Mössbauer Effect in Metallic Iron

The Mössbauer effect in metallic iron has been studied from 4 to 1300°K, with particular emphasis on the region near the Curie temperature at 1043°K.

Measurements of the internal field $H_n$ at the nucleus agree with nuclear magnetic resonance measurements and follow closely, but not exactly, the saturation magnetization curve of iron. No internal field is observed above the Curie temperature. At room temperature $H_n=330\mathrm{}\pm 3$ kOe. The ratio of magnetic moments of the two lowest levels of ${\mathrm{Fe}}^{57}$ is $\frac{{\mu }_1}{{\mu }_0}=-1.715\mathrm{}\pm 0.004$.

The observed temperature shift in the energy of the resonant radiation may be attributed chiefly to relativistic time dilation. In the low-temperature region the variation is nonlinear and compatible with a Debye temperature $\theta =400\mathrm{}\pm 30°$ K, although the data indicate that $\theta$ is not strictly independent of temperature. At high temperatures, the classical limit $\frac{\left(\frac{1}{E}\right)\partial E}{\partial T}=-\frac{3k}{2M{c}^2}$ for the relativistic shift is attained and perhaps exceeded. Disagreement with the classical limit would indicate a temperature variation in the isomer shift. At the Curie point and at the transition from $\alpha$ to $\gamma$ iron, the discontinuities observed in the temperature shift are too great to be attributed to the relativistic shift. If attributed to the isomer shift, these discontinuities indicate that the electron density at the nucleus increases at the transitions from the ferromagnetic to the paramagnetic state and from $\alpha$ to $\gamma$ iron.

The strength of the resonant absorption was also determined as a function of temperature. These measurements are compatible with a Debye temperature that falls from about 400 to 300°K in passing from low to high temperatures over the range studied.