Physical properties of MgB₂ superconducting films grown on Cu-impurity layers

The physical properties of several MgB₂/Cu (0, 1, 4 and 8 nm)/Al₂O₃ superconducting films were investigated. The MgB₂ films were fabricated on Cu (0, 1, 4 and 8 nm)/Al₂O₃ substrates by employing the hybrid physical-chemical vapor deposition (HPCVD) method, after depositing Cu-impurity layers of various thicknesses were deposited on Al₂O₃ substrate by using pulsed laser deposition (PLD). The measurements of the magnetic moment m(H) for the MgB₂/Cu/Al₂O₃ films were carried out at temperatures between 5 K and 50 K in fields up to 6 T by using a physical property measurement system (PPMS) 9 T system. The MgB₂/Cu/Al₂O₃ films with Cu-impurity layers of 0, 1, 4 and 8 nm thicknesses showed values of the onset temperature (T_c) of 39.5, 38.5, 39.0 and 39.0 K, respectively. For the MgB₂/Cu/Al₂O₃ superconducting films, the α values in the relation J_c (∝ ΔM ∝ Δm) = H^-α were estimated to be 0.888 ∼ 0.987 at 10 K and 0.650 ∼ 0.897 at 30 K for external fields applied parallel to the c-axis. The MgB₂/Cu (4 nm)/Al₂O₃ film showed a notably low value of α = 0.650 and remarkably high magnetic properties over all magnetic fields used in their study because of the introduction of the flux-pinning effect.