Avalanche of flux jumps in polycrystalline MgB2 superconductor

Abstract

Dc magnetization studies were carried out on a polycrystalline MgB2 superconductor, which was synthesized at 750°C in Fe tube encapsulation under a vacuum of 10−5Torr. Zero resistances at H=0 and 8T are obtained at 38 and 22K, 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 20K and 2T, estimated by the Bean model is higher than 105A∕cm2. Due to the strong pinning, we observed the presence of flux avalanches below H=2T at temperatures below 20K. 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 increasing∕decreasing the field in all four quadrants of the M(H) loops. The dynamics behavior of sinusoidal-like symmetric reproducible flux avalanches is discussed.