First Principles Theory of the Pressure Induced Invar Effect in FeNi Alloys

TL;DR

This paper presents a first-principles theoretical investigation into the pressure-induced invar effect in FeNi alloys, linking magnetic transitions and entropy contributions to explain anomalously low thermal expansion under pressure.

Contribution

It introduces a comprehensive first-principles model that explains the pressure-induced invar effect through magnetic state transitions and entropy effects in FeNi alloys.

Findings

Anomalies in the equation of states at different compositions.

Magnetic transitions from ferromagnetic to complex states under pressure.

Magnetic entropy increases with pressure, explaining the invar effect.

Abstract

The Fe$_{0.64}$Ni$_{0.36}$ alloy exhibits an anomalously low thermal expansion at ambient conditions, an effect that is known as the invar effect. Other Fe$_{x}$Ni$_{1-x}$ alloys do not exhibit this effect at ambient conditions but upon application of pressure even Ni-rich compositions show low thermal expansion, thus called the pressure induced invar effect. We investigate the pressure induced invar effect for Fe$_{x}$Ni$_{1-x}$ for x = 0.64, 0.50, 0.25 by performing a large set of supercell calculations, taking into account noncollinear magnetic states. We observe anomalies in the equation of states for the three compositions. The anomalies coincide with magnetic transitions from a ferromagnetic state at high volumes to a complex magnetic state at lower volumes. Our results can be interpreted in the model of noncollinear magnetism which relates the invar effect to increasing contribution of magnetic entropy with pressure.