【作者】 余功奇；

【导师】 袁松柳；

【作者基本信息】 华中科技大学 ， 材料物理与化学， 2007， 博士

【摘要】 双层锰氧化物不仅具有一些与三维钙钛矿结构锰氧化物相似的性质,而且由于维度的降低,以及自身特别的结构,使得它又表现出了许多独特的性质。因此,它不仅为研究和认识混合价锰氧化物的本质特性提供了又一个独特的研究平台,而且通过它,可以探索一些在三维锰氧化物中观察不到的新现象。本文以双层锰氧化物La_2-2xSr_1+2xMn₂O₇为研究对象,选择以La_1.4Sr_1.6Mn₂O₇为母体化合物,重点对其A位和Mn位掺杂效应进行了系统地研究。主要内容和结果如下:介绍了混合价锰氧化物中所具有的一般性质和一些重要的理论模型与观点,接着介绍了双层锰氧化物具有自身特点的一些性质和特征,包括磁结构与相图、特殊的晶体结构、磁和电输运性质、磁电阻效应、轨道自由度等。在此基础上,提出了本论文的选题和研究内容的基本思路。对用固相反应法和溶胶-凝胶法来制备双层锰氧化物的最佳工艺条件进行了探索,分别用这两种方法成功制备出了高质量的样品,比较了这两种方法的优缺点。以名义组分为La4/3Sr5/3Mn2O7的样品为例,研究了烧结温度对其结构、电输运性质及磁电阻效应的影响,观察到了双层锰氧化物的电阻率随烧结温度的升高而增大的现象。用固相法成功制备出了La_2-2xSr_1+2xMn₂O₇（ 0 .3≤x≤0.5）多晶样品,并对它们的结构、电、磁性质进行的探讨。在保持载流子浓度不变的前提下,特意选择Y和Ba这一小一大两种离子,对La_1.4Sr_1.6Mn₂O₇的A位进行掺杂研究,主要考察A位尺寸和失配效应对系统性质的影响。研究内容包括:La_1.4-zY_zSr_1.6Mn₂O₇中的Y掺杂效应,La_1.4Sr_1.6-yBa_yMn₂O₇中的Ba掺杂效应,La_1.4-zY_zSr_1.6-yBa_yMn₂O₇（z=y）中等量的Y和Ba共掺杂效应。结果表明,当z < 0.08、y≤0.3或者z=y < 0.08时,样品属于单相的双层结构,掺杂抑制的磁转变和绝缘体-金属转变,导致电阻率增大,转变处的磁电阻效应有所增强,当z≥0.08、y > 0.4或者z=y≥0.08时,样品不再是单相,而是出现了ABO3型钙钛矿相,使得绝缘体-金属转变逐渐向高温移动,同时电阻率下降。这说明离子大小失配使得双层四方相不易形成,较小离子的存在更不利于四方相的形成。比较了z=0.05, y=0; z=0,y=0.05; z=y=0.05三个样品的实验结果,发现Y的掺杂效应强于Ba的掺杂效应,而Y和Ba共掺杂效应最明显。因为Y掺杂增加的晶格畸变,而Ba掺杂减小了晶格畸变,所以Y掺杂比Ba掺杂有更大的掺杂效应。等量的Y和Ba共掺杂虽然能保持A位的平均离子半径几乎不变,但却进一步增大了离子大小的失配,导致最大的掺杂效应,证明了失配效应在这里起主导作用。研究了在La_1.4Sr_1.6Mn₂O₇的Mn位上的系列过渡元素掺杂效应,掺杂元素包括:Cr、Fe、Co、Ni、Cu、Zn、Ti、Nb、Mo等,掺杂浓度固定为2%。掺杂引起了系统的电、磁性质（诸如:三维磁转变温度TC、二维磁转变温度T*、绝缘体-金属转变温度TIM、电阻率、磁电阻、高场磁化强度,高温导电方式等）明显的变化。从掺杂离子的尺寸效应、与Mn离子之间不同的磁交换作用、Mn3+/Mn4+比的变化以及Mn-O-Mn网络和双交换作用的破坏等角度对实验结果进行了分析和讨论。Cr掺杂有明显不同的掺杂效应。在锰氧化物中,首次观察到Cr掺杂使得系统的TC、TIM同时升高,且电阻率下降这种现象。对La_1.4Sr_1.6(Mn_1-yCr_y)₂O₇ （0≤y≤0.1）系列的电、磁性质,特别是磁性质进行了研究。我们发现Cr较低掺杂和较高掺杂对电、磁性质有明显不同的影响。随着Cr掺杂的增加,TIM先升后降,峰值电阻率先降后升。TC也先升高,然后几乎保持不变。TC和T*的变化也不一致,随着Cr掺杂,T*单调地下降,且与掺杂浓度几乎成线性关系。这些结果仅仅用双交换不能很好地理解。通过综合考虑Cr掺杂对双层内及双层间磁相互作用的不同影响,我们较合理地解释了其掺杂效应。更多还原

【Abstract】 The bilayer manganites not only possess some similar properties with the three-dimensional perovskite manganites, but also show many own unique properties due to the decrease of dimension and particular structure. So, the bilayer manganites offer the opportunity not only to investigate the generic features of the mixed-valence manganites , but also to explore phenomena that are not found in the 3D manganites. In this thesis, we focus on the bilayer manganites La_2-2xSr_1+2xMn₂O₇, and choose La_1.4Sr_1.6Mn₂O₇ as parent compound to investigate systematically it’s A-site and Mn-site doping effect. The major investigations and results are presented as follow:The introduction on the generic properties of the mixed-valence manganites and some important theory model and viewpoints has been presented. Then some features of the bilayer manganites are introduced, and these features include magnetic structure and phase diagram, crystal structure, magnetic and electrical transport properties, magnetoresistance effect, the orbital degrees of freedom etc. On the basis of the review, the basic thinking of subject selection and investigation significance is put forward.The high quality samples have been successfully synthesized by the solid state reaction method and sol-gel method, respectively. The virtue and shortcoming of the two methods is compared. For the sample with nominal composition La4/3Sr5/3Mn2O7, the effect of sintering temperature on the structure and transport properties and magnetoresistance are investigated, and we find the resistivity of the bilayer manganites increases with the increase of sintering temperature. The polycrystalline La_2-2xSr_1+2xMn₂O₇ （0.3≤x≤0.5） samples were successfully synthesized by the solid state reaction method and their structure and electrical and magnetic properties were studied and discussed.With the carrier concentration kept the constant, the small ion Y and the big one Ba are chosen to study the A site doping of La_1.4Sr_1.6Mn₂O₇ and the main study focus is on the influence of A-site size and mismatch on the system properties. The research includes: Y doping in La_1.4-zY_zSr_1.6Mn₂O₇, Ba doping in La_1.4Sr_1.6-yBa_yMn₂O₇ and the co-doping of Y and Ba of the same content in La_1.4-zY_zSr_1.6-yBa_yMn₂O₇（z=y）. The result shows that when z < 0.08、y≤0.3 or z=y < 0.08, the sample is a single phase of bilayer structure. The doping suppresses the magnetic transition and insulator-metal transition （IMT） which results in the increases of the resistivity and the magnetoreaiatance increases near IMT. When z≥0.08、y > 0.4 or z=y≥0.08, the sample is no longer in single phase but includes the perovskite phase of ABO3 type, which shifts IMT towards higher temperature and decreases resistivity. This shows that ionic size mismatch makes it difficult to form bilayer phase and the existence of smaller ions makes it more difficult for tetragonal phase to form. Through the comparison of the experiment results of three samples which are z=0.05, y=0; z=0, y=0.05; z=y=0.05, it is discovered that Y doping has greater doping effect than Ba doping and the co-doping of Y and Ba has the most obvious effect. Because Y doping increases lattice distortion and Ba doping decreases it, Y doping has greater doping effects than Ba doping. Though co-doping of Y and Ba of the same content can keep the average A-site ionic radius nearly constant, it further increases ionic size mismatch and results in the greatest doping effect, which proves that the size mismatch plays a dominant role here.The thesis also studied the doping effects of series transition element on Mn-site of La_1.4Sr_1.6Mn₂O₇. The transition elements include Cr、Fe、Co、Ni、Cu、Zn、Ti、Nb、Mo, etc and the doping concentration is fixed as 2%. Doping causes obvious changes of electronic and magnetic properties （such as the three-dimensional magnetic transition temperature TC, the two-dimensional magnetic transition temperature T*, and the insulator-metal transition temperature TIM, resistivity, magnetoreaiatance, high field magnetization, and high temperature conductance）. The thesis also analysed and studied the experiment results by considering the size effect of doping ions, the different magnetic interaction between doping ions and Mn ions, the change of Mn3+/Mn4+ ratio and the damage of Mn-O-Mn network and double-exchange interaction etc. Cr has obvious different doping effects. It is observed for the first time that Cr doping increases Tc and TIM at the same time in manganites and resistivity decreases.The thesis studied the electrical and magnetic properties of the La_1.4Sr_1.6(Mn_1-yCr_y)₂O₇ （0≤y≤0.1） series and studied magnetic properties especially. We have found that lower Cr doping and higher Cr doping has obviously different effects on electrical and magnetic properties. With the increase of Cr doping, TIM increases first and then decreases while peak resistivity decreases first and then increases. TC also increases first and then keeps almost the same. The change of TC does not correspond to that T*. With Cr doping T* decreases monotonously and almost follows linear relation with the doping concentration. All these results can not be explained well by double-exchange interaction. Through considering the different influences of Cr doping on intrabilayer and interbilayer magnetic interaction, we have explained reasonably its doping effect.更多还原