase.

Abstract

Rare earth free alloys are in focus of permanent magnet research since the accessibility of the elements needed for nowadays conventional magnets is limited. Tetragonally strained iron-cobalt (Fe-Co) has attracted large interest as promising candidate due to theoretical calculations. In experiments, however, the applied strain quickly relaxes with increasing film thickness and hampers stabilization of a strong magnetocrystalline anisotropy. In our study we show that already 2 at% of carbon substantially reduce the lattice relaxation leading to the formation of a spontaneously strained phase with 3 % tetragonal distortion. In these strained (Fe$_{0.4}$Co$_{0.6}$)$_{0.98}$C$_{0.02}$ films, a magnetocrystalline anisotropy above 0.4 MJ/m$^3$ is observed while the large polarization of 2.1 T is maintained. Compared to binary Fe-Co this is a remarkable improvement of the intrinsic magnetic properties. In this paper, we relate our experimental work to theoretical studies of strained Fe-Co-C and find a very good agreement.