Title: Vortex Pinning Landscape in YBa ₂Cu ₃O ₇ Films Grown by Hybrid Liquid Phase Epitaxy

The inductance of vortices {ital L}{sub {ital V}}({ital T},{ital B}) in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} films is studied for temperature 8{lt}{ital T}{lt}85 K, and magnetic field 0{lt}{ital B}{lt}6 T with the goal of identifying the morphology of the vortex pinning sites and the type of the glass phase below the vortex lattice melting transition {ital B}{sub {ital g}}({ital T}). The key feature that distinguishes point, linear, and planar pinning sites is how rapidly {ital L}{sub {ital V}} increases when thresholds in {ital T} and {ital B} are crossed. We find that, below a threshold field {ital B}{sub {ital p}}({italmore » T}){lt}{ital B}{sub {ital g}}({ital T}), the effective pinning force on each vortex is independent of {ital B}. At {ital B}{sub {ital p}}{lt}{ital B}{lt}{ital B}{sub {ital g}}, {ital L}{sub {ital V}} increases as {ital B}{sup 2}, whereas at {ital B}{approx_equal}{ital B}{sub {ital g}}, {ital L}{sub {ital V}} diverges faster than {ital B}{sup 2} following a scaling law on {ital f}, consistent with a phase transition between a glass and a liquid vortex state. Although the value of the threshold field, {ital B}{sub {ital p}}{approx_equal}8(1{minus}{ital T}/{ital T}{sub {ital C}}) T, is consistent with any type of pinning defect, the increase of {ital L}{sub {ital V}}{proportional_to}{ital B}{sup 2} for {ital B}{approx_gt}{ital B}{sub {ital p}} is much less rapid than predicted for collective pinning of vortices by point defects. Linear and planar defects are possible explanations, but the necessary theoretical calculations are currently absent. {copyright} {ital 1996 The American Physical Society.}« less

In both crystals at 4.2 K, a trapped flux along the [ital c] axis is seen to inhibit the initial production of vortex flux by external fields ([ital H][sub [ital e]]) in the [ital a]-[ital b] plane and to increase the retention of this vortex flux during subsequent cycling of [ital H][sub [ital e]]. Thus, the cross flux along [ital c] acts consistently as a pinning agent for the vortices aligned along [ital a]-[ital b]. The cross-flux effects in both these crystals are as pronounced as those observed in grain-oriented YBa[sub 2]Cu[sub 3]O[sub 7] [S. J. Park and J. S.more » Kouvel, Phys. Rev. B 48, 13 995 (1993)] and they persist undiminished to much larger cycling ranges of [ital H][sub [ital e]] due to the stronger pinning of the cross flux itself.« less

The interaction between magnetic flux vortices and twin boundaries is investigated in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} thin films. Films grown to various thicknesses, and cooled along different paths in the YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} phase diagram, are shown to have various twin boundary microstructures and densities. It is demonstrated that changes in processing result in differences in the vortex-pinning behavior of the films, but that these differences are not due to changes in the microstructure of the twin boundaries. Further, observed differences in the twin spacing do not result in changes in the volume pinning force. This is found tomore » be true in the magnetic-field regime spanning the range from where the flux vortex spacing is large relative to the twin density to where the vortex spacing is small relative to this density.« less

Nanostructural modulation in the cap layer used in coated conductors can be a potential source for nucleating microstructural defects into the superconducting layer for improving the flux-pinning. We report on the successful fabrication of phase separated, epitaxial, nanostructured films comprised of LaMnO{sub 3} (LMO) and MgO via pulsed laser deposition (PLD) on biaxially-textured MgO metallic templates with a LMO buffer layer. Scanning Auger compositional mapping and transmission electron microscopy cross sectional images confirm the nanoscale, spatial modulation corresponding to the nanostructured phase separation in the film. YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} films (0.8 {micro}m thick) grown using PLD on such phasemore » separated, nanostructured cap layers show reduced field dependence of the critical current density with an ? value of -0.38 (in J{sub c}-H{sup -{alpha}}).« less