Temperature dependence of filament-coupling in Bi-2223 tapes: magneto-optical study

Coupling through random superconducting bridges between filaments in a multifilamentary Ag-sheathed Bi2Sr2Ca2Cu3O10+ tape has been investigated by magneto-optical imaging at temperatures from 20 K up to Tc. Magnetic flux distributions have been measured on the surface of an intact tape in the remanent state on applying a strong perpendicular magnetic field. The flux distributions observed at low temperatures reflect the arrangement of individual filaments. At high temperatures, the distribution becomes more similar to that for a uniform monocore tape, indicating that superconducting connections appear between the filaments. To discuss the relative contributions of the intra- and inter-filament currents, a simple model based on the Bean critical state was proposed and applied to analyse the temperature dependent behaviour. The inter-filament coupling, increasing with temperature, reaches at 77 K a point where the currents flowing in large inter-filament loops are roughly equal to the intra-filament currents.