The development of bi-epitaxial texture and high grain boundary Jc values in Tl-2212 films on MgO substrates

We have found that naturally occurring grain boundaries in Tl2Ba2CaCu2O8 (Tl-2212) thin films grown on MgO substrates have significantly higher critical current values (Jcgb) than expected. In particular, films grown on clean MgO are bi-epitaxial, containing almost exclusively 450 tilt grain boundaries with Jcgb values as high as 106 A cm-2 at 77 K. We have used high resolution electron backscatter diffraction (EBSD) to analyse the structure of both 'natural' grain boundaries in Tl-2212 films grown on MgO substrates, and 'artificial' grain boundaries forced to form in Tl-2212 films grown on lattice-matched bicrystal substrates such as LaAlO3. Polycrystalline, c-axis aligned Tl-2212 films on 'dirty' MgO contain diffuse or highly dissociated grain boundaries, thus explaining their high Jc values. Artificial grain boundaries, however, show a much more abrupt change in orientation at the grain boundary. The bi-epitaxial 450 grain boundaries are also abrupt; therefore, the high Jcgb values suggest that the local structure or chemistry at these grain boundaries is different from those of both artificial and other natural grain boundaries in polycrystalline films