Relation between critical current densities and epitaxy of YBa2Cu3O7 thin films on MgO(100) and SrTiO3(100)

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Abstract
The preferred orientation of YBa2Cu3O7 thin films grown on single‐crystal MgO(100) and SrTiO3(100) substrates has been investigated using x‐ray diffraction and transmission electron microscopy. Various types of microstructural defects were identified in the films and correlated with a lowering of the critical current density Jc. On MgO substrates it was found that a film with 5% of its grains rotated 45° with respect to the dominant orientation and a high density of small‐angle grain boundaries had Jc reduced by 75% compared to a film with no 45° grain boundaries and a low density of small‐angle grain boundaries. On SrTiO3 substrates it was found that an increase in the amount of a‐axis oriented material from 0.6% to 8.3% lead to a reduction in Jc by 70% . In general, higher critical current densities were found on SrTiO3 than on MgO substrates. This fact is attributed to the closer lattice match of YBa2Cu3O7 with SrTiO3, resulting in an in‐plane mosaic spread of only 0.38° for the best film on SrTiO3 compared to 0.86° for the best film on MgO.