Flux pinning characteristics in ultrafine multifilamentary NbTi superconductors with different artificial pin materials

In order to improve and design critical current densities in commercial superconductors, the establishment of an artificial pinning center composite technique based on the flux pinning mechanism is desired. For that purpose, we studied the influence of different kinds of artificial pin materials (Nb, Nb-7.5wt.%Ta, Ta) on the flux pinning in multifilamentary NbTi superconductors. It was found that Nb pins act as the strongest pinners among them as predicted by difference of free energy between different kinds of superconductors estimated from the Ginzburg-Landau theory. As a result, the pinning scaling law holds true in a wide range of temperatures and magnetic fields. However, the contribution of artificial pins gradually decreased with reducing pin size. This is thought to be mainly caused by the degradation of upper critical field due to the proximity effect as well as the reduction in pin size.