Thermal Expansion Coefficient and Spontaneous Volume Magnetostriction of Fe-Ni (fcc) Alloys

The thermal expansion coefficient α of Fe-Ni ( fcc ) alloys was measured in the range from 800°C to room temperature. Below the Curie temperature T c , the α– T curve exhibits an anomaly corresponding to the spontaneous volume magnetostriction ω s . The `paramagnetic' thermal expansion coefficient α p below T c was determined by the extrapolation of the α– T curve above T c and the value of ω s was estimated from the difference between α p and α below T c . The relation between ω s and the change of T c with pressure was discussed. The α p vs. composition curve at a fixed temperature shows a minimum around the invar alloys. This minimum corresponds to the anomaly in the elastic moduli vs. composition curves. It is thus pointed out that the lattice energy must be considered in addition to the magnetic energy in discussing the origin of the invar properties.