Critical Current Densities Of Films Research Articles

MgB2 thin film fabrication with excess Mg by sequential e-beam evaporation and transport properties under magnetic fields

Superconducting MgB2 thin films with different thicknesses were fabricated by excess Mg using sequential electron beam evaporation technique. The flux rate of Mg and B were chosen as 3 nm/sn and 0.5 nm/sn respectively. The thickness of Mg/B layer was determined as 3/1 which causes an excess Mg for MgB2 formation. After two-step preparation process, the superconducting transition temperatures were found in the range of 32–37 K with a sharp transition to zero resistance state. The irreversibility fields, upper critical fields and activation energies were derived from magneto resistivity curves. The transport critical current densities of films were determined from I–V characteristics under different applied fields and the data was extrapolated for determination of value of the 0 K.

Twin boundaries and critical current densities of YBa2Cu3O7 thick films fabricated by the BaF2 process

2 µm thick epitaxial YBa2Cu3O7, YBCO, films on SrTiO3 substrates were prepared at the reaction temperatures of 725, 750 and 800 °C by the BaF2 process. The dc critical current densities of these films were found to vary by a factor of 2 at 77 K in perpendicular dc magnetic fields measured up to 1.5 T. The twin boundary spacing was essentially unchanged among these films while the size of Y2O3 precipitates in the films varied by a factor of ∼5. Also, the measured ratios of the lattice parameters, a/b, of the films were surprisingly larger than those found in standard fully oxygenated YBCO. This unusual property of the films appeared to be caused by the presence of the Y2O3 precipitates which provided a strain constraint in limiting the full tetragonal-to-orthorhombic phase transformation due to the oxygenation. Differences in the degree of the constraint possibly led to the observed difference in Jc among these films.