The purpose is to make clear the fundamental physical nature of the Invar problem, disregarding its details. After a brief discussion of the practical applications of the Invar alloys, the large magnetovolume effects and the anomalous magnetic properties are reviewed. The basic ideas of the various interpretations of the Invar effects, as well as the microscopic properties of the Invar alloys, are summarized. The physical background of the Elinvar alloys is also presented. Recent observations of Invar-type effects in new materials suggest that the Invar effect is rather common in itinerant ferro- and antiferromagnets.

The invar problem

Invar-type alloys - magnetomechanical effects

The sound velocity of a Fe 36.6 at.% Ni and of an ordered Fe 28.2 at.% Pt alloy have been measured between 4.2K and room temperature. Shear waves revealed an anomalous temperature dependence which is due to spin-phonon coupling. For the longitudinal mode a relaxation effect was found at low temperature for the FeNi alloy but not for the FePt alloy. It is shown that the relaxation mechanism is related to the inhomogeneous magnetic structure of the FeNi alloy. The relaxation is ascribed to a stress-induced change of fluctuations of the Fe magnetic moment. The variations of the sound velocities with magnetic field are reported which yield information on the spin-phonon coupling mechanism.

Low temperature acoustic relaxation effect in FeNi and FePt invar alloys

Elastic properties of magnetic materials

To specify the condition for the anomalous thermal expansion due to a large spontaneous volume magnetostriction in ferromagnetic Laves phase intermetallic TFe 2 compounds, thermal expansion measurements for some TFe 2 compounds were made. The obtained results suggest that the anomaly is sensitive to the interatomic distance between neighboring iron atoms.

M. Shiga

Papers

Invar effects in Zr0.7Nb0.3Fe2