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

TL;DR

This study investigates the coexistence and phase separation of magnetism and superconductivity in La$_{0.7}$Y$_{0.3}$FeAsO$_{1-x}$F$_{x}$ using muon spin rotation and NMR, revealing macroscopic phase segregation at certain doping levels.

Contribution

It provides detailed experimental evidence of phase separation between magnetic and superconducting regions in this material, differing from nanoscopic coexistence seen in other compounds.

Findings

Magnetism and superconductivity coexist at x=0.065.

Phase segregation occurs in macroscopic regions.

Coexistence scenario differs from that in fully magnetic rare-earth substituted compounds.

Abstract

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.