Onset of magnetism in optimally electron-doped LnFe1-xRuxAsO1-yFy (Ln = La, Nd or Sm) superconductors around x=1/4

The appearance of static magnetism, nanoscopically coexisting with superconductivity, is shown to be a general feature of optimally electron-doped LFe1−xRuxAsO1−yFy superconductors (L represents a lanthanide ion) upon isovalent substitution of Fe by Ru. The magnetic ordering temperature TN and the magnitude of the internal field display a domelike dependence on x, peaked around x=1/4, with higher TN values for those materials characterized by a larger z cell coordinate of As. Remarkably, the latter are also those with the highest superconducting transition temperatures (Tc) for x=0. The reduction of Tc(x) is found to be significant in the x region of the phase diagram where the static magnetism develops. Upon increasing the Ru content superconductivity eventually disappears, but only at x≃0.6.