Paramagnetic Meissner effect in high-temperature superconductors

Abstract

We have studied the low-field Meissner effect of polycrystalline Bi high-temperature superconductors using a special superconducting-quantum-interference-device magnetometer. In certain samples a surprising feature was observed: Instead of the usual diamagnetic moment a paramagnetic moment develops in the field cooling mode below ${\mathit{T}}_{\mathit{c}}$ for fields H1 Oe. The data are consistent with orbital paramagnetic moments due to spontaneous currents. Such currents may originate in so-called \ensuremath{\pi} contacts in the weak-link network of polycrystalline material. In some of these samples also an anomaly in the low-field microwave absorption was observed, which is obviously correlated with the existence of spontaneous currents.