Magnetization and Mössbauer studies of Y1-xCaxSr2Cu3-yFeyO6+z.

Abstract

ac-susceptibility, dc-magnetization, and $^{57}\mathrm{Fe}$ M\ossbauer spectra of ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{Sr}}_{2}$${\mathrm{Cu}}_{3\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Fe}}_{\mathit{y}}$${\mathrm{O}}_{6+\mathit{z}}$ (x=0, 0.1, and 0.2; y=0.3 and 0.4) are presented. Our data indicate bulk superconductivity in these compounds. In the case of y=0.4, the substitution of x=0.1 Ca increases both ${\mathit{T}}_{\mathit{c}}$ and flux pinning. The M\ossbauer parameters of the sample containing y=0.4 Fe and no Ca in comparison to those of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Fe}}_{\mathit{y}}$${\mathrm{O}}_{6+\mathit{z}}$ indicate that at least 85% of Fe substitutes the Cu(1) sites and about 15% goes to Cu(2) sites. The existence of superconductivity at such high concentrations of Fe and the revival of superconducting properties by Ca substitution are discussed.