Influence of Sm-doping on the structural, magnetic, and electrical properties of La0.8−xSmxSr0.2MnO3 (0 < x < 0.45) manganites

Structural, magnetic, and electrical properties of the La0.8−xSmxSr0.2MnO3 (0 ⩽ x ⩽ 0.45) manganites prepared by a solid-state reaction technique was studied systematically. It was found that with increase in the Sm content, the crystal structure transformed from rhombohedral (x < 0.3 samples) to orthorhombic (x > 0.3 samples). The ac magnetic susceptibility measurements show that all samples undergo a transition from paramagnetic (PM) to ferromagnetic (FM) phase at the Curie temperature, TC, which decreases from 296 K down to 165 K with increase in the Sm doping level from x = 0 to x = 0.45. In addition, the glassy state exists in the x = 0.15-0.45 samples, which is stronger in higher doped compounds (x = 0.30 and x = 0.45). This behavior indicates that the substitution of Sm weakens the double exchange (DE) process. The field dependence of magnetization for the samples shows a soft FM nature with a small hysteresis loop and a low coercive field, Hc, for the doped samples. The irreversibility in the magnetization for increasing and decreasing the applied field is due to the glassy behavior observed in highly doped samples. The temperature dependence of resistivity, ρ(T), measurement indicates that by increasing the Sm doping level, the metal-insulator transition temperature decreases, and the heavily doped samples become insulators. The metallic region of the ρ(T) curve for the x = 0-0.10 samples was fitted with the model of electron-electron and electron-magnon scattering, while the insulating region was fitted with the small polaron hopping (SPH) at T > θD/2 (θD, Debye temperature) and the variable range hopping (VRH) models at T < θD/2.