Pinning characteristics of (Nb,Ta)3Sn superconductors produced by Nb/Ta-Sn composite process

Pinning properties in (Nb,Ta)₃Sn superconductors produced by a Nb/Ta-Sn composite process are studied through the measurement of the temperature and magnetic field dependences of J_c and its anisotropy with respect to the field direction, referring to the grain morphology and the compositional analysis of the (Nb,Ta) ₃Sn phase. The experimental result of the anisotropy in J_c and the observation of the grain morphology suggest that the contribution of the grain boundary to the pinning force density is not large. Two regions having different grain morphologies can be seen in the (Nb,Ta)₃Sn layer, where the boundary of the two regions roughly corresponds to the boundary between the Nb-Ta sheath and the Ta-Sn core in the initial Nb-Ta/Ta-Sn mono-core composite before reaction. Such a boundary between the two regions inside the (Nb,Ta)₃Sn phase and/or a S-N interface of the boundary between the Nb-sheath and the (Nb,Ta)₃Sn superconducting phase would influence the pinning property. Both are possible reasons for the anisotropy in J_c. On the other hand, the so-called “peak effect” is recognized in the flux pinning, suggesting that another pinning mechanism acts in high field, differing from that in low field