Coexistence of different pinning mechanisms in Bi-2223 superconductor and its implications for using the material for high current applications

Abstract

We investigate the pinning mechanism in polycrystalline samples of Bi-2223. Using the differential magneto-optical (DMO) imaging technique, we track the magnetic field penetration in our sample. DMO imaging studies show circular regions with an average diameter of 20 μm with dark contrast appearing at temperatures near Tc. We identify these as strong-pinning regions with a substantially higher local penetration field than the surrounding regions. A unique feature of these strong-pinning centers is that they survive high temperatures (near Tc) and produce a non-Gaussian distribution of the penetration field strength. Analysis of the magnetic field dependence of the pinning force shows two distinct pinning mechanisms: a predominantly surface pinning mechanism is active at low temperatures well below Tc, while at higher temperatures near Tc, there is a crossover into a purely δTc pinning. Our studies show that surface pinning effects are most likely related to grain alignment, grain boundary, and voids in the sample. The strong δTc pinning is related to local stoichiometric fluctuations in the sample. One can potentially exploit this for enhancing the high T and Jc values of superconductors. We investigate the impact of these pinning centers on the current distribution in a macroscopic Bi-2223 superconducting cylindrical tube. We map the current distribution using an array of hall sensors distributed around the cylinder. The map reveals a non-uniform current distribution across the tube at high currents. This study suggests an inhomogeneous distribution of strong-pinning centers across large length scales in superconductors which are used for current lead applications.