Flux patterns of monofilamentary Bi₂Sr₂Ca₂Cu₃O_10+δ tapes at various temperatures

By means of magneto-optic imaging, flux patterns of monofilamentary, silver-sheathed Bi₂Sr₂Ca₂Cu₃O_10+δ (Bi-2223) tapes are obtained at various temperatures between 12 K and 77 K. Above 50 K, the flux distributions are found to be nearly homogeneous, implying a uniform current flow. On decreasing the temperature, the observed flux patterns develop indications of granularity. This effect is most pronounced at the lowest temperature investigated. From these observations we can deduce the temperature dependence of the transport current density, j_trans, and of the current density of the grains, j_grain. We show that the appearance of granularity in the flux patterns at low temperatures can be explained by assuming a very steep temperature dependence of the intragranular current density, being large at low T and decreasing rapidly with increasing T until at about 50 K, j_grain ≈ j_trans. The absence of granularity above 50 K shows that the current flow is here dominated by the transport currents.