The determination of flux density gradients in Nb3Sn diffusion layers by means of the magneto-optical faraday effect

By means of the magneto-optical Faraday effect the penetration of magnetic flux into poly-crystalline diffusion layers of Nb3Sn is observed. For magnetic fields Hext ⪆ H′p (H′p denotes the penetration field) flux density profiles with constant flux gradients, depending only on the microstructure of the layer, are found in striking contrast to similar investigations on low-x type II superconductors like Nb or NbTa. In polycrystalline layers the grain boundaries are found to be the essential pinning centres. The volume pinning force respectively the critical current density and the mean grain size, d, are related by the well known 1/d-relationship. The direct summation of the elementary pinning force yields critical current densities in good agreement with the experimental results.