Magnetovoloume Effects of Itinerant Electron Ferromagnets. I. Thermal Expansion Anomaly of the Invar Alloy

Abstract

The spontaneous volume magnetostriction of metallic ferromagnets is discussed based on the band theory. Dependences on lattice parameter of the band width and of the molecular field coefficient are taken into account. The difference between the lattice parameters in the ferromagnetic and paramagnetic states at 0°K is expressed in terms of the difference between the energies in the ferromagnetic and paramagnetic states. It is understood qualitatively why the absolute value of negative spontaneous volume magnetostriction ( e.g. in Ni) is generally small while that of positive one ( e.g. in Invar) is sometimes large. By assuming a simple but reasonable density of states curve for fcc Fe–Ni alloys the spontaneous volume magnetostriction of the alloys is calculated. The calculated value for Invar alloy is seven times larger than that for Ni metal in absolute value; the agreement with experimental results is satisfying.