Doping Effects In Double Perovskite Y₂MnCrO₆Compound

Transition metal oxides, usually with a perovskite structure ABO3, have attracted a great deal of attention over the past decades, largely because of their unique physical properties such as superconductivity, colossal magnetoresi stance, metal-insulator transition, ferroelectricity and multiferroicity properties. Double perovskite oxides (A2BB’O6) are the unique kind of perovskite oxide. Because B and B’site ions have various electron configurations, these oxides can have a deeper physical intension, and their electromagnetic properties show complex and diverse characteristics (such as charge ordering, magnetic order, magnetocrystalline anisotropy, spin-glass and so on), which have important academic significance and broad application prospects.Recently it has been gradually understood that double perovskite oxide (Y2MnCrO6) with B-site ordering displays multiferroics and ferrimagnetism. In this paper, we doped Y2MnCrO6in A-site, B/B’-site with chemical elements, synthesized Y2-2yLa2yMnCrO6、Y2-2yBi2yMnCrO6、Y2Mn1-yGayCrO6using solid-state reaction method. Then we chiefly studied their structure and magnetic properties of Y2-2yLa2yMnCrO6and Y2Mn1-yGayCrO6, and preparation conditions for the Y2-2yBi2yMnCrO6. There are five chapters in this thesis, and the main contents of each chapter is introduced as fellow:In Chapter1, we first give a brief introduction on the research history of perovskite manganite oxides, and we review some physical concepts such as crystal structure, superexchange interaction, double exchange interaction, Jahn-Teller effect, magnetic ordering and ordering effect. Then, we give a brief introduction to the structure and electromagnetic properties of double perovskite oxides (A2BB’O6). Next, we introduce the magnetoelectric properties of A-or B-site doped A2BB’O6compounds. At last, we describe the related properties of Y2MnCrO6.In chapter2, we first provide the preparation method of polycrystalline samples, and then introduce the analysis method of crystal structure, we mainly give the related principle of polycrystalline powder X-yay diffraction, and also provide a brief introduction to the Rietveld refinement and GSAS software. Finally we introduce the Vibrating Sample Magnetormeter (VSM), with which we can measure magnetic properties, and homemade variable temperature resistance test system.In chapter3, we first study the evolution in La3+doped Y2-2yLa2yMnCrO6(y= 0-0.4,0.9) polycrystalline samples. For lightly doped samples with y≤0.3, the refined structural parameters of XRD patterns display a decrease tendency of structural anisotropy with increasing La content, suggesting the gradual suppression of layered B-site cation ordering in the compounds with increasing A-site cation size. The temperature dependence of magnetization gives that the ferrimagnetic state at temperatures below-75K for the prototype Y2MnCrO6remains till y=0.3, while the samples with y-0.9show a much higher transition temperature at～120K. For y between0.4and0.8, XRD patterns give that all the samples are two-phase mixtures. For the ferrimagnetic samples with y≤0.2, the detailed analysis on the high-temperature paramagnetic susceptibility reveals that the antiferromagnetic coupling within the Cr3+sublattice is obviously enhanced with increasing La3+content, which is an unfavourable factor for the ferrimagnetism. The electrical transport properties of the samples are dominated by the thermal activation mechanism, and the activation energy decreases distinctly with increasing y (y≤0.3). The results indicate that the ferrimagnetism and layered B-site cation ordering in Y2MnCrO6oxide is suppressed with increasing the A-site average cation size. In addition, we preliminarily investigated preparation conditions of the Y2-2yyBi2yMnCrO6. For lightly Bi-doped samples, sintering temperature has little effect on sample; for highly Bi-doped samples, high-temperature sintering degrades the quality of the samples. With increasing Bi content, low-temperature sintering is favorable.In chapter4, by means of XRD and magnetization measurements, we have characterized the effects of structure and magnetic properties of Y2Mn1-yGayCrO6(y=0,0.2,0.4,0.6). The crystal structure analysis shows that the samples remain monoclinic structure for the prototype Y2MnCrO6till y=0.3. The temperature dependence of magnetization gives a ferromagnetic state at low temperatures with increasing Ga content.The magnetocrystalline anisotropy and the number of magnetic ions decreases, and the exchange interactions change with increasing Ga content.In chapter5, a conclusion is given for the whole thesis and then we put forward some worthy ideas for further research.