Absence of large nanoscale electronic inhomogeneities in the Ba(Fe _1-xCo _x) ₂As ₂ pnictide

⁷⁵As NMR and susceptiblity were measured in a Ba(Fe _1-xCo _x) ₂As ₂ single crystal for x = 6% for various field H values and orientations. The sharpness of the superconducting and magnetic transitions demonstrates a homogeneity of the Co doping x better than ±0.25%. On the nanometer scale, the paramagnetic part of the NMR spectra is found very anisotropic and very narrow for H||ab which allows to rule out the interpretation of reference [J. Phys. Soc. Jpn 78, 013711 (2009); Phys. Rev. B 79, R 140506 (2009)] in terms of strong Co induced electronic inhomogeneities. We propose that a distribution of hyperfine couplings and chemical shifts due to the Co effect on its nearest As explains the observed linewidths and relaxations. All these measurements show that Co substitution induces a very homogeneous electronic doping in BaFe ₂As ₂, from nano to micrometer lengthscales, on the contrary to the K doping.