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学术活动 |
清华大学钟��博士学术报告通知
发表日期：2014-04-16
报告题目：Quantitative experimental determination of site-specific magnetic structures by transmitted electrons
报告人：钟��博士（Xiaoyan Zhong）
Beijing National Center for Electron Microscopy
School of Materials Science and Engineering
The State Key Laboratory of New Ceramics and Fine Processing
Laboratory of Advanced Materials (MOE)
Tsinghua University, Beijing 100084, People’s Republic of China
地点：电镜中心会议室
时间：4月17日下午三点（周四）
In this work, we have developed the site-specific electron energy-loss magnetic chiral dichroism (EMCD) method for magnetic structure determination in magnetic materials with non-equivalent crystallographic sites at a nanometer scale. It’s the first work to experimentally demonstrate that the fast electron as a new source can be used to determine magnetic structure for a wide range of materials, which is generally considered to be accomplished by neutron diffraction. Compared with previous EMCD works in which EMCD was just used for detecting the ferromagnetic signals of materials, we fundamentally raise the EMCD technique to the new level of magnetic structure determination.
In the example of NiFe2O4, we achieve comprehensive magnetic structure information using the site-specific EMCD method under the assumption of no magnetic information known previously. The magnetic structure information we obtain includes site-specific total magnetic moment, site-specific orbital to spin magnetic moment (mL/mS) ratio and total magnetic moment of a unit cell. Our method is testified to be valid by comparing our results with those obtained by theoretical calculations and other experimental techniques such as XMCD and neutron diffraction.
The site-specific EMCD method we developed has its unique values compared with previous magnetism characterization techniques such as XMCD and neutron diffraction. In comparison, using transmitted electron in site-specific EMCD method, we can reach a high spatial resolution, and get site-specific and element-specific magnetic information, as well as distinguish the orbital and spin magnetic moments. For example, using site-specific EMCD method our work first reports the experimentally determined mL/mS ratios of Fe atoms in octahedral and tetrahedral sites.
In the technical aspects, the extremely strong EMCD signals acquired using site-specific EMCD method are at least three times high than the previous reported EMCD spectra, which allow us to do quantitative analysis. We first did the quantitative works on EMCD spectra to obtain total magnetic moments (the sum of spin and orbital magnetic moments) for atoms at different sites.
In sum, our work opens the door of using fast electrons to determine magnetic structures for a wide range of magnetic materials in a nanometer scale. Site-specific EMCD may benefit much not only to the fundamental research of magnetic states and behavior in complex magnetic materials, but also to revealing the magnetic structure in nanostructures or interface of the composite magnetic films.
