1209.6512 (G. Prando et al.)

Phase separation at the magnetic-superconducting transition in
La$_{0.7}$Y$_{0.3}$FeAsO$_{1-x}$F$_{x}$    [PDF]

G. Prando, S. Sanna, G. Lamura, T. Shiroka, M. Tropeano, A. Palenzona, H. -J. Grafe, B. Büchner, P. Carretta, R. De Renzi

In this paper we report a detailed $\mu^{+}$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 $0.025 \leq x \leq 0.15$. 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.

View original: http://arxiv.org/abs/1209.6512