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 T-N and the magnitude of the internal field display a domelike dependence on x, peaked around x = 1/4, with higher T-N 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 (T-c) for x = 0. The reduction of T-c (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 similar or equal to 0.6. DOI: 10.1103/PhysRevB.87.134518