Effects of Fission-Fragment Damage on Vortex Dimensionality in Silver-sheathed Bi2Sr2Ca2Cu3Ox Tapes

Abstract

We report on the vortex dimensionality of uranium-doped Ag/Bi2223 tapes, before and after irradiation to a thermal-neutron fluence. The effective activation energies as a function of current density and applied field were calculated from dynamic magnetisation relaxation measurements. A dimensional crossover from a 3D elastic creep regime to 2D plastic creep was observed in the nonirradiated tape at an applied magnetic field Hcr = 0.37 T, with an associated change in the flux hop velocity and temperature dependence of the activation energy. After introduction of the fission-fragment damage by irradiation, a shift in the crossover to Hcr = 0.65 T was observed. These results indicate an enhancement of the c-axis vortex coherence by the introduced splayed columnar defects, explaining the greater pinning efficiency of the uranium-fission method in Bi2223 rather than the less anisotropic Y123. Conflicting results obtained for the irradiated tape in the absence of any temperature scaling of the activation energies demonstrate the importance of the inclusion of scaling in the magnetisation relaxation analysis.