  High resolution Neutron Spin Echo (NSE) spectroscopy was used to investigate
the dynamics of an 3D Heisenberg ferromagnet in the exchange-controlled regime
over a broad range of temperatures and momentum transfer. These results allow
for the first time an extensive comparison between the experimental dynamical
critical behavior and the predictions of the Renormalization Group (RG) theory.
The agreement is exhaustive and surprising as the RG theory accounts not only
for the critical relaxation but also for the shape crossover towards an
exponential diffusive relaxation when moving from the critical to the
hydrodynamic regime above $T_C$.
