Temperature Dependence of the Isomer Shift and the Hyperfine Field Near the Curie Point in Iron

Mössbauer measurements have been made on ${\mathrm{Fe}}^{57}$ in iron near the Curie temperature. The discontinuity in the isomer shift near $T_c$ is found to take place within a temperature range of less than 0.3°C, which suggests that this magnetic transition could be of first order. The "critical index" $\beta$ for the variation with temperature of the spontaneous magnetization in the range $0.98T_c<T<\mathrm{}0.9992\mathrm{}{T}_c$ is found to be definitely greater than $\frac{1}{3}$.