Phase separation at the magnetic-superconducting transition in La0.7Y0.3FeAsO1-xFx

In this paper we report a detailed µ+SR and 19F NMR study of the La0.7Y0.3FeAsO1−xFx class of materials. Here, the diamagnetic La1-yYy substitution increases chemical pressure and, accordingly, sizeably enhances the optimal superconducting transition temperature. We investigate the magnetic–superconducting phase transition by keeping the Y content constant (y = 0.3) and by varying the F content in the range equation image. Our results show how magnetism and superconductivity coexist for x = 0.065. Such coexistence is due to segregation of the two phases in macroscopic regions, resembling what was observed in LaFeAsO1-xFx materials under applied hydrostatic pressure. This scenario is qualitatively different from the nanoscopic coexistence of the two order parameters observed when La is fully substituted by magnetic rare-earth ions like Sm or Ce.