We will first present the basic principles and the uses of Nuclear Magnetic Resonance in Solid State Physics and especially in strongly correlated materials based on a few examples. NMR is a local probe which enables to get unique informations about magnetism, superconductivity, spin or charge distribution and local static and dynamic susceptibilities. In addition, t is not sensitive to spurious phases and probes the full volume of the sample.
The second part of the talk will be devoted to recent NMR results obtained in pnictide superconductors. In the Co-doped BaFe2As2, we will show how NMR unambiguously demonstrates the atomic coexistence of superconductivity and an incommensurate spin density wave. In comparison, we will show the peculiar case of Ru doping. Ru and Co apparently induce the same phase diagram despite the fact that Ru is isovalent while Co gives one electron to the FeAs layer. However, from the NMR local point of vue, the situation is very different: Ru induces an intrinsic spatial distribution in the FeAs layer which results in a strong distribution of the local properties, such as the ordered magnetism or the superconductivity, resulting in a segregated situation.
