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

Abstract

We have found that naturally occurring grain boundaries inTl2Ba2CaCu2O8 (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 exclusively45° tilt grainboundaries 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 filmsgrown on MgO substrates, and ‘artificial’ grain boundaries forced to formin Tl-2212 films grown on lattice-matched bicrystal substrates such asLaAlO3.Polycrystalline, c-axis aligned Tl-2212 films on ‘dirty’ MgO contain diffuse orhighly dissociated grain boundaries, thus explaining their highJc values. Artificial grain boundaries, however, show a much moreabrupt change in orientation at the grain boundary. The bi-epitaxial45° grain boundaries are also abrupt; therefore, the highJcgb values suggest that the local structure or chemistry at these grain boundaries is differentfrom those of both artificial and other natural grain boundaries in polycrystalline films.