HIGH-FIELD CRITICAL CURRENT DENSITIES AND RESISTIVE TRANSITION OF NBCN SUPERCONDUCTING FILMS

We studied the influence of the carbon density on the critical current densities of niobium carbonitride films in high magnetic fields. The samples were about 2 200 Å thick and were obtained by reactive sputtering either from a Nb-C target in a residual Ar + N2 atmosphere or from a bulk Nb target in a residual Ar + N2 + CH4 atmosphere. They all had sufficiently high critical temperature, around 15 K, which implies that the ratio of carbon atoms over the total supplementary nitrogen and carbon atoms lay in the range 0.06 to 0.35. The samples were immersed in the liquid helium at 4.2 K. The registered V(I) curves either showed an hysteretic feature or not, depending on the composition and/or the applied magnetic field. The best critical current densities, about several 1010 Am-2 in a zero field were still more than 3 × 108 in a 14 T field, irrespective of direction, transverse or perpendicular. In the former case, some samples were tested up to 18 T, a value for which the best critical density was 107 Am-2. Heterogeneous samples which have periodic variations in carbon concentration, have still greater critical densities in high transverse fields. The Jc(B) curves show a plateau or a slight hump when the field is such that the flux line lattice parameter fits the period of the carbon ratio variations. The resistivities of the different films are also high enough, more than 10-6 Ωm. So these alloys might be of interest in a superconducting switchgear, using the resistive transition to the normal state in order to limit a short circuit current. © 1979.