Superconducting critical current of Nb-Ti wire with anisotropic defect structure. II.

Abstract

Anisotropy of the global pinning force Fp was inductively measured for Nb-49 wt. %Ti monofilamentary wire with highly anisotropic pinning structures in magnetic field parallel and transverse to the wire axis. According to the recent models of Evetts and Plummer [Proceedings of the International Symposium on Flux Pinning and Electromagnetic Properties in Superconductors (Matsukuma, Fukuoka, 1985)] and of Dew-Hughes [Philos. Mag. B 55, 459 (1987)], the morphology of pins is assumed to determine the flux pinning characteristic by the occurrence of plastic shear of the fluxoid lattice. The saturation characteristic of Fp as in Nb3 Sn is expected to be obtained if the field is parallel to the wire axis, while the nonsaturation one, as in commercial Nb-Ti, is expected in the transverse geometry. However, the results obtained show similar nonsaturation characteristics in both geometries and are contradictory to expectation. These results suggest that the pinning characteristic is mainly determined by the pinning parameters as the elementary pinning force and the pin concentration, but is not affected by the simple morphology of pinning centers.