Abstract
It has been shown by small-angle neutron scattering that the magnetic structure of Invar alloys in the ground state is characterized by the occurrence of static magnetic fluctuations on the scale of 10–12 Å. Centres of such fluctuations are iron atoms surrounded by the same sort of atoms in the nearest coordination sphere. These fluctuations are due to antiferromagnetic iron pair interactions. The magnetic part of thermal expansion coefficient of Fe-Ni alloys is evaluated within the framework of the molecular field approximation. The Invar effect is brought about by a mixed exchange interaction and by a strong dependence of the exchange integral J FeFe on the interatomic distance.
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