Spin fluctuations in Ni above T_C: comparison with RG

Constant energy scans in neutron scattering provide a very sensitive tool for comparing theoretical lineshapes with experiment. In order to verify the predictions of asymptotic renormalization group theory (aRG) for isotropic ferromagnets above T_C we measured the temperature dependence of the paramagnetic scattering from the itinerant 3-d ferromagnet Ni at small q values. The agreement with the theory is good near T_C for excitation energies h {combining short solidus overlay}ω <k_BT_C. Further away from T_C serious discrepancies occur, mainly because the spin fluctuations slow down more quickly than expected on the basis of asymptotic renormalization group theory and mode-mode coupling theory.