"Anomalous" Mössbauer spectra characterized by the simultaneous presence of a pronouned central peak indicative of the paramagnetic state, and hyperfine structure characteristic of the ordered state, have been obtained in magnetically ordered compounds of iron close to their disordering temperature $T_N$. We adduce evidence supporting the hypothesis that the spectra found near $T_N$ are the result of collective spin flips of small regions (volume $v_{\mathrm{eff}}$) of the material, corresponding to the onset of superparamagnetic effects as the temperature approaches $T_N$ from below. A good fit is obtained between experimental spectra of Fe${\mathrm{F}}_3$, and the spectral shape computed using reasonable distributions of $v_{\mathrm{eff}}$ and $T_N$. The measured effect of an applied magnetic field upon the spectra near $T_N$ is explained in terms of our model of "critical superparamagnetism." Finally, the relevance of our results to general measurements of critical phenomena is discussed.

• Received 16 July 1968

DOI:https://doi.org/10.1103/PhysRev.177.864