【作者】 刘爽；

【导师】 王云鹏；

【作者基本信息】 中南大学 ， 凝聚态物理， 2024， 硕士

【摘要】 磁子自旋电子学是最近兴起的学科,其旨在研究使用磁子来传输和处理信息,被认为是最有希望的下一代信息技术。在磁子自旋电子学这一交叉学科领域中,如何理解磁子与其所在固体环境之间的相互作用关系是最核心的问题,其中最具代表性的关系主要有磁子和晶格之间的关系,磁子和光之间的关系,磁子和其他元激发之间的关系等,但是理论上对于这些关系的研究仍然欠缺,尤其是缺少系统性的第一性原理研究。本文基于密度泛函理论,运用第一性原理的方法针对性地研究了磁子和光子、晶格和声子的相互作用,获得的主要研究成果如下:（1）对磁子和光相互作用的理论研究,分别研究了磁子的拉曼效应和磁-拉曼效应。对单层和双层Cr I₃中磁子所导致的拉曼散射进行了计算。分析了对称性在磁子拉曼散射中的作用。理论分析和计算结果表明磁子模式的奇偶性决定了是否有一阶磁子拉曼散射,晶体的旋转对称性决定了拉曼散射的选择定则。第一性原理计算表明在单斜双层Cr I₃中,由于层间耦合破坏了对称性,因而其磁子拉曼的选择定则也被破坏。在磁-拉曼效应的部分,我们研究了在自旋轨道耦合的作用下,磁性对声子的拉曼效应产生的影响。我们计算了单层Cr I₃中磁矩沿+（-）z方向排列时A_1g声子的拉曼张量和拉曼强度,计算得到的线偏振拉曼偏转符合实验观测。探讨了线偏振拉曼的偏转程度和圆偏振拉曼的极化率和光子能量以及第一性原理计算中交换关联近似以及U值之间的关系。预测了Cr Br₃和Cr Cl₃中的线偏振拉曼偏转。（2）对磁子和晶格相互作用的理论研究。传统的固体热膨胀理论考虑了声子,但在很大程度上忽略了其他类型的元激发。一般来说,仅考虑声子的贡献就足以描述材料的热膨胀行为。最近的一项实验报道了二维磁体Cr Br₃在低温下的负热膨胀。然而,通过第一性原理计算,我们发现Cr Br₃的声子在低温下引起的负热膨胀比实验低几个数量级。考虑到Cr Br₃是一种磁性半导体,我们认为磁子对Cr Br₃的负热膨胀有重要贡献。因此,我们在热膨胀理论中加入了磁子对亥姆霍兹自由能的贡献,证实了磁子在Cr Br₃负热膨胀中的主导作用,并预测了Cr I₃和Cr Cl₃在低温下的热膨胀行为。（3）对磁子和声子的非线性耦合的理论研究。我们通过第一性原理的方法模拟了Co F₂中Co上磁矩的进动对周围原子的影响,发现其近邻的F原子的受力主要为双倍频,并且受力方向主要为B_1g声子的振动方向,还原了实验上的非线性耦合现象,并且确定了这种耦合来源于自旋轨道耦合。我们还研究了Fe F₂,Mn F₂,Ni F₂材料,发现晶体ab轴平面内的各项异性与这种非线性耦合呈正相关。图26幅,表4个,参考文献155篇更多还原

【Abstract】 Magnon spintronics is a newly emerging discipline,which aims to study using magnons to transmit and process information.Magnon spintronics is considered to be the most promising next generation information technology.How to understand the interaction between magnons and their solid environment is the most core problem in magnon spintronics,the most representative relationships include the relationship between magnons and lattice,the relationship between magnons and light,and the relationship between magnons and other excitation,etc.However,theoretical research on these relationships is still lacking.In particular,there is a lack of systematic first-principles research.In this thesis,based on density functional theory,the interaction between magnons and photons,lattice and phonons is studied using the method of first principles.The main research achievements are as follows:（1）Theoretical study of the interaction of magnons and light.Raman effect due to magnon and magneto-Raman effect are studied respectively.The Raman scattering caused by magnons in monolayer and bilayers Cr I₃is calculated.The role of symmetry in magnon Raman scattering is analyzed.The theoretical analysis and calculation results show that the one magnon Raman scattering is determined by the parity of the magnon modes,and the selection rule of Raman scattering is determined by the rotational symmetry of the crystal.The first principles calculation shows that in the monoclinic bilayer Cr I₃,the selection rule of magnon Raman is broken because the interlayer coupling breaks the rotational symmetry.In the section of magneto-Raman effect,we study the influence of magnetism on the Raman effect due to phonons with spin-orbit coupling.We calculated the Raman tensor and Raman intensity of A_1g phonons in monolayer Cr I₃with the magnetic moment is aligned in the+（-）z direction,and the calculated rotation of linear polarization Raman accords with the experimental observation.The rotation angle of linearly polarized Raman and the circular polarization show strong dependence on the photon energy as well as on the exchange-correlation functional.（2）Theoretical study of the interaction between magnon and lattice.Traditional theories of thermal expansion of solids take into account phonons,but largely ignore other types of fundamental excitation.In general,the contribution of phonons is sufficient to describe the thermal expansion behavior.A recent experiment reported the negative thermal expansion of a two-dimensional magnet Cr Br₃ at low temperatures.However,through first-principles calculations,we found that the phonon induced negative thermal expansion at low temperatures is several orders of magnitude lower than experimentally observation.Considering that Cr Br₃ is a magnetic semiconductor,we believe that magnons have important contribution for the negative thermal expansion of Cr Br₃.Therefore,we added the contribution of magnons to the Helmholtz free energy to the theory of thermal expansion;confirmed the leading role of magnons in the negative thermal expansion of Cr Br₃;and predicted the thermal expansion behavior of Cr I₃ and Cr Cl₃ at low temperatures.（3）Theoretical study of nonlinear coupling of magnon and phonon.The effect of the precession of the magnetic moment on Co in Co F₂ on the surrounding atoms is simulated by first principles.Our calculation results show that within one period of the magnon,the force projected onto the B_1g phonon mode changes by two periods.The nonlinear coupling phenomenon in the experiment is reproduced,we found that this coupling comes from the spin-orbit coupling.We also used this method to Fe F₂,Mn F₂ and Ni F₂,we found that the anisotropy in the ab plane are positively correlated with this nonlinear coupling.更多还原