Nanostructure characterization of Ni and B layers as artificial pinning centers in multilayered MgB₂/Ni and MgB₂/B superconducting thin films

Research on the MgB₂/Ni and MgB₂/B multilayer films fabricated by an electron beam (EB) evaporation technique have been extensively carried out. The critical current density, J_c of MgB₂/Ni and MgB₂/B multilayer films in parallel fields has been suggested to be higher than that of monolayer MgB₂ film due to introducing the artificial pinning centers of nano-sized Ni and B layers. Nanostructure characterization of the artificial pinning centers in the multilayer films were examined by transmission electron microscopy (TEM) and scanning TEM (STEM-energy dispersive X-ray spectroscopy (STEM-EDS))-EDS to understand the mechanism of flux pinning. The growth of columnar MgB₂ grains along the film-thickness direction was recognized in the MgB₂/Ni multilayer film, but not in the MgB₂/B multilayer film. Nano-sized Ni layers were present as crystalline epitaxial layers which is interpreted that Ni atoms might be incorporated into the MgB₂ lattice to form (Mg,Ni)B ₂ phase. On the other hand, nano-sized B layers were amorphous layers. Crystalline (Mg,Ni)B₂ layers worked more effectively than amorphous B-layers, providing higher flux-pinning force that resulted in higher J_c of the MgB₂/Ni multilayer film than the MgB ₂/B multilayer film.