Speaker: Xiaoyan Zhong University of Hong Kong, People's Republic of China
Abstract:
Energy-filtered transmission electron microscopy (EFTEM) is an effective method to acquire chemical information from samples with high spatial resolution in the transmission electron microscope (TEM) with parallel beam illumination. However, the major obstacles in the application of EFTEM are related to its low dose efficiency, poor energy resolution (which is related to the slit width of the energy window), poor signal-to-noise ratio, and limited spatial resolution caused by chromatic/spherical aberration. Spatially resolved energy loss electron spectroscopy (SR-EELS) which collects the energy-loss and spatial information at the same time could help to overcome these disadvantages. Thanks to the development of spherical/chromatic aberration (Cs/Cc) corrector, the resolution of SR-EELS is pushed to the atomic level and makes it possible to acquire atomic-scale elemental signals [1]. With a parallel incoming beam, we acquired Cs/Cc corrected atomic plane resolved (APR) imaging of EELS on SrTiO3. With dynamic diffraction calculation and experiment data analysis, we explored the optimized experimental conditions to access atomic-plane-resolved chemical information with APR-EELS, which is also important for collecting magnetic circular dichroism with atomic plane resolution.
The atomic-level knowledge of the local spin configuration of magnetic materials is of great importance to predict and control their physical properties. However, it is highly challenging to experimentally characterize the magnetic properties of such materials with atomic-scale spatial resolution. One of the best options to push the spatial resolution of magnetic imaging lies in the electron energy-loss magnetic chiral dichroism [2], which is also called electron magnetic circular dichroism (EMCD). Physically, X-ray magnetic circular dichroism (XMCD) and EMCD share 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 the 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 [3].
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. Recently by using EMCD and achromatic electron microscopy, we can access the magnetic circular dichroism with atomic plane resolution [1]. Combined with the 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. In the examples of complex oxides e.g. Sr2FeMoO6 [1], nanocomposite Sr2Fe1+xRe1-xO6 [4], and antiphase boundary of NiFe2O4 [5], we would like to show how to achieve atomic-scale magnetic, chemical and structural information and understand the structure-property relationship of these magnetic materials at the atomic level.6
References
1 Wang, Z. C., et al., Nature Materials 17 (2018) 221-225.
2Schattschneider, P. et al., Nature 441 (2006) 486-488.
3Wang, Z.Q., et al., Nature Communications 4 (2013) 1395.
4Ho, P.-L., et al., Ultramicroscopy 193 (2018) 137-142.
5Li, Z., et al., Advanced Functional Materials 31 (2021) 2008306.
6This work was financially supported by NSFC (52171014), Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20210324134402007), Guangdong Provincial Department of Science and Technology (2024A1515012303), Sino-German Center for Research Promotion (M-0265), RGC (C1013-23EF, C1018-22E, CityU 11302121, CityU 11309822), European Research Council (856538, “3D MAGiC”), and CityU (9610484, 9680291, 9678288, 9610607，7020043).
Host: Vincenzo Grillo