Structural And Magnetic Evolution Of A-cationic Doping In Double Perovskite Sr₂FeMoO₆

In recent years,scientists have done a lot of research on the structure and magnetic and electrical transport properties of the double perovskite oxide materials,and made a great progress onmaterials function.the double perovskite oxides have been widely used in the manufacture of magnetoresistance materials,It is worth noting that the double perovskite oxide has a significant effect of tunnellingmagnetoresistance effect,making this material one of the magnetoresistance materials most likely to be used at room temperature.However,there are still a lot of problems which attract us to do further study.In order to further study the double perovskite oxide,it is necessary to improve the preparation process of the material,So as to solve some problems of the double perovskite oxide material.Since the Sr₂FeMoO₆ is one of the typical double perovskite oxides A₂BB'O₆,the crystal structure,magnetic properties and electrical transport structure of the Sr₂FeMoO₆ have been studied and discussed by the combination of theory and experiment in this thesis.The crystal structure and properties of the Sr₂FeMoO₆ was studied by improving the preparation process of the material.The Cation doping on A site can effectively modulate crystal structure and properties,the structure and physical prope rties of the obtained samples were measured.1)The double solute oxide Sr₂FeMoO₆ was synthesized by the sol-gel method.One of the samples was calcined and then quench-treatment,and the other samples were automatically cooled.The lattice structure and the magnetic properties of the two samples were measured.X-ray powder diffraction analysis reveals that the sample has a single phase and exhibits I4/m symmetry.The cation order ? of the sample increases to 98.9?2?% from 94.2?3?%,which is prepared by traditional sol–gel method.The initial magnetisation isotherm of the sample is detected at 300 K.Unit-cell magnetisation for the current sample is 1.332 ?B at 300 K,and that for the traditional sol–gel method sample is 0.946 ?B.Unit-cell magnetisation is enhanced to 40.80% by quench-treatment technique.Quench treatment is an effective method of enhancing the Fe/Mo order and magnetic properties of double perovskite SFMO.2)The double perovskite compounds (Sr_2-3x Gd_xBa_2x)FeMoO₆?x=0.00,0.05,0.10,0.20 and 0.25?were prepared by standard solid-state reaction.samples was sintered in a flow of 5% H₂/Ar atmosphere.The(Sr_2-3xGd_xBa_2x)FeMoO₆ samples was studied by X-ray diffraction.All samples were single phase.Order degree of Fe/Mo decreased from 92.07% to 84.99%.Gd and Ba co-doping on Sr site had a similar effect mechanism on the distortion size of MoO₆ and FeO₆ octahedral?The magnetic and electrical transport properties of these samples were measured by a vibrating sample magnetometer.TC decreased from 394 K to 331 K for samples x=0.00 to x=0.25.The saturation magnetization decreases with the increase of the doping amount x in the range of 1.68 ?B/unit cell to 2.52 ?B/unit cell at 300 K.All samples exhibited ferromagnetic characteristics by detection of the hysteresis loops of(Sr_2–3xGd_xBa_2x)FeMoO₆ at 5 K.Magnetic measurements showed that all samples underwent paramagnetic?PM?to ferromagnetic?FM?phase transition near the Curie temperature?Tc?.Antisite defect could be the main reason for Tc reduction?X-ray photoelectron spectroscopy results show the effects of Gd and Ba co-doping on the proportion of Fe³⁺/Fe²⁺ and Mo⁵⁺/Mo⁶⁺.The zero-field-cooling and field-cooling processes are found to be irreversible,which is a typical cluster glass behavior.Isothermal magnetization curves versus magnetic field were compared at around the Curie temperature.The M²–H/M curve analyses indicate a second-order magnetic transition.At x?0.15,the samples with show a semi-conducting behavior from 50 K to 300 K.However,at x=0.20 and x=0.25,the samples exhibit a semi-conducting to metallic transition at TS-M=175 K and 115 K,respectively.