Manganese Doping Of La <sub> 0.64 </ Sub> Of (sr / Ca) Of Mno <sub> <sub> 0.33 </ Sub> </ Sub> Material Structure And Electrical Transport Mechanism

Since the discovery of colossal magnetoresistance (CMR) in perovskite manganese oxides RE_1-xAE_xMnO₃ (where RE is a trivalent rare-earth element such as La,Pr,Nd,Sm or Bi, and AE is a divalent metal element such as Ca, Sr, Ba, or Pb), much theoretical and experimental work has been done to investigate the physical mechanism of the CMR effect because of its physical interest and application potential in aspect of the magnetic sensor, the read-out magnetism, as well as the stochastic memory and so on. The colossal magneto-resistance appears at temperature close to the Curie temperature, and resistance will reduce with the temperature being increased or decreased quickly.In previous work, large resistance changes are achieved only in a strong magnetic field of Tesla range and in a narrow temperature range around Curie temperature, severely limiting their practical applications. Thus, reducing the field scale and increasing the operation temperature are the key points to be studied widely. One way to promote CMR effect at room temperature is to choose the parent materials with high Curie temperature, by tuning the composition of these compounds, e.g, doping ions into the AE or B (Mn) sites with different ionic sizes as well as adjusting the appropriate dopant amount, the CMR effect can be improved a lot under feild of low Tesla range and room temperature. La_1-xAE_xMnO₃ (AE=Sr,Ca,Ba) materials have the Curie temperature near room temperature, which promotes the research and development of this material systems, and in recent years becomes a very interested area in condensed matter physics and materials science.The main purpose of this paper, we use La-Vacancy samples of La_0.64(Sr/Ca)_0.33MnO₃ as the parent materials, and the substitution with Ti on Mn sites, a series of La_0.64Ca_0.33Mn_1-xTi_xO₃ and La_0.64Ca_0.33Mn_1-xTi_xO₃ (x=0.0, 0.05, 0.1, 0.15, 0.2, 0.3) polycrystalline ceramic samples were prepared by solid-state-reaction method. Then the microstructure, surface morphology and electrical properties have been investigated. The main works of this whole thesis are as follows:Chapter one and two present a review of the history and progress of magneto-resistance effect, and give the definition of magneto-resistance. It is mainly about the physical properties such as crystal structure, electronic structure and some of the ordered phases, and theoretic explanation of perovskite oxide which include double-exchange model, Jahn-Teller distortion effect. Chapter three introduces the conventional solid-state-reaction method, and relevant measurement principles and methods. Chapter four and five are mainly about the specific preparation of samples, some important results obtaind by X-ray diffraction (XRD) using Cu-Kαradition and microscope at room temperature. XRD paterns show a single phase with perovskite structure for all the samples. A series of La_0.64Ca_0.33Mn_1-xTi_xO₃ (x=0.0, 0.05, 0.1, 0.15, 0.2, 0.3) samples are orthorhombic symmetry, and space group Pnma. A series of La_0.64Ca_0.33Mn_1-xTi_xO₃ (x=0.0, 0.05, 0.1, 0.15, 0.2, 0.3) samples show crystal structure changes from orthogonal to rhombohedral symmwtry, and space group^R(?)c.phe electrical propertie of the samples were measured in zero magnetic fields between 80K- 350K. The insulator-metal transition temperature (about 160K) of La_0.64Ca_0.33MnO₃ sample is lower than La_0.67Ca_0.33MnO₃ samples, which indicates La-vacancy destroy the double exchange interaction. From the Resistance-Temperature chart of La_0.64Sr_0.33Mn_1-xTi_xO₃ samples, with increasing TiO₂ doping level, the insulator-metal transition temperature T_p decreases. We find that a transition temperature T_p, from metal to insulator exists obviously for Ti doping level with x=0.05, 0.1, corresponding T_p=250K, 160K. Substituted 0.05 Ti for the sample, we guess a new transition point happend at 340K.