Lattice Distortion and Magnetic Inhomogeneity in Fe-Ni Invar Alloy

X-ray diffraction and magnetization measurements were carried out on Invar alloy with magnetic inhomogeneity over the temperature range from room temperature to 400°C. The temperature dependence of the lattice strains on the (311), (220) and (200) planes were calculated from the diffraction line breadths. It was found that the average lattice strain ( Δ d / d ) at room temperature amounts to 1.17×10 -3 and almost vanishes above the Curie temperature, and furthermore the temperature dependence is similar to the temperature dependence of the magnetization. The origin of the large lattice-distortion is discussed on the basis of the anomalous magnetovolume effect and the related lattice instability in Invar composition.