Competing effects of Mn and y doping on the low-energy excitations and phase diagram of La1-y YyFe1-xMnxAsO0.89 F0.11 iron-based superconductors

Muon spin rotation (?SR) and F19 nuclear magnetic resonance (NMR) measurements were performed to investigate the effect of Mn for Fe substitutions in La1-yYyFe1-xMnxAsO0.89F0.11 superconductors. While for y=0 a very low critical concentration of Mn (x=0.2%) is needed to quench superconductivity, as y increases the negative chemical pressure introduced by Y for La substitution stabilizes superconductivity and for y=20% it is suppressed at Mn contents an order of magnitude larger. A magnetic phase arises once superconductivity is suppressed both for y=0 and for y=20%. Low-energy spin fluctuations give rise to a peak in F19 NMR 1/T1 with an onset well above the superconducting transition temperature and whose magnitude increases with x. Also the static magnetic correlations probed by F19 NMR linewidth measurements show a marked increase with Mn content. The disruption of superconductivity and the onset of the magnetic ground state are discussed in the light of the proximity of LaFeAsO0.89F0.11 to a quantum critical point.