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

摘要

Anisotropy of upper critical fields BC2 and irreversibility fields Bi 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 BC2|| exceeded the BC2⊥ below a certain temperature T* and the temperature dependence of BC2|| was close to (1-T/Tc)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. Bi|| 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.