Magnetic Impurities in the Pnictide Superconductor Ba₁₋ₓKₓFe₂As₂

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NMR measurements have been performed on single crystals of Ba₁₋ₓKₓFe₂As₂ (x = 0, 0.45) and CaFe₂As₂ grown from Sn flux. The Ba-based pnictide crystals contain significant amounts of Sn in their structure, 1%, giving rise to magnetic impurity effects evident in the NMR spectrum and in the magnetization. Our experiments show that the large impurity magnetization is broadly distributed on a microscopic scale, generating substantial magnetic field gradients. There is a concomitant 20% reduction in the transition temperature which is most likely due to magnetic electron scattering. We suggest that the relative robustness of superconductivity (x=0.45) in the presence of severe magnetic inhomogeneity might be accounted for by strong spatial correlations between impurities on the coherence length scale.