| The study of the synthetication and performance of the A2BB'O6 type double perovskite 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 Sr2YRu1-xCuxO6 (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 Sr2FeMoO6 at room temperature (300K), which was as high as 10%. A2BB'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 perovskite is very attractive and has extreme values.We first reviewed the research achievements of A2BB'O6 and new progress; then we synthesized Sr2YRu1-xZnxO6,Sr2FeRuO6,and Sr2FeMnO6 with solid-state reaction method. Then we chiefly studied their structure and magnetic propertied of the Sr2YRu1-xZnxO6 and the magnetic properties and magnetoresistance effect of the Sr2FeRuO6 and Sr2FeMnO6. Main results of my work are shown as following: First, Zn-doping Sr2YRu1-xZnxO6 (x=0,0.1,0.2) system with peroviskite 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.Sr2YRuO6 is found to be of a cubic lattice cell with a=b=c=8.164?,where Y and Ru ions occupy the B site of double perovskite cell in orderly way. Its single-phase characteristic can be maintained up to Zn-doping content x=0.2.The electrical resistivity of the sample has been measured by traditional four-probe technique.Sr2YRuO6 is found to exhibit the typical electrical transport behavior of an antiferromagnetic insulator as reported. As a whole, a transition from a paramagnetic insulator to a paramagnetic semiconductor caused by Zn doping was observed in Sr2YRu1-xZnxO6 (x=0,0.1,0.2) double perovskite system.Second, Polycrystalline double perovskite Sr2FeRuO6 has been synthesized by solid-state reaction method. X-ray diffraction (XRD) and subsequent Rietveld refinement confirm that Sr2FeRuO6 is of double perovskite structure with a triclinic cell (a=5.508 ? ,b=5.474 ?, c=7.861 ?,β=90.127) and space group I2/c, where Fe and Ru randomly occupy the center of oxygen octahedron (B-sites). The magnetization curve at 300 K is measured by vibrating sample magnetometer (VSM) up to 9 T and reveals a very small magnetic susceptibility (χ=2.99 1 0-6) and an effective magnetic moment Peff=0.5397μB, which implies that Sr2FeRuO6 is likely antiferromagnetic with its Neel temperature far above room temperature. The temperature dependences of electrical resistivity are measured by standard four-probe DC technique under 0 T and 7 T respectively, which indicate that Sr2FeRuO6 exhibits typical semiconductor behavior with an estimated energy gap Eg~0.23 eV. Under 7T and 115 K, the magnetoresistance (MR) is estimated to be as large as ~25%.Third, Sr2FeMnO6 has been synthesized and identified to be an ordered double perovskite compound, which belongs to a cubic system (a=7.726 ?) with a space group Fm3m (No. 225). From ESR measurements, an antiferromagnetic to paramagnetic phase transition for Sr2FeMnO6 was discovered for the first time, and the Neel temperature was determined to be TN ~475K. In paramagnetic phase above TN, the value of gyromagnetic factor g was found to be very close to 2, which indicates that the Fe ions are in trivalent state with a electronic configuration ( t2g3eg2 for 3d5:S = 5/2, L=0, J=5/2, g=2) splitted by the crystal field of oxygen octahedron and the Mn ions are in pentavalent state (t2g2 eg0 for 3d2: S = 1, L=3, J=2, g=2/3). Sr2FeMnO6 and Sr2FeMoO6 are not only of similar ordered double perovskite structure, but also of similar magnetic behavior. Both of them exhibit a magnetic transition far above room temperature. The temperature dependences of electrical resistance are measured by standard four-probe DC technique under 0 T and 1T respectively, which indicate that Sr2FeMnO6 exhibits typical semiconductor behavior. Under 1T and 200 K, the magnetoresistance (MR) is estimated to be as large as ~4.5%. |
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