Direct Visualization and Quantification of Exchange Bias Coupling of Magnetic Heterojunctions at the Atomic Scale via Spatially-resolved Electron Magnetic Circular Dichroism

The atomic-level knowledge of local magnetic, structural and chemical information of materials with exchange-bias coupled heterointerfaces is of great importance to predict and control their physical properties and performance in logic and memory devices, in order to meet the challenges of ever-increasing demands in new energy-efficient information and communications technologies. However, it is highly challenging to directly obtain magnetic information at the atomic scale owing to the limitation of spatial resolution of the currently available magnetic characterization techniques. To break this bottleneck we have recently developed the atomic-plane resolved electron magnetic circular dichroism (EMCD) method, which allow us access quantitative spin and orbital magnetic moments at the atomic level, and nanoscale magnetic hysteresis loop measurement in a transmission electron microscope (TEM). In this proposal, based on our recent methodological developments we aim to directly visualize the atomic scale magnetic information across the interfaces between antiferromagnetic and ferromagnetic materials in oxide heterojunctions and quantitatively determine the exchange bias field from nanoscale sample regions with specific interfacial configurations, which has not been achieved yet with any other techniques currently available. In addition to magnetic information, we can also provide atomic-scale information of atomic positions, elemental composition and chemical bonding by combining scanning transmission electron microscopy with electron energy-loss spectroscopy and energy dispersive X-ray spectroscopy. The structural, compositional, bonding and magnetic information can be correlated from the very same region, providing deep insight into structure-property relationships in exchange-bias-coupled heterojunctions at the atomic scale. The methodological approach in this proposal is a general and universal solution for other magnetically-coupled interfaces. Additionally, theTime-ResolvedAberration-CorrectedEnvironmental (TRACE) TEM equipped with highly coherent pulse electron gun, double aberration correctors, an electron energy loss spectrometer and an ultra-fast and low-dose-sensitive camera, which has been delivered to City University of Hong Kong and will be installed in late 2020, provide the unique capabilities to significantly suppress the radiation damage and noise level during data acquisition, which is extremely important to improve the signal to noise ratio of experimental EMCD spectra for quantitative analysis. Considering the wide applications of exchange-bias-coupled heterojunctions to various devices in information technology, this project will not only contribute to a fundamental understanding of the cooperative interplay between charge, spin, orbital and lattice degrees of freedom at the atomic level, but also pave the way for new designs of magnetic heterostructures for future applications with improved device performances.