Origin of Anomalies and Thermodynamic Aspects in Invar Alloys II. Effect of Magnetic Phase Mixing

Abstract

The origin of the magnetic inhomogeneity in Fe–Ni Invar alloys is attributed to the first-order magnetic transition, which was predicted before in the itinerant electron model. The large effect of the magnetovolume coupling on various thermodynamic properties is examined in the model of magnetic mixing of the ferromagnetic phase with larger atomic volume and the paramagnetic phase with smaller atomic volume. At low temperatures the mixing is assumed to be frozen in. The mutual transformation between the ferromagnetic and paramagnetic phases due to the changes of temperature, magnetic field and pressure give additional contributions to the respective thermodynamic properties. The difference between the observed values of the field derivative of the volume and the pressure derivative of the magnetization is discussed. The experimental results of various thermodynamic properties in Fe–Ni Invar alloys are analyzed in connection with the present theoretical results and are satisfactorily explained.