Magnetism of γ Fe-Ni Invar Alloys with Low Nickel Concentration

Abstract

Magnetization, Mössbauer absorption spectra and lattice constants have been measured on the γ phase (f.c.c.) Fe-Ni alloys of 20∼30 at% Ni using finely grained samples in which the γ→α martensitic transformation can be suppressed even at liq. He temperature. The magnetic moment decreases very sharply from 1.3µ B in 30 at% Ni to 0.2µ B in 20 at% Ni. The Mössbauer absorption spectra observed at liq. He temperature show the appearance of antiferromagnetism in the low nickel concentration range. These experimental facts indicate that in these composition range the alloys consist magnetically of two phases, namely ferromagnetic and antiferromagnetic ones. This magnetic heterogeneity is caused by the statistical concentration fluctuation and is discussed quantitatively by considering a Gaussian type concentration distribution. The temperature and composition variations of the lattice constant and the thermal expansion coefficient can also be interpreted quantitatively on the basis of the same Gaussian type concentration fluctuation.