  We investigate the ultra-sharp jump in the isothermal magnetization and the
resistivity in the polycrystalline $Sm_{0.5}(Ca_{0.5-y}Sr_{y})MnO_3$ $(y = 0,
0.1, 0.2, 0.25, 0.3, 0.5)$ compounds. The critical field $(H_{cr})$, required
for the ultra-sharp jump, decreases with increase of `Sr' concentration, i.e.
with increase of average A-site ionic radius $\langle r_A\rangle$. The
magnetotransport data indicate that the phase separation increases with the
increase of $\langle r_A\rangle$, i.e. with $y$. The dependency of $H_{cr}$
with magnetic field sweep rate reveals that the ultra-sharp jump from
antiferromagnetic (AFM) state to the ferromagnetic (FM) state is of martensitic
in nature. Our two-band double exchange model Hamiltonian calculations show
that the `Sr' doping induces the ferromagnetic clusters in the
antiferromagnetic insulating phase and in turn reduces the critical field. In
the end we present a phenomenological picture obtained from our combined
experimental and theoretical study.
