Superconducting properties of High-J_c MgB_2 Coatings

摘要

We report the fabrication and superconducting properties of ～0.5 μn thick, fine-grained polycrystalline coatings of MgB_2 on single-crystal substrate surfaces. The films exhibit large critical current densities, implying little effect from the grain boundaries. Analyses for thermal activation effects are inconclusive, and evidence is presented that the irreversibility line is dominated by the combined influences of H_(c2) anisotropy and polycrystallinity. Comparative studies of the magnetic persistent currents and electrical transport properties reveal excellent agreement over a wide range of temperature and magnetic field. this result is contrary to similar comparisons on high-temperature cuprates, where disparities arise from the effects of large flux creep and the diverse electric field regimes probed by the two techniques. The MgB_2 films exhibit extremely sharp voltage-current relations away from the irreversibility line, in qualitative agreement with observed large J_c values and low rates of magnetic flux creep.