Evidence for local moments by electron spin resonance study of polycrystalline LaFeAsO1-xFx (x=0 and 0.13)

The temperature dependence of electron spin resonance was studied in the oxypnictide superconductors LaFeAsO1-xFx (x=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 the bottleneck effect due to coupling of local moments and itinerant electrons. In addition, a Curie-Weiss-type behavior is also observed in temperature-dependent spin susceptibility. Our results strongly support the existence of local moments in these materials, while their origin is still unclear. The results also indicate strong magnetic frustration in this system, and the magnetic fluctuation mechanism for superconductivity is suggested.