Superconducting properties of Cu-NbTi composite wires with fine filaments

Copper matrix NbTi wires with ultrafine filaments were fabricated by the multiple stacking technique, and the NbTi-filament diameter dependences of the critical temperature, the upper critical magnetic field and the critical current density were measured. The filament diameter ranged down to 0.008 μm. As the diameter was reduced, the critical temperature began to decrease at a diameter of m. This depression was attributed to the proximity effect of the copper matrix on the NbTi filaments. The critical field measurement was carried out at 4.2 K under external magnetic fields applied both transverse and parallel to the wire axis. These critical fields were also lowered by the critical temperature depression due to the proximity effect. Furthermore, it was observed that the parallel critical fields were larger than the transverse ones. The critical current density at 4.2 K under the transverse magnetic field increased with decreasing filament diameter. This behavior was ascribed to the surface pinning at the boundary between the filaments and the matrix.