Fabrication of Alternately-layered MgB₂/B Thin Films and Their Superconducting Properties

Abstract

Alternately-layered MgB₂/B thin films were prepared on silicon (100) substrates by an electron-beam evaporation technique without any post-annealing. The thickness of each MgB₂-layer was designed to be 42, 24 or 15 nm. Cross-sectional transmission electron microscopy observations confirmed that the layered structure was successfully obtained. The critical temperature T_c of the MgB₂/B thin film decreased as the MgB₂-layer became thinner. We then compared the magnetic field dependence of the critical current density J_c and the global pinning force F_p between the alternately-layered MgB₂/B and pure MgB₂ thin films. In the MgB₂/B film, J_c was higher in fields parallel to the substrate than in perpendicular fields. The F_p -B curve in parallel fields had a clear peak at 4 T, indicating that the maximum F_p was achieved when the quantized fluxon line spacing matched the B-layer spacing. The results suggest a high possibility that the alternately-layered nanostructure improves the superconducting performance of MgB₂.