Microscopic study of the effect of impurities on the first order spin   density wave transition in BaFe$_2$As$_2$
  We report an $^{75}$As NMR study of BaFe$_2$As$_2$ in both single crystals and polycrystal forms. We find that Sn impurities in the single crystal dramatically alter the low energy spin fluctuations and suppress the ordering temperature from 138 K to 85 K. In contrast to Sn-free samples, we find that the temperature dependence of the $^{75}$As NMR spectra and spin lattice relaxation rates reveal a second order phase transition to a state of incommensurate magnetic order. 
S.-H. Baek, T. Klimczuk, F. Ronning, E. D. Bauer, J. D. Thompson, and   N. J. Curro