  We have investigated the effect of Ti doping on the transport properties
coupled with the magnetic ones in
Sm$_{0.55}$Sr$_{0.45}$Mn$_{1-\eta}$Ti$_{\eta}$O$_3$ ($0 \leq \eta \leq 0.04$).
The parent compound, Sm$_{0.55}$Sr$_{0.45}$MnO$_3$, exhibits a first-order
paramagnetic-insulator to ferromagnetic-metal transition just below $T_{\rm c}$
= 128 K. With substitution of Ti at Mn sites ($B$-site), $T_{\rm c}$ decreases
approximately linearly at the rate of 22 K$\%^{-1}$ while the width of thermal
hysteresis in magnetization and resistivity increases almost in an exponential
fashion. The most spectacular effect has been observed for the composition
$\eta$=0.03, where a magnetic field of only 1 T yields a huge
magnetoresistance, $1.2 \times 10^7$ $\%$ at $T_c\approx$ 63 K. With increasing
magnetic field, the transition shifts towards higher temperature, and the
first-order nature of the transition gets weakened and eventually becomes
crossover above a critical field ($H_{cr}$) which increases with Ti doping. For
Ti doping above 0.03, the system remains insulting without any ferromagnetic
ordering down to 2 K. The Monte-Carlo calculations based on a two-band double
exchange model show that the decrease of $T_{\rm c}$ with Ti doping is
associated with the increase of the lattice distortions around the doped Ti
ions.
