Flux pinning in the weak layers of superconducting heterogeneous structures

The critical current density in the weak layer (J(cn)) of superconducting composites has been found to be strongly dependent upon the pinning either at the strong/weak layer interface, or within the strongly superconducting layer itself. The particular composites investigated in this work were NbTi(Nb)-Cu multifilamentary (MF) composites, although analogous results would be expected for any heterogeneous superconducting material if the pin spacing is small as compared to the layer thickness. The pinning strength for the weak layer has been calculated under various conditions and as a function of position around the M-B loop. Pinning was investigated for various flux configurations. In particular, shear and bowing pinning mechanisms were described, and the associated J(cn) were calculated. These values were differentiated from the 'depairing' current density (J(Dn)). S-N-S boundary conditions (BCs) were reformulated in terms of an anisotropic N-material. Pinning calculations were compared with experimental results and reasonable agreement was seen. In addition, these theories lead naturally to an M-B loop asymmetry frequently seen in many superconducting composites. Finally, some comments have been made on the application of these models to superconducting composites generally, and high T-c superconductors specifically. (C) 1999 Elsevier Science B.V. All rights reserved.