HIGH IRREVERSIBILITY FIELDS AND CURRENT DENSITIES IN NDBA2CU3O7-DELTASINGLE-CRYSTALS AND MELT-TEXTURED SAMPLES

NdBa2Cu3O7-delta single crystals and melt-textured samples were prepar ed in reduced atmospheres starting from the Nd-poor side of the phase diagram in order to minimize the Nd/Ba substitution and the second pha se content. Current density j and irreversibility field B-irr were mea sured as a function of temperature and angle between the c axis and ap plied field direction in samples with different impurity and oxygen co ntents. Current density vs field shows a pronounced peak effect which results from small clusters of oxygen vacancies, whereas a second peak is related to the twin structure. The current density at 77 K is for fields up to 4 T above 4X10(4) A/cm(2) and reaches values of 1.2x10(5) A/cm(2) in the peak at 2.1 T. The irreversibility fields parallel to the c axis reach 13.4 and 11.8 T at 77 K in purl and homogeneous singl e crystals and melt-textured samples, respectively. These extremely hi gh values, which are caused by the twin structure, decrease to 10 and 8 T, respectively, if the current is optimized. The correlation and in terference between j and B-irr in the presence of weakly and strongly interacting defects such as small clusters of oxygen vacancies and Nd- 422 precipitates, respectively, as well as the influence of the twin s tructure are analyzed and discussed.