A study of coupling loss on bi-columnar BSCCO/Ag tapes through ac susceptibility measurements

Coupling losses were studied in composite tapes containing superconducting material in the form of two separate stacks of densely packed filaments embedded in a metallic matrix of Ag or Ag alloy. This kind of sample geometry is quite favourable for studying the coupling currents and, in particular, the role of superconducting bridges between filaments. By using the ac susceptibility technique, the electromagnetic losses as a function of ac magnetic field amplitude and frequency were measured at the temperature T = 77 K for two tapes with different matrix composition. The length of the tapes was varied by subsequent cutting in order to investigate its influence on the dynamics of magnetic flux penetration. The geometrical factor χ0 which takes into account the demagnetizing effects was established from ac susceptibility data at low amplitudes. Losses versus frequency dependencies have been found to agree nicely with the theoretical model developed for round multifilamentary wires. Applying this model, the effective resistivity of the matrix was determined for each tape by using only measured quantities. For the tape with a pure silver matrix its value was found to be larger than that predicted by the theory for given metal resistivity and filamentary architecture. In contrast, in the tape with a Ag/Mg alloy matrix, an effective resistivity much lower than expected was determined. We explain these discrepancies by taking into account the quality of the interface between the superconducting filaments and the normal matrix. In the case of a soft matrix of pure Ag, this is of poor quality, while the properties of the alloy matrix seem to provoke the extensive creation of intergrowths which can be actually observed in these kind of tapes