MAGNETIC-RELAXATION OF HIGHLY TEXTURED BI2SR2CACU2O8+DELTA POLYCRYSTALLINE FIBRES

The magnetic relaxation of Bi2Sr2CaCu2O8+delta superconducting highly textured polycrystalline materials (fibres) grown by laser induced flo ating zone melting methods have been studied using ac susceptibility a nd de magnetisation techniques. Effective pinning energies, U-eff, hav e been derived for a wide range of current densities, J, and magnetic fields both parallel and perpendicular to the fibre axis. In agreement with the microstructure U-eff,U-parallel to(J) > U-eff,U-perpendicula r to(J) and approximately follows for both orientations the phenomenol ogy observed on single crystals for c-axis parallel fields. Near the i rreversibility line U-eff is J independent, expressed as: U-eff(T,B) = U-eff(0 K,1 T)B--0.5(1 - T/T-c). Moreover, both U-eff,U-parallel to(J ) and U-eff,U-perpendicular to(J) values are higher than analogous U-e ff,U-c values on single crystals. Accordingly, for a given temperature the irreversibility line appears at larger fields, about ten times fo r fields perpendicular to the fibre axis and even higher in the parall el case. A careful analysis of the pinning energy distributions in fib res has been undertaken showing the presence of a high energy shoulder , absent in unirradiated single crystals.