Microscopic Coexistence of Superconductivity and Antiferromagnetism in Underdoped $\mathrm{Ba}({\mathrm{Fe}}_{1\mathbf{-}x}{\mathrm{Ru}}_x{)}_2{\mathrm{As}}_2$

We use ${}^{75}\mathrm{As}$ nuclear magnetic resonance to investigate the local electronic properties of $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Ru}}_x{)}_2{\mathrm{As}}_2$ ($x=0.23$). We find two phase transitions: to antiferromagnetism at $T_N\approx 60\mathrm{K}$ and to superconductivity at $T_C\approx 15\mathrm{K}$. Below $T_N$, our data show that the system is fully magnetic, with a commensurate antiferromagnetic structure and a moment of $0.4{\mu }_B/\mathrm{Fe}$. The spin-lattice relaxation rate $1/{}^{75}T_1$ is large in the magnetic state, indicating a high density of itinerant electrons induced by Ru doping. On cooling below $T_C$, $1/{}^{75}T_1$ on the magnetic sites falls sharply, providing unambiguous evidence for the microscopic coexistence of antiferromagnetism and superconductivity.

Figure 1

(a) In-plane resistivity of the $\mathrm{Ba}({\mathrm{Fe}}_{0.77}{\mathrm{Ru}}_{0.23}{)}_2{\mathrm{As}}_2$ crystal on cooling at zero field. Inset: Schematic phase diagram of $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Ru}}_x{)}_2{\mathrm{As}}_2$, where the dotted line represents the doping of our crystals. (b) dc susceptibility during field cooling (FC, with a 10 Oe field applied in the crystalline $ab$ plane) and zero-field cooling (ZFC). Inset: Resonance frequency of the NMR circuit as a function of temperature, with a 10 T field applied in two orientations.

Figure 2

${}^{75}\mathrm{As}$ NMR spectra with a 10 T magnetic field applied (a) along the $c$ axis and (b) in the $ab$ plane of our single crystal. Inset of (a): Static hyperfine field at ${}^{75}\mathrm{As}$ sites as a function of temperature. Inset of (b): Representation of one magnetic configuration in the presence of Ru dopants and the resulting ${}^{75}\mathrm{As}$ hyperfine field.

Figure 3

(a) ${}^{75}K(T)$ as a function of temperature for $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Ru}}_x{)}_2{\mathrm{As}}_2$ ($x=0$, 0.23) with two field orientations. The solid lines are a fit to the function ${}^{75}K(T)=a+b{T}^2$. (b) $1/{}^{75}T_1$ as a function of temperature with two field orientations. For each doping, the solid line above $T_N$ is a fit to $1/{}^{75}T_1=AT/(T-\theta )+BT$.

Figure 4

(a) $1/{}^{75}T_{1}T(T)$ on semilog axes for $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Ru}}_x{)}_2{\mathrm{As}}_2$ ($x=0$, 0.23). Solid lines below $T_N$ indicate the trend towards a low-temperature constant. (b) ${}^{75}\mathrm{As}$ spectrum at $T=2$ and 80 K, shown with $1/{}^{75}T_{1}T$ at $T=20$ and 2 K. (c) ${}^{75}\mathrm{As}$ magnetization recovery curve at three frequencies. The solid line represents a fit using a single $T_1$ component. (d) $1/{}^{75}T_{1}T$ at low temperatures for the three frequencies $f_1$, $f_2$, and $f_3$ (see text).