Pinning strength and irreversibility fields of Nb-Ti multifilamentary composites with different artificial pins

Irreversibility fields B_irr and upper critical fields B_C2 for Nb-Ti multifilamentary composites having artificial pinning centers (APC) were investigated to clarify the relationship between their degradation and high-field flux pinning. APC wire specimens having Nb as strong pins and Ta as weak pins, with the different filament diameter from 1.07 to 0.27 μm, were prepared for measurements. Comparatively large difference between B_irr and B_C2 was observed for all of the APC specimens, even for the specimens with the high flux pinning strength. The temperature dependence of B_irr was explained well by the flux creep theory in which the size of superconductors was taken into account. Furthermore, magnetic relaxation for the specimens was measured to estimate the apparent pinning potential U₀*. The difference of U₀* among the specimens was also explained qualitatively by the flux creep theory. These results suggest that the large filament diameter in the bulk limit is quite important to improve the high-field flux pinning when using APC method more effectively.