Effect of B-site Ti doping on the magnetic, low field magnetocaloric and electrical transport properties of La0.7Sr0.3Mn1-xTixO3 perovskites

We report the effect of Ti doping on the magnetic, magnetocaloric and transport properties of La0.7Sr0.3Mn1-xTixO3 (0 <= x <= 0.3) polycrystalline compounds. Magnetic studies indicate that Ti doping weakens the double exchange ferromagnetic-paramagnetic transition and lowers the paramagnetic-ferromagnetic transition temperature. Also the results show that by increasing Ti doping level, the metal-insulator transition temperature decreases and the system with x >= 0.2 becomes an insulator. The magnetocaloric effect of 0 <= x <= 0.1 samples was studied using phenomenological method. With the increase in Ti-doping level, the Delta S-max/Delta H x 10(2) value decreases from 16.2 J/kg K kOe (x = 0.0) to 4.2 J/kg K kOe (x = 0.05). However, the peak of the magnetic entropy change versus temperature plot for the sample with x = 0.05 broadens significantly. These characteristics, together with the critical transition temperature being near room temperature, indicate that this sample is promising as an efficient magnetic refrigerant at around room temperature. Magneto-resistance MR% data shows that its peak value increases with Ti-doping and MR reach highest value at x = 0.1. Moreover, the isothermal field dependent magnetoresistance in the temperatures range from 20 K to 150 K has been analyzed using a model based on spin polarized tunneling at the grain boundaries and it shows that there is a good agreement with our experimental data. The samples with 0 <= x <= 0.1 exhibit metallic behavior, which fits well to the resistivity rho = rho(0) + rho T-2(2) + rho T-4.5(4.5), implying that combination of electron-electron, electron-magnon, and electron-phonon scattering processes contribute to the electrical resistivity in these samples. The electrical resistivity data in the insulating region of other samples (x >= 0.1) is well described using small polaron hopping and Mott's variable range hopping models. The non-adiabatic hopping conduction mechanism seems to be the most appropriate model explaining the conduction mechanism in La0.7Sr0.3Mn1-xTixO3 (0.1 <= x <= 0.3). (C) 2015 Elsevier B.V. All rights reserved.