| The magnetoresistance materials Sr2FeMoO6 with double perovskite structure and series of A site doped compounds were successfully synthesized by Sol-Gel method and microwave method. X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM) were used to analyze and characterize the phase structure, the size and morphology of particles, magnetic and electric properties of the samples.First, with cheap activated carbon for reducing agent, the sol-gel method was used to prepare the giant magnetoresistance materials Sr2FeMoO6, which is the double perovskite oxide. The influence of process conditions on the structure and magnetic properties of the samples was investigated in detail and the appropriate conditions were determined. The as-synthesized sample is Sr2FeMoO6 with tetragonal crystal structure and I4/mmm space group, and unit cell parameter is a = 5.580?, c = 7.882?. The grain size is below 100nm. The Curie temperature is above room temperature and the saturation magnetization of the sample is 13.59A·m2/kg under 1T at room temperature. The room temperature magnetoresistance of the sample under 1T field is up to -10.02%. The sample exhibit typical semiconductor behavior and the conductive mechanism can be described by small polaron variable range hopping model.Then, the series of Sr2-xAxFeMoO6 (A = Li+, Na+, K+ or Gd3+) were synthesized by sol-gel method, in order to investigate the influence of A site substitution on structure and magnetic properties of the samples. Because the contribution of the steric effect and electron-doping effect are different in different systems, the unit cell parameters of Li-, Na-, K-doped samples are less than those of undoped sample, while the unit cell parameters of Gd-doped samples are more than those of undoped sample. With the increase of doping amount, the saturation magnetization of Li-, Na- or K-doped samples increases firstly and then decrease, while that of Gd-doped samples decreases with the increase of doping amount. Meanwhile, the primary mechanism of the synthesis of Sr1.9K0.1FeMoO6 by sol-gel method is proposed according to the analysis of TG-DTA, XRD, infra-red spectrum and X-ray energy disperse spectrum.Finally, a new process named microwave sintering method is explored to prepare double perovskite Sr2FeMoO6 with MnO2 for microwave absorbent and activated carbon for reducing agent. Compared with the traditional solid state reaction and sol-gel technique, this method has many advantages of short reaction time, saving energy and high efficiency. The as-synthesized sample has tetragonal crystal structure and I4/mmm space group, and unit cell parameter is a = 5.571?, c = 7.872?. The investigation of magnetic and electric properties shows that microwave sintering method is favor to improve the saturation magnetization and room temperature magnetoresistance of the sample. The Curie temperature is above room temperature and the saturation magnetization of the sample is17.949 A·m2/kg under 1T at room temperature. The room temperature magnetoresistance of the sample under 1T field is up to -15.63%, which is larger than that of the sample prepared by sol-gel technique. The sample exhibit typical semiconductor behavior and the conductive mechanism can be described by adiabatic small polaron model. According to XRD and infra-red spectra analysis, the reaction mechanism is proposed, as follows: firstly, under the radiation of microwave, the reaction of SrCO3,Fe2O3 and MoO3 produces the precursor Sr2Fe2O5 and SrMoO4; then, the precursor is reduced by activated carbon in order to facilitate the formation of Fe-O-Mo bond, i.e., the generation of final product Sr2FeMoO6.
|
PDF Full Text Request