Flux jumping and a bulk-to-granular transition in the magnetization of a compacted and sintered MgB2 superconductor

In this paper, we report the results of field (H) and temperature (T) dependent magnetization (M) measurements of a pellet of uniform, large-grain sintered MgB2. It was found that iron is compatible with MgB2 and can be used as sheath material for fabrication of Fe-clad MgB2 wires. 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, 0 T) 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. (C) 2001 Elsevier Science B.V. All rights reserved.