2-16.Atomic scale co-located characterization of structure, composition and magnetic moments of materials

Xiaoyan Zhong

National Center for Electron Microscopy in Beijing, Key Laboratory of Advanced Materials (MOE), The State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China.

Abstract: The atomic-level knowledge of local spin configuration of the magnetic materials is of great importance to predict and control their physical properties, in order to meet the challenges of ever-increasing demands on performance of functional materials. However, it is highly challenging to experimentally characterize magnetic properties of such materials with atomic scale spatial resolution.

      The leading techniques in spatially resolved magnetic imaging are magnetic exchange force microscopy and spin polarized scanning tunneling microscopy. However, as they are surface sensitive, very little information can be obtained regarding bulk or buried materials. The X-ray magnetic circular dichroism (XMCD) combined with photoelectron emission microscopy technique is a very attractive alternative because it has the spatial resolution as high as the polarized x-ray beam size besides element specific feature, as it is less surface sensitive and can be used to look at the interior of the thin films. However, the length scale of magnetic contrast using highly brilliant left and right circularly polarized X-ray beams is around 15nm.

       The best option to push the spatial resolution of the spectromicroscopies lies in the electron beam equivalent technique electron energy-loss magnetic chiral dichroism (EMCD), which is also called electron magnetic circular dichroism. Physically, XMCD and EMCD shares the same underlying physics in which the angular momentum transferred during X-ray absorption or inelastic electron scattering can selectively excite magnetic sublevels in atoms. The structured electron beams generated through interference of suitably phased plane waves can produce beams with orbital angular momentum. Electron beams can be easily focused compared with X-rays, allowing for atomic scale magnetism to be probed. Previously, we have found a strong EMCD signal in transition metal oxides allowing them to use standing wave methods to identify the different spin states of Fe atoms with site specificity.

       In principle EMCD can offer higher spatial resolution and greater depth sensitivity due to the short de Broglie wavelength and penetration of high-energy electrons compared to XMCD. Our approach combines spatially-resolved EMCD with the latest developments in chromatic aberration corrected electron microscopy, which reduces the focal spread of inelastically scattered electrons by orders of magnitude when compared with the use of spherical aberration correction alone. In the example of complex oxide of ferromagnetic Sr2FeMoO6 with a Curie temperature of ~420 K and a tetragonal double-perovskite structure, magnetic circular dichroism spectra of Fe are imaged atomic plane by atomic plane as shown in Figure 1, which can provide quantitative information of element-selective orbital and spin magnetic moments at the atomic level. Combining with advanced capability of structural and chemical imaging by using aberration-corrected transmission electron microscopy, all the information including magnetic polarization, atomic configurations, and chemical states can be simultaneously accessed from the very same sample region.
        The spatial resolution of atomic-plane resolved EMCD method goes beyond that of any currently available technique, including XMCD and neutron diffraction. The structural, compositional and bonding information can be correlated with local spin configurations on the atomic scale, providing deep insight into structure-property relationships in magnetic materials at the atomic level. These informations are not only contribute to a fundamental understanding of the local interplay between charge, spin, orbital and lattice degrees of freedom in magnetic functional materials, but also pave the way for new designs of magnetic materials for future applications with improved device functionality.

Figure 1. (a) Crystallographic and magnetic structure of ferromagnetic double-perovskite Sr2FeMoO6. (b) High-angle annular dark field scanning transmission electron microscopy image of Sr2FeMoO6 along a pseudo-cubic [110] zone axis reveals the positions of Sr, Fe and Mo atoms. (c) Atomic scale energy-dispersive spectroscopy mapping shows each columns of Sr atoms in green, Fe atoms in red and Mo atoms in blue. (d) Atomic plane resolved electron magnetic circular dichroism of Fe in Sr2FeMoO6.

       原子尺度协同表征材料结构成分磁矩

钟虓䶮

           清华大学北京电子显微镜中心、先进材料教育部重点实验室、
新型陶瓷与精细工艺国家重点实验室、材料学院，中国北京。

摘要:磁性材料被广泛应用于国民经济和国家安全中的各个领域，信息科技的高速发展尤其对 磁性材料的先进性能研发提出了迫切需求。实现自旋构型与材料结构的原子尺度协同定量表征， 是理解、预测与调控磁性材料的物理性能的关键。实现自旋构型原子尺度成像仍然是材料表征 领域的一大挑战，在当今材料科学基础研究中具有重大的科学意义，在设计制造高密度、低功 耗、快速的存储器件、推进信息与通讯技术方面有广阔的应用前景。

      具有高空间分辨的磁成像技术目前应用较多的有磁交换力显微学与自旋极化扫描隧道显微 学等的扫描探针显微学，它们是以扫描隧道显微镜、原子力显微镜为基础发展而来的，具有原 子尺度分辨率。然而这些磁性测量的方法都是基于对材料表面原子磁性信息的测量，难以测量 材料内部的磁性能，且无法获得磁圆二色性谱。在同步辐射源的软 X 射线显微术中发展起来的 X 射线磁圆二色性谱(X-ray Magnetic Circular Dichriosm, XMCD)技术，它的原理是磁性材料 在左旋圆偏振 X 射线和右旋圆偏振 X 射线的照射下，其 X 射线吸收谱在磁性元素的电离边处不同，两个谱图之差即为磁圆二向色性信号。XMCD 技术具有优异的能量分辨率和信噪比。目前 国际上同步辐射 X 射线的空间分辨率最高能达到 10nm，通常在几十个纳米到亚微米尺度。因此， XMCD 技术也很难实现在原子尺度上测量材料磁圆二色性信号。在测量材料微观磁结构和磁性 能方面，传统的透射电子显微学(Transmission Electron Microscopy, TEM)方法是洛伦兹电子显 微技术和电子全息技术，目前这两种技术空间分辨率最高可达到 1nm，但只能给出材料内部净 磁矩信息，无法实现原子分辨和元素分辨，不能区分自旋和轨道磁矩。

      近些年以来，随着电子磁圆二色性谱(Electron Magnetic Circular Dichroism, EMCD),也称 电子能量损失磁手性二向色性谱(Electron Energy-loss Magnetic Chiral Dichroism)的出现，电子 显微学在微观磁结构与磁性能研究领域有了突破性的进展。EMCD 技术的基本物理原理类似于 XMCD 技术，可获得材料的本征磁圆二色性(Magnetic Circular Dichroism, MCD)谱，进而定 量计算得到材料中不同元素各自的总磁矩、轨道磁矩、自旋磁矩等磁学信息。较之于 XMCD 所 使用的 X 射线与中子衍射所使用的中子源，EMCD 所使用的高能电子具有更短的波长，EMCD 可以提供更高的空间分辨率和更高的深度灵敏度。理论上可实现在原子尺度上测量磁圆二色性谱解析磁结构，这也是目前 XMCD 和中子衍射所无法达到的空间分辨率。

      近期我们通过利用色差校正的透射电子显微镜，我们能够实现原子面分辨的磁圆二色谱成 像技术。通过使用先进的像差校正透射电子显微镜的结构和化学成像技术，可以从同一样品区 域在原子尺度同时获得材料的自旋构型，原子排布，化学信息等。以 Sr2FeMoO6 和 Sr2Fe1+xRe1-xO6 氧化物为例，我们将展示如何获得原子尺度磁矩，化学和结构信息，为原子尺度了解磁性材料 的构效关系奠定了基础。

      我们发展了原子尺度高通量结构-成分-磁炬多参量协同表征技术。该技术实现在原子尺度理 解自旋、晶格、电荷、轨道等多个自由度的结构参量与材料磁性能之间的相互关联，推进对先 进磁性材料的表征与调控。本技术是应用色差球差校正电子显微术与空间分辨电子磁圆二色谱， 从原子尺度获得的磁圆二色谱中提取定量的轨道与自旋磁矩，并结合像差校正电子显微学结构 表征手段，将材料的轨道自旋磁矩分布磁信息与其原子构型、元素组成、化学键合等结构信息 在原子层次上一一对应，从实验方法上建立原子尺度结构-成分-磁炬的空间映射模型。

关键词:电子磁圆二色谱;透射电子显微学;定量轨道自旋磁矩;原子尺度;结构-成分-磁炬协 同表征