Single crystals of Ba(Fe1−xNix)2As2 have been produced for systematic magnetic measurements of the critical current density (Jc) over a range of dopings from x = 0.025 to 0.066 and a range of temperatures from 2 K. Analysis of the field dependent critical current density, Jc, shows strong evidence pointing to a flux pinning mechanism dominated by local variation in the mean free path for all dopings. The values of Jc measured indicate a peak at approximately x = 0.049 on the pseudobinary phase diagram, close to a proposed quantum critical point at x = 0.05. Pressure dependent measurements of this sample show anomalous behaviour including a negative pressure relationship and a peak in Jc around 0.65 GPa.

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.

We report the results from a comprehensive study of the effect that annealing FeSe 0.35 Te 0.65 single crystals in different atmospheres has on the surface chemistry, the superconducting transition temperature, T c , and the critical current density, J c . A review of the literature and our data shows that annealing in oxygen, nitrogen, air or vacuum atmosphere leads to a universal superconducting phase diagram that is independent of the variance in the as-grown properties. We show that atmospheres with an oxygen partial pressure of p > 10 −3 hPa during annealing are necessary to improve the superconducting properties in this compound. This is demonstrated by Raman and magnetisation measurements on air, nitrogen, or low vacuum annealed FeSe 0.35 Te 0.65 samples, which show that the improvement in the superconducting properties is strongly correlated to the formation of a thin iron oxide surface layer. However, annealing under high vacuum does not lead to the formation of an iron oxide layer and no improvement in the superconducting properties is observed. Our findings show that the previously reported improvements in the superconducting properties after annealing under vacuum or in nitrogen are thus likely to have been caused by residual oxygen. Furthermore, we propose a diffusion model for the formation of the iron oxide layer as the driving force for the reduction of interstitial excess iron that suppresses superconductivity in this compound. Overall, our results show that the presence of an iron reactant, such as oxygen, during annealing is necessary to improve the superconducting properties of FeSe 1−x Te x .