The Invar Problem

Abstract

Anomalous thermal expansion and its relation to magnetic properties are surveyed for various Invar alloys. Saturation magnetization was measured for alloy systems (FexNi1−x)92M8, where M=Cu, Co, Mn, Cr, and V, and also for Fe70(RexPt1−x)30 at 4.2°K. Sharp drop of the saturation moment occurs at the average electron concentration, N=8.13 to 8.52, for all the Invar alloys including the above-mentioned alloy systems and N=8.21 to 8.52 for all the Fe–Ni base alloys. When, however, it was plotted against the electron concentration of the matrix surrounding less electropositive impurity atoms such as Mn, Cr, or V, all the alloy systems were found to exhibit a sharp drop of the moment in narrower concentration range of 8.48 to 8.60 for all the Fe–Ni base alloys. Saturation moment and a reversible susceptibility in the saturation range were also measured at room temperature for (110) [001] and (001) [110] rolled single crystals of the composition 29 at.% Ni–Fe. It was found that saturation moment was reduced from 79 to 72 emu/g, and the reversible susceptibility increased from 4.0 to 4.6×10−4 emu/g by 70% reduction of the thickness. The mechanism was discussed in relation to the well-known effect of cold rolling on the thermal expansion coefficient of Invar.