Dynamical critical fields, aging effects, and restoring in some YBa2Cu3Ox compounds studied by complex susceptibility measurements

The ac initial complex susceptibility of sintered samples of high temperature superconducting YBaCuO compounds is studied in detail. The importance of using very low external magnetic ac fields (hac < 10−3 Oe) is pointed out in order to prevent dynamical saturation (flux-pinning) when reaching the critical ac field h which is much lower than the first critical (static) field Hc1. The field, temperature, and frequency dependences of both real part (diamagnetic shielding) and imaginary part (power losses) are consistent with a rather large spread of critical temperatures over the volume: the highest (Tc = 90 K) at the surface and the lowest (50 < Tc < 80 K) in the inner part of the volume. It is also shown that this technique is a very useful tool for studying and/or controlling aging effects, water contamination, and thermal restoring treatments.