Invar effect by local moment model

Abstract

On the basis of the molecular field approximation the magnetic part of the thermal expansion coefficient of fcc Fe-Ni alloys is evaluated. The main possibility of explaining the invar effect is shown and its physical matter is revealed. The invar phenomenon is caused by mixed exchange interaction and by strong dependence of the exchange interaction J FeFe on the interatomic distance. The latter circumstance gives rise to a large spontansous magnetostriction which compensates the normal thermal expansion of these alloys.