Role of interdomain and interplatelet boundaries on magnetic flux penetration in melt grown YBCO

The microstructure of a melt processed YBCO sample reveals the presence of random domains with aligned platelets separated by microcracks inside the domains. The lower critical field (H/sub c1/) and critical current density (J/sub c/) of these boundaries will be less than that of the grains, and their role in determining the bulk properties cannot be neglected. We have employed a sensitive lock-in flat-band detection technique and recorded the low field ac M-H loops at 33 Hz and 77 K and derived the flux profiles. The results show an enhancement in H/sub c1/ due to melt processing. Inclusion of Y/sub 2/BaCuO/sub 5/, and Ag enhances the H/sub c1/ further. The flux profiles show distinct slope changes at certain fields which reflect different coupling strengths of weaklinks that become operative for flux entry at different applied fields. While the low field flux profiles are found to reflect the effect of residual weaklinks (like liquid phase and micropores) and domain boundaries, those at higher fields might reflect the effect of platelet boundaries.