Thermal post processing of FeSe1−xTex: formation of surface iron oxides and enhancement of Jc

Abstract

We present the results from structural analysis of single crystalline FeSe1−xTex and magnetisation measurements of as-grown FeSe0.35Te0.65 samples and after they have been annealed in air. Raman spectroscopy revealed that air annealing leads to the formation of hematite (α-Fe2O3) and magnetite (Fe3O4) on the surface of FeSe1−xTex, with the bulk beneath being free of iron oxide. Furthermore, we observe an increase in the superconducting transition temperature, Tc, from 7 to 14 K, and in the critical current density, Jc, by one order of magnitude for all applied magnetic fields. A comparison of two FeSe0.35Te0.65 samples of different thickness indicates a possible size dependence for the optimal annealing conditions. Annealing for a second time (prolonged annealing) was found to induce a surface barrier, indicated by the transition from a symmetric to a highly asymmetric magnetic hysteresis loop.