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Three factors control the transition from paramagnetic insulator to ferromagnetic metal. The first is the hole doping. The second factor is the average ionic radius of the A site cation r_A. The last one concerns the ionic size mismatch σ~2 at the A site. In order to study the effect of σ~2, a series of samples were prepared with constant value of x and r_A. The measurement of resistivity versus temperature dependence under magnetic field (0 and 5 T) indicates that the insulator-metal transition temperature moves towards the low temperature region with increasing σ~2. The magnetoresistance of La_ 0.53 Sm_ 0.17 Sr_ 0.3 MnO_3 obtains its maximum at room temperature. The experimental observation was discussed based on the measurement of the transport properties.
Three factors control the transition from paramagnetic insulator to ferromagnetic metal. The second factor is the average ionic radius of the A site cation r_A. The last one concerns the ionic size mismatch σ ~ 2 at the A site. The order of study the effect of σ ~ 2, a series of samples were prepared with constant value of x and r_A. The measurement of resistivity versus temperature dependence under magnetic field (0 and 5 T) indicates that the insulator-metal transition temperature moves towards the low temperature region with increasing σ ~ 2. The magnetometer of La_ 0.53 Sm_ 0.17 Sr_ 0.3 MnO_3 rendering its maximum at room temperature. The experimental observation was discussed based on the measurement of the transport properties.