The deliberate insertion of magnetic Mn dopants in the Fe sites of the optimally doped SmFeAsO0.88F0.12 iron-based superconductor can modify in a controlled way its electronic properties. The resulting phase diagram was investigated across a wide range of manganese contents (x) by means of muon-spin spectroscopy (μSR), both in zero and in transverse fields, to probe the magnetic and the superconducting order, respectively. The pure superconducting phase (at x<0.03) is replaced by a crossover region at intermediate Mn values (0.03≤x<0.08), where superconductivity coexists with static magnetic order. After completely suppressing superconductivity for x=0.08, a further increase in Mn content reinforces the natural tendency towards antiferromagnetic correlations among the magnetic Mn ions. The sharp drop of Tc and the induced magnetic order in the presence of magnetic disorder/dopants, such as Mn, are both consistent with a recent theoretical model of unconventional superconductors (M. N. Gastiasoro et al., Phys. Rev. Lett. 117, 257002 (2016)10.1103/PhysRevLett.117.257002), which includes correlation-enhanced RKKY couplings between the impurity moments.
Bibliographical noteFunding Information:
This work was performed at the Swiss Muon Source Paul Scherrer Institut (PSI, Switzerland) and was in part supported by the Schweizerische Nationalfonds zur Farderung der Wissenschaftlichen Forschung (SNF) and the NCCR research pool MaNEP of SNF. G.L. and M.P. thank M. Lucaccini and G. Tavilla for their valuable technical support and C. Robustelli for the supply of cryogenic liquids. We acknowledge financial support by FP7-EU project SUPER-IRON (No. 283204) (M.P. and G.L.), by MIUR project FIRB2012-HybridNanoDev (Grant No. RBFR1236VV) (G.L.), and by MIUR project PRIN2012 (Grant No. 2012X3YFZ2) (M.P., G.L., S.S., M.M., P.C., R.D.R., and S.B.). M.N.G. and B.M.A acknowledge support from Lundbeckfond fellowship (Grant No. A9318).