Atomic scale imaging of magnetic circular dichroism, chemistry and structure of magnetic oxides

Magnetics Society

Speaker – Xiaoyan Zhong – TRACE EM Unit and Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong SAR, P. R. China

The atomic-level knowledge of 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 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 [1], 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 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 [2].

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 are able to access the magnetic circular dichroism with atomic plane resolution [3]. 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. In the examples of complex oxides e.g. Sr₂FeMoO₆ [3], nanocomposite Sr₂Fe_1+xRe_1-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.

Dr. Xiaoyan Zhong is currently an associate Professor in the Department of Materials Science and Engineering at City University of Hong Kong. He received his B.S. degree in Materials Science and

Engineering in 2001 and PhD degree in Materials Science and Engineering in 2007 at Tsinghua University. After three-year postdoctoral research at Argonne National Laboratory in USA, he began his independent academic career as assistant professor and associate professor at Tsinghua University from 2010 to 2020. He joined Department of Materials Science and Engineering at City University of Hong Kong since May 2020. His current research interests involve methodology development of transmission electron microscopy and spectroscopy and their application in solving new challenges in magnetic materials. Recently Zhong's group has developed the quantitative atomic-plane resolved electron magnetic

circular dichroism method and pushed spatial resolution of magnetic circular dichroism into atomic level by achromatic electron microscopy, which was published in the peer-reviewed journals such as Nature Materials, Nature Communications and Advanced Functional Materials. He received “Ten Major Scientific and Technological Progress of China's Colleges and Universities” awarded by Ministry of Education of the People’s Republic of China and the Excellent Young Scholar

References

[1] Schattschneider, P., et al., Detection of magnetic circular dichroism using a transmission electron microscope. Nature, 2006, 441, 486-488.

[2] Wang, Z.Q., et al., Quantitative experimental determination of site-specific magnetic structures by transmitted electrons. Nature Communications, 2013, 4, 1395.

[3] Wang, Z.C., et al., Atomic scale imaging of magnetic circular dichroism by achromatic electron microscopy. Nature Materials, 2018, 17, 221-225.

[4] Ho, P.-L., et al., Effect of cation ratio and order on magnetic circular dichroism in the double perovskite Sr₂Fe_1+xRe_1-xO₆. Ultramicroscopy, 2018. 193. 137-142.

[5] Li, Z., et al., Atomic Structure and Electron Magnetic Circular Dichroism of Individual Rock Salt Structure Antiphase Boundaries in Spinel Ferrites. Advanced Functional Materials