Fast Spin Fluctuation Viewed by Muon Spin Relaxation in Optimally Doped and Overdoped Iron-Based Oxypnictide Superconductors LaFeAsO1-xFx

Zero-field and longitudinal-field muon spin relaxation ($\mu$SR) measurements have been performed in optimally doped and overdoped iron-based oxypnictide superconductors LaFeAsO 1-x F x in order to investigate the spin dynamics over a wide temperature range. We have observed no sign of the slowing-down behavior of the spin fluctuations in all samples below and above the superconducting transition temperature. The results of the present study suggest that spins, which would play an important role in the appearance of superconductivity with the characteristics of the Fermi surface, fluctuate rapidly from the viewpoint of $\mu$SR and their fluctuation frequency is beyond the upper limit of the characteristic time window of $\mu$SR (${\sim}10^{-11}$ s) in optimally doped and overdoped LaFeAsO 1-x F x .