Enhancing magnetism in Fe-Al alloys by increasing chemical disorder

Abstract

We present a quantum-mechanical study of a B2-phase of Fe70Al30 alloy with and without antiphase boundaries (APBs) with the {001} crystallographic orientation of APB interfaces. Our motivation is to explain experimental findings demonstrating a significantly higher magnetic flux density from A2-phase interlayers at the thermally-induced APBs in this material and suggesting that the ferromagnetism is stabilized by the disorder in the A2 phase. Our investigation of sharp APBs (without any A2-phase interlayer) indicates that they have moderate APB energies (~0.1 J/m2) and cannot explain the experimentally detected increase in the ferromagnetism because they often induce a ferro-to-ferrimagnetic transition. When studying thermal APBs, we introduce a few atomic layers of A2 phase of Fe70Al30 into the interface of sharp APBs. The averaged computed magnetic moment of Fe atoms in the whole B2/A2 nanocomposite is then increased by 11.5% w.r.t. the B2 phase. The A2 phase itself (treated separately as a bulk) has the total magnetic moment even higher, by 17.5%, and this increase also applies if the A2 phase at APBs is sufficiently thick (the experimental value is 2–3 nm). We link the changes in the magnetism to the facts that (i) the Al atoms in the first nearest neighbor (1NN) shell of Fe atoms nonlinearly reduce their magnetic moments and (ii) there are on average less Al atoms in the 1NN shell of Fe atoms in the A2 phase. These effects synergically combine with the influence of APBs which provide local atomic configurations not existing in an APB-free bulk. The identified mechanism of increasing the magnetic properties by introducing APBs with disordered phases can be used as a new designing principle when developing new magnetic materials.