NMR study of nematic spin fluctuations in a detwinned single crystal of underdoped $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$

We report the experimental details of how mechanical detwinning can be implemented in tandem with high-sensitivity nuclear magnetic resonance measurements and use this setup to measure the in-plane anisotropy of the spin-lattice relaxation rate in underdoped $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ with $x=0.048$. The anisotropy reaches a maximum of 30% at $T_N$, and the recovery data reveal that the glassy behavior of the spin fluctuations present in the twinned state persist in the fully detwinned crystal. A theoretical model is presented to describe the spin-lattice relaxation rate in terms of anisotropic nematic spin fluctuations.

Figure 1

(a) Strain device with single crystal of $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.952}{\mathrm{Co}}_{0.048}\right)}_2{\mathrm{As}}_2$ under strain. (b) $ab$ plane, showing the Fe atoms and spin orientation in the ordered magnetic phase, with the orthorhombic $a_{\mathrm{O}}$ and $b_{\mathrm{O}}$ axes shown as dotted lines, and the tetragonal axes $\left(a_T\right)$ shown as dashed lines. (c) Close-up image of the crystal oriented such that the applied field, ${\mathbf{H}}_0$, is along the $b_{\mathrm{O}}$ (perpendicular to the direction of applied strain) and (d) along $a_{\mathrm{O}}$ (parallel to the direction of applied strain). For the latter case, the coil was rotated by approximately ${30}^{\circ }$ so that a component of the radio-frequency field ${\mathbf{H}}_1$ lies perpendicular to ${\mathbf{H}}_0$.

Figure 2

(a) Resistance and (b) derivative of resistance versus temperature for the $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.952}{\mathrm{Co}}_{0.048}\right)}_2{\mathrm{As}}_2$ crystal measured with and without strain in zero field. Resistivity reaches a minimum at $T_S$, and the $dR/dT$ curve exhibits a minimum at $T_N$ [1].

Figure 3

(a) ${\left(T_{1}T\right)}^{-1}$ and (b) the stretching exponent $\beta$ vs temperature for $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.952}{\mathrm{Co}}_{0.048}\right)}_2{\mathrm{As}}_2$ for field oriented along either the $a_{\mathrm{O}}$ or $b_{\mathrm{O}}$ directions. The data in panel (a) have been offset vertically by 1 and 2 ${\mathrm{s}}^{-1}{\mathrm{K}}^{-1}$ for clarity and in panel (b) by 0.5 and 1.0.

Figure 4

(a) ${\left(T_{1}T\right)}^{-1}\left(\epsilon \right)-{\left(T_{1}T\right)}^{-1}\left(0\right)$ for field along both the $a$ and $b$ directions, and (b) ${\left(T_{1}T\right)}_{\mathrm{anis}}^{-1}$ and ${\left(T_{1}T\right)}_{\mathrm{iso}}^{-1}$ vs temperature for $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.952}{\mathrm{Co}}_{0.048}\right)}_2{\mathrm{As}}_2$.