Influence of oxygen deficiency and of neutron-induced defects on flux pinning in melt textured bulk YBa₂Cu₃O_7-x samples

Critical current densities (J_c), irreversibility lines (IL) and the time dependence of magnetic moments in melt-textured YBa₂Cu₃O_7-x with different oxygen contents (92.4 K < T_c < 27.4 K) were investigated. The samples show an increasing tendency towards two-dimensional pinning with decreasing oxygen content. The values of Jc and the effective activation energy decrease systematically and all the ILs shift to lower magnetic fields and temperatures with decreasing oxygen content. The ILs show a crossover from a power law dependence H_irr = β(1 - T_irr/T_c)^α with α ∼ 3/2 to a more rapid temperature dependence at higher fields. The field H_cr, at which the crossover occurs, is lower for samples with higher oxygen deficiency. After neutron irradiation J_c increases in all cases except for the material with T_c = 27.4 K, and the temperature dependence of Jc becomes flatter than in the unirradiated state. After irradiation the ILs change differently depending on oxygen content, i.e. they shift markedly to higher fields and temperatures at intermediate oxygen content, change only slightly or shift to lower fields and temperatures at high and low oxygen contents. For all the samples the crossover field H_cr increases after irradiation.