  This paper reports on peculiar metamagnetic transitions which take place in
antiferromagnetic, charge and orbitally ordered (AFCOO) manganites. At very low
temperatures, the virgin magnetization curves of some of these compounds
exhibit several, sharp steps giving rise to a staircase-like shape. This
staircase effect is shown to be sensitive to various experimental parameters.
Several potential interpretations of the staircase effect are discussed in
relation to this set of results. A martensitic-like scenario, involving a
leading role of the structural distortions associated to the collapse of the
orbital ordering, is found to be the most plausible interpretation.
