The temperature dependence of the initial susceptibility ${\chi }_0$ of nickel above the Curie point $T_c$ and the field dependence of its magnetization at $T_c$ are deduced from the data of Weiss and Forrer and found to be at variance with the simple molecular-field model. Instead, the experimental ${{\chi }_0}^{-1\mathrm{}}$-versus-$T$ curve just above $T_c$ is shown to follow the simple relation ${{\chi }_0}^{-1\mathrm{}}=A{(T-T_c]}^{\gamma }$, with $\gamma =1.35\mathrm{}\pm 0.02$, in excellent agreement with the $\frac{4}{3}$-power relation recently predicted from the exact series for the Heisenberg model. From the coefficient $A$, it is deduced that ${\mu }_0$, the average atomic moment, is 0.642 ${\mathrm{\mu }}_{\mathrm{B}}$ and that the individual electron moments are in a state corresponding to $S=\frac{1}{2}$. At higher temperatures, the ${{\chi }_0}^{-1\mathrm{}}$-versus-$T$ curve deviates from the Heisenberg-model predictions, possibly because of a gradual rise in ${\mu }_0$ with increasing temperature. Up to the highest field $H$ of measurement, the magnetization at $T_c$ is shown to vary as $H^{\epsilon }$ with $\epsilon =0.237\mathrm{}$, which is consistent with the exponent values for an analogous empirical relationship between the density and pressure of several different gases at their critical points.

• Received 15 July 1964

DOI:https://doi.org/10.1103/PhysRev.136.A1626