铁性序演化及其热效应

IMR OpenIR

	铁性序演化及其热效应
	李昺
学位类型	博士
导师	张志东 ; 任卫军
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	磁热效应电热效应磁-结构耦合超结构金属-绝缘体转变 Magnetocaloric Effect Electrocaloric Effect Magneto-structural Superlattice Metal-insulator Transition
其他摘要	" 随着对高温超导体、庞磁电阻锰酸盐及多铁性材料的深入研究，晶格、自旋、电荷及轨道各个自由度（序）间的强烈耦合及其多场交叉调控的物理效应颇受关注。晶格和广义极化（磁化强度或铁电极化）的强烈耦合导致了一级相变，伴随着较大的热效应。由于潜在的高效固态制冷应用，这种外场调控的热效应成为研究界和工业界共同重视的课题。然而，晶格和广义极化的强烈耦合致使以前对于二级相变热效应的理解和认识在一级相变体系中失效。显然，如何理解晶格和广义极化的强烈耦合、耦合序的演化规律及其对热效应的影响对于认识一级相变热力学和制冷应用有着重要的意义。本文正是围绕晶格-广义极化耦合，利用X射线衍射、透射电镜、X射线光电子谱、磁化率、应变、电输运等实验手段以及热力学数值方法和第一原理计算，研究了代表性体系在有序相-无序相转变和有序相-有序相转变过程中耦合序的演化规律及其对热效应的影响： 1. 用Landau-Devonshire相变理论设计了一个可逆过程，克服了实验上测试等温磁化曲线所带来的不利影响和计算磁熵变的不确定性。基于这个可逆过程，我们发现，材料的磁性仅仅由四个本征参数：Curie温度TC，Curie-Weiss温度T0，Curie常数C和Curie-Weiss温度的自发磁化强度Ms0所决定。推导出了一个简单的计算磁熵变的公式：ΔS = −1/2 [M(T,H)2 −Ms(T)2]。将这一计算磁熵变的方法应用于LaFe11.5Si1.5，得到的结果与从比热得到的熵变一致。 2. 将Landau-Devonshire相变理论应用于具有代表性的电热效应铁电聚合物体系P(VDF-TrFE)，研究了相变对电热效应的影响。进一步从应用角度出发，研究了材料从块体向薄膜演化过程中应变对电热效应调控。发现一级相变体系中具有远大于二级相变体系的电热效应，其等温熵变大约是二级相变体系的2倍，和磁-结构耦合体系中的结论一致，说明晶格-电极化的耦合同样对电热效应起到了重要作用。具有同样成分的材料，薄膜的电热效应要略小于块体。具有不同应力的薄膜，其等温熵变相等，但等温熵变的峰值会随应力的性质不同出现不同方向的偏移。说明可以利用应力去调控电热效应材料的服役温区。 3. 导出了一个表征磁-结构耦合的公式，得到了实验结果的直接证实。又利用两个有序相的德拜温度与应变的关联，实现了磁-结构解耦合，分离得到了纯粹的磁熵变和结构熵变，发现结构自由度大致贡献了50%的熵变，从而进一步认识到磁-结构耦合在磁热效应中的重要作用。 4. 通过仔细分析反铁磁-铁磁这种有序相-有序相转变的特点，归纳体系的相图。基于平均场模型，利用场致相变的临界场将这两个有序相关联起来，确定了低温有序相的“有序温度”。提出通过比较两个有序相的有序温度大小的方式来理解磁热效应的方法：从有序温度高的相到有序温度低的相之转变将导致反常磁热效应，反之则是正常磁热效应。这一观点在主要的材料体系中都得到了证实。 5. 用高压合成方法制备了F掺杂的CrO2样品，系统地研究了体系的结构、磁性、输运性质、电子结构等随成分、温度和磁场的变化关系。F−的引入带来了晶格畸变能和电子关联效应的相互竞争。低F−含量时，F−成有序排列而降低的库仑排斥能远大于F−的集中带来的晶格畸变能，所以出现了具有F−有序排列形成的超斑点和条纹相的铁磁性半金属；而高F−含量时，形成F−条纹相所带来的库仑排斥能的降低远不能补偿由于巨大的应力集中造成的晶格畸变能，甚至结构发生了明显的畸变，所以形成F−无序排列的反铁磁绝缘体。 6. 报道了三个基分别针对近液氢、液氮和室温温区于二级相变的新磁热效应材料：TbCoC2，Tb3Co和CrO2−xFx。磁化强度-温度关系、磁化强度-磁场关系以及Arrott图均表明它们具有二级相变的行为。50 kOe磁场变化下的最大磁熵变分别为：-15，-18及-3.8 J kg−1K−1。" ; In the present work, we focus on the coupling between lattice and generalized polarizations and perform a systematical and detailed study on the order to disorder transitions and the transitions between ordered phases, and the concomitant caloric effects, by employing a lot of complementaryexperimental ways like X-ray diffraction, X-ray photoelectron spectroscopy, electronic probes microanalysis, transmission electronic microscopy, magnetic, transport, strain etc., and theoretical approaches including thermodynamic numerical calculation and ab initio calculation. This research is summarized as follows: 1. The Landau-Devonshire phase transition theory is used to deal with the firstorder transition and derive a reversible process to calculate the magnetic entropy change, by which one can avoid the undesirable influences from the measurement of magnetic isotherms and the uncertainty. Properties of a material are found to be dependent only on four intrinsic quantities This kind of method is applied to deal with LaFe11.5Si1.5 and consistent result with experiments is obtained. 2. We apply the Landau-Devonshire phase transition theory to a famous electrocaloric polymer poly(vinylidene fluoride-trifluoroethylene). Just changing the nature of the phase transition from the first-order to the second-order reduces the isothermal entropy change, adiabatic temperature change and refrigerant capacity. Converting to be film also leads to the reduction in electrocaloric effects, and positive (negative) strains shift the entropy curves to higher (lower) temperatures, which suggests the working temperature window of electrocaloric materials could be adjusted by introducing different strains. 3. A well-defined linear relationship is found between the quantity characterizing magnetic degree of freedom and the thermal expansion on behalf of structural degree of freedom, which demonstrates the magneto-structural coupling. Within the exchange-inversion model, such a linear relationship is theoretically derived and the magnetostructural correlation is elucidated. 4. Keeping the features of this ordered to ordered transition in mind, we summarize the several related phase diagrams. Within the mean field model, the critical magnetic field correlates the ordering temperatures of these two phases and the ordering temperature of the ordered phase at lower temperatures is determined. We propose that the normal or inverse magnetocaloric effects could be understood by comparing the two ordering temperatures. 5. We prepare F-doped CrO2 samples with different compositions by high pressure synthesis and study the variation of crystalline structure, magnetism, transport, electronic structure etc with doping, fields and temperatures. F-doping introduces two effects: distortion of lattice and electron correlation. The competition between them just underlies the exotic properties. At composition with lower F−，the ordering arrangement of F−forms superstructures with vectors of (1/3, 0, 1/3) and (-1/3, 0, 1/3), as seen from electronic diffraction superlattice spots and high-resolution stripes. The energy gain comes from the lowering of Coulomb repulsion. The system shows ferromagnetic half-metal features. With doping increasing, intensive lattice strain leads to a lattice distortion of previous rutile structure. To disperse the strain, the superstructure of F−disappears. The correlation effect and distortion localize the electrons and an antierromagnetic insulator with a gap of 0.2 eV is generated. 6. We reported three new magnetocaloric materials: TbCoC2, Tb3Co and CrO2−xFx, active in near liquid-hydrogen, liquid-nitrogen and room temperatures windows, respectively. The temperature and magnetic-field dependencies of magnetization and Arrott plots suggest they behave reversibly in the vicinity of ordering temperatures. Their magnetic-entropy changes are -15, -18 and -3.8 J kg−1 K−1 under magnetic-field changes of 50 kOe, respectively."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64451
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	李昺. 铁性序演化及其热效应[D]. 北京. 中国科学院金属研究所,2012.