Structure and magnetic properties of nonsuperconducting doped Co and Fe ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Cu}}_{1-x}{M}_x{\mathrm{O}}_y$ phases

The structure and magnetic properties of the ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Cu}}_{1-x}{M}_x{\mathrm{O}}_y$ ($M=\mathrm{Co}\mathrm{and}\mathrm{Fe}$) materials were studied. The limits of solid solution formation are at $x=0.5\mathrm{}$ for the Fe system and $x=1\mathrm{}$ 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 $x=1\mathrm{}$, but with disorder at the oxygen positions. The formal valence of Co in this compound is about 2.5±0.2 as deduced from structural and chemical measurements, whereas Fe adopts the oxidation state +3 as deduced by Mössbauer 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.