Magnetization distribution in the ordered Fe72Pt28 Invar alloy

The distribution of magnetization in the ordered Invar alloy Fe72Pt28 has been studied as a function of temperature in the range 100-520 K using polarized neutron diffraction. The temperature range covers both the Invar region and the paramagnetic regime in which the thermal expansion has normal Gruneisen behaviour. The polarized neutron data have been analysed in terms of localized iron 3d and Pt 5d moments allowing local anisotropy of the magnetization distribution on the Fe atoms. The results suggest that the Pt atoms carry a small negative moment (0.3 mu(B)) in the magnetically ordered state. The fraction of unpaired electrons with e(g) symmetry is approximately 48% 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 e(g) and t(2g) carriers.