【作者】 於乾英；

【导师】 张金仓；

【作者基本信息】 上海大学 ， 无线电物理， 2008， 博士

【摘要】 在强关联钙钛矿结构锰氧化物体系中，由于电荷、轨道、自旋自由度之间的相互耦合和高的自旋极化特征，使得该体系表现出诸多奇异的物理性质和丰富的物理内涵，涉及到强关联和多体系统、金属-绝缘体（M-I）转变等一系列凝聚态物理的基本问题，向现代物理学和信息科学的传统概念提出了挑战，从而给物理学家、材料科学家和信息科学家们提出了新的研究课题，使得强关联钙钛矿锰氧化物体系的研究成为二十一世纪科学技术特别是凝聚态物理强关联电子系统的主要研究热点之一。本文以结构分析和物性测量为主要手段，系统研究了多自旋态Co离子诱导的La_2／3Ca_1／3Mn_1-xCo_xO₃（0≤x≤0．15）体系的二次金属-绝缘体（M-I）转变行为，多自旋Co离子替代对La_2／3Ca_1／3MnO₃体系低温电阻最小行为的影响规律，以及磁场对La_2／3Ca_1／3Mn_1-xCo_xO₃体系输运行为的反常调制现象，给出了该行为与自旋有序性之间的关联，探讨了低温电阻最小现象对磁场反常依赖的物理机制。全文共分六章，主要内容为：第一章主要对钙钛矿型锰氧化物研究的历史、现状及发展趋势做了简单的论述。着重对稀土锰氧化物体系的Mn位掺杂效应以及本工作中所采用的掺杂元素Co在掺杂中表现出的特性进行了综述，并对该体系所涉及的其他相关理论进行了简单介绍，最后给出了本论文工作的出发点和主要研究内容。第二章阐述了实验样品的制备方法、结构表征、物性测量的方法和基本原理，主要包括电、磁输运性质等。第三章系统研究了多自旋Co离子诱导的La_2／3Ca_1／3Mn_1-xCo_xO₃（0≤x≤0．15）体系的二次金属-绝缘（M-I）转变行为。通过结构、磁的和电输运实验测量，结果表明，样品具有很好的单相正交O’，晶格参数随替代量的增大呈减小趋势。所有样品都呈现出CMR体系典型的顺磁-铁磁（PM-FM）转变，铁磁转变居里温度T_C随Co替代量的增加而降低。在顺磁区域，不同样品在T_C以上温度范围内遵守居里-外斯定律。磁测量发现场冷（FC）和零场冷（ZFC）热磁曲线的分叉现象，证明随着Co的掺入，体系内部出现复杂的铁磁／反铁磁（FM／AFM）混合并竞争的相互作用，导致了团簇玻璃行为，说明Co的掺入诱导了系统磁无序的增强效应，也证明了由Co掺杂所导致体系的本征不均匀性。体系高温区域电输运行为较符合Mott的可变程跃迁模型。Co替代导致体系出现电输运反常，具体表现为在居里温度T_C以下电阻-温度曲线的二次M-I行为，且随Co替代量的增大，无论是高温M-I转变峰还是低温M-I转变峰，其峰值温度均向低温区移动；所不同的是，对Co替代样品而言，随外加磁场增加，高温M-I转变峰值温度T_PH向高温区移动，而低温M-I转变峰值温度T_PL则保持不变，表现出磁场无关的特征。相应的M-I转变峰值电阻率对Co替代和外加磁场表现出很强的依赖关系：随Co替代含量的增加，各M-I转变峰值电阻率增加，而低温M-I转变峰值电阻对Co替代更为敏感。对照样品磁特性测量结果，证明高温M-I转变峰对应于未替代体系的M-I转变，低温峰对应的反常变化则与Co³⁺离子替代Mn⁴⁺后在体系中引入氧缺位和高的自旋态相关联。对应电阻二次M-I转变，MR曲线也呈现双峰现象，这种双峰共存现象可以使MR效应在较宽的温度范围内存在，这一现象将可能对该类材料的未来开发与应用产生有益影响，该研究为CMR体系物理机制的理解提供了重要的实验证据第四章研究了多自旋Co离子替代对La_2／3Ca_1／3MnO₃体系低温电阻最小行为的影响规律，并给出了该行为与自旋有序性之间的关联。结果表明，各样品呈现出的电阻最小行为受到Co替代含量很强的影响。实验给出了低温区不同磁场下样品的电输运结果，发现磁场对样品的低温电阻上翘现象具有很强的调制作用，但响应规律与常规的稀磁合金有所不同。结合该系列实验样品的磁结构特征，我们认为随Co含量的增加，AFM团簇增多，体系无序度增强，可能在这种很强的无序条件下，电阻上翘不能被明显的反转过来。磁场一方面抑制杂质磁矩的自旋散射，另一方面由无序导致的加强的电子电子相互作用又使得电阻上翘在磁场的影响下得到加强。为了解释这种低温下的类Kondo电阻异常上升输运行为，结合Kondo散射、e-e相互作用和e-p散射等理论模型，我们对实验数据进行了数值拟合，证明这里的低温输运行为可以从自旋无序散射、e-e相互作用、电子之间的强关联效应，以及e-p相互作用予以解释。结合本课题组前期首次在铁磁金属锰氧化物体系中发现存在的类Kondo散射的实验证据后，再次证明了Kondo反常行为不仅仅存在于传统的稀磁合金中，而且也存在于包含有自旋无序团簇的导电铁磁金属化合物中，这种低温反常输运行为可能反映了锰氧化物强关联电子系统的普遍特征，对强关联CMR锰氧化物体系物理机制的理解具有重要作用。第五章研究了磁场对La_2／3Ca_1/3Mn_0．9Co_0.1O样品输运行为的反常调制现象，讨论了低温电阻最小现象对磁场依赖关系及其物理机制。结果表明，电阻双峰效应具体表现为除了居里温度T_C附近T_PH处的M-I转变以外，在低温区域T_PL处出现二次M-I转变，但磁特性测量结果却显示T_PL表现出某种磁场无关的特征。另外在50K以下，样品在不同磁场下均表现出电阻低温最小值行为。基于晶界效应的考虑，通过磁测量结果表明，低场下的反常输运行为同时证明了体系中存在的自旋无序散射，包括自旋极化和晶界隧穿效应，表现为多晶样品所固有的典型本征特性之一。同时，我们认为低场下磁场对电阻最小行为的轻微调制和较高场下对电阻最小行为的加强这两种截然不同的变化表明了存在于体系中既有自旋无序散射、e-e相互作用、又有自旋极化和晶界隧穿效应，也是引起低温下电阻率异常上升的主要原因。低温下反常金属-半导体转变反映了存在于多晶锰氧化物体系中的类Kondo自旋无序散射，e-e相互作用的存在反映了锰氧化物体系电子之间的强关联相互作用特征，为锰氧化物体系的普遍特征之一。第六章对本论文工作给予了总结，并对目前工作的发展进行了讨论和展望。更多还原

【Abstract】 The strongly correlated manganites with prevoskite structure have a strong coupling between the spin, orbit, charge and lattice freedom of degree which lead to a series of novel physical behaviors related to the basic conception and important questions in modern physics and materials science, for example, strongly correlated electrons, metal-insulator （M-I） transition, ordering parameters, spintronics and devices physics etc. All these challenge the traditional understanding of physics and information science and will provide newly subject for the physicists, material scientists and information scientists. This probably makes strongly correlated manganites the one of the hotspots in the field of condensed matter physics in the 21^st century. In this paper, two metal-insulator （M-I） transitions induced by multi-spin Co ion, and Co-doping effect on the resistivity minimum behavior as well as the modulation of these abnormal transport properties by the magnetic field in the La_2/3Ca_1/3Mn_1-xCo_xO₃ （0≤x≤0.15） system were systematically studied by XRD structural analysis technique and physical transport measurements. The correlation between these behaviors and spin ordering character was given. And the related abnormal field dependence of low temperature minimum and its physical mechanism were also discussed. The whole content of this thesis consists of six chapters as following:In chapter one, we introduce the history of the study on manganites and give a summary of the research on the manganites’ physics properties. The doping effect at Mn site was presented in details, especially in Co doping. It also introduced some related theories and models for colossal magnetoresistivity （CMR） manganites which were used in this thesis. The motivations, significance and main content of the present work are given at the end of this chapter.In chapter two, the main methods and principles in our experiment are involved, including the preparation of samples, structural analysis and physical property measurements such as electron transport, magnetization on the study of CMR manganites.In chapter three, we study systematically the two metal-insulator （M-I） transition properties in La_2/3Ca_1/3Mn_1-xCo_xO₃ （0≤x≤0.15） system by electrical, magnetic transport measurements and XRD structural analysis technique. The results showed that all the samples are good single phase and the lattice parameters decrease with the increasing of doping concentration x. All the samples display a single paramagnetic-ferromagnetic （PM-FM） phase transition, which is the typical character of CMR manganites. The Curie temperature T_C （defined as the inflection point in the M-T curve） shifts to low temperature with the increasing x. In addition, all the samples follow a Curie-Weiss behavior to some extent in the PM region .The results of magnetization show field-cooled （FC） curve didn’t coincide with the zero-field-cooled （ZFC） one below T_C, which indicates the coexistence of and competition between ferromagnetic （FM） and antiferromagnetic（AFM） interaction and is a signature of cluster-glass-like（CG） behavior in highly doped Co samples. All these may indicate the substitution of Co could enhance the magnetic disorder in the system and reflect the general intrinsic characteristic. The transport behavior in the higher temperature satisfied with the model of variable-range-hopping （VRH） model very well. The abnormal transport properties were induced by Co doping. In details, the resistivity exhibited two M-I transitions. The high M-I transition peak (T_PH) and low M-I transition peak (T_PL) all shifted to lower temperature with the increasing of x. While the T_PH shifted to high temperature but the T_PL was almost unchanged under the external field, respectively. The corresponding M-I transition peak values of resistivity showed strong dependence on the external field and Co doping concentration x. All the peak values of resistivity increased with the increases of x, and the low temperature peak value seemed more sensitive than that of high temperature peak. Compared with the magnetization properties, it could be concluded that the high-temperature M-I transition peak should be related with the undoped M-I transition, and the abnormal behavior of low-temperature M-I transition peak have some relationship with the oxygen vacancy and high spin state induced by Co³⁺ substituting Mn⁴⁺ in the present system. Corresponding to the double M-I transitions in resistivity, MR also exhibits two peaks, and the coexistence of two MR peaks gives rise to the MR effect in a broad temperature range, which is very useful for practical application. Present study may provide abundant experimental information for understanding the nature of the strongly CMR manganites.In chapter four, the multi-spin Co ion doping effect on low-temperature resistivity minimum for La_2/3Ca_1/3Mn_1-xCo_xO₃ （0≤x≤0.15） were systemically studied, and the correlation between this behavior and the spin ordering characteristic is also given. The novel resistivity minimum appears at low temperature for all samples, which is very sensitive to the content of Co. Moreover the abnormal electrical transport can be tuned strongly by applied magnetic field, which is very different from that of conventional dilute alloys. Combining with the magnetic properties of the samples, we think the abnormal behavior is mainly related to Co substitution. AFM clusters increase as the Co increases, which may result in magnetic disorder in the system, and it is under such strong disorder system that the resistivity upturn can not be returned. On one hand, the magnetic field restrains the spin related scattering, but on the other hand, in such a disorder system in which enhanced e-e interaction exists, the magnetic field may make the abnormal electrical transport more obvious. In order to explain the novel electrical transport we fit the present experimental data in the framework of Kondo-like spin scattering, electron-electron （e-e） and electron-phonon （e-p） interactions. It is seen that all the data are fitted very well. The fitting results show that this kind of low-temperature abnormal transport can be explained by spin disorder scattering （Kondo-like spin scattering）, e-e and e-p interactions, and antiferromagnetic clusters on a nano/micro-scale. Combining with the early studies in our group which firstly showed the appearance of Kondo-like scattering in ferromagnetic metal oxides, our work again indicate that the novel resistivity minimum at low temperatures may reflect the general intrinsic character of the strongly correlated electron system. It may play a important role to reveal the nature of the strongly CMR manganites.In chapter five, the modulation of these abnormal transport properties by the magnetic field for the polycrystalline bulk La_2/3Ca_1/3Mn_0.9Co_0.1O₃ sample were systematically studied. We also discussed the abnormal field dependence of low temperature minimum and its physical mechanism. The results showed that besides the first M-I transition at T_PH corresponding to T_C on the electric transport curve, a secondary M-I transition is found at lower temperature T_PL .But the magnetic results indicated the secondary M-I transition peak has an abnormal characteristic with magnetic field-free on temperature dependency. The electrical resistivity minimum phenomenon was observed under various magnetic fields below 50K. Based on the effect of grain boundary, the magnetization results indicate the existence of the spin disorder scattering, including the spin polarization and the grain boundary （GB） tunneling effect, which should reflects the typical intrinsic property in the polycrystalline bulk samples. Moreover, the low field can do a little suppression on the resistivity minimum, but the high field can enhance the resistivity upturn. The different behaviors between the low field and high field may indicate there should exist not only the spin disorder scattering, including the spin polarization and the grain boundary （GB） tunneling effect, but also the enhanced e-e interaction, and all are the origin of the low-temperature resistivity upturn. The resistivity minimum at low temperatures proves the existence of Kondo-like spin disorder scattering and e-e interaction due to the strongly correlated interaction between the electrons. These should be a general character in the bulk manganites.In chapter six, we give a conclusion of the whole thesis and discuss the open questions on the work.更多还原