Small-angle scattering of Mössbauerγrays near magnetic phase transitions

The small-angle scattering of Mössbauer $\gamma$ rays of the canted antiferromagnet Fe${\mathrm{F}}_3$, the ferromagnet ${\mathrm{Fe}}_3$C, and the ferromagnetic alloy Ni + 2-at.%. ${\mathrm{Fe}}^{57}$ was studied as a function of the ambient temperature. A pronounced increase in the small-angle scattering of the recoilless 14.4-keV $\gamma$ rays of these substances is observed as the temperature increases toward $T_c$. This phenomenon exhibits features similar to those which were observed by x-ray scattering of binary alloys, light scattered by fluids, and neutrons of ferromagnets in the neighborhood of the critical temperature. The scattering intensity begins to increase around $0.9T_c$ where the Mössbauer spectrum shows a central peak coexisting with hyperfine splitting. The critical scattering is angle dependent and suggests a relation between the "superparamagnetic,"-type coherent spin fluctuations suggested in the literature and the onset of criticality near second-order magnetic phase transitions.