交换弹簧超晶格耦合机制的透射电镜高分辨定量研究

Accurately adjusting the macro-properties of the hard/soft magnetic exchange spring multilayer and improving the accuracy of its related spintronic devices largely depend on the correct understanding of its interfacial magnetic coupling mechanism. Single crystalline DyFe2/YFe2 superlattice is a model system to study the mechanism of hard/soft exchange spring. With this model, it is possible to exclude some influential factors that are difficult to distinguish and control, such as interface roughness and grain size, and focus on more controllable single factor such as the thickness of each layer. The inevitable crystal defects and strain variation in real thin films, however, may introduce nonideal and nonhomogeneous magnetic coupling behavior, which requires further investigation. In this project, we will apply the energy-loss magnetic chiral dichroism(EMCD) technique, which is a high-resolution and quantitative measurement technique of the element-specific magnetic moments with transmission electron microscopy(TEM), to investigate the nonhomogeneous interfacial coupling behavior. The morphology, crystal structure and element-specific magnetic moment information will be simultaneously obtained at nanometer scale in TEM, and the dynamic diffraction and first-principles calculations will also be performed accordingly. Thus the lateral and longitudinal nonhomogeneity of the interfacial coupling behavior related to crystal defects and strain variation respectively will be revealed. The research results may provide a deep understanding of the effects of defects and strain on the interfacial magnetic coupling in the exchange spring system, and may also present a feasible method to investigate the magnetic coupling of multilayers in high resolution TEM.

深刻理解硬磁/软磁交换弹簧系统中界面磁耦合的影响因素及机制，是精确调控其宏观性能、提高相关自旋电子器件精度的基础和关键。该系统的模型材料DyFe2/YFe2超晶格独具单晶属性，便于在研究中排除界面粗糙度、晶粒尺寸等难以准确区分和控制的因素。但材料中的晶体缺陷和应力变化仍可引起层间界面的非均匀耦合，而从微观上建立缺陷和应力与磁耦合变化的对应关系是理解这两种影响因素的关键。新兴透射电镜磁表征技术“电子能量损失谱磁手性各向异性(EMCD)”具备高空间分辨、元素分辨和磁矩定量的优势，可用于实现该微观研究。本项目将基于EMCD技术，同时获取DyFe2/YFe2超晶格中纳米尺度的形貌、晶体结构和界面磁结构，结合动力学衍射和第一性原理计算，定量研究分别由缺陷和应力引起的界面磁结构的横向和纵向非均匀性。研究结果将揭示缺陷和应力对交换弹簧界面耦合的影响机制，同时也为多层膜磁耦合的高分辨研究提供一套新的方法。

准确理解多层膜界面磁耦合的影响因素及机制，是实现精确的性能调控、提高相关自旋电子器件精度的基础和关键。本项目基于新兴高分辨定量磁表征谱技术——电子能量损失谱磁手性各向异性(EMCD)，发展了一种基于透射电镜的多层膜界面磁结构二维成像方法，可用于薄膜材料体系界面磁耦合相关的研究。该成像方法克服此前磁结构研究中观测方法空间分辨率不足等局限，可获取纳米分辨率的形貌、晶体结构以及磁矩信息，其中磁矩信息具有自旋取向分辨和元素分辨的特性。项目将这种方法应用在DyFe2/YFe2超晶格多层膜中，成功地观察到该多层膜界面Fe 3d磁矩的交换弹簧自旋构型及其在二维空间的非均匀变化，并观察到Fe磁矩的自旋取向在晶体缺陷的周围发生了明显的变化。另外，在EMCD谱技术的定量分析方面取得了两项成果。第一，实验发现了薄膜材料中下层非磁性层的存在及其厚度的变化均会显著影响其上层的磁性膜的各向异性信号强度，进而会影响磁矩的定量结果。第二，将EMCD谱技术成功应用到磁性纳米嵌埋颗粒膜材料中，通过探测和比较不同区域中数个纳米颗粒在薄膜面外方向的总自旋和轨道磁矩，观察到不同取向的铁磁颗粒之间的磁耦合。上述研究成果为薄膜材料磁耦合的高分辨研究提供一套可行的方法，可广泛应用于对薄膜材料界面磁耦合影响因素及机制的深入研究。