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

Anisotropy of upper critical fields B/sub C2/ and irreversibility fields B/sub 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/sub C2/spl par// exceeded the B/sub C2/spl perp// below a certain temperature T/sup */ and the temperature dependence of B/sub C2/spl par// was close to (1-T/T/sub c/)/sup 0.5/. These results suggest that dimensional crossover occurs at T/sup */, 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/sub i/spl par// 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.

I. Introduction

We HAVE been applying the artificial pinning center (APC) method to create new Nb–Ti multifilamentary composites with higher critical current performance [1], [2]. Nb APC composites have achieved excellent critical current densities at magnetic fields up to 5 T. However, in high fields remains at lower level than that of conventional Nb–Ti composites with ribbon-shaped -Ti pins. In our previous work [3], to improve properties in high fields, the relationship between irreversibility fields and the filament size for Nb–Ti composites with several kinds of APC was investigated, and it revealed that it is very effective to keep filament diameter within the bulk limit as well as to introduce strong pinning centers for increasing , as predicted by the flux creep theory [4]. However, for further improvement of the flux pinning in high fields, other means are still desired.