Proximity effect current strengths in NbTi multifilamentary samples with and without Nb barriers and processed under various conditions

Abstract

Conventional magnetization measurements of proximity effect (PE) coupling in superconducting (S) multifilamentary (MF) strands have been made, and a previously developed technique has been used to quantify the coupling strength in terms of a PE coupling supercurrent, J/sub cn/. This quantity has been determined for a number of MF strand types processed under various conditions. All are NbTi/Cu MF composites in some of which the NbTi filaments were surrounded by Nb barriers, while in the others they were not. The magnetic field dependence of J/sub cn/ was also explored. The measured differences in J/sub cn/ are attributed to differences in the properties of the filament/matrix (normal metal, N) interface treated as a S1-S2-N trilayer. These results, together with those of an earlier series of experiments confirm that PE coupling between the filament and the matrix, and hence the normal-state coherence length, /spl xi//sub N/, is enhanced by the presence of a Nb (S2) barrier; and furthermore that the field-dependent coupling strength, gauged in terms of J/sub cn/ or /spl xi//sub N/, depends on the thickness of that barrier. It is demonstrated that magnetic PE-coupling studies not only provide a technique for extracting /spl xi//sub N/, a quantity central to artificial pinning center (APC) design, but also may help in the investigation of S-N boundary conditions in the clean limit.