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The extended defects produced by the fission of neutron-irradiated U-235 inclusions in Ag/Bi2223 superconducting tapes act as very effective pinning centres. Substantial improvements in the magnetic field dependence of the critical current densities, J(c), and in the irreversibility line of treated tapes have previously been observed. The magnetoresistance of Ag/Bi2223 tapes treated with the doping and irradiation technique has been measured, and activation energies calculated from the Arrhenius plots. The findings corroborate the previous J(c), results, confirming the strong pinning action of the extended columnar defects, particularly in fields aligned along the c-axis of the tapes.

Systematic study of the flux pinning at 77 K in $^{235}$U-oxide doped Ag-sheathed Bi2223 tapes irradiated by thermal neutrons has been performed. Within the explored range of $^{235}$U contents ($c\leq 0.75$ at\%) and neutron fluences ($\phi \leq 6\cdot 10^{19}$ m$^{-2}$) pinning capacity increases linearly with fission track density (proportional to $c\phi $). In particular, field $B^{\ast }$ for the maximum volume pinning force ($% F_{p\max }=J_{c}B^{\ast }$) and the irreversiblity field $B_{irr}$ increase linearly with $c\phi $ both for fields parallel and perpendicular to the tape plane. Since $J_{c}$ decreases with $c$ in tapes doped with $^{235}$% U-oxide, $B_{irr}$(77 K, perpendicular field) seems limited to about 4 T. However, more subtle addition of $^{235}$U atoms can produce tapes with flux pinning exceeding that in YBCO type of materials.

We examined the distribution of vortices in both Bi-2223 Ag-sheathed tape and U-doped Bi-2223 Ag-sheathed tape applying the high-resolution Bitter method. In the low temperature region (T/spl Lt/T/sub c/), we found that the vortices in the undoped Bi-2223 Ag-sheathed tape developed into either a triangular arrangement or a heterogeneous arrangement along the grain boundaries of the Ag sheath, which form weak pinning centers. The vortex distribution for U-doped Bi-2223 samples was slightly different from the distribution for the undoped material. We observed some movement of the vortices in both the U-doped and the undoped Bi-2223 Ag-sheathed tapes at 50 K. At higher temperatures, T>75 K, the vortices in the Bi-2223 tapes moved even faster so that no deposition pattern, indicating the location for vortices, formed.

Critical currents J c ( T , B ) for virgin and four 235 U doped Bi2223/Ag tapes irradiated with thermal neutrons (U/n treated) were measured in the temperature range T ⩾50 K and magnetic field B ⩽1 T. Below 97 K vortex pinning in U/n treated tapes increases with matching field B φ , as evidenced by a weaker B dependence of J c on increasing B φ . Hence, the field B max ( F p max =J c B max ) and the irreversibility field B irr (F p → 0) of U/n treated tapes are enhanced compared to those for virgin tape. Since the rate of the enhancement of B max and B irr with B φ depends on T , one can tune the maximum enhancement at desired T .

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 magnetization relaxation measurements. A dimensional crossover from a three-dimensional (3D) elastic creep regime to a 2D plastic creep was observed in the nonirradiated tape at an applied magnetic field ${\ensuremath{\mu}}_{0}{H}_{\mathrm{cr}}\ensuremath{\approx}0.37\mathrm{T},$ with an associated change in the flux hop velocity and temperature dependence. After the introduction of the fission-fragment damage by irradiation, a shift in the crossover to ${\ensuremath{\mu}}_{0}{H}_{\mathrm{cr}}\ensuremath{\approx}0.65\mathrm{T}$ 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 magnetization relaxation analysis.

The magnetoresistance R(T, B) of virgin and four U-235 doped Bi2223/Ag tapes irradiated with thermal neutrons (U/n treated) has been measured in the temperature range T greater than or equal to 50 K and magnetic field B less than or equal to 1 T. For B greater than or equal to 20 mT the pinning potentials U-0 of U/n treated tapes were enhanced compared to that of virgin tape. The relative enhancement U-0(B, U/n)/U-0(8, virg.) increased with matching field B-phi and persisted to higher B at higher B-phi. For U/n treated samples, U-0 showed universal variation with B/B-phi : U-0(B less than or similar to B-phi) proportional to (B-phi/B)(proportional to) with alpha approximate to 0.5 and faster variation at higher B. The resistive irreversibility fields B-irr(T) of all U/n treated samples for T less than or similar to 100 K were also enhanced: B-irr (T, U/n)/B-irr (T, virg.) increased with B-phi and the enhancement persisted to lower T for higher B-phi. The maximum enhancements of B-irr scale well with the corresponding enhancements of U-0(B).