Slow magnetic fluctuations and superconductivity in fluorine-doped NdFeAsO

Among the widely studied superconducting iron-pnictide compounds belonging to the Ln1111 family (with Ln a lanthanide), a systematic investigation of the crossover region between the superconducting and the antiferromagnetic phase for the Ln = Nd case has been missing. We fill this gap by focusing on the intermediate doping regime of NdFeAsO1-xFx by means of dc-magnetometry and muon-spin spectroscopy (μSR) measurements. The long-range order we detect at low fluorine doping is replaced by short-range magnetic interactions at x=0.08, where also superconductivity appears. In this case, longitudinal-field μSR experiments show clear evidence of slow magnetic fluctuations that disappear at low temperatures. This fluctuating component is ascribed to the glassy-like character of the magnetically ordered phase of NdFeAsO at intermediate fluorine doping.