  The recent discovery of intermediate-temperature superconductivity (ITC) in
MgB2 by Akimitsu et al. and its almost simultaneous explanation in terms of a
hole-carrier-based pairing mechanism by Hirsch, has triggered an avalanche of
studies of its structural, magnetic and transport properties. As a further
contribution to the field we report the results of field (H) and temperature
(T) dependent magnetization (M) measurements of a pellet of uniform,
large-grain sintered MgB2. We show that at low temperatures the size of the
pellet and its critical current density, Jc(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 Jc s (e.g. 105 A/cm2, 20 K, 0T) and hence that for each
temperature of measurement Jc(H) decreased monotonically with H over the entire
field range, except for a gap within the grain-decoupling zone.
