Relation between critical current and exponent n in Bi(2223)/Ag tapes

Transport critical currents Ic at 1 μV cm-1 and exponents n of the power law characteristics (E ∼ jn) of single-filament Bi(2223)/Ag tapes were measured at 77 K in the self-field and in external magnetic fields up to 1.2 T. The effects of tape orientation in a transverse magnetic field from -120 deg. to +120 deg. (related to the basic state at B parallel c-axis) were investigated as well. The values of the transport critical current Ic and exponent n as well their mutual relation reflect the quality of the Josephson-linked current network connectivity at B < B* (B* is the cross-over field at which the weak-link currents cease to contribute to the transport current) and the ability of pinning centres to pin flux lines in the whole magnetic field region. The weak-link connectivity and the pinning potential are directly related to the structural state of the samples resulting from their different thermomechanical treatments. The better the structural quality, the higher the Ic/n ratio. This ratio seems to reflect the effective number of intergrain connections and pinning centres. (author)