Critical spin dynamics of a Heisenberg ferromagnet

The spin dynamics of a Heisenberg ferromagnet at the critical temperature is reviewed. Particular attention is given to the interpretation of recent experiments on EuO, by both neutron spin-echo and standard neutron-scattering spectroscopy. The observed spin correlation function is calculated by a simplified version of the coupled-mode theory. The findings are compared with earlier calculations and results obtained with the renormalisation group technique. Neither of these theories is found to be satisfactory, and several reasons for the inadequacies are discussed. Finally, a semi-phenomenological model is shown to give a tolerable description of the experimental data.