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

Magnetic measurements on a high-Tc superconductor YBa2Cu3O7 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 differs substantially from that observed in spin glasses.