A common magnetic origin for the Invar effects in fcc iron-based ferromagnets

TL;DR

This study uses first-principles calculations to reveal that magnetic interactions fundamentally cause the Invar effect in disordered fcc iron-based ferromagnets, linking thermal expansion anomalies to magnetic fluctuations.

Contribution

It demonstrates that magnetic contributions to free energy explain Invar effects and proposes magnetic fluctuation rates as key to thermal expansion behavior in these alloys.

Findings

Magnetic contributions account for Invar effects in iron-nickel alloys.

The relationship between thermal expansion and magnetism is consistent across studied alloys.

Magnetic fluctuation rates determine whether an alloy exhibits Invar or normal expansion.

Abstract

Using first-principles calculations, in conjunction with Ising magnetism, we undertake a theoretical study to elucidate the origin of the experimentally observed Invar effects in disordered fcc iron-based ferromagnets. First, we show that our theory can account for the Invar effects in iron-nickel alloys, the anomalies being driven by the magnetic contributions to the average free energies. Second, we present evidence indicating that the relationship between thermal expansion and magnetism is essentially the same in all the studied alloys, including those which display the Invar effect and those which do not. Hence we propose that magnetism plays a crucial role in determining whether a system exhibits normal thermal expansion, the Invar effect, or something else. The crucial determining factor is the rate at which the relative orientation of the local magnetic moments of nearest-neighbor iron atoms fluctuates as the system is heated.