Avalanche of flux jumps in polycrystalline MgB₂superconductor

Dc magnetization studies were carried out on a polycrystalline Mg B2 superconductor, which was synthesized at 750 °C in Fe tube encapsulation under a vacuum of 10-5 Torr. Zero resistances at H=0 and 8 T are obtained at 38 and 22 K, respectively. The material possesses a sharp diamagnetic transition in the zero-field-cooled branch, whereas the field-cooled branch exhibits the paramagnetic-Meissner-effect-like phenomenon below Tc, indicating strong pinning in this compound. The critical current density at 20 K and 2 T, estimated by the Bean model is higher than 105 A cm2. Due to the strong pinning, we observed the presence of flux avalanches below H=2 T at temperatures below 20 K. The magnetization M (H) grows (as usual) slowly with H and falls sharply to near zero moment value, and further grows again in a common way. The flux avalanches were seen quite symmetric in both increasingdecreasing the field in all four quadrants of the M (H) loops. The dynamics behavior of sinusoidal-like symmetric reproducible flux avalanches is discussed.