Magneto-optical studies of magnetization of melt-processedYBa2Cu3O7−δ

Magnetization patterns on different faces of long bars cut from melt-processed ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (YBCO) samples were studied using advanced magneto-optical techniques. It is shown that with an external magnetic field perpendicular to the long side of the bar the magnetization front is described by a cylindrical surface having a cross section shaped as a higher-order ellipse, i.e., the front has a sharper bending near the sample edges than a second-order ellipse. Similar to bicrystal boundaries, the boundaries of structure domains consisting of $c$ axis-aligned crystalline plates are revealed to be weak links in a wide temperature range for misorientation angles between domains exceeding 10°. The temperature dependence of the critical currents inside the domains are determined by fitting induction profiles measured across the bars. From direct flux pattern observations and measurements of the anisotropy for currents flowing along and across the crystalline plates, it follows that the crystallite boundaries also become effective weak links at higher temperatures and fields. Possible reasons for why the observed anisotropy is much less than that for $c$ axis and $\mathrm{ab}$ plane currents in YBCO single crystals are discussed.