Anomalies in the magnetization of melt-processed YBCO superconductors

Anomalies have been observed in the magnetization curves of melt-processed polycrystalline YBCO materials containing certain dopants such as Pr and Pt. Such anomalies are not produced, and are even suppressed, by other dopants, such as Nb. The anomaly consists of the appearance of a second, higher-field peak in the irreversible region of the magnetization plot. It can be attributed to uncoupling of flux-pinning centers from the superconducting matrix at high fields. Creep measurements show that the activation energy for flux creep exhibits a maximum at the vicinity of the anomalous second peak. (orig.)