Abstract. Nuclear magnetic resonance (NMR) measurements have been performed on single crystals of Ba_1−xK_xFe₂As₂ (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, such as clustering on the coherence length scale.

Received 19 February 2009
Published 5 May 2009