RKKY coupled local-moment magnetism in NaFe_1-xCu_xAs

Using high resolution ²³Na NMR measurements on lightly Cu-doped metallic single crystals of NaFe_1-xCu_xAs (x ≈ 0.01) and numerical simulation, we present direct evidence of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in this pnictide system. The magnetic Cu²⁺ impurities are polarized by external field, inducing a spin polarization of the conduction electrons. Aided by computer simulation, we identify the ²³Na NMR satellite resonances with the RKKY oscillations of this spin polarization coupled to loal moments at Fe sites via exchange interaction. Our ²³Na spin-lattice and spin-spin relaxation data exhibits signature of an itinerant and inhomogeneous AFM phase in this system. Our NMR results indicate coexistence of local and itinerant magnetism in lightly Cu-doped NaFe_1-xCu_xAs.