  We have investigated the magnetic properties of Bi0.5Ca0.5MnO3 under
ultrahigh magnetic fields by magnetization measurements. This compound shows
the metamagnetic transition which indicates the magnetic field induced charge
order melting at the room temperature T = 293 K. We reveled that the
temperature dependence of the critical field, where the highest critical field
is higher than 100 T. At low temperatures, it has been found that the
metamagnetic transitions become broad and vanish in the field ascending process
below 70 K. On the other hand, in the field descending process, a clear
metamagnetic transition is observed even below 70 K. This unusual behavior can
be understood as the gradual charge order melting by magnetic field, and its
simultaneous reformation. A coexistence of multiple order parameters can
account for the observed characteristic dynamics of the first order phase
transition. Finally, we constructed the B-T phase diagram with the unusual
critical feature at ultrahigh magnetic fields.
