Temperature and polarization dependence of low-energy magnetic fluctuations in nearly-optimal-doped NaFe${}_{0.9785}$Co${}_{0.0215}$As

Abstract

We use unpolarized and polarized neutron scattering to study the temperature and polarization dependence of low-energy magnetic fluctuations in nearly-optimal-doped NaFe${}_{0.9785}$Co${}_{0.0215}$As, with coexisting superconductivity ($T_{\mathrm{c}}\approx 19$ K) and weak antiferromagnetic order ($T_{\mathrm{N}}\approx 30$ K, ordered moment $\approx 0.02$ ${\mu }_{\mathrm{B}}$/Fe). A single spin resonance mode with intensity tracking the superconducting order parameter is observed, although energy of the mode only softens slightly on approaching $T_{\mathrm{c}}$. Polarized neutron scattering reveals that the single resonance is mostly isotropic in spin space, similar to overdoped NaFe${}_{0.935}$Co${}_{0.045}$As but different from optimal electron-, hole-, and isovalent-doped BaFe${}_2$As${}_2$ compounds, all featuring an additional prominent anisotropic component. Spin anisotropy in NaFe${}_{0.9785}$Co${}_{0.0215}$As is instead present at energies below the resonance, which becomes partially gapped below $T_{\mathrm{c}}$, similar to the situation in optimal-doped YBa${}_2$Cu${}_3$O${}_{6.9}$. Our results indicate that anisotropic spin fluctuations in NaFe${}_{1-x}$Co${}_x$As appear in the form of a resonance in the underdoped regime, become partially gapped below $T_{\mathrm{c}}$ near optimal doping and disappear in overdoped compounds.

arXiv commentto be published in PRB

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• Spin order and excitations in the collinear kagome antiferromagnet Y${}_{0.5}$Fe${}_3$Sn${}_3$