Irreversibility fields and upper critical fields in multifilamentary Nb-Ti composites having artificial pins with anisotropic microstructure

Anisotropy of upper critical fields B_C2 and irreversibility fields B_i was investigated for multifilamentary Nb-Ti composites having artificial pinning centers in perpendicular and parallel magnetic fields to the filament axis. It was observed that the B_C2|| exceeded the B_C2⊥ below a certain temperature T^* and the temperature dependence of B_C2|| was close to (1-T/T_c)^0.5. These results suggest that dimensional crossover occurs at T^*, i.e., the magnetic flux system in the parallel field changes from 3D to 2D state as predicted by the Takahashi-Tachiki theory for multilayer superconductors. B_i|| is also enhanced due to no restriction of the superconductor size, as explained by the flux creep theory. Consequently, the flux pinning strength in the parallel magnetic field to the filament axis increased at high fields. Furthermore, tape specimens with arranged plate-like pins fabricated by rolling process indicated expected enhancement of flux pinning.