Comparative study of sequential neutron irradiation and annealing effects in superconducting YBa₂Cu₃O_7-δ, Y₂Ba₄Cu₈O₁₆ and NdBa₂Cu₃O_7-δ single crystals

NdBa₂Cu₃O_7-δ (Nd-123) and Y₂Ba₄Cu₈O₁₆ (Y-124) single crystals were sequentially irradiated with fast neutrons and annealed in air at 250 °C. After each treatment the transition temperature (T_c) and the field dependence of the magnetic moment with the field applied parallel to the crystallographic c-axis were measured at various temperatures by SQUID magnetometry. The results are compared with those on a previously investigated YBa₂Cu₃O_7-δ (Y-123) single crystal. The behaviour of the transition temperature is well established for Y-123, i.e. T_c decreases due to irradiation and increases due to annealing. We find a similar effect for Y-124, but a completely different behaviour of Nd-123. The critical current density (J_c) increases for all samples after an irradiation step and becomes smaller after annealing. We show that the influence of the defects created by irradiation on T_c and J_c strongly depends on the as-grown defect structure. J_c after irradiation is rather a function of the neutron fluence since the last annealing step than a function of the overall dose.