Flux pinning in the weak layers of superconducting heterogeneous structures

The critical current density in the weak layer (J(cn)) of superconducting c omposites has been found to be strongly dependent upon the pinning either a t the strong/weak layer interface, or within the strongly superconducting l ayer itself. The particular composites investigated in this work were NbTi( Nb)-Cu multifilamentary (MF) composites, although analogous results would b e expected for any heterogeneous superconducting material if the pin spacin g 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 des cribed, and the associated J(cn) were calculated. These values were differe ntiated from the 'depairing' current density (J(Dn)). S-N-S boundary condit ions (BCs) were reformulated in terms of an anisotropic N-material. Pinning calculations were compared with experimental results and reasonable agreem ent was seen. In addition, these theories lead naturally to an M-B loop asy mmetry frequently seen in many superconducting composites. Finally, some co mments 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.