Muon spin rotation (mu SR) and F-19 nuclear magnetic resonance (NMR) measurements were performed to investigate the effect of Mn for Fe substitutions in La1-y YyFe1-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 F-19 NMR 1/T-1 with an onset well above the superconducting transition temperature and whose magnitude increases with x. Also the static magnetic correlations probed by F-19 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.
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