  Substitutions at the Mn-site of the charge-ordered Pr0.5Ca0.5MnO3 manganite
is an effective way to induce abrupt jumps on the magnetic field driven
magnetization curve. In order to get new insights into the origin of this
remarkable feature, the Pr0.5Ca0.5Mn0.97Ga0.03O3 perovskite manganite has been
studied by neutron diffraction, versus temperature and at 2.5K in an applied
magnetic field up to 6 Tesla. A weak and complex antiferromagnetic order is
found for the low temperature ground-state. Magnetic transitions, associated
with structural ones, are evidenced for certain strengths of magnetic field,
which gives rise to the step-like behavior corresponding to the magnetization
curve. Small angle neutron scattering provides evidence for a nucleation
process of micron size ferromagnetic domains which follows the magnetization
behavior.
