Fast suppression of superconductivity occurs in optimally electron doped LaFeAsO0.89F0.11 by substituting Fe by Mn at a remarkablylow critical content xc=0.2% [1,2].
Here we show that as soon as superconductivity is destroyed the parent undoped LaFeAsO ground state is recovered: both the same stripe ordered magnetic structure and tetragonal to orthorhombic transition are detected in optimally electron doped LaFeAsO0.89F0.11 with 0.5% of Mn .
Recent theoretical calculations  display that enhanced spin correlations developing around Mn severely speed up the reduction of Tc driven by the magnetic disorder, and may quench the entire superconducting phase already at Mn concentrations below 1%. The magnetic order generated by Mn doping is efficiently stabilized due to correlation-enhanced RKKY exchange couplings between neighbouring Mn ions [3,4].
Interestingly we show that the Mn critical threshold xc for the suppression of superconductivity is rapidly increased to few percent of Mn by squeezing the volume cell by substituting La by Y or Sm (xc = 8%) [5,6]. This behavior can be explained by considering the role of the electronic correlations, which are increased in isostructural compounds with bigger unit cell volume. Correlations enhance the RKKY exchange interactions between Mn ions driving the compound with La at the verge of an electronic instability, i.e. xc tends to 0.
The LnFeAsO (Ln=lanthanide) can be considered as a formidable example of how the electronic properties of strongly correlated systems can be significantly affected by fine-tuning the correlation strength with impurities and chemical pressure.
 F. Hammerath, P. Bonfà, S. Sanna, G. Prando, R. De Renzi, Y. Kobayashi, M. Sato, and P. Carretta, Phys. Rev. B 89, 134503 (2014).
 F. Hammerath, M. Moroni, L. Bossoni, S. Sanna, R. Kappenberger, S. Wurmehl, A. U. B. Wolter, M. A. Afrassa, Y. Kobayashi, M. Sato, B. Buchner, and P. Carretta, Phys. Rev. B 92, 020505(R) (2015).
 M. Moroni, P. Carretta, G. Allodi, R. De Renzi, M. N. Gastiasoro, B. M. Andersen, P. Materne, H.-H. Klauss, Y. Kobayashi, M. Sato, and S. Sanna, Phys. Rev. B 95, 180501(R) (2017).
 M. N. Gastiasoro, F. Bernardini and B. M. Andersen, Phys. Rev. Lett. 117, 257002 (2016).
 M. Moroni, S. Sanna, et al. Phys. Rev.B 94, 054508 (2016).
 G. Lamura, T. Shiroka, S. Bordignon, S. Sanna et al., Phys Rev. B 94, 214517 (2016).