  Muon Spin Rotation ($\mu$SR) and $^{19}$F Nuclear Magnetic Resonance (NMR)
measurements were performed to investigate the effect of Mn for Fe
substitutions in La$_{1-y}$Y$_{y}$Fe$_{1-x}$Mn$_x$AsO$_{0.89}$F$_{0.11}$
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 $^{19}$F 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 $^{19}$F 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 LaFeAsO$_{0.89}$F$_{0.11}$ to a quantum critical point.
