Mn local moments prevent superconductivity in iron pnictides Ba(Fe _1xMn _x) ₂As ₂

⁷⁵As nuclear magnetic resonance (NMR) experiments were performed on Ba(Fe _1xMn _x) ₂As ₂ (x _Mn=2.5%, 5% and 12%) single crystals. The Fe layer magnetic susceptibility far from Mn atoms is probed by the ⁷⁵As NMR line shift and is found similar to that of BaFe ₂As ₂, implying that Mn does not induce charge doping. A satellite line associated with the Mn nearest neighbours (n.n.) of ⁷⁵As displays a Curie-Weiss shift which demonstrates that Mn carries a local magnetic moment. This is confirmed by the main line broadening typical of a RKKY-like Mn-induced staggered spin polarization. The Mn moment is due to the localization of the additional Mn hole. These findings explain why Mn does not induce superconductivity in the pnictides contrary to other dopants such as Co, Ni, Ru or K.