Magnetic impurities in the pnictide superconductor Ba_1−<i>x</i>K_<i>x</i>Fe₂As₂

Abstract

NMR measurements have been performed on single crystals of\nBa$_{1-x}$K$_{x}$Fe$_2$As$_2$ (x = 0, 0.45) and CaFe$_2$As$_2$ grown from Sn\nflux. The Ba-based pnictide crystals contain significant amounts of Sn in their\nstructure, $\\sim 1$%, giving rise to magnetic impurity effects evident in the\nNMR spectrum and in the magnetization. Our experiments show that the large\nimpurity magnetization is broadly distributed on a microscopic scale,\ngenerating substantial magnetic field gradients. There is a concomitant 20%\nreduction in the transition temperature which is most likely due to magnetic\nelectron scattering. We suggest that the relative robustness of\nsuperconductivity ($x=0.45$) in the presence of severe magnetic inhomogeneity\nmight be accounted for by strong spatial correlations between impurities on the\ncoherence length scale.\n