  The effects of magnetic field on the martensitic transition have been studied
in Ni45Co5Mn38Sb12. We find a large field-induced irreversibility in this
system, as revealed by the field dependence of resistivity, magnetization, and
heat capacity data. At the critical temperature, the field-induced conversion
of the martensitic to austenite phase is not reversible under any field
variation. At this temperature any energy fluctuation induces nucleation and
growth of the equilibrium austenite phase at the expense of the metastable
martensitic phase and gets arrested. All these three measurements completely
rule out the coexistence of austenite and martensitic phases in the
irreversibility regime.
