| The pervoskite manganese oxide compounds have attracted an intense interest in recent years since the GMR effect, which have strong potention technological application, found in La0.67Ca0.33MnO3thin film. Many works were carried out about the properties of magnetic and electrical transport, and the interaction among spin, orbital, lattice and electron for this system. It was found that the Seebeck coefficient of CaMnO3is very high at room temperature (about350μV/K), so, it can be regarded as the candidate for the n-type oxide thermoelectric material. In this dissertation, first, we investigated the thermoelectric performance of Yb0.1Ca0.9Mn1-xNbxO3at high temperature; second, we studied the magnetic behavior of YxCa1-xMnO3by ESR between140K and470K; third, we studied the influence of the nanoparticle size to the magnetic behavior of Y0.1Ca0.9MnO3; finally, we investigate the EB effect of the Ln0.2Ca0.8MnO3(Ln=Rare earth). Details are as follows:In chapter one, we briefly introduce the structure of crystalline and electrical, the concepts of double exchange and superexchange interaction, the magnetic structure and CMR effect, and so on. Besides, the properties and developments of electron-doped CaMnO3are presented particularly.In chapter two, the thermoelectric performance of Yb0.1Ca0.9MnO3doped with Nb5+at B-site is investigated in this chapter. It is found that there is a phase transition from O-type to O*-type orthorhombic structure with increasing of Nb doping content, which indicates that the structure distortion becomes more seriously. Since the electron-phonon interaction can be enhanced by the structure distortion, the small polaron formation is promoted in Yb0.1Ca0.9Mn1-xNbxO3with increasing Nb content. In the whole measured temperature range, the electrical conductivity can be fitted very well by the adiabatic small polaron hopping model. The activated energy Ea is ascending with increasing Nb content. The temperature dependence of Seebeck coefficient S of Yb1.1Ca0.9Mn1-xNbxO3shows that the S is basically inversive to the charge carrier concentration. S(T) can be fitted well by Cutler and Mott model which indicates that the density of state around the Fermi level is strongly affected by Nb-doping at B-site. It is contrary to those of CaMnO3and RE0.1Ca0.9MnO3, when Nb content x>0.05, the|S|and σ show a same tendency of the temperature dependence. In chapter three, the magnetic property of YxCa1-xMnO3was investigated by electron-spin resonance (ESR) under the temperature from140K to470K. It is found that with temperature increasing, the ESR linewidth reaches the maximum at Tmax (=200K,220K and270K for x=0.08,0.10and0.12, respectively) and then decreases in the temperature range from140K to470K for x≥0.08, while it decreases with temperature increasing in the whole temperature range for x=0.05. The spin-lattice relaxation plays the main role for x≥0.08at the temperature range from140K to Tmax in which the ESR linewidth can be described very well by the adiabatic small polaron hopping model. The ESR line intensity decreases with temperature increasing for all the samples between140K and470K except300K≤T≤330K in which it increases with temperature increasing. Meanwhile, the g-factor reaches the minimum value at about T=300K, and then increases with temperature increasing for all the samples between140K and470K. The behaviors of the ESR linewidth and intensity versus temperature indicate the appearance of the short range magnetic correlations. In addition, the electron transport properties of YxCa1-xMnO3reveal a metal-insulator transition at TMI which varies from254K to293K with Y3+content increasing. The resistivity versus temperature between140K and TMI can be fitted well by the adiabatic small polaron hopping model. It is suggested that both the metal-insulator transition of YxCa1-xMnO3and the change of magnetic interactions are closely related with the small polaron collapses at high temperature.In chapter four, the Griffith phase of YxCa1-xMnO3was investigated. It can be found that the1/χ-T curve follows the Curie-Weiss law at high-temperature, but exhibits a Griffith phaselike downturn below a certain temperature. The onset of this downturn is denoted as TG below which the ferromagnetic clusters emerge in the paramagnetism matrix, as is described in a Griffith phase system. The downturn tendency can be depressed by doped content or magnetic field. And the1/χ-T curve can be well described by the theory of Griffith phase between Tc and TG, The appearance of Griffith phase in this system even doped by2%Y3+ions, indicates furtherly that the carriers in this system play a important roles.In chapter five, different size nanoparticles of Y0.1Ca0.9MnO3were prepared successfully by sol-gel method. The size dependence of the magnetic was investigated. It is found that the magnetization decreases with the nanoparticle size increasing at10K, which is generally contrary to the behaviors of the compound with antiferromagnetic matrix. Since weak ferromagnetic embedded in Y0.1Ca0.9MnO2 antiferromagnetic matrix at low temperature, so, we can understand the abnormal behavior by the core-shell model easily. |
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