Electron doping evolution of the magnetic excitations in NaFe1-xCoxAs

We use time-of-flight (TOF) inelastic-neutron-scattering (INS) spectroscopy to investigate the doping dependence of magnetic excitations across the phase diagram of NaFe1-xCoxAs with x=0, 0.0175, 0.0215, 0.05, and 0.11. The effect of electron doping by partially substituting Fe by Co is to form resonances that couple with superconductivity, broaden, and suppress low-energy (E≤80 meV) spin excitations compared with spin waves in undoped NaFeAs. However, high-energy (E>80 meV) spin excitations are weakly Co-doping-dependent. Integration of the local spin dynamic susceptibility χ′′(ω) of NaFe1-xCoxAs reveals a total fluctuating moment of 3.6 μB2/Fe and a small but systematic reduction with electron doping. The presence of a large spin gap in Co-overdoped nonsuperconducting NaFe0.89Co0.11As suggests that Fermi surface nesting is responsible for low-energy spin excitations. These results parallel the Ni-doping evolution of spin excitations in BaFe2-xNixAs2 in spite of the differences in crystal structures and Fermi surface evolution in these two families of iron pnictides, thus confirming the notion that low-energy spin excitations coupling with itinerant electrons are important for superconductivity, while weakly doping-dependent high-energy spin excitations result from localized moments.