Structure, magnetic, and transport properties of Ti-substituted La_0.7Sr_0.3MnO₃

Abstract

Ti-substituted perovskites La_0.7Sr_0.3Mn_1-xTi_xO₃ with 0≤x≤0.20, were investigated by neutron diffraction, magnetization, electric resistivity, and magnetoresistance (MR) measurements. All samples show a rhombohedral structure (space group R3^-c) from 10 K to room temperature. At room temperature, the cell parameters a,c and the unit cell volume increase with increasing Ti content. However, at 10 K, the cell parameter a has a maximum value for x=0.10, and decreases for x>0.10, while the unit cell volume remains nearly constant for x>0.10. The average (Mn,Ti)-O bond length increases up to x=0.15, and the (Mn,Ti)-O-(Mn,Ti) bond angle decreases with increasing Ti content to its minimum value at x=0.15 at room temperature. Below the Curie temperature T_C, the resistance exhibits metallic behavior for the x≤0.05 samples. A metal (semiconductor) to insulator transition is observed for the x≥0.10 samples. A peak in resistivity appears below T_C for all samples, and shifts to a lower temperature as x increases. The substitution of Mn by Ti decreases the 2p-3d hybridization between O and Mn ions, reduces the bandwidth W, and increases the electron-phonon coupling. Therefore, the T_C shifts to a lower temperature and the resistivity increases with increasing Ti content. A field-induced shift of the resistivity maximum occurs at x≤0.10. The separation of T_C and the resistivity maximum temperature T_ρ,max enhances the MR effect in these compounds due to the weak coupling between the magnetic ordering and the resistivity as compared with La_0.7Sr_0.3MnO₃.