Invar anomalies

Abstract

Mössbauer results for iron nickel alloys and for iron in copper‐based alloys suggest the physical concepts used in formulating a model for the magnetic properties of iron nickel alloys. A change in the magnetic state of an iron atom when it has more than five iron near neighbors leads to giant moment behavior for alloys at the Invar composition. The falloff in the saturation magnetization occurs because the cluster centers get further apart as the iron concentration is increased, and at the Invar composition the clusters on average just touch. Estimates yield more than 17μ_B for these clusters, and they include about 25 atoms around the magnetic nucleus. The formation and magnetization of these spin clusters result in the iron atoms which are not magnetic centers being magnetized with an increase in volume due to exchange interactions. By using the dependence on the concentration and temperature of the expansion anomaly, it is deduced that interatomic exchange between iron atoms which have lost their ferromagnetic tendencies produces the positive volume magnetostriction and the expansivities, lattice parameters, and forced volume magnetostrictions calculated on this model. The Mössbauer results show that in some regions of the sample the giant moments are weakly coupled to the mean magnetization, and it is the polarizing of the other iron atoms by these giant moments which produces the unusual temperature dependence of the low‐temperature forced volume magnetostriction observed by Schlosser et al. A calculation using couplings derived from the Mössbauer results gives excellent agreement with experiment. It is obvious from this work that Invar is best considered as an inhomogeneous ferromagnet, analogous to PdFe or critical CuNi alloys