Effect of interstitial iron on Jc and Hc2 anisotropy and magnetic relaxation of Fe1+yTe0.6Se0.4 single crystal

Abstract

For a long time, the second peak effect in FeTe0.6Se0.4 has attracted the interest of researchers. The role of interstitial iron which is unique to FeTe0.6Se0.4 compared to FeSe superconductors, in the second peak effect remains unknown. In this study, we controlled the content of interstitial iron through flowing oxygen and prolonged vacuum annealing, and subsequently conducted investigations on the magnetic flux dynamics and superconducting properties with anisotropies. After reducing the interstitial iron content, Tc increased from 12.6 K to 13.8 K. The critical current density at 4.5 K increased from 1.2×105 A/cm2 to 1.76×105 A/cm2. At low fields (H<Honset), interstitial iron as weak pinning centers can rapidly trap magnetic flux and exhibit fast relaxation through elastic motion. At high fields, strong pinning centers lead to collective pinning of magnetic flux and demonstrate a reduction in relaxation rate through plastic motion. The presence of interstitial iron hinders the transition from elastic to plastic motion for magnetic flux. Furthermore, reducing the interstitial iron expands the field range of the second peak effect and leads to a noticeable change in the pinning behavior, with a transition from δl-type to δTc-type pinning and enhances the anisotropy of the system. Finally, our study provides a reference for understanding the origin of the second peak in FeTe0.6Se0.4 and offers guidance for the practical application of FeTe0.6Se0.4.