Studies Of Microstructures And Electromagnetic Transport Properties In The Ca-Doped Manganites

The La1-xCaxMnO3ceramics were prepared by the solid-state reaction method. The microstructures of the samples were characterized by XRD, FE-SEM, positron annihilation techniques, and the electromagnetic transport properties of the samples were tested by the PPMS. The influences of different process conditions (sintering temperature, holding time, pressure) on the physical properties of the La0.67Ca0.33MnO3ceramic, the formation mechanism of electromagnetic transport properties of the La1-xCaxMnO3samples, and the effects of Y3+、Cu2+and Fe3+doping on the electromagnetic properties of the La1-xCaxMnO3samples were investigated. The main experimental results are as follows:(1) The La1-xCaxMnO3sample shows a perovskite structure. On increasing the Ca concentration in the La1-xCaxMnO3samples, the b-axis and the cell volume decrease.(2)The ferromagnetic-paramagnetic transiton and metal-insulator transition take place in La1-xCaxMnO3sample, and its Curie temperature Tc is almost the same as its metal-insulator transition temperature TMI. (3) For La1-xCaxMnO3samples prepared under the pressure of30MPa, sintered at1400℃for20h, the double exchange interaction, the magnetization, Tc and TM1of La0.67Ca0.33MnO3reach maximum values, while the resistivity of it reaches a minimum value.(4) For the La1-xCaxMnO3sample, when T higher than Tc (paramagnetic insulator region), the conduct electrons of the samples are controlled by the mechanism of the carrier thermal activation (ρ=ρ0exp(-Eα/(kBT))); while T lower than Tc (ferromagnetic metal region), the conduct electrons of the samples are controlled by the compete of three kinds of scattering mechanisms (ρ=ρα+ρ2.5T2.5).(5) The perovskite structure of the Ca-doped maganites remains unchanged after being doped by small amount of Y3+、Cu2+and Fe3+(6) For the La0.67-x Yx Ca0.33MnO3samples, the addition of Y3+into the sample gives rise to the decrease of the ferromagnetic and the magnetization of the sample, and the Tc, TM1of the sample move toward low temperature. Under the applied magnetic field of1T, the maximum magneticresistances of La0.67Ca0.33MnO3, La0.60Y0.07Ca0.33MnO3and La0.57Y0.10Ca0.33MnO3are30.92%,133.25%and229.19%, respectively.(7) For the La0.67Ca0.33Mn1-xCuxO3samples, the addition of Cu2+into the sample leads to the increase of the cell volume due to the radius of Cu2+larger than that of Mn+3. On increasing the content of Cu in the sample, the electron density in the bulk of the sample increases, while the electron density of the defect decreases, the value of Tc decreases from244K to42K, the magnization and the ferromagnetic of the samples reduce sharply.(8) For the La0.67Ca0.33Mn1-xFexO3samples, the addition of Fe into the sample, not only reduces the magnetization of the sample, but also leads to the transition temperature Tc of the sample moving toward lower temperature. The Tc of La0.67Ca0.33MnO3, La0.67Ca0.33Mn0.95Fe0.05O3, La0.67Ca0.33Mn0.9Fe0.1O3and La0.67Ca0.33Mno.85Fe0.15O3are244K,132K,117K and99K, respectively. With the increasing of Fe content, the low-temperature magnetization and the magnitude of the magnetic irreversibility (at temperature lower than Tc) of the sample increase. At low temperatures, the spin glass state exists in these samples.