Intralayer doping effects on the high-energy magnetic correlations in NaFeAs

We have used resonant inelastic x-ray scattering (RIXS) and dynamical susceptibility calculations to study the magnetic excitations in ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$ ($x=0$, 0.03, and 0.08). Despite a relatively low ordered magnetic moment, collective magnetic modes are observed in parent compounds ($x=0$) and persist in optimally ($x=0.03$) and overdoped ($x=0.08$) samples. Their magnetic bandwidths are unaffected by doping within the range investigated. High-energy magnetic excitations in iron pnictides are robust against doping and present irrespectively of the ordered magnetic moment. Nevertheless, Co doping slightly reduces the overall magnetic spectral weight, differently from previous studies on hole-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, where it was observed constant. Finally, we demonstrate that the doping evolution of magnetic modes is different for the dopants being inside or outside the Fe-As layer.

Figure 1

(a) Fe $L_{2,3}$ x-ray absorption spectra for Par (black line), Opt (red line), and Over (green line). (b) Experimental configuration and Fe $L_3$ RIXS spectra for Par at (0.44, 0) and $h{\nu }_{\mathrm{in}}=707.3$ eV.

Figure 2

(a) Experimental data (black dots) and relative fitting (blue solid line) of resonant emission of Par at (0.44, 0) and $\pi$ polarization. The incoming energy was tuned on the maximum of the Fe $L_3$ XAS. (b) Zoom into the low-energy-loss region of (a) and fitting of background (gray solid line), elastic (pink shaded), and magnetic peak (purple shaded). (c) Momentum and polarization dependence of the RIXS spectra of Par along $(0,0)\to (0.44,0)$. Solid line is $\pi$ polarization and dashed line is $\sigma$ polarization. (d) Comparison of RIXS spectra at (0.44, 0) and $\pi$ polarization for Par, Opt, and Over. All the data were collected at 13 K.

Figure 3

Momentum dependence of RIXS spectra along $(0,0)\to (0.44,0)$ and $(0,0)\to (0.32,0.32)$ for Par, Opt, and Over. Spectra are recorded in $\pi$ polarization at the maximum of the Fe $L_3$ absorption edge. We show experimental data (green dots), background and elastic (purple dotted line), and magnetic peaks (black dotted line). The sum of background, elastic, and magnetic peaks is depicted as a red solid line. At low $q_{//}$ a fitting is unreliable, so no fitting was attempted.

Figure 4

Energy dispersion curves for Par, Opt, and Over vs transferred momentum. Left panel: (1, 0) direction; right panel: (1, 1) direction. At $q=0.06$, 0.11, and 0.2 along (1, 0) and $q=0.04$, 0.08, and 0.15 along (1, 1) the fitting was not possible because of overlap of the magnetic peak with the elastic line. Only an estimation of the energy range is provided for these values, as depicted by error bars. The points have been slightly shifted along the $x$ axis for better visualization.

Figure 5

(a) Fitting results. Widths of magnetic peaks vs transferred momentum. Left side: (1, 0) direction; right side: (1, 1) direction. (b) Sum of the area of magnetic peaks for (0.44, 0), (0.41, 0), (0.37, 0), (0.32, 0.32), (0.29, 0.29), and (0.27, 0.27) normalized to Fe content vs doping ($x$). (c) Sum of the area of magnetic peaks for (0.44, 0), (0.41, 0), (0.37, 0), (0.32, 0.32), (0.29, 0.29), and (0.27, 0.27) rescaled per formula unit vs doping ($x$). These peaks were selected because they are the most clear and with the highest intensity. The blue dotted lines represent a guide for the eye.

Figure 6

Calculated dynamical susceptibility (color plot) overlaid with experimental data (black and white dots with error bars) for Par, Opt, and Over. Top row: Calculated spin susceptibility (${\chi }_S$). Bottom row: Calculated charge susceptibility (${\chi }_C$). Both crystallographic directions (1, 0) and (1, 1) are shown.