Muon spin relaxation studies of magnetic order and superfluid density in antiferromagnetic NdOFeAs, BaFe2As2 and superconducting (Ba,K)Fe2As2

Zero-field (ZF) muon spin relaxation (\(\mu\)SR) measurements have revealed static commensurate magnetic order of Fe moments in NdOFeAs below \(T_{N} \sim 135\) K, with the ordered moment size nearly equal to that in LaOFeAs, and confirmed similar behavior in BaFe\(_{2}\)As\(_{2}\). In single crystals of superconducting (Ba\(_{0.55}\)K\(_{0.45}\))Fe\(_{2}\)As\(_{2}\), \(\mu\)SR spectra indicate static magnetism with incommensurate or short-ranged spin structure in \(\sim\) 70 % of volume below \(T_{N} \sim\) 80 K, coexisting with remaining volume which exhibits superfluid-response consistent with nodeless gap below \(T_{c}\sim 30\) K.