Spin reorientation in ${\mathrm{Ba}}_{0.65}{\mathrm{Na}}_{0.35}{\mathrm{Fe}}_2{\mathrm{As}}_2$ studied by single-crystal neutron diffraction

We have studied the magnetic ordering in ${\mathrm{Ba}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ with $0.25\le x\le 0.4$ by unpolarized and polarized neutron diffraction using single crystals. Unlike most FeAs-based compounds that magnetically order, Na-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ exhibits two successive magnetic transitions: For $x=0.35$, upon cooling, magnetic order occurs at $\sim 70$ K with in-plane magnetic moments being arranged as in pure or Ni-, Co-, or K-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ samples. At a temperature of $\sim 46$ K a second phase transition occurs, which the single-crystal neutron-diffraction experiments can unambiguously identify as a spin reorientation. At low temperatures, the ordered magnetic moments in ${\mathrm{Ba}}_{0.65}{\mathrm{Na}}_{0.35}{\mathrm{Fe}}_2{\mathrm{As}}_2$ point along the $c$ direction. The two nearly degenerate magnetic states document orbital degeneracy to persist in the superconducting phase.

Figure 1

Magnetic structure of ${\mathrm{Ba}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ in the AFM1 phase $46<T<70$ K and in the AFM2 phase at low temperature, $T<46$ K. Note that the fourfold symmetry axis is broken in both cases by the in-plane component of the magnetic propagation vector chosen as $(0.5,0.5,1)$.

Figure 2

(a) Temperature dependence of magnetic and nuclear Bragg reflection intensities in ${\mathrm{Ba}}_{0.65}{\mathrm{Na}}_{0.35}{\mathrm{Fe}}_2{\mathrm{As}}_2$ measured on the PANDA spectrometer. Note that the magnetic, $(0.5,0.5,1)$ and $(0.5,0.5,3)$, and nuclear, $\left(112\right)$ and $\left(002\right)$, Bragg peaks refer to different scales. $\left(112\right)$ and $\left(002\right)$ intensities were measured upon heating and cooling, respectively. (b) and (c) show an enlargement of the low-temperature range, illustrating the reduction of magnetic intensities in the SC state.

Figure 3

(a) shows the $l$ dependence of magnetic integrated intensities in the AFM1 and AFM2 phases. The lines are model calculations for the magnetic structures shown in Fig. 1 with moments parallel to the $a$ and $c$ directions. (b)–(e) Rocking scans across the magnetic Bragg peaks $(0.5,0.5,l)$ with $l=1$, 3, 5, 7, measured on the PANDA spectrometer at 5 and 50 K, in the AFM2 and AFM1 phases, respectively.

Figure 4

Results of polarized neutron-diffraction experiments on the 4F spectrometer. Scans were performed across the $(0.5,0.5,1)$ magnetic Bragg peak in both the SF and NSF channel for the neutron polarization along the $x,y$, and $z$ directions. There is no indication of any scattering associated with a magnetic moment pointing in the $[1,-1,0]$ direction.