Atomic coexistence of superconductivity and incommensurate magnetic order in the pnictide $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$

${}^{75}\text{A}\text{s}$ nuclear magnetic resonance (NMR) and susceptiblity were measured in a $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ single crystal for $x=6\%$. NMR spectra and relaxation rates allow to show that all Fe sites experience an incommensurate magnetic ordering below $T=31\text{K}$. Comparison with undoped compound allows to estimate a typical moment of $0.05{\mu }_B$. Anisotropy of the NMR widths can be interpreted using a model of incommensurability with a wave vector $\left(1/2-\epsilon ,0,l\right)$ with $\epsilon$ of the order of 0.04. Below $T_C=21.8\text{K}$, a full volume superconductivity develops as shown by susceptibility and relaxation rate, and magnetic order remains unaffected, demonstrating coexistence of both states on each Fe site.

Figure 1

(Color online) Left panel: data for $T_{\text{SDW}}$ (circles) and $T_C$ (triangles) for $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ taken from Ref. 8, where the stars represent the $x=6\%$ composition used here. The moment amplitude (diamonds) corresponds to the right axis, using values of Ref. 15 and this study. Right panel: macroscopic susceptibility at $H=1\text{G}$, after zero-field cooled.

Figure 2

(Color online) Panels a and b: typical NMR spectra for the two orientations of the applied field. A strong broadening is observed below $T_{\text{SDW}}$ (break on the intensity axis was introduced to artificially enlarge quadrupolar satellites at high temperature). Panel c: zoom on the $H\parallel ab$ central line for various temperatures (spectra were measured by sweeping the frequency at $H=7.5\text{T}$, and converted in field units).

Figure 3

(Color online) Upper panel: NMR field distribution for the two field orientations shows apparition of the SDW magnetic ordering below $T=31\text{K}$. Lower panel: $1/T_{1}T$ measured for $H\parallel ab$ (left axis) shows two peaks at both SC and SDW transitions, while detuning of the NMR cavity (right axis) demonstrates the apparition of SC.

Figure 4

(Color online) Left panel: the commensurate AF order in ${\text{BaFe}}_2{\text{As}}_2$ induces two values for the local field $h_{loc}$ on ${}^{75}\text{A}\text{s}$ nucleus along the $c$ axis only. Right panel: any incommensurate order such as the example displayed distributes $h_{loc}$ value along $c$ and along $\left(a,b\right)$ (moments are arbitrary scaled and only As atoms above the Fe layer have been shown).