Fluctuating magnetism of Co- and Cu-doped NaFeAs

We report an x-ray emission spectroscopy (XES) study of the local fluctuating magnetic moment ( µ bare ) in NaFe 1 − x Co x As and NaFe 1 − x Cu x As. In NaFeAs, the reduced height of the As ions induces a local magnetic moment higher than Ba 2 As 2 , despite lower T N and ordered magnetic moment. As NaFeAs is doped with Co µ bare is slightly reduced, whereas Cu doping leaves it unaffected, indicating a different doping mechanism: based on electron counting for Co whereas impurity scattering dominates in the case of Cu. Finally, we observe an increase of µ bare with temperature in all samples as observed in electron- and hole-doped BaFe 2 As 2 . Since both Co and Cu doping display superconductivity, our findings demonstrate that the formation of Cooper pairs is not connected with the complete loss of fluctuating paramagnetic moments.

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