Nuclear magnetic resonance studies of coexisting antiferromagnetism and superconductivity in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

We present $^{75}$As nuclear magnetic resonance (NMR) spectra and spin lattice relaxation data from Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for $x=$ 0.05757, 0.05898, and 0.06163, with $T_C=$ 21.5 K, 22 K, and 22.7 K respectively. The spectra become broadened below the antiferromagnetic (AFM) phase transition. Spin lattice relaxation was measured by inversion recovery at the central line with $H \perp \hat{c}$ down to 4.5 K into the coexistence region. As temperature is decreased toward the AFM phase transition, the exponential inversion recovery curve begins to deviate from the theoretical prediction. The curves were fit to a stretched exponential to characterize this deviation as a function of temperature. This behavior persists into the coexistence region and may be related to nematic fluctuations.