Thermodynamic Properties of Invar at Low Temperatures

The low-temperature thermodynamic data on an iron-nickel alloy (Carpenter Steel N-37) are presented and reduced to the six coefficients needed to express the pressure, magnetic moment, and entropy changes in terms of changes of volume, magnetic field, and temperature. At low temperatures, the $T=0\mathrm{}$ saturation magnetic moment $M_0$ varies as $V^{11\pm 1}$ where $V$ is the volume, and the entropy $S$ varies as $M_0^{}{}_{}{}^{-\frac{5}{2}\pm 1}{V}^{5\pm 3}T$. A band model appears to offer the simplest explanation of these variations, and the detailed implications of such a model are pointed out. In particular, the anomalously large volume dependences in Invar are explicable in terms of the approach to instability of the band splitting, occasioned by the shape of the density of states of the single-particle energies. A simple model indicates that a weak volume dependence of the exchange is sufficient to produce the large effects observed.