Flux pinning in Nb-Ti composite superconductors having artificial normal pins with different coherence length

Recently, improvement of the flux pinning properties in several kinds of superconductors has been achieved by introducing artificial pinning centers (APC). The artificial pinning technique is very useful not only to increase critical current densities J_c but also to study the pinning mechanism and its optimization. We have been developing APC composites for the purpose of improvement in magnetic field dependence of J_c for Nb-Ti superconductors. In this study, we investigate the temperature and magnetic field dependence of the pinning force density F_p for Nb-Ti multifilamentary composites having superconducting pins (Nb) and normal conducting pins (Cu, Cu-Ni) with different coherence length ξ_n. The F_p obtained for all APC wires is larger than that for conventional wire without APC, and the F_p increases as the pin size decreases. In spite of the same APC arrangement in the design stage, F_p for Cu-Ni pin wire is larger than that of Cu pin and also the F_p´s values for the normal pins are smaller than that of superconducting Nb pin wire. Furthermore, T_c for the normal pin wires reduces drastically with reducing pin size, and the peak field of the F_p dose not shift to higher field, while that for Nb pin dose. We evaluate the elementary pinning force f_p for these APC wires using the G-L theory taken into account the proximity effect