Vortex-flux anomalies in MgB2 polycrystals

For two MgB2 polycrystalline samples prepared differently, magnetic hysteresis loops at 4.2 K revealed some striking magnetization (M) anomalies at low magnetic fields (H). In one sample, M drops abruptly by an amount that diminishes to zero as the maximum M, reached just before the drop, is reduced below a critical value (by reducing the slowly cycled H). The remanent magnetization (Mrem), measured as the temperature (T) was slowly raised from 4.2 K to Tc (39 K), was seen to decrease very rapidly just above 4.2 K and then remain constant up to ∼20 K, where it starts to descend gradually to zero at Tc. A similar set of anomalies was observed with the second sample, but where the hysteretic drop of M is rapid but not abrupt. However, both samples showed that the vortex-flux component of M remains nearly constant over a range of H just before the rapid drop of M, indicating that the vortices in the sample stay fairly constant in number before many of them exit the sample rapidly. For both MgB2 samples, this whole set of anomalous properties disappears at temperatures above 5 K, where the slow steady descent of Mrem as T increases to Tc indicates that the size distribution of vortex pinning forces extends up to very high values.