Origin of Anomalies and Thermodynamic Aspects in Invar Alloys

Abstract

The effect of the forced magnetostriction on various thermodynamic properties of Invar alloys is examined. The temperature variations of the thermal expansion coefficient and bulk modulus defined at constant magnetization and the concentration dependences of this bulk modulus and the high-field susceptibility at constant volume are shown to be normal. There is an enhancement due to the magnetovolume coupling in the high-field susceptibility, compressibility and forced magnetostriction. The various anomalies are attributed to the large magnetovolume coupling in Fe–Ni Invar alloys. It is seen that Fe–Pt alloys are rather normal at low temperature. The magnetovolume coupling term is calculated in a simple itinerant electron model for fcc Fe–Ni alloys. To explain the large magnetovolume coupling in Fe–Ni alloys, the homogeneous itinerant electron model is not sufficient and a magnetic inhomogeneity with different atomic volumes will explain this discrepancy.