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 $T_capprox$ 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.
تحميل البحث