Weak links and the poor transport critical currents of the 123 compounds

Abstract

The results of recent transport measurements, DC and AC magnetization measurements, and investigations of the grain boundary morphology and composition of 123 compounds which have been conducted by our group are described. The samples studied have low normal state resistivities (200–600 μΩcm at 100 K) and largely magnetic-field-independent transport critical current densities (J ct) in the range 2–10 T at temperatures of 4–77 K, indicating that they are of relatively high quality. Nevertheless, the J ct values measured on these samples are still very low (4–100 A/cm 2, 2–20 T, 4.2 K), and the large values of J cm/J ct (J cm is the magnetization critical current density) determined for these specimens indicate that weak links still limit J ct. Critical current measurements have been made under different relative orientations of applied field, crystal axes, and measuring current on sintered textured samples (c axes of all of the grains nearly parallel). Anisotropy of J cm similar to that observed in single crystals was found. By comparing the ratio J cm/J ct for different orientations of B relative to the c axis, it is concluded that local composition variations must act as efficient pinning centers at 4.2 K. A very low and strongly-field-dependent J cm for B ⊥ c at 77 K suggests, however, that these 4.2 K pinning centers become weak links at 77 K. Evidence is presented for the occurrence of weak links at regions internal to the grains. Detailed microstructural investigations using conventional and analytical transmission electron microscopy and scanning Auger microanalysis on fracture surfaces did not provide any definitive information about the nature of the weak links.