【作者】 刘新军；

【导师】 姜恩永； 李志青；

【作者基本信息】 天津大学 ， 材料物理与化学， 2007， 博士

【摘要】 自从庞磁电阻（CMR）物理现象发现以来,钙钛矿型锰氧化物的研究就成为了当今凝聚态物理学和材料物理学研究的热点之一。这不仅是因为CMR效应在磁传感器、磁记录信号读出磁头和磁存储技术领域具有潜在的应用价值,而且这类氧化物本身也蕴含着丰富的物理现象,如外加磁场引起的绝缘体–金属转变,电子/结构相分离等现象。本论文通过对钙钛矿型锰氧化物的A位和B位掺杂研究,对其结构、磁和电输运性质进行了研究探索,主要工作概括如下:采用Pechini方法制备了铁磁绝缘体Nd_0.85Na_0.15MnO₃多晶样品。磁性研究表明该样品在居里温度附近呈现出短程铁磁有序相与顺磁相共存现象,不同温度下的电子自旋共振（ESR）谱线证实了这一相分离现象随温度变化的动态过程。用小极化子跃迁模型分析了高温区的ESR参数和电阻率,得到了与小极化子产生和跃迁相关的激活能。低温下Fe掺杂锰氧化物LaMn_0.7Fe_0.3O₃多晶样品处于类团簇玻璃态,交流磁化率的测量证实了这一点。当从高温向低温方向趋近于有序温度时,样品中出现了铁磁团簇,这些铁磁团簇是由Mn³⁺/Mn³⁺或者Mn³⁺/Fe³⁺离子之间的铁磁相互作用构成的;当温度进一步降低时,这些铁磁团簇与Fe³⁺/Fe³⁺离子之间的反铁磁相互作用竞争,形成了类团簇玻璃态。另一方面研究了LaMn_0.7Fe_0.3O₃样品的低温动力学特性,观察到有趣的现象,即加场冷却和零场冷却时的磁记忆效应。在30 K附近,时效在负温度循环和正温度循环中都表现出记忆效应,而在61 K附近,时效只在负温度循环中表现出记忆效应,反映了这个系统存在着两种不同的非平衡态。多晶CaMn1-xFexO3 （0≤x≤0.35）系列样品呈现出正交畸变的钙钛矿结构,随着Fe含量的增加,晶胞体积线性增大,反铁磁性逐渐趋于弱化。对于x = 0.08和0.10的样品,在外加1 kOe磁场冷却的磁化强度曲线上观察到磁化强度反转现象,这与反铁磁母体中的自旋倾斜排列密切相关。而对于x≥0.15的样品,负磁化强度现象消失,铁磁组份与反铁磁组份共存,且反铁磁相互作用占主导地位。虽然电输运测量表明所有CaMn1-xFexO3样品都表现出绝缘体行为,但是随着Fe含量的增加样品电阻率的变化呈现出非线性趋势:当2%的Fe掺入CaMnO₃时,电阻率增加了近三个数量级,这是由Fe随机取代Mn带来的无序分布引起的;随着Fe含量的进一步增加,载流子浓度增大,电阻率降低。更多还原

【Abstract】 The manganite with a perovskite structure has been attracted a lot of attention in condensed matter physics and material physics since the discovery of the colossal magnetoresistance （CMR） phenomenon, not only because of the potential applications of CMR effect in magnetic field sensors, magnetic reading heads and magnetic random access memories, but also because of many intriguing physical properties observed in this type of oxide, such as the insulator-metal transition induced by the applied magnetic field, the electronic/structural phase separation. In this thesis, we have investigated the structural, magnetic and electrical transport properties of manganites through doping on the A- and B-site in the perovskite manganites. The main results are generalized as follows:We have prepared the ferromagnetic insulating manganite Nd_0.85Na_0.15MnO₃ polycrystalline sample by Pechini process. The magnetic researches indicate that the short-range ferromagnetic order and paramagnetic state coexist around the Curie temperature （TC）. The electron spin resonance （ESR） lines at various temperatures demonstrate the dynamic process of the phase separation phenomenon with decreasing temperature. In the higher temperature range, the temperature dependence of ESR parameters and electrical resistivity are discussed at the frame of the adiabatic small polaron hopping model, and the activation energies related to the creation of the polarons and activating the hopping of the polarons were obtained.Ac susceptibility measurements have demonstrated that polycrystalline Fe-doped manganite LaMn_0.7Fe_0.3O₃ is a typical cluster-glass-like state in the low temperature range. Combined with all results, we conclude that magnetic clusters, contributed from ferromagnetic interaction between Mn³⁺ and Mn³⁺/Fe³⁺ ions, develop as temperature approaches the ordering temperature, and compete with antiferromagnetic interaction between Fe³⁺ ions in character of cluster-glass-like state in lower temperature. We have also performed a series of measurements to study the low temperature dynamics of LaMn_0.7Fe_0.3O₃ sample. The results, obtained in the zero-field-cooled and field-cooled dc magnetization and magnetic relaxation, demonstrate striking memory effects. It was also found that, in the aging experiments, there are symmetrical response on negative and positive temperature change around 30 K, and there are not symmetrical response around 61 K, representing two different non-equilibrium states of this system.The polycrystalline CaMn1-xFexO3 （0≤x≤0.35） phases adopt an orthorhom- bically distorted perovskite structure. The lattice expansion and weakening antiferro- magnetism are observed with increasing Fe concentration. Magnetization reversal is observed in the field-cooled magnetization curves under the field of 1 kOe for x = 0.80 and 0.10, which can be attributed to a spin-canting arrangement in the antiferro- magnetic matrix. For x≥0.15, the negative magnetization phenomenon disappears, and ferromagnetic component coexists with antiferromagnetic one, but antiferromagnetic interaction still dominates in these compounds. Electrical transport measurements show the insulating behavior in the studied temperature range for all compositions. Fe doping, even at a level as low as x = 0.02, causes a marked resistivity increase, which can be ascribed to the disordered arrangement of transition metal cations. Further increasing Fe content causes resistivity to gradually decrease due to the increasing carriers.更多还原