Artificial pinning centres in jelly-roll processed niobium-copper composites.

In order to clarify the mechanism concerning the enhancement of critical-current density due to artificial pinning centres, nanometer scale niobium-copper multilayer composites have been prepared from niobium and copper foils by the jelly-roll process. The layer thickness of the specimen was chosen using a calculation based on the phenomenological Ginzburg-Landau theory. The microstructure of the composite was investigated by scanning electron microscopy. The critical-current density and critical temperature were measured at various magnetic fields. A remarkable increase in the critical-current density was found when the layer thickness was approximately the same scale as the vortex lattice parameter. However, samples with thinner layers showed lower critical-temperature values due to the proximity effect.