Magnetic Impurities in the Pnictide Superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

TL;DR

This study investigates magnetic impurities in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ superconductors using NMR, revealing impurity effects, magnetic inhomogeneity, and their impact on superconducting transition temperature.

Contribution

It provides new insights into how magnetic impurities and inhomogeneities affect superconductivity in pnictide materials.

Findings

Magnetic impurities cause broad spectral features and magnetic field gradients.

Impurities lead to a 20% reduction in the superconducting transition temperature.

Superconductivity remains relatively robust despite severe magnetic inhomogeneity.

Abstract

NMR measurements have been performed on single crystals of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ (x = 0, 0.45) and CaFe$_2$As$_2$ grown from Sn flux. The Ba-based pnictide crystals contain significant amounts of Sn in their structure, $\sim 1$%, 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.