A superconducting-to-magnetic transition is reported for $\mathrm{La}\mathrm{Fe}\mathrm{As}{\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}$ where a per-thousand amount of Mn impurities is dispersed. By employing local spectroscopic techniques like muon spin rotation ($\mu$SR) and nuclear quadrupole resonance (NQR) on compounds with Mn contents ranging from $x=0.025$% to $x=0.75\%$, we find that the electronic properties are extremely sensitive to the Mn impurities. In fact, a small amount of Mn as low as 0.2% suppresses superconductivity completely. Static magnetism, involving the FeAs planes, is observed to arise for $x>0.1$% and becomes further enhanced upon increasing Mn substitution. Also a progressive increase of low-energy spin fluctuations, leading to an enhancement of the NQR spin-lattice relaxation rate $T_1^{-1}$, is observed upon Mn substitution. The analysis of $T_1^{-1}$ for the sample closest to the crossover between superconductivity and magnetism ($x=0.2$%) points toward the presence of an antiferromagnetic quantum critical point around that doping level.

• Received 29 January 2014
• Revised 17 March 2014

DOI:https://doi.org/10.1103/PhysRevB.89.134503