The epitaxy of a thin-film Y-Ba-Cu-O (YBCO) superconductor deposited on a single-crystal [001] MgO substrate was examined by transmission electron microscopy. The large lattice mismatch (8–10%) in the basal plane of YBCO and MgO is accommodated mainly by the formation of a polycrystalline, mosaic structure. The grain boundaries correspond to unique crystallographic interfaces, determined by the crystal symmetry of the substrate and the thin film.

The structure and magnetic properties of the ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Cu}}_{1\ensuremath{-}x}{M}_{x}{\mathrm{O}}_{y}$ ($M=\mathrm{Co}\mathrm{and}\mathrm{Fe}$) materials were studied. The limits of solid solution formation are at $x=0.5$ for the Fe system and $x=1$ for the Co system. Crystals of the new ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Co}{\mathrm{O}}_{y}$ phase were grown and the structure established by x-ray crystallography. The subcell is the same as that of the 10-K superconductor, ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Co}{\mathrm{O}}_{y}$, but the superstructure is different, as it exhibits a commensurate modulation of periodicity 4 instead of 5. Extra oxygen is accommodated in the Bi layers, as in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{3}{\mathrm{Fe}}_{2}{\mathrm{O}}_{y}$, and the structure of the Bi-O layers can be described as 50% rocksalt type and 50% oxygen-deficient perovskite for $x=1$, but with disorder at the oxygen positions. The formal valence of Co in this compound is about 2.5\ifmmode\pm\else\textpm\fi{}0.2 as deduced from structural and chemical measurements, whereas Fe adopts the oxidation state +3 as deduced by M\"ossbauer measurements. ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Co}{\mathrm{O}}_{y}$ is an antiferromagnetic insulator with the spins lying within the Co${\mathrm{O}}_{2}$ sheets and the antiferromagnetic ordering temperature (${T}_{N}$) is sensitive to processing conditions and composition changes. The high anisotropy of the susceptibility suggests that ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Co}{\mathrm{O}}_{y}$ may be an Ising or $\mathrm{XY}$ antiferromagnet.