The appearance of static magnetism, nanoscopically coexisting with superconductivity, is shown to be a general feature of optimally electron-doped $L$Fe${}_{1-x}$Ru${}_x$AsO${}_{1-y}$F${}_y$ 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 $T_c\left(x\right)$ 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 $x\simeq 0.6$.

• Received 20 March 2013

DOI:https://doi.org/10.1103/PhysRevB.87.134518