From soft to hard magnetic Fe-Co-B by spontaneous strain: A combined first principle and thin film study

TL;DR

This study combines first-principles calculations and thin film experiments to demonstrate that interstitial boron induces spontaneous tetragonal strain in Fe-Co alloys, transforming them from soft to hard magnets with significant anisotropy.

Contribution

It introduces a novel approach of using boron-induced strain to enhance magnetic hardness in Fe-Co alloys, supported by both theoretical predictions and experimental validation.

Findings

Boron causes spontaneous tetragonal strain in Fe-Co alloys.

Uniaxial anisotropy constants exceed 0.5 MJ/m^3 with B content up to 4 at%.

Excess boron leads to amorphization and degradation of magnetic properties.

Abstract

In order to convert the well-known Fe-Co-B alloy from a soft to a hard magnet, we propose tetragonal strain by interstitial boron. Density functional theory reveals that when B atoms occupy octahedral interstitial sites, the bcc Fe-Co lattice is strained spontaneously. Such highly distorted Fe-Co is predicted to reach a strong magnetocrystalline anisotropy which may compete with shape anisotropy. Probing this theoretical suggestion experimentally, epitaxial films are examined. A spontaneous strain up to 5 % lattice distortion is obtained for B contents up to 4 at%, which leads to uniaxial anisotropy constants exceeding 0.5 MJ/m^3. However, a further addition of B results in a partial amorphization, which degrades both anisotropy and magnetization.