Microscopic 75As NMR study of the effect of impurities on the first-order spin-density-wave transition in BaFe2As2

We report an As-75 NMR study of BaFe2As2 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 to 85 K. In contrast to Sn-free samples, we find that the temperature dependence of the As-75 NMR spectra and spin-lattice relaxation rates reveal a second-order phase transition to a state of incommensurate magnetic order.