Sr₂Mn_1-xGa_xMoO₆及Sr₂FeMoO₆/La_0.67Sr_0.33MnO₃的制备、晶体结构和电磁性质研究

【作者】 仝阳；

【导师】 郭光华；

【作者基本信息】 中南大学 ， 凝聚态物理， 2010， 硕士

【摘要】 近年来,人们发现双钙钛矿型化合物Sr2FeMoO6、Sr2FeRe06具有高的电子自旋极化率、高的居里温度和室温低场巨磁电阻效应,从而使它们成为了最有可能在室温下应用的磁电阻材料之一,也进一步激发了人们对有序的双钙钛矿化合物A2BB＇O6的研究热情。本文采用固相反应法、溶胶-凝胶法和物理共混法制备了Sr2FeMoO6、掺杂体系的Sr2Mn1-xGaxMoO6以及复合系的Sr2FeMoO6/La0.67Sr0.33MnO3化合物,通过XRD、SEM、AFM、PPMS等测量手段对所制备样品的晶体结构、电磁特性进行了系统的研究,得到了如下主要结论：(a)分别采用固相反应法和溶胶-凝胶法制备了多晶Sr2FeMoO6化合物。XRD分析表明,固相反应法的前驱体包含SrMoO4和SrFeO3-x两种物相,Sr2FeMoO6是由这两种前驱体反应获得；而溶胶-凝胶制备的前驱体中已有Sr2FeMoO6物相形成,通过烧结可去除前驱体中的SrMoO4和Sr3MoO6杂相。通过研究烧结温度和有序度的关系,我们发现烧结温度的提高可有效提高有序峰的强度比值,即有序度得到增强。结构精修分析表明由两种制备方法获得的样品都为四方结构,空间群为I4/m,B/B＇位阳离子占位率都大于90%。SEM表征发现溶胶-凝胶获得的样品颗粒均匀,而固相法制备的样品颗粒分布范围较大。(b)采用固相反应法制备了B位掺杂化合物Sr2Mn1-xGaxMoO6(x=0.0-1.0)。XRD分析表明,Sr2MnMoO6具有单斜晶体结构,空间群为P21/n。Ga的掺杂没有改变化合物的晶体结构,但衍射峰整体向高角度漂移。结构精修分析表明,Sr2Mn1-xGaxMoO6(x=0.0-1.0)样品的晶胞体积随Ga含量的增加而逐渐减小；同时,B/B＇位离子占位有序度伴随Ga的掺入而逐渐降低；此外,Ga的引入导致<B-O>键长的缩短，<B＇-O>键长的伸长。(c)采用物理共混方法制备了Sr2FeMoO6/xLa0.67Sr0.33MnO3(x=0.1-0.5)复合体系。晶体结构分析表明,随着复合量x的增加Sr2FeM006和La0.67Sr0.33MnO3的晶体结构并没有改变,两相是各自以独立相的形式存在的。电磁测量发现,随着复合量x的增加,饱和磁化强度Ms呈线性增加趋势,电阻率急剧增加几个量级,低场磁电阻达到了14.2%,即磁性La0.67Sr0.33MnO3的复合有效地提高了样品的磁电阻。更多还原

【Abstract】 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 <B-O> and shrinkage of bond <B’-O>.(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.更多还原