Abstract
In the present work we have studied polycrystalline samples of the GdO1−x F x FeAs superconductor by means of high field electron spin resonance (HF-ESR) spectroscopy. A set of the samples with different levels of fluorine doping was measured in the frequency range from 10 GHz to 400 GHz in magnetic fields up to 15 T. Surprising results have been obtained on the GdO0.85F0.15FeAs sample with the superconducting transition temperature T c =20 K. Gd ESR gives clear indications of the enhancement of (quasi)-static magnetic correlations which set in below ∼80 K and continue to develop even in the superconducting state. This suggests an occurrence of an intimate interplay between magnetism and superconductivity in the FeAs planes which evolves in GdO1−x F x FeAs compound upon the fluorine doping.
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