【作者】 高湉；

【导师】 曹世勋；

【作者基本信息】 上海大学 ， 无线电物理， 2009， 博士

【摘要】 以钙钛矿结构锰氧化物为代表的磁电阻材料,由于它们所表现出的庞磁电阻效应(CMR, Colossal Magnetoresistance)在提高磁存储密度以及磁敏感探测元件等领域具有十分广阔的应用前景,因而受到人们的广泛关注。同时,这类体系还表现出诸如磁场或电场等诱导的绝缘体—金属转变,电荷/轨道/自旋有序,以及相分离等十分丰富的物理内容,涉及到凝聚态物理的许多基本问题,对这些问题的微观物理机制的正确认识,必将对凝聚态物理的发展起到巨大的推动作用。本论文工作以具有ABO3典型钙钛矿结构的锰氧化物La-Mn-O体系为具体研究对象,采用宏观物性测量与微观结构分析相结合的方法,详细研究了LaMnO3母体中进行B位替代的LaMn1-xCuxO3体系和在此基础上进行A位替代的La1-yCayMn0.90Cu0.10O3体系中的元素替代效应、微观电子向列有序与宏观物性的关联、以及纳米尺度的畴壁钉扎效应,并对相分离锰氧化物强关联体系中的纳米尺度磁畴/畴壁结构、微观电子有序/无序转变、及其与材料的电/磁输运行为的关联效应等问题的理解有了更深层次的发展。本论文共分为六章,主要内容包括:第一章综述了磁电阻效应研究的历史、发展与现状,并简单介绍了庞磁电阻锰氧化物体系中丰富的物理现象及其潜在的应用前景。通过本章的介绍,我们将对磁电阻效应以及磁电子学有一个概括的了解,并对元素替代后生成的掺杂锰氧化物的基本物理性质,包括晶体结构、电子结构、磁特性、输运机制、有序相等基本物理问题有所认识;最后我们还就本文工作的研究目的和出发点进行了概括性描述。第二章介绍了实验样品的制备方法、结构分析与表征、物性测量的方法和基本原理,主要包括电、磁输运测量和样品形貌/粒度的观察与比较等。第三章研究了A位和B位元素替代对锰氧化物电、磁输运性质以及磁电阻效应的影响。主要包括两部分内容:第一部分研究了Cu替代Mn位对LaMnO3反铁磁母体性质的改变,研究了B位非磁性元素替代以及离子价态变化对锰氧化物磁性和磁电阻效应的影响。实验结果证明通过直接在反铁磁LaMnO3母体的锰位进行Cu替代可以导致双交换作用,并出现强铁磁性和大的磁电阻效应,并且当Cu的替代浓度为10%时,双交换作用和磁电阻效应最为明显。第二部分是在B位最佳掺杂的基础上,研究了La位被Ca替代的影响。重点讨论了相分离体系中不同的相以及近邻畴/团簇间的摩擦效应,以及由此引发的热磁曲线中大的热滞现象。这些研究成果对理解巨磁电阻的微观机制以及探索新型磁电阻材料都提供了重要的信息。第四章研究了La1-yCayMn0.90Cu0.10O3体系中的变磁性磁化台阶跳跃现象,并用电子向列有序理论对它进行了很好的解释,首次有针对性地从实验上证实了电子向列有序理论的有效性,这对于变磁性转变行为的研究以及重新认识相分离机制具有重要意义。第五章对普遍存在于微量元素替代锰氧化物体系中100 K以下的低温磁化反常现象进行了深入细致的研究,我们选取LaMn1-xCuxO3体系为具体研究对象,实验证实了纳米尺度畴壁钉扎效应的存在,并且发现畴壁钉扎现象消失的单畴颗粒的临界尺寸RC约为100 nm,这对于进一步明确低温下强关联锰氧化物体系中的复杂磁结构及其低温相图是很有帮助的。第六章是对本论文工作的总结,并简要叙述了工作的意义。更多还原

【Abstract】 Since the discovery of colossal magnetoresistance effect (CMR) in perovskite manganites, it has sparked considerable renewed interests in these long-known materials with an eye towards both an understanding of the CMR mechanism and related properties and potential applications in magnetic information storage and low-field mangetic sensors. Beside the CMR effect, these materials also exhibit intriguing physical properties such as insulator-metal transition and structure transition induced by applied magnetic-field or electric-field, charge/orbital/spin ordering, and phase separation etc. The full understanding of these properties will definitely stimulate the progress of condensed matter physics.In this thesis, the element-substitution effects, microcosmic electronic nematic order, nano-scaled domain wall pinning and their correlated effects are minutely studied, by both the measurements of macroscopical physical properties and the analysis of microstructure, in phase separated B-site Cu substituted LaMn1-xCuxO3 and the related A-site Ca substituted La1-yCayMn0.90Cu0.10O3 perovskites. A further understanding of nano-scaled magnetic domain (wall) structure, microcosmic electronic ordering/disordering transition and their correlated effects in magnetic/electrical transport properties is obtained. The whole thesis consists of six chapters.The first chapter aims at a brief overview of the history, progress, and current status of magnetoresistance effect. The abundant physical contents and potential applications in CMR manganites are also introduced, including crystal structure, electronic structure, magnetic/electrical transport mechanism, all kinds of ordering states, etc. By these illustrations, one may acquire a basic sight on magnetoresistance effect as well as magnetoelectronics. Meanwhile, the aims and motivations of the present work are synoptically given.In Chapter Two, the main methods and principles in our experiments are involved, including the preparation of samples, structural analysis and physical property measurements such as electrical transport, AC/DC magnetization, observation of crystal grains and their boundaries by electron microscopy, and so on.In Chapter Three, the effects of A-site and B-site substitution by foreign elements are studied by preparing LaMn1-xCuxO3 and La1-yCayMn0.90Cu0.10O3 samples. Firstly, the effects of nonmagnetic Cu substitution for Mn are studied in LaMn1-xCuxO3 system. The results confirm that the strong double-exchange ferromagnetism and large CMR effect can be achieved by directly Cu substitution into B-site. And when the Cu content is at 10%, both the double-exchange and CMR are optimal. Then, the effects of A-site Ca substitution for La are also studied at a fixed Cu substitution level x=0.10, especially the friction effects between differect phases or neighbouring domains/clusters, the large thermal hysteresis is also discussed. This part is helpful for the understanding of CMR mechanism and exploring new CMR materials.The fourth chapter is mainly about the study of metamagnetic step-like transition in La1-yCayMn0.90Cu0.10O3 manganites. A new theory of electron nematic ordering model is approved to be effective rather than phase separation, which is important for metamagnetism study and a further understanding of the phase separation model.In Chapter Five, the anomalous magnetic behavior below 100 K in dilute Cu substituted LaMn1-xCuxO3 system is studied systematically, and the domain wall pinning effect is confirmed by our measurements. The TEM pictures show that the critical size of single domain particle is at ablout RC ~ 100 nm. These results are useful to get to the bottom of the perplexing magnetic domain structure and build up a clear phase diagram at low temperatures in manganites.In Chapter Six, a summarization of the present work is given, altogether with the significance of this work.更多还原