The temperature dependence of the magnetization distribution in Fe65Ni35 invar: incompatibility of the two-state model

The distribution of the magnetization in Fe65Ni35 invar has been studied as a function of temperature in the range 100 to 600 K using polarized neutron diffraction. The range in temperature covers the region over which the anomalous thermal expansion giving rise to the invar effect occurs. The polarized neutron results which have been analysed in terms of localized iron and nickel 3d moments with cubic anisotropy show that the fraction of unpaired electrons with eg symmetry is approximately 47% and does not change with temperature. This is in contradiction to models of the invar effect which invoke the thermal population of two states with a different number of eg and t2g carriers.