Observation of isotropic critical spin fluctuations in Gd

Perturbed γγ angular-correlation experiments on oriented single crystals of Gd, containing 1 ppb of ${}_{}{}^{111}{}_{}{}^{}$In, demonstrate that critical spin fluctuations are isotropic to within about 1 K above the Curie temperature. Closer to $T_C$ we find evidence that the critical spin fluctuations fall increasingly along the c axis. Analysis of nuclear relaxation times in the isotropic region leads to spin autocorrelation times that diverge with an exponent w=0.53(8). This value is not consistent with either spin-nonconserved Heisenberg behavior or Ising behavior, but agrees with Mössbauer data for Gd ${}_{}{}^{161}{}_{}{}^{}$Dy.