  The temperature dependence of electron spin resonance (ESR) was studied in
the oxypnictide superconductors LaFeAsO$_{1-x}$F$_x$ (x = 0.0 and 0.13). In the
samples, the ESR signal indicates that the g factor and peak-to-peak linewidth
strongly depend on temperature, especially at low temperatures. It indicates a
strong coupling picture with existence of local moment. The dependence
mentioned above gradually attenuates, and tends to saturation around
room-temperature. This behavior could be ascribed to "bottleneck" effect due to
coupling of local moment and itinerant electron. In addition, a Curie-Weiss
like behavior is also observed in temperature dependent integral intensity for
the two samples. Our results strongly support the existence of local moments in
these materials while its origin is still unclear. The results also indicate
strong magnetic frustration in this system, and magnetic fluctuation mechanism
for superconductivity is suggested.
