磁光材料是一类兼具磁学和光学性质的材料，可广泛应用于光学存储、光学通信及光子芯片等领域。在过去半个多世纪，大量的基础研究集中在如何通过物理化学方法提高材料的磁光响应，普遍认为元素掺杂和新材料设计是提升磁光材料性能的主要途经。在研究稀土及其它重元素的引入如何提高材料的磁光响应机制方面，还缺少深入到从原子尺度来理解其中机理的实验研究工作。这也是受限于以往材料表征方法的空间分辨率限制和缺少多种序参量协同测量的研究。本学位论文以先进电子显微学方法为主要研究手段，利用其高空间分辨率和高能量分辨率等特点从实空间、动量空间以及能量空间等多个角度对磁光材料中的多种序参量进行测量，在原子尺度上理解铋、稀土元素的掺杂对铁氧体中不同量子序参量（包括点阵、电荷、自旋和轨道序参量等）之间强耦合作用的影响。此外通过对铋掺杂石榴石磁光材料的量子序参量的认识，进一步利用铋元素和磁性铁氧层组合的方式实现了新型磁光材料的设计，结合宏观物性表征和电子显微结构的分析验证了这一新型外延薄膜材料具有室温下的磁光响应。在原子尺度上直接测量了铋掺杂镥铁石榴石中铋元素的晶格占位、磁性离子的电荷分布-自旋构型和轨道劈裂等行为，结合理论计算揭示了铋元素降低了磁光材料中不同占位铁离子的晶体场劈裂能大小，从而改变磁光跃迁过程的能量大小和低能端的磁光跃迁强度。在对铈掺杂钇铁石榴石原子尺度的研究中，揭示了外延生长薄膜中铈元素的畴界偏析，并诱导出Fe(3d) - Ce(4f)轨道之间的电荷转移。结合理论计算，进一步揭示了该电荷转移行为与晶格序参量之间存在强耦合作用，并阐明了一直存在争议的电荷转移行为的结构起源，该行为的产生是来自于铈掺杂磁光材料中的界面效应。从原子尺度上实现了对元素掺杂铁石榴石材料中反相畴界区域的电荷测量和结构解析，该协同测量手段从微观结构上理解了这些反相畴界对磁光性质具有抑制作用。基于对元素掺杂铁石榴石序参量耦合作用的理解，我们进一步利用铋元素和亚铁磁铁氧层组合的方式，成功地在该种层状外延材料中观测到室温下的磁光响应，为磁光材料的设计提供了新思路。本论文通过对复杂元素掺杂磁光材料的多种序参量的协同测量及关联性研究，为元素掺杂和磁光响应的关联效应以及新的磁光材料设计提供了一些依据和指导性意见。本论文也展示了先进电子显微学方法在序参量的协同测量方面的能力和可行性，为磁光功能材料的研发和机理研究起到了一定的促进作用。

Magneto-optical materials exhibit fruitful magnetic and optical properties, leading to its wide application in optical storage, optical communications and optical chips in the future. In the past few years, a large amount of works had been focusing on how can improve the magneto-optical responses in ferrite by trying the different physical or chemical methods. In particular, the element substituting and the development for new magneto-optical materials had been considered as main methods to tune the properties of magneto-optical materials, however, there were less insights into the relationship between the element substitution and improved magneto-optical responses at atomic scale, which was partially limited to the available characterization technique with high spatial resolution in the past few years. Here, we investigate the magneto-optical materials in real space, momentum space and energy space by using the advanced electron microscopy with high spatial and energy resolution, to give atomic-scale insights into the local variations of multiple order parameters (such as lattice, charge, spin and orbital order parameters) due to the substitution of bismuth and rare earth elements. Besides, inspired by the investigation on bismuth substituted iron garnet, the combination of bismuth and ferromagnetic iron oxides are proposed to design new magneto-optical materials, and further confirmed to exhibit the obvious room-temperature magneto-optical response after being investigated by macro-properties characterization and electron microscopy analysis.(i) With the combination of theoretical calculation, the direct investigation at atomic scale on local multiple order parameters reveals how the bismuth substitution can modulate the local lattice structure, charge distribution, spin and orbital splitting effect of iron in different lattice sites, and further demonstrate bismuth substitution can decrease crystal field splitting energy in iron sites, leading to the modulation of magneto-optical transition in the range of low photonic energy. (ii) The atomic-scale investigation on cerium substituted iron garnet reveals that the cerium substituent tends to segregate across the boundaries in the prepared films, lead to the charge transfer between iron and cerium. Further investigation reveals that the observed charge transfer is strongly related to the different lattice sites across the boundaries, which is verified by theoretical calculation. These results help to clarify the origin of charge transfer in cerium substituted iron garnet. (iii) The charge states and structural variation in the regions close to the antiphase boundaries (APBs) in element substituted iron garnet are investigated at atomic-scale. The synergistical characterization reveals that the formation of APBs in element substituted iron garnet can inhibit the magneto-optical responses. (iiii) Inspired by the studies on the bismuth substituted iron garnets, new strategy that the magnetic layers are coupled with bismuth element is proposed to design new magneto-optical materials. This proposed magneto-optical material exhibits layered structure and obvious magneto-optical responses at room temperature. These studies give a new idea for the design of magneto-optical materials. Through the synergistical investigation on element substituted magneto-optical materials at atomic scale, we provide the guidance of the relationship between multiple order parameters and the new design for magneto-optical materials. The results presented in this dissertation also demonstrate that the capabilities of the synergistical investigation at high resolution, by the use of advanced electron microscopy, can promote the research of the functional materials and contribute to the development of new magneto-optical materials.