The magnetic moment of several ferromagnetic face-centered-cubic iron–nickel alloys in the invar region have been investigated at low temperatures, as a function of both the magnetic field and the temperature. A vibrating-sample magnetometer was used, for which relative changes in the moment of 3 parts in 105 could be resolved. Such resolution has permitted a detailed analysis of the temperature dependence of ΔM/M, the relative deviation of the magnetic moment from its value at 0 K, in terms of a contribution arising from spin-wave excitations, varying as T3/2, and one from single-particle excitations, varying as T2, both of these at constant volume, together with a term describing the effect of volume dilation on the moment. The fits to the data for H = 10 kOe yield, for 34 at. % Ni:[Formula: see text]and for 40 at. %Ni:[Formula: see text]
