  The deliberate insertion of magnetic Mn dopants in the Fe sites of the
optimally-doped SmFeAsO0.88-F0.12 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 (muSR), both in zero- and in transverse fields,
respectively, to probe the magnetic and the superconducting order. 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. Gastiasoro et al., ArXiv
1606.09495], which includes correlation-enhanced RKKY-couplings between the
impurity moments.
