Muon-spin-relaxation studies of magnetic order and superfluid density in antiferromagnetic NdFeAsO, BaFe2As2, and superconducting Ba1−xKxFe2As2

Zero-field (ZF) muon-spin-relaxation (μSR) measurements have revealed static commensurate magnetic order of Fe moments in NdFeAsO below TN∼135 K, with the ordered moment size nearly equal to that in LaFeAsO, and confirmed similar behavior in BaFe2As2. In single crystals of superconducting Ba0.55K0.45Fe2As2, μSR spectra indicate static magnetism with incommensurate or short-ranged spin structure in ∼70% of volume below TN∼80 K, coexisting with the remaining volume which exhibits a superfluid response below Tc∼30 K.