【作者】 凌浪生；

【导师】 张裕恒；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2008， 博士

【摘要】 对钙钛矿结构过渡金属氧化物的研究始于上个世纪五六十年代。近十几年来对该体系研究的复兴，主要是由于高温超导氧化物和CMR效应的发现。以锰氧化合物为代表的巨磁阻材料，由于他们所表现出的超大巨磁阻效应（ColossalMagnetoresistance）在提高磁存储密度、研究磁致冷器件以及磁敏探测元件上具有十分广泛的应用前景，因而受到人们的广泛关注。更重要的是，在物理学基础研究上，这类材料表现出丰富的物理内容，比如由磁场或光诱导的绝缘体-金属转变、电荷有序、轨道有序、相分离等，这些都激发着人们去探索并成为了当前物理研究中的热点之一。对超大磁电阻效应微观机制的研究，将会对凝聚态物理的许多领域的发展和完善起到重要的推动作用。在本论文中，我们通过对钙钛矿锰氧化物进行A位掺杂研究，对这类材料的物性以及电荷有序行为进行了一些探索。本论文分为五章。第一章综述了磁电阻效应的历史与研究进展。介绍了钙钛矿锰氧化物材料丰富的物理内容，包括晶体结构、电子结构、磁性质、输运性质、电荷有序态、相分离现象、掺杂效应等奇特的物理现象。通过本章，我们将了解到钙钛矿锰氧化物的基本物理性质，同时对诸如双交换作用，Jahn-Teller效应等物理概念有所认识，为进入该研究领域作好准备。第二章中我们深入地研究了Nd_0.6Ln_0.1Sr_0.3MnO₃（Ln=La，Pr，Gd and Dy）体系的磁性质。A位平均离子半径<r_A>被认为对体系的PM-FM相变有着深刻的影响。实验结果指出随着掺杂元素Ln从La变到Gd，相变温度T_C逐渐降低。这一结果是由于<r_A>的减小所引起的。对于具有最小<r_A>和最大σ²的Dy掺杂体系，相变温度T_C突然大幅上升。我们把这一现象归结于Dy³⁺离子大的磁矩。6T磁场下测量得到的M（T）曲线在30 K以下磁化强度的迅速增加直接说明了A位稀土离子的磁有序状态。第三章中我们系统研究了具有较小<r_A>的电子型掺杂锰氧化物Gd_0.4Ca_0.6MnO₃体系的电输运和磁性性质。通过M-T、M-H和BSR测量发现M-T曲线中的磁化强度起源于B位无序Mn离子和A位Gd³⁺离子的顺磁贡献。M-T曲线在T_CO以下的奇特行为起源于在形成电荷有序态之后锰氧化物中剩余的无序Mn离子，并且这一反铁磁电荷有序态被认为是一种局域的短程有序态。第四章中我们研究了Pr_0.5-xNd_xSr_0.5MnO₃（x=0，0．1，0．2，0．3，0．4，0．5）体系的磁性性质。在掺杂两端的样品Pr_0.5Sr_0.5MnO₃和Nd_0.5Sr_0.5MnO₃由于不同的轨道序d_x²-y²和d_3x²-r²／d_ey²-r²，他们在低温下分别为A型反铁磁和CE型反铁磁。因此，Pr_0.5-xNd_xSr_0.5MnO₃的磁结构被认为是A型和CE型反铁磁结构的混和。我们的实验结果给出随着pr³⁺离子逐渐被Nd³⁺离子替代，T_C几乎保持不变而T_N显著降低。我们认为这一现象起源于不同磁结构类型的轨道序的不稳定性而不是A位离子半径的影响。第五章中我们研究了Ln_0.4Ca_0.6MnO₃（Ln=La，Pr，Nd，Sm）的磁性及电输运性质。<r_A>对锰氧化物中电荷有序行为的影响是特别深远的。电荷有序温度T_CO随着<r_A>的减小而增大。反铁磁电荷有序态从长程有序转变为局域的短程有序态，并且Ln=La，Pr，Nd，Sm掺杂样品的磁行为各不相同。通过M-T、M-H和ESR测量发现M-T曲线中的磁化强度起源于B位无序Mn离子和A位磁性离子的顺磁贡献。T_CO的上升归因于<r_A>的减小所导致的e_g电子的局域化，并且不同样品的M-T曲线在T_CO以下的不同行为起源于在体系形成电荷有序态之后剩余的无序Mn离子的数量各不相同。本博士论文工作得到了国家自然科学基金（No．10334090，No．10504029）和国家重点基础研究项目（No．2007CB925001，No．001CB610604）的支持。更多还原

【Abstract】 The studies of transition-metal oxides with perovskite structure started in 1950’s. In the recent ten.years, the renaissance of these materials is due to the exciting discovery of high- T_C （high temperature superconductivity） in cuprates and CMR （colossal magnetoresistance ） effect in manganites. The CMR materials represented by manganites have attracted the major attention of researchers in recent years due to the fascinating properties such as colossal magnetoresistance, and the potential applications in magnetic devices, e.g., read and/or write heads for magnetic disk drives, magnetic refrigeration, magnetic random access memories as well as magnetic field sensors. These inspire people to investigate it and become one of the hot topics of present physical research. Most importantly, as a strongly correlated electron system, these materials also exhibit intriguing physical properties such as insulator-metal and/or structure transition induced by applied field or photo radiation, charge ordering, orbital ordering and phase separation etc. Once the micro mechanism of the CMR effect is fully elucidated, the progress in many fields of condensed matter physics will be definitely stimulated. In this dissertation, the author devoted his effort to the study of the properties and charge ordering in doped perovskite manganites by doping at A-site. The whole dissertation consists of five chapters.The chapter one: a brief overview of the progress of the perovskite manganites and related properties. This chapter devotes to a review of the magnetoresistance phenomena and related physical properties of perovskite manganites, such as the crystal structural, electronic configuration, magnetic property, electronic transport, charge ordering, phase separation. Some physics concept, such as double-exchange, Jahn-Teller effect, etc. are interpreted. This part is helping to build up a background for the research.The chapter two: The magnetic properties of Nd_0.6Ln_0.1Sr_0.3MnO₃ compositions （Ln = La, Pr, Gd, Dy） have been investigated thoroughly. The effect of （r_A） is considered to be profound on the PM-FM phase transition in the manganites. The experiment results indicate that T_C decreases with Ln changing from La to Gd due to the decreases of <r_A> The abnormal increase of T_C in Dy-composition with the smallest <r_A> and largestσ² is attributed to the large magnetic moment of Dy³⁺ ions. The rapid increase of magnetization below 30 K indicates directly the magnetic ordering of rare earth ions at A-site.The chapter three: The electrical and magnetic properties of highly doped manganite Gd_0.4Ca_0.6MnO₃ with a relatively small <r_A> have been investigated thoroughly. Through the M-T, M-H, and ESR measurements it is found that the magnetization in M-T curve originates from the paramagnetic contribution of disordered Mn ions at B-site and Gd³⁺ ions at A-site. The peculiar behavior of M-T curve below T_CO originates from the remaining spin disordered Mn ions after the formation of charge-ordering phase in the manganite, and the AFM charge-ordering state is supposed to be a local short-range ordering state.The chapter four: We investigated the magnetic properties of the Pr_0.5-xNd_xSr_0.5MnO₃ （x = 0, 0.1, 0. 2, 0. 3, 0. 4, 0. 5） system. Since the two end samples Pr_0.5Sr_0.5MnO₃ and Nd_0.5Sr_0.5MnO₃ are A-type and CE-type AFM at low temperatures, respectively, due to their different orbital ordering d _x²-y² and d_3x²-r²/d（3y²-r²）, the magnetic structure of Pr_0.5-xNd_xSr_0.5MnO₃ is expected to be a mixture of A- and CE-type AFM. Our experiment results show that T_C remains almost constant, while T_N decreases dramatically as Pr³⁺ ions are replaced by Nd³⁺ ions. We suggest that this originates from the orbital ordering instability of the different magnetic structure type rather than from the ionic radius at the A-site.The chapter five: The electrical and magnetic properties of Ln_0.4Ca_0.6MnO₃ compositions （Ln = La, Pr, Nd, Sm） have been investigated. The effect of <r_A> on the charge-ordering behavior of the manganites is considered to be profound. The charge-ordering temperature T_CO increases with decreasing <r_A> , the antiferromagnetic charge-ordering state transforms from long-range into local short-range ordering and the magnetic behaviors for Ln = La, Pr, Nd, Sm are all different. Through the M-T, M-H, and electron spin resonance measurements, it is found that the magnetization in M-T curves comes from the paramagnetic contribution of disordered Mn ions at the B-site and magnetic ions at the A-site. The rise of T_CO is attributed to the fact that e_g electrons are localized by decreasing <r_A>, and the different behavior of M-T curves below T_CO originates from the number of remaining spin disordered Mn ions after the formation of the charge-ordering phase in the manganites.This work is supported by National Natural Science Foundation of China through Grant No. 10334090, No. 10504029, and the State Key Project of Fundamental Research, China, No. 2007CB925001, No. 001CB610604.更多还原