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

Abstract

Using sources of $^{161}\mathrm{Dy}\mathrm{Gd}$ produced directly by thermal neutron irradiation of $^{160}\mathrm{Gd}$-enriched Gd metal, we have observed M\"ossbauer line broadening in the reduced temperature range $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}<t<3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ above ${T}_{c}$. The excess linewidth diverges according to the power law $\ensuremath{\Delta}{\ensuremath{\Gamma}}_{c}=D{t}^{\ensuremath{-}w}$, with $w$ in the range $0.21<w<0.28$ 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 $1.3<z<1.52$. This range is anomalously low, since even with dipolar or pseudodipolar perturbations of isotropic short-range exchange interactions one expects $z=2.0$.