Anomalous critical slowing down of spin fluctuations in Gd observed withDy161Mössbauer effect

Using sources of ${}_{}{}^{161}{}_{}{}^{}\mathrm{Dy}\mathrm{Gd}$ produced directly by thermal neutron irradiation of ${}_{}{}^{160}{}_{}{}^{}\mathrm{Gd}$-enriched Gd metal, we have observed Mössbauer line broadening in the reduced temperature range $3\times {10}^{-4\mathrm{}}<t<\mathrm{}3\mathrm{}\times {10}^{-2\mathrm{}}$ above $T_c$. The excess linewidth diverges according to the power law $\Delta {\Gamma }_c=D{t}^{-w}$, with $w$ in the range $0.21<w<\mathrm{}0.28\mathrm{}$ depending on fitting procedure. On the assumption that the spin fluctuations are isotropic and describable by dynamical scaling theory, we conclude that the dynamical exponent $z$ lies in the range $1.3<z<\mathrm{}1.52\mathrm{}$. This range is anomalously low, since even with dipolar or pseudodipolar perturbations of isotropic short-range exchange interactions one expects $z=2.0\mathrm{}$.