Sr <sub> 2 </ Sub> Of Mn <sub> 1-x </ Sub> Ga <sub> The X </ Sub> Of Moo <sub> 6 </ Sub> And Sr <sub> 2 </ Sub> Femoo 6 <sub> </ Sub> / La <sub> 0.67 </ Sub> Of Mno <su

The high degree of spin polarization,high Curie temperature and giant magnetoresistance in low magnetic fields at room temperature have recently been reported in Sr2FeMoO6,Sr2FeReO6with double perovskite structure, which makes them as good candidates for the application of giant magnetoresistance at room temperature. These findings are further attracting considerable interest to the research of ordering double perovskite A2BB'O6 compound. The Sr2FeMoO6 compounds, Sr2Mn1-xGaxMoO6 compounds and Sr2FeMoO6/La0.67Sr0.33MnO3 composites have been prepared by sol-gel method,solid state method and physically mixed processes. Crystal structure,magnetic and electrical-transport properties of these samples have been investigated by XRD, SEM, AFM, PPMS, etc. Conclusions from our research are shown as follows:(a) Polycrystalline samples of Sr2FeMoO6 are synthesized by two different methods:solid state method and sol-gel method. The analysis of XRD indicates that two precusor phases SrMoO4 and SrFeO3-x are acquired by solid state method, and the formation of Sr2FeMoO6 is via the reaction of these two precursor phases; and however by sol-gel method, precusor phases include Sr2FeMoO6, and minor phases SrMoO4 and Sr3MoO6 can be removed after sintering. Through investigating the relation between the sintered temperature and degree of ordering, we found the ratio of intensity of the ordering profile would increase under higher sinterted temperature. In other words, the degree of ordering will be improved. The analysis of structural refinement indicates that both samples are tetragonal, space group isⅠ4/m and the occupancy of cation in the sites is more than 90%. According to SEM, grain size of the sol-gel method-made samples is homogeneous; however, grain size of solid-state method-made sample has large range of distribution.(b) The B-site doped Sr2Mn1-xGaxMoO6(x=0.0-1.0) compounds are prepared by solid state method. A room temperature analysis of high-resolution X-ray powder diffraction patterns indicates that Sr2MnMoO6 compound crystallize in the monoclinic space group P21/n; crystal structure is not changed by the Ga doping, but the whole X-ray powder diffraction profiles shift to higher 2θdegree. The structural refinement has shown that unit cell volume of Sr2Mn1-xGaxMoO6 (x=0.0-1.0) compounds decreases with the Ga concentration; B/B' site cationic ordering also decreases by Ga doping; the introduction of Ga leads to extension of bond and shrinkage of bond .(c) Sr2FeMoO6/xLa0.67Sr0.33MnO3(x=0.1-0.5) composites are prepared by physically mixed processes. The analysis of crystal structure indicates that both structures of these two phases:Sr2FeMoO6 and La0.67Sr0.33MnO3, are not changed dependent on the amount of La0.67Sr0.33MnO3, and these two phases are existed separately. The electromagnetic measurement of the composite system shows that the strength of the samples' saturation magnetization increases with the increase of x, and the ratio of resistance will increase in magnitude, with its magnetoresistance in low magnetic fields up to 14.2%. This proved that the introduction of magnetic La0.67Sr0.33MnO3 phase can enhance magnetoresistance effectively.