Magnetic order and superconductivity in YBa2Cu3-yMyOz (M=Fe,Co).

Abstract

M\ossbauer studies, magnetic susceptibility, and powder x-ray diffraction were used to study the magnetic properties and superconductivity of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3\mathrm{\ensuremath{-}}\mathit{y}}$${\mathit{M}}_{\mathit{y}}$${\mathrm{O}}_{\mathit{z}}$ (M=Fe, Co) in both oxygen-rich and oxygen-deficient samples. ${\mathit{T}}_{\mathit{c}}$ is reduced by increasing the dopant concentration and the analysis of M\ossbauer spectra at different temperatures shows that antiferromagnetism in the Cu(2) site is found at concentrations where superconductivity disappears. It means that the materials exhibit either superconductivity or antiferromagnetic order in the Cu(2) site, depending on composition. The phase diagrams obtained are very similar to the well-known oxygen content phase diagram. For z=6 and y\ensuremath{\ge}0.5, and for M=Co also with z=7, two independent magnetic transition temperatures have been observed. Both ${\mathit{T}}_{\mathit{N}1}$\ensuremath{\sim}400 K and ${\mathit{T}}_{\mathit{N}2}$\ensuremath{\sim}220 K, which correspond to the Cu(2) and Cu(1) sites, respectively, change very little with y.