Magneto-optical study of flux penetration and critical current densities in [001] tilt ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin-film bicrystals

Magneto-optical (MO) imaging has been used to visualize and calculate magnetic flux and current distributions at temperatures T ranging from 7 to 80 K in thin-film [001] tilt ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ bicrystals with misorientation angles 3°≤θ≤10°. A characteristic cusp in the flux distribution ${\mathit{B}}_{\mathit{z}}$(x,y) was observed for 5°≤θ≤7°, which is shown to indicate that the critical current density ${\mathit{J}}_{\mathit{b}}$ across the boundary is smaller than the intragrain ${\mathit{J}}_{\mathit{c}}$. We use the Bean model for thin-film superconductors to calculate the observed features of the ${\mathit{B}}_{\mathit{z}}$(x,y) distribution and to separate both the intragrain ${\mathit{J}}_{\mathit{c}}$ and intergrain ${\mathit{J}}_{\mathit{b}}$(θ) independently from the MO data. The study of angular and temperature dependencies of ${\mathit{J}}_{\mathit{b}}$(T,θ) in bicrystals with different θ shows that ${\mathit{J}}_{\mathit{b}}$(θ) strongly decreases with θ above θ≊5°. The decrease of ${\mathit{J}}_{\mathit{b}}$(T,θ) with temperature becomes weaker as the misorientation angle θ is increased, so the substantial difference in ${\mathit{J}}_{\mathit{b}}$ for 5° and 7° boundaries at low T turns out to be less pronounced at liquid-nitrogen temperatures. In addition, the ratio ${\mathit{J}}_{\mathit{b}}$(θ,T)/${\mathit{J}}_{\mathit{c}}$(T) for low-angle grain boundaries is shown to exhibit an anomalous increase with T, thus indicating that the grain boundaries can provide additional flux pinning. This is plausibly associated with the grain boundary dislocations that accommodate the misorientation of the grains. © 1996 The American Physical Society.