Flux pinning strength of different superconducting artificial pins in Nb-Ti multifilamentary composites

Abstract

To improve the critical current density (J/sub c/) of multifilamentary Nb-Ti superconductors, the pinning characteristics of wires having different superconducting artificial pinning materials and pin sizes were studied. Nb, Nb-7.5wt.%Ta and Ta pins were introduced into each Nb-Ti filament as an artificial pinning center (APC). Magnetization was measured to obtain the magnetic field dependence of the pinning force density (F/sub P/) at various temperatures. A significant enhancement of F/sub P/ has been observed for APC wires in comparison to conventional wires. F/sub P/ increases in ascending order of Ta, Nb-Ta and Nb pins. This result reflects the elementary pinning force (f/sub P/) calculated by the Ginzburg-Landau (G-L) theory. The slope of the F/sub P/ curve for Ta pins changes at 4.4 K, where a super-normal transition occurs. We also investigated the F/sub P/ properties in a magnetic field parallel to the wire axis. In this case, the peak of F/sub P/ shifts to a higher magnetic field compared with that of a perpendicular magnetic field. This result suggests that the number of effective pinning sites in a parallel magnetic field is larger than in a perpendicular magnetic field due to the anisotropic microstructure of the pins. Furthermore, we report on their flux pinning scaling behaviors.