The recent discovery of superconductivity at a rather high temperature in the iron pnictides (Tc,max=56K) has revived some fundamental questions about the existence and the nature of the superconducting phase : in particular, the existence of an antiferromagnetic phase that is in vicinity of the superconducting phase in their phase diagram, as in other unconventional superconductors, raises questions about the link between magnetism and superconductivity. In this thesis, we studied the normal state as well as the antiferromagnetic and superconducting phases of the iron pnictides on a local scale with Nuclear Magnetic Resonance (NMR). Starting from the same parent compound BaFe2As2, we studied heterovalent Co substitution in Fe site realizing an electron doping and isovalent Ru substitution in Fe site. The normal state is shown to display important qualitative differences with the normal state of cuprates superconductors: disorder induced substitutions in electronically active layers is weak and we show the absence of a PseudoGap phase from spin susceptibility measurements. Whereas the phase diagram is similar for Co and Ru substitutions, we show that the nature of the antiferromagnetic and the superconducting phases is qualitatively different on a local scale in the two cases. For Co substitution leading to electron doping, the electronic phases are homogeneous and we demonstrate in particular the homogeneous coexistence of antiferromagnetism and superconductivity down to an atomic scale for some compositions: this suggests a magnetism of itinerant nature and an unconventional superconducting order parameter for the superconducting phase. For the isovalent Ru substitution, the electronic phases are inhomogeneous at a scale surprisingly low of the order of the nanometer scale, leading to a coexistence that is very distributed spatially. This works shows the possibility to induce an unconventional superconducting phase by the weakening of an antiferromagnetic phase made possible with very different means : either in reciprocal space with electron doping or in real space with isovalent substitution. Moreover, this is shown to lead to different kinds of coexistence between these phases in the two cases.
