Improving flux pinning at high fields in intermetallic superconductors: clues from MgB2 and MgCNi3

We discuss flux pinning and nanostructural analyzes of two intermetallic superconductors that exhibit substantial deviations from the usual flux-shear behavior. Kramer plots for a MgB₂ thin film, which contained a substantial fraction of MgO nanoprecipitates, show an additional component that is attributed to core pinning by the precipitates. Also, polycrystalline MgCNi₃ displays a crossover from flux-shear to core pinning behavior as the temperature is reduced. At the same time, the flux line core diameter becomes comparable to the length scales of nanoprecipitates found by high-resolution electron microscopy. Thus, both experiments suggest that flux shear, and its low-field pinning characteristic, can be exceeded by incorporating nanoprecipitates in an intermetallic superconductor. However, too many precipitates within the grain boundaries can block uniform current flow between grains.