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

Abstract

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 ΔSmax/ΔH×102 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 ρ=ρ0+ρ2T2+ρ4.5T4.5, 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).