Fluctuating magnetism of Co- and Cu-doped NaFeAs

TL;DR

This study uses x-ray emission spectroscopy to investigate the local magnetic moments in NaFeAs doped with Co and Cu, revealing different doping mechanisms and the persistence of magnetic fluctuations in superconducting states.

Contribution

It provides new insights into how Co and Cu doping affect magnetic fluctuations and the mechanisms behind superconductivity in NaFeAs.

Findings

Co doping slightly reduces magnetic moments; Cu doping does not.

Magnetic moments increase with temperature in all samples.

Superconductivity occurs despite persistent magnetic fluctuations.

Abstract

We report an x-ray emission spectroscopy (XES) study of the local fluctuating magnetic moment ($\mu_{bare}$) in $\mathrm{NaFe_{1-x}Co_{x}As}$ and $\mathrm{NaFe_{1-x}Cu_{x}As}$. In NaFeAs, the reduced height of the As ions induces a local magnetic moment higher than $\mathrm{Ba_2As_2}$, despite lower T$_N$ and ordered magnetic moment. As NaFeAs is doped with Co $\mu_{bare}$ 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 $\mu_{bare}$ with temperature in all samples as observed in electron- and hole-doped $\mathrm{BaFe_2As_2}$. 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.