Structural And Magnetic Properties On Several Perovskite-type Oxides

Perovskite oxides have been receiving special attentions in the community of condensed matter physics because of their fruitful physical properties, such as ferromagnetism, ferroelectricity, Piezoelectricity, colossal magnetoresistances, and so on. In this thesis we have performed extensive studies on several selected perovskite oxides, such as BiFeO, Bi0.9La0.1FeO3, Bi0.8La0.2-xPbxFeO3 (x= 0-0.20). Bi0.8Lao0.2-xBaxFeO3 (x= 0-0.20), La0.67Sr0.15?0.18MnO3-?and investigated the synthesized method, structure, magnetic and electrical properties by proper doping. The major interesting results can be summarized as follows.1. The crystalline structure, magnetic and ferroelectric properties have been studied for the polycrystalline Bi0.8La0.2-xPbxFeO3 (BLPFO, x= 0-0.20) ceramic samples, in which x changes from 0 to 0.2. Rietveld refinements with the X-ray diffraction (XRD) data show that the compound crystal structure changes gradually from pseudotetragonal (x= 0) to pseudocubic (x= 0.20) with increasing Pb concentration. Accompanied with the structural and compositional changes, magnetic ordering of the samples exhibits significantly non-monotonical variation corresponding to x. The compound remnant magnetization (Mr) and coercivity (He) both reach minimum values close to zero at x= 0.07. Unlike magnetic responses, the ferroelectric measurements show that the compounds have monotonical change in the remnant electric dipole polarization (Pr). In order to confirm these above results in co-dopped BiFeO3 ceramics, we prepared Bi0.8La0.2-xBaxFeO3 (BLBFO, x= 0-0.20) ceramics and made similar studies on the structural and magnetic properties of BLBFO ceramics. Similarly, X-ray diffraction (XRD) spectra show that the compound crystal structure changes gradually from pseudotetragonal (x= 0) to pseudocubic (x= 0.20) with increasing Ba concentration. Although it also shows clearly that the magnetization varies non-monotonically with the increase of x, the remnant magnetization (Mr) decreases fast and reaches minimum values close to zero at x= 0.07. In addition, compared with that of La and Pb co-doing, the value of Mr is much larger when the concentation of Ba is 20%.2. We have investigated the structural, magnetic and electrical properties in three representative nonstoichiometric La0.67Sr0.15?0.18Mn3-?(LSMO) samples, initially prepared by a sol-gel method but sintered with varying treatment conditions. XRD analysis based on Rietveld refinements shows that all these samples have single perovskite phase with a rhombohedral symmetry. Although no distinct change can be observed in structural symmetry upon the treatments, the oxygen content shows significant influences on the magnetic properties of the samples. The corresponding value of TC increases systematically from?280 K (sintered in air) to?303 K (sintered in oxygen) with the increase of oxygen content. The obtained samples exhibit good magnetocaloric effect (MCE), which can be attributed to the high magnetization and its sharp drop near TC as well. A peak value of |?SM(T,H)| exceeding 5 J/kg·K near TC has been observed indicating promising application potentials in room temperature magnetic refrigeration.The electrical properties of LSMO vary with the increase of oxygen content in our samples, producing a decrease of resistance and high TMI·The transport results also show that every sample is metallic because of single magneton scattering in low temperature, on the other hand every sample is an insulator because of small polar hopping model in high temperature. The two transport properties of small polaron and single magneton scattering coexist at the temperature rang of phase transition. 3. Mn ions were tried to dope into LaAlO3 single crystal by thermal diffusion method in order to investigate its magnetism. Two representative samples, i.e., LaAlO3 single crystals surrounding with MnO2 powders annealed at 750?and 1300?respectively, were examined by structural and magnetic characterizations. Ferromagnetism can be observed in both of them, while the magnetization of the latter is much stronger than that of the former at the same temperature. Oxygen vacancies and Mn cations play important roles in the origin of the ferromagnetism, which can be understood within the framework of a bound magnetic polaron model.