Dependence of magnetic properties of pellets of nominal composition YBa2Cu3O7−x on processing conditions

Abstract

We have studied the dependence of the magnetic properties of pellets of nominal composition $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ on variations in preparation conditions. We find that rapid quenching of the pellets from annealing temperatures greater than 500-600°C causes the formation of significant amounts of secondary phases. Two of these phases, ${\mathrm{Y}}_2$BaCu${\mathrm{O}}_5$ and BaCu${\mathrm{O}}_2$, have magnetic moments of approximately $1.8{\mu }_B$ per Cu atom. We find that ${\mathrm{Y}}_2$BaCu${\mathrm{O}}_5$ orders antiferromagnetically below 30 K whereas ${\mathrm{BaCuO}}_2$ is paramagnetic down to at least 6 K. Eliminating these secondary phases by optimizing the processing conditions gives single-phase material (> 99.8% by mass) for which the magnetic susceptibility above the superconducting transition temperature is nearly independent of temperature. The magnitude of this susceptibility, after correction for ioncore diamagnetism, is enhanced by a factor of approximately 2 over the Pauli susceptibility estimated from the band-structure density of states, indicating the importance of electron-electron correlation effects.