Electrical And Magnetic Properties Of La_1-x-yCa_xK_y MnO₃ Compounds

Because of the complex interplay and competition among charge, spin, orbital, and lattice degrees, perovskite manganites exhibit fascinating physical properties and phenomena, such as colossal magnetoresistance (CMR) effect, metal-insulator transition, the complicated magnetic behavior and charge/orbital ordering, phase separation, etc. Up to now, they have attracted enormous interest worldwide in condensed physics and material science.LaMnO3, the parent compound of the perovskite manganites, is an antiferromagnetic insulator with T_N = 140K and has an distorted orthorhombic structure with a Pbnm space group. Doping the La site with different metal ion (such as alkali metals or alkali earth metals with different valence) results Mn ion with mixed valence coexisting in one compound. Due to the double exchange theory and the Jahn-Teller effect, the coexistence of Mn ion with mixed valence can dramatically change the interaction of charge, spin, orbital, and lattice in the compounds and then supply them with enormous and potential application in magnetoelectronics. These physical properties have great relations with the structure of the compounds. Conventional solid state reaction is the most common method in the preparation of these compounds. The doping element have been alkali metals, alkali earth metals, Ag, Hg, and etc.. The doping compounds crystallize in a distorted rhombohedral or orthorhombic perovskite structure.For the compounds with Ca²⁺ in La site, when 0.2 < x< 0.5, the compounds La1-xCaxMnO3 crystallizes in the orthorhombic system with a Pbum space group and exhibit colossal magnetoresistance effect around T_C, corresponding to a transition from the paramagnetic insulating (PMI) phase to the ferromagnetic metallic (FMM) phase upon cooling. When 0.5 < x< 0.8, the compound exhibit charge-ordering state above T_C instead of paramagnetic-insulated state, and then change to antiferromagnetic state with the decreasing temperature. It is because partial substitution of Ca²⁺ for La³⁺ make the conversion of Mn³⁺ ( t₂g³g_e¹ ) into Mn4+ ( t₂g³ e_g⁰) which induces the double exchange interaction between the Mn³⁺ and Mn4+ through O₂^-, Jahn-Teller effect and intrinsic inhomogeneities phase separation. Up to now, the research in the physical properties and phenomena of doping A-site with different divalent ions have been widely investigated, but work on substituting A-site with monovalent alkali elements and divalent alkaline-earth elements at the same time was not so much reported. Substitution by monovalent ion causes to inject as two times holes as divalent substituent do in to the system, hence, leads to more interesting changes in both structural and physical properties. Taking into account of this reason, we make research on the electrical and magnetic properties of perovskite manganites, using various characterization methods in this thesis.1. The temperature dependence of resistivity of the samples have been measured by standard four-probe method using a PPMS. The measurement was performed in zero field, and an applied field of 2T and 5T between 4 and 350K. With the decreasing temperature, the resistivities of the compound exhibit a peak value at T_P, corresponding to the transition from semiconduting to metallic conducting. The resistivities of the samples decreases with applying magnetic field and show negative magnetoresistance, which is in inverse proportion to the temperature. In order to understand the nature of the transport mechanism in the whole measured temperature, we attempt to fit theρ-T curves both in the high temperature ( T > T_P) and low temperature (T < T_P) phases according to any possible models. The results reveal that the semiconducting behavior can be well described by VRH model, while the resistivty in low temperature is governed by the electron-electron scattering process.2. The magnetic measurements were made using a SQUID magnetometer from 4K to 300K. We get the ZFC/FC curves in different applied field of 100Oe, 500Oe and 1000Oe, and the field dependence of magnetization were obtained at different temperatures. The compounds exhibit complex magnetic phenomena and undergo the paramagnetic to ferromagnetic, then ferromagnetic to antiferromagnetic transitiongs. Taking into account the electrical results, we suppose spin-canted antiferromagnetic state in the compounds.3. From room temperature down to the liquid nitrogen temperature, the capacitance, the loss tangent and the resistance are measured as a function of frequency using LCR magnetometer. The real part of the complex relative dielectric permittivity varies slightly with the fluctuation of temperature, which make it great possible in actual application. We used the Zview software to fit the Nyquist plots and obtained the equivalent circuit in intrinsic compounds.We report for the first time here the detailed study of magnetic and electrical transport properties of the ABO₃-type perovskite manganites La_1-x-yCa_xK_yMnO₃ with cubic shape. The so-obtained compounds exhibit different magnetic properties from those reported. Structure controls the properties and the structure was determined by the synthesis method. The intension of our work is to determine the suitable approach through the understanding of the relationship between the structure and properties.