  Low energy spin fluctuations are studied for the electron-doped Fe-based
superconductor LaFeAsO(1-x)F(x) by inelastic neutron scattering up to the
energy transfer of w = 15 meV using polycrystalline samples. Superconducting
samples (x=0.057, Tc=25 K and x=0.082, Tc=29 K) show dynamical spin
susceptibility chi"(w) almost comparable with the parent sample's. However
chi"(w) is almost vanished in the x=0.158 sample where the superconductivity is
highly suppressed. These results are compatible with the theoretical
suggestions that the spin fluctuation plays an important role for the
superconductivity.
