Perovskite Manganese Oxide Nanomaterials Preparation And Related Materials Research

The perovskite-type manganties have attracted considerable attention in the past decades because of their rich physics as well as application potential in magnetic sensors and recording materials. Recently, with the development of the nanomaterials, much attention has been paid to the nanoscaled manganites. Some methods, such as hydrothermal method, sol-gel method and pulse laser deposition method, have been developed to prepare manganite nanomaterials. Studies on the nanoscaled manganites revealed that they exhibit a number of novel physical properties such as low field high magnetoresistance, superparamagnetism, and large coercivity and more promising applications as compared to the corresponding bulk compounds. Although the former studies have reached some interesting results, the following issues still need to be further investigated: the traditional sol-gel template method can prepare the multi-components oxide nanowires, but it is limited for fabricating the large scale and small diameter (< 60 nm) multi-component manganite nanowire arrays because the driven forces is only capillarity; the understanding of size effect on the ferromagnetic transition temperature (enhanced or weaken or unchanged) and its origin are far from unity; at present, the intensive studies on the nanoscale manganites are focused on the low doping manganites with ferromagnetic ground state, whereas for the high doping manganites with more rich physics, their nanomaterials have been seldom studied, for example, the size effects on the charge ordering and magnetic properties and their origins are still unclear. This dissertation mainly focuses its attention on above issues and gives a systematic study. The main contents in the dissertation are presented as follows:In chapter 1, the research history of the manganites is briefly introduced: the crystal structure and lattice distortion, the electronic and magnetic phase diagrams of La_1-xCa_xMnO₃ and other systems, the essential physical mechanism such as the double exchange interaction, the Jahn-Teller effect and the superexchange interaction, the charge ordering and its modulation, the preparation of manganite nanomaterials and related physical properties.In chapter 2, the traditional sol-gel template method was improved to fabricate large scale polycrystalline La_0.62Pb_0.38MnO₃ nanowire arrays with a diameter of about 50 nm by combing the vacuumed affusion with sol-gel alumina template. The growth mechanism of the nanowires is also briefly discussed. Magnetic measurement results indicate the nanowire arrays have an obvious shape anisotropy resulting from the large aspect ratio. The ferromagnetic transition temperature T_C of the nanowires is lower than that of the corresponding bulk material due to the reduction of e_g electron bandwidth W.In chapter 3, the particle size effect on the structure and magnetic properties of La_0.6Pb_0.4MnO₃ compounds with average particle diameters varied from 5 to 100 nm prepared by sol-gel method was studied. With decreasing particle size, the Mn-0 bond length increases while the Mn-O-Mn bond angle decreases, implying the increase of rhombohedral distortion. Magnetization measurements indicate that the ferromagnetic transition temperature T_C of La_0.6Pb_0.4MnO₃ decreases with decreasing particle size, which originates from the narrowed bandwidth W weakening the double exchange interaction. At the same time, the magnetic domain structure evolves from multi-domain to single domain and finally a superparamagnetic behavior is detected. The critical single domain size determined by experiment is about 25 nm, and the critical superparamagnetic behavior size obtained by calculation is 5.8 nm. The saturation magnetization at low temperature monotonously decreases with decreasing particle size, while the covercivity exhibits a non-monotonous variation with a maximum at about 25 nm.In chapter 4, single-crystalline La_0.5Ca_0.5MnO₃ nanowires have been successfully prepared by hydrothermal method at 270℃. The result of magnetic measurement indicates that the nanowires have an enhanced T_C compared with that of the corresponding bulk compound. This is related to the increased bandwidth W which enhances the double exchange interaction and favors the delocalization of e_g electrons. In addition, the magnetic and transport measurements indicate that there is no charge ordering transition characterization in La_0.5Ca_0.5MnO₃ nanowires.In chapter 5, the charge ordering (CO) and magnetic properties of La_0.25Ca_0.75MnO₃ compounds with average particle sizes ranging from 40 to 2000 nm have been studied. With decreasing particle size the CO transition gradually shifts to lower temperatures and becomes increasingly wide and weak, meanwhile the ferromagnetic cluster glass state appears and the temperature of the appearance of the ferromagnetic clusters T_C shows a non-monotonous variation. The magnetizations at low temperature show a relative complex change with particle size. The anomalous behaviors can be explained in terms of a core-shell model and the temperature-particle size phase diagram of La_0.25Ca_0.75MnO₃ is established on the basis of the magnetic and structure analysis. In addition, magnetic measurements for La_0.25Ca_0.75MnO₃ nanowires and La_0.35Ca_0.65MnO₃ nanoparticles also confirm the above results in...