NMR study of nematic spin fluctuations in a detwinned single crystal of underdopedBa(Fe1−xCox)2As2

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 $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_x)}₂{\mathrm{As}}₂$ 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.