In this letter, we report the results of field (H) and temperature (T) dependent magnetization (M) measurements of a pellet of uniform, large-grain sintered MgB 2 . We show that at low temperatures the size of the pellet and its critical current density, J c (H) --i.e. its M(H) --ensure low field flux jumping, which of course ceases when M(H) drops below a critical value. With further increase of H and T the individual grains decouple and the M(H) loops drop to lower lying branches, unresolved in the usual full M(H) representation. After taking into account the sample size and grain size, respectively, the bulk sample and the grains were deduced to exhibit the same magnetically determined J c s (e.g. 10 5 A/cm 2 , 20 K, 0T) and hence that for each temperature of measurement J c (H) decreased monotonically with H over the entire field range, except for a gap within the grain-decoupling zone.
The fabrication, characterisation, and superconductivity of MgB 2 thick films grown on stainless steel substrate were studied. XRD, SEM, and magnetic measurements were carried out. It was found that the MgB 2 thick films can be fast formed by heating samples to 660 o C then immediately cooling down to room temperature. XRD shows above 90% MgB 2 phase and less than 10 % MgO. However, the samples sintered at 800 o C for 4 h contain both MgB 4 and MgO impurities in addition to MgB 2 . The fast formed MgB 2 films appear to have a good grain connectivity that gives a J c of 8 x 10 4 A/cm 2 at 5 K and 1 T and maintained this value at 20 K in zero field.2
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