  The 57Fe M\"ossbauer spectroscopy was applied to an iron-based layered
superconductor LaO0.89F0.11FeAs with a transition temperature of 26 K and its
parent material LaOFeAs. Throughout the temperature range from 4.2 to 298 K, a
singlet spectrum with no magnetic splitting was observed as a main component of
each M\"ossbauer spectrum of the F-doped superconductor. No additional internal
magnetic field was observed for the spectrum measured at 4.2 K under a magnetic
field of 7 T. On the other hand, the parent LaOFeAs shows a magnetic transition
at around 140 K, and this temperature is slightly lower than that of a
structural phase transition from tetragonal to orthorhombic phase, which
accompanies the resistivity anomaly at around 150 K. The magnetic moment is
estimated to be ~0.35 $\mu$B/Fe at 4.2 K in the orthorhombic phase, and the
spin disorder remains in the magnetic ordered state even at 4.2 K. The fact
that no magnetic transition in LaO0.89F0.11FeAs was observed even at 4.2 K
under 7 T implies a strong spin fluctuation above Tc or small magnetic moment
in this system. Therefore, the present results show that the F-doping
effectively suppresses the magnetic and structural transitions in the parent
material and the suppression leads to emergence of superconductivity in this
system.
