HIGH CRITICAL FLUX-DENSITY GRADIENTS NEAR THE SURFACE OF SUPERCONDUCTING NIOBIUM

The profiles of the magnetic induction near the surface of cylindrical niobium samples were measured by the method of differential susceptibility in minor hysteresis loops. The flux density gradient near the surface generally is considerably higher than in the bulk. This may be interpreted in terms of a surface barrier or of a surface pinning force. The experimental results can be summarized as follows: (1) Surface pinning always increased with surface roughness. (2) When the surface was polished the barrier was only observed in a plastically deformed sample. (3) The influence of the surface seems to extend down to a depth of some tenth of a millimeter. © 1979 Plenum Publishing Corporation.