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

Abstract

The Fe0.64Ni0.36 alloy exhibits an anomalously low thermal expansion at ambient conditions, an effect that is known as the invar effect. Other FexNi1−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 FexNi1−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.