Phase separation at the magnetic-superconducting transition in La$_0.7$Y$_0.3$FeAsO$_1-x$F$_x$
In this paper we report a detailed $μ^+$SR and $^19$F-NMR study of the La$_0.7$Y$_0.3$FeAsO$_1-x$F$_x$ class of materials. Here, the diamagnetic La$_1-y$Y$_y$ 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 $0.025 ≤ x ≤ 0.15$. 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 LaFeAsO$_1-x$F$_x$ 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.