Theory of the Invar Effect in FeNi Alloy

Anomalous thermal expansion of FeNi alloy is calculated on the basis of Liberman-Pettifor's virial theorem and the full single site approximation in the functional integral method. Results explain well the Invar effect. In general, the electronic contribution to the thermal expansion consists of three terms, i.e. the positive term proportional to the specific heat, the local moment term caused by the temperature variation of the amplitudes of the local moments and the term due to the s - d charge transfer. It is verified that the large negative thermal expansion of the electron system is caused by the local moment term. The lattice parameter, the bulk modulus at T =0 K and other magnetic quantities are also calculated, which are consistent with the experimental results.