Atomic coexistence of superconductivity and incommensurate magnetic order in the pnictide Ba (Fe_1-xCo_x)₂As₂

A 75 s nuclear magnetic resonance (NMR) and susceptiblity were measured in a Ba ((Fe_1-xCo_x)₂As₂ single crystal for x=6%. NMR spectra and relaxation rates allow to show that all Fe sites experience an incommensurate magnetic ordering below T=31 K. Comparison with undoped compound allows to estimate a typical moment of 0.05_μB. Anisotropy of the NMR widths can be interpreted using a model of incommensurability with a wave vector (1/2-ε,0,l) with ε of the order of 0.04. Below T_C =21.8 K, a full volume superconductivity develops as shown by susceptibility and relaxation rate, and magnetic order remains unaffected, demonstrating coexistence of both states on each Fe site.