Magnetic properties of a high-Tc superconductor YBa2Cu3O7: Reentry-like features, paramagnetism, and glassy behavior.

Magnetic measurements on a high-T, superconductor YBa2Cu30q are reported. Diamagnetism sets in, at low fields, below a temperature T=90 K. At low T the field-cooled magnetization becomes positive again. We show that the low-T phase is still superconducting and the apparent reentry behavior is due to strong paramagnetic contributions. Glassy features appear below a temperature whose field dependence divers substantially from that observed in spin glasses. Last year Bednorz and Muller ' reported on the possibility of a breakthrough in producing superconductors with high transition temperature T,. Since then the highest T, has advanced substantially, and very recently Wu et al. reported on a Y-Ba-Cu-0 system with T, &90 K. In this paper we present magnetic measurements on a supercon-ductor of the latter family, a YBa2Cu307 sample. Both the zero-field-cooled (ZFC) and the field-cooled (FC) magnetizations of this sample exhibit a diamagnetic behavior which sets in, at low fields, below 90 K. The FC diamagnetic susceptibility at low temperatures and fields is of the order of — 20% of the complete Meissner eAect (I/4tr). The ZFC magnetization decreases monotonically with decreasing T. The FC branch, on the other hand, exhibits a minimum and starts increasing upon further cooling , reaching a positive value at low temperature. Glassy features such as a large diA'erence between the ZFC and the FC susceptibilities and time-dependent remanent magnetization appear below a "glass" temperature Tg.