Properties of superconducting NbTi superfine filament composites with diameters ≲0.1 μm

Abstract

The critical temperature and the electromagnetic properties of NbTi-Cu composites with ≈9.4 × 10 6 very fine superconducting filaments, prepared by the multiple compaction process, were measured. The corresponding critical current density, volume pinning force, effective transverse resistivity, hysteresis and eddy current losses at different field amplitudes and frequencies up to 50 Hz, were determined and some of these quantities were compared with theoretical calculations including proximity, size and surface effects. The J c ( d) dependence on the filament diameter, d, is explained by the change of the upper critical field, B c2, with T c, where the T c( d) dependence is taken from experimental values. The unexpected very high critical current densities at low fields (> 10 11 A m −2 at B ≲0.5 T) and the maximum volume pinning force ( ⋍5 × 10 10 N m −3 ) obtained at unusually low fields of B ≈ 1 T were explained theoretically by the existence of a one-dimensional flux line lattice in the filaments. The magnetization curves, the a.c. losses, the ratio of J cM from magnetization and J cT from resistive measurements, as well as the theoretical calculations show that the first compaction level strands behave like monofilaments with strong surface current contribution. This is mainly due to the proximity effect which renders the copper between the filaments to be essentially superconducting up to applied fields of ≲5 T. To eliminate the increase of hysteresis losses due to this effect, some highly resistive material, e.g. cupronickel, should be introduced at the basic compaction of the filaments. A peculiar minimum on the magnetization curves appears that is not caused by flux jumps on reversing the magnetic field direction.