Dynamical critical behavior of isotropic ferromagnets

Experiments measuring the dynamical exponent $z$ in the isotropic ferromagnets EuS, EuO, Ni, Fe, and Co are reviewed. Our recent hyperfine interaction experiments demonstrating crossover in $z$ are described in detail. In contrast to early neutron experiments on isotropic ferromagnetis it is found that with the exception of Co, pure Heisenberg behavior, i.e., $z=2.5\mathrm{}$, is not observed in any of the materials surveyed. Instead, when either the wave number or the reduced temperature is sufficiently small, isotropic ferromagnets exhibit asymptotic behavior characterized by $z=2.0\mathrm{}$. The most likely theoretical explanation is that significant spin-nonconserving forces are perturbing the Heisenberg exchange interaction. For ESR and neutron studies of EuS and EuO it has been shown that the observed behavior can be explained by dipolar forces. For hyperfine interaction experiments on Ni and Fe, the observed crossover to $z=2.0\mathrm{}$ must be attributed to other, stronger spin-nonconserving forces. In the exceptional case of Co, where crossover to $z=2.0\mathrm{}$ has not yet been observed, it is expected that future, more nearly asymptotic experiments, will detect crossover.