On the magnetic properties of Invar alloys

It is pointed out that the face-centred cubic Fe 0·7 Ni 0·3 alloy, which lies in the Invar alloy region, has a number of holes which just half fills the upper sub-band of the 3d band, i.e. two per atom, and it is suggested that the unusual magnetic behaviour at the Invar alloy region of face-centred cubic Fe-Ni alloys is due to the change of sign of D , the coefficient in the dispersion relation for spin waves with long wavelength ( h ω q = Dq 2 ), near that alloy. The typical magnetic properties of these alloys, i.e. the composition dependence of D , of spontaneous magnetization M and of Curie temperature T c , are explained consistently in the framework of the model.