【作者】 樊济宇；

【导师】 查朝征；

【作者基本信息】 新疆大学 ， 理论物理， 2003， 硕士

【摘要】 近年来，以钙钛矿结构氧化物为代表的巨磁电阻材料，由于它们所表现出来的超大磁电阻效应（Colossal Magnetoresistance）在提高磁存储密度及磁敏感探测元件上具有十分广阔的应用前景，因而受到人们的广泛关注。同时，这类材料还表现出诸如磁场或光诱导的绝缘体-金属转变，电荷有序、轨道有序、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，一旦解决了这些问题的微观物理机制，必将对凝聚态物理的发展和完善起到巨大的推动作用。 目前，普遍认为双交换作用对钙钛矿结构氧化物中存在的CMR效应起到非常重要的作用，因而本论文通过对Mn位的元素替代来考察掺杂后对双交换作用的影响。在本论文中，对于体系中普遍存在的在铁磁背景中没有金属—绝缘体转变，我们从能带论的观点提出了一种新的理论解释，并且通过相应的实验而得到证实，从而推翻了前人将这一现象简单地归于双交换作用由于掺杂后而遭到破坏的观点。 本论文分为四章。 第一章综述了磁电阻效应研究的历史、发展与现状。介绍了各种磁电阻材料的物理化学性质，如磁性多层膜、钙钛矿结构氧化物等。通过本章的介绍，我们将对磁电阻效应以及磁电子学都有一个概括的了解。 第二章介绍了超大磁电阻材料锰氧化物丰富的物理性质。包括晶体结构、电子结构、磁性质、输运性质、电磁相图、有序相的物理现象。通过本章，我们将了解到掺杂锰氧化物的基本物理性质，并对诸如双交换作用、Jahn-Teller效应、电荷有序等物理概念有所认识，为进入该研究领域作好了准备。 第三章双交换作用和CMR效应的机制。在本章中除分析双交换作用的过程，同时介绍两种关于产生CMR效应的机制理论。 第四章利用Mg对Mn位掺杂，来分析掺杂后巨磁电阻材料La_0.67Sr_0.33MnO₃的变化。通过磁化强度，电阻率，电子自旋共振，红外光谱等测量。结果发现在低掺杂区有金属—绝缘体相变而高掺杂区体系完全变成绝缘体。通过红外光谱分析，我们提出这是由于邻近Mg的和非邻近Mg的锰氧八面体Jahn-Teller畸变不同造成的。实 yll验结果很好地证明了这一观点。更多还原

【Abstract】 Recently, 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 and related properties and potential applications in magnetic information store and low-field magnetic sensors. Beside the CMR effect, these materials also exhibit intriguing physical properties such as insulator-metal and/or structure transition induced by applied magnetic-field or photo radiation, charge ordering, orbital ordering, and phase separation etc. The full understanding of these properties will definitely stimulate the progress of condensed matter physics.Nowadays, it has been extensively accepted that the double exchange interaction plays a key role in perovskite manganites. Therefore, in this thesis the effect of the double exchange is studied by doping on Mn sites. To the universal behavior that there is not insulating-metallic transition in ferromagnetic background, we proposed a new view to explain it. Furthermore, this opinion had also been proved by some experiments. Therefore, the previous view about this phenomenon was overthrown.The whole thesis consists of four chapters.1.A brief overview of magnetoresistance effect. This chapter aims at a brief overview of the history, progress, and current status of all kinds of magnetoresis-tanc materials, such as magnetic multilayers, perovskite oxides, magnetite, and so on. By these illustration, we may acquire a basic sight on magnetorresistance as well as magnetoelectronics.2. An introduction to the physical properties of perovskite manganites. This chapter deals with the influent physics properties and some spectacular phenomenon observed in perovskite manganites, including the structural, magnetic, electronic transport, phase diagram, charge and orbital ordering. Some special physics concepts,suchas double exchange, Jahn-Teller effect, electron-phonon coupling, are interpreted. This part is helping to build up a background for the research on colossal magnetore-sistance.3.The theory of double exchange and mechanism of CMR effectexchange.In this chapter, The theory of double exchange will be analyzed in detail. In addition, two kinds of theory about CMR Mechanism are also dicussed.4.The effect on the perovskite CMR material Lao.erSro.asMnOs by doping Mg on Mn site. This chapter is focused on the change of CMR material Lao.eTSro.saMnOa by doping Mg on the Mn site. We observed that there exits insulating-metallic transition in the low doping region, but without that in the heavy doping zone, by the measurement of magnetization, resistivity, and electron spin resonance. We suggest that this behavior attributes to the difference between the Jahn-Teller distortion of the Mn3+ ion neighboring on Mg2+ and that of the Mn3+ion non-neighboring on Mg2+, which is confirmed by the IR and Raman spectra. The anomalous transport behavior is dominated by the different Jahn-Teller distortion. This opinion is also proved well by our experimental results.更多还原