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1、中国科学技术大学硕士学位论文自旋冰材料R2Ti2O7(R=Dy,Ho)的单晶生长和低温物性研究姓名：吕艳艳申请学位级别：硕士专业：凝聚态物理指导教师：孙学峰2011-05-08摘 要 i 摘 要 摘 要 阻挫磁性材料目前作为凝聚态物理研究领域的一个重要分支， 其中自旋的几何阻挫，即由晶体结构的几何对称性造成的阻挫尤其引人注目。由于阻挫效应的存在，自旋之间的相互作用互不相容，从而导致了基态的宏观简并，展现出丰富的量子磁性。烧绿石结构的 R2Ti2O7（R = Dy，Ho）是一种典型的几何阻挫磁性材料， 强的晶体场效应使得正四面体顶点的磁性离子只能沿着晶体轴111方向成Ising 模型排列。由于阻
2、挫效应的存在，材料中自旋与自旋之间的近邻反铁磁相互作用和长程铁磁的磁偶极相互作用之间相互竞争， 导致了材料在低温下具有六重简并的“两进两出”自旋冰基态。另外，外加不同方向的磁场会不同程度地解除简并，表现出丰富的磁相变行为。然而目前对自旋冰基态以及磁相变行为的研究仍然存在很多争议。加之最近基于 R2Ti2O7（R = Dy，Ho）材料提出的磁单极子模型从另外一个角度解释了自旋冰体系的很多现象， 与之前的磁偶极矩模型相比哪一种更加完善目前也不是很清楚。 热传导测量被公认为是研究磁性材料的一种有效手段，它不仅能很好地探测元激发，比如声子，磁振子等的热输运性质，还能很好地研究自旋声子耦合，磁结构及转变
3、以及磁场诱导的量子相变等行为。本文针对以上问题，首先对该材料的单晶生长进行了深入研究，在获得更高品质的单晶的基础上，细致研究了磁化率，比热以及热传导等性质，希望能够为自旋冰体系的物理机制理解提供可靠的实验依据。 第一章主要综述了对自旋冰材料 R2Ti2O7（R = Dy，Ho）的一些研究进展，包括磁化率，比热，中子衍射等各种实验结果，介绍了目前对该体系的磁性质和磁结构的理解，并简单描述了磁偶极矩模型和磁单极子模型。从中可以看出现阶段对自旋冰材料的研究还存在着很多问题和争议。 第二章重点介绍了我们在浮区法生长 R2Ti2O7（R = Dy，Ho）单晶方面的探索，经过多次尝试，我们得到了比较理想的
4、生长条件，获得了高品质的单晶。磁化率和比热的结果验证了自旋冰的基态性质，并与之前的工作符合很好。另外，我们用低温热传导行为对不同条件下退火的 Dy2Ti2O7单晶进行了深入研究，发现单晶中的氧缺陷对声子峰的影响非常小，从而认为 R2Ti2O7单晶所具有的较小的声子热导峰主要是由于材料中较强的磁涨落对声子产生的磁散射造成的。 关键字：自旋阻挫 自旋冰 R2Ti2O7 热传导 摘 要 ii Abstract Physics of the frustrated magnetic materials is considered as an important branch in condensed m
5、ater physics. The geometric spin frustration, which is caused by the frustration of the geometric symmetry of the crystal structure, is particularly notable. The presence of frustration effect and the incompatible interactions among spins result in the macroscopic ground state degeneracy, showing ri
6、ch quantum magnetic properties. Pyrochlore structure R2Ti2O7 (R = Dy, Ho) is a typical geometrically frustrated of magnetic materials. Its crystal field effect is so strong that the magnetic ions with Ising anisotropy lie on the tetrahedron vertices only along the crystal axis of 111 direction. The
7、competition among frustration effect, spin antiferromagnetic interaction between the nearest neighbor, and the long-range ferromagnetic interaction, which is originated from the magnetic dipole interaction, result in the spin-ice ground state with a six-fold degenerate and the “two-in-two-out“ spin
8、configuration at low temperatures. In addition, the magnetic field in different directions can lift the degeneracy of different levels, showing rich magnetic phase transition behaviors. However, there are still many controversies on the spin-ice ground state and magnetic phase transition behaviors.
9、One of the most attractive progress in this field is the discovery of magnetic monopoles in spin-ice R2Ti2O7, which can explain a lot of exotic phenomena based on the magnetic monopole model. It is still not very clear which method is more perfect between magnetic dipole model and the magnetic monop
10、ole model. Recently, thermal conductivity measurement is recognized as an effective method in studying the magnetic materials. It can be not only used to detect the excitations, such as phonon, magnon, but also to study the spin-phonon coupling, magnetic structure and the quantum phase transitions.
11、In this thesis, we explore the methods of the crystal growth of R2Ti2O7 in order to get the higher quality single crystals, and then we carefully study the magnetic susceptibility, specific heat, thermal conductivity and other properties of the spin-ice materials. The first chapter reviews the previ
12、ous works on the research of spin-ice materials R2Ti2O7 (R = Dy, Ho), including magnetic susceptibility, specific heat, neutron diffraction etc.，which describe the magnetic properties and the magnetic structure, and give a brief description of the magnetic dipole moment model and the model of magnet
13、ic monopoles. It can be seen that there are still many problems and disputes on the spin ice materials. 摘 要 iii The second chapter focuses on the grow conditions of the R2Ti2O7 (R = Dy, Ho) single crystals using the floating-zone method. After many attempts, we succeed in getting some ideal growing
14、conditions, and obtain high-quality single crystals. The low-temperature data of magnetic susceptibility and specific heat verified the spin-ice ground state, which is consistent with the previous works. In addition, we study the low-temperature heat transport behaviors of Dy2Ti2O7 single crystals,
15、which are annealed under different conditions. It is found that the oxygen defects have little effect on the phonon heat transport. The small phonon peak observed in R2Ti2O7 single crystals is mainly due to strong magnetic scattering induced by the magnetic fluctuations of the spin-ice ground state.
16、 Key words: spin frustration，spin ice，R2Ti2O7，thermal conductivity 中国科学技术大学学位论文原创性声明 本人声明所呈交的学位论文,是本人在导师指导下进行研究工作所取得的成果。除已特别加以标注和致谢的地方外，论文中不包含任何他人已经发表或撰写过的研究成果。 与我一同工作的同志对本研究所做的贡献均已在论文中作了明确的说明。 作者签名：_ 签字日期：_ 中国科学技术大学学位论文授权使用声明 作为申请学位的条件之一， 学位论文著作权拥有者授权中国科学技术大学拥有学位论文的部分使用权， 即： 学校有权按有关规定向国家有关部门或机构送交论文的复印件和电子版，允许论文被查阅和借阅，可以将学位论文编入有关数据库进行检索，可以采用影印、缩印或扫描等复制手段保存、 汇编学位论文。 本人提交的电子文档的内容和纸质论文的内容相一致。 保密的学位论文在解密后也遵守此规定。 公开 保密（_年） 作者签名：_
