Anomalous Low-Field Magnetic Behaviour of High-TC Superconductors

Abstract

ABSTRACTWe report on the studies of low-field magnetic properties in the well-characteristic superconducting LaCaBaCu3Oy, YBa2Cu3Oy, YaBa4Cu8Oy, (La1–85Sr0–15)CuO4-y, (Tl0–5Pb0–5)Ca2Sr2Cu3Oy and (Bi2−xPbx)Ca2Sr2Cu3Oy oxides. The studies of magnetic properties include ac zero-field cooled (ZFC), field-cooled (FC), and ac susceptibilities, and their dependences on the applied magnetic field and temperature. As the applied dc magnetic field is low enough, the FC susceptibility varies flat with the increasing of temperature from low temperature, but the diamagnetic signal becomes increasing at some temperature and reaches to the maximum values, and then continously decreases with temperature, at last the diamagnetism rapidly transfers to paramagnetism with temperature going to above Tc. The magnetization reduction at low temperature significantly appears at the low dc magnetic field and exists within a lot of single-phase high-Tc superconductors. However, this phenomenon is not observed in the measurements of low-frequency ac complex susceptibility for the ac applied fields from 0.001 G to 10 G and disappears for higher dc applied field. But, there exists a two-step drop in ac and ZFC susceptibilities curve and the drop correlates with the de FC magnetization dip. The magnetic field range of the dip occurrence is dependent on specimens. The strange magnetic property may be regard with superconducting coupling link in grain boundaries and interpreted by the grain-cluster model associated with Josephson effect.