The Structure And Magnetic Properties Of Double Perovskite Oxide System

The study of the synthetication and performance of the A2B'B"O6 type double peroviskite has been reported since 20th century 50's.At the end of the 90's, a series of special character such as non-conventional high temperature superconductivity and the room temperature tunneling magnetoresistance effect in this system had been discovered, thus caused the research of double perovskite oxide compound system got high attention both in experiment and the theory, and became an extremely active role of front research area in condensed matter physics.In 1997 Maokun Wu has reported a non-cuprate-planes Sr₂YRu_1-xCu_xO₆ (Tc up to 30K), which belongs to high temperature superconductor and has some interesting magnetism properties such as magnetic moments ordered at low temperature and Spin-glass behavior;In 1998 Kobayashi etc. discovered the tunneling magnetoresistance effect in Sr2FeMoO at room temperature (300K), which was as high as 10%. A2B'B"O6 double perovskite material was considered as one of the most applied magnetic resistance materials immediately. In theory, cause B' and B" site ions have varied electron configuration; these oxides can have more abound reforming unit, which provide theoreticians abundant model building space. In one word, the probe into the electric and magnetic property of this double peroviskite is very attractive and has extreme values.By comparing the double perovskite structure oxide with the simple perovskite structure oxide in this thesis, we first reviewed the research achievements of A2B'B"O6 and new progress; then we synthesized Ba₂YRu_1-xCu_xO₆, YBa₂Cu₃O₇-δand Ba₂NiMoO₆ with solid-state reaction method. Then we chiefly studied their structure and magnetic propertied of the Ba₂YRu_1-xCu_xO₆ and YBa₂Cu₃O₇-δt,he magnetic properties and structure of the Ba₂NiMoO₆ Main results of my work are shown as following:First, YBa₂Cu₃O₇-δsystems with perovskite structure have been synthesized by solid-state reaction method. The structure and the surface morphology of the sample are investigated by x-ray diffraction and scanning electron microscope. The result of YBa₂Cu₃O₇-δsample XRD shows that single-phase has already formed with the theory simulation polytropism sample XRD The sample belongs to the rhombic system, the crystal space group is Pmmm (a=3.817 (?), b=3.883 (?), c=11.633 (?)). The result of YBa₂Cu₃O₇-δsample XRD under the oxygen atmosphere annealing shows cell parameter change little. The resistance of the sample has been measured by traditional four-probe technique. The experimental result indicates that the oxygen content of the sample plays a crucial role to the superconductivity transition temperature. Finally the magnetic susceptibility with changed temperature curve of sample is measured by using America Quanta Design Content Measure System(PPMS). The experimental result indicates that the sample has Meissner effect and enters perfect absoluteness dimagnetism macrography quanta state step by step under superconductivity transition temperature. The electrical resistivity of the sample has been measured by standard four-probe technique in a temperature range from 10 K to 300 K. Ba2YRuO6 is found to exhibit the typical electrical transport behavior of an insulator with a large resistance; Cu doping considerably lower the resistance of the sample ,a transition from an insulator to semiconductor caused by Cu doping was observed in Ba₂YRu_1-xCu_xO₆ (x=0, 0.1, 0.15) double perovskite system.Second, Polycrystalline double perovskite Ba₂NiMoO₆ have been synthesized by solid-state reaction method. The structure and the surface morphology of the sample are investigated by x-ray diffraction and scanning electron microscope.X-ray diffraction (XRD) and subsequent Rietveld refinement confirm that Ba₂NiMoO₆ is of ordered double perovskite structure with a cubic lattice cell with a=b=c=8.045(?), where Ni and Mo ions occupy the B site of double perovskite cell oderly. And the Bragg reflection (111) and(311) was identified to be a characteristic feature. The magnetization curve at room temperature has been measured by Vibrating Sample Magnetometer(VSM) and Electron Spin Resonance(ESR). The results indicate that the sample exhibits the paramagnetic behavior.The remanent polarization (2Pr) increases by La-doping .