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.