Spatially Resolved Electron Magnetic Circular Dichroism of Magnetically Coupled Interfaces and Antiphase Boundaries

Abstract

The presence of interfaces and defects in crystals can cause disruptions in the otherwise perfect three-dimensional periodic crystalline structure. Therefore, they have always been prominent subjects of research in the field of materials science. With the increasing demand for high-performance memory and logic devices, the need for advanced magnetic materials is continuously growing. Additionally, introducing magnetically coupled interfaces and defects into magnetic functional materials has become popular in the pursuit of self-controlling material properties. Thus, scholars researching magnetic functional materials, such as spintronics, seek in-depth disclosure of the relationship between interface/defect structure and magnetic properties. In this thesis, we utilize the electron magnetic circular dichroism (EMCD) technique based on the transmission electron microscopy to achieve high spatial resolution characterization of the magnetically coupled interfaces within the magnetic exchange spring multilayer system DyFe₂/YFe₂, and the magnetically coupled antiphase boundaries in Sr₂FeReO₆.

In the DyFe₂/YFe₂ multilayers, the magnetic coupling between the hard magnet DyFe₂ and soft magnet YFe₂ can enhance the magnetic energy product. When an external magnetic field is applied to superlattices with large individual thicknesses, the exchange magnetic spring can be formed. However, there has been no direct visualization of the magnetic profile of YFe₂ and DyFe₂ layers at a high resolution. To achieve this purpose, we conducted spatially resolved EMCD measurements in DyFe₂/YFe₂ multilayers using the parallel beam illumination mode in a spherical and chromatic aberration-corrected transmission electron microscope. To better interpret the recorded spatially resolved electron energy loss spectroscopy (SREELS) images, we have developed automated scripts for processing SREELS images at both the low-loss region and the high-loss region. We have discussed the extension of the EMCD processing method from one-dimension to two-dimension systematically. Inelastic scattering cross-sections have been systematically calculated as a reference for computational optimization and quantitative EMCD analysis. We have experimentally investigated the spatially resolved Fe EMCD signals in the layers of YFe₂ and DyFe₂. Our observations have revealed the reversal of Fe EMCD signals in the DyFe₂/YFe₂ multilayers. Our study have also provided detailed information on the magnetic profile within each layer, and we suggest a possible magnetic configuration based on our experimental results and previous research. Chemical and structure information of the DyFe₂/YFe₂ multilayers have been characterized through electron energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDX).

In the double perovskite Sr₂FeReO₆ with magnetically coupled antiphase boundaries, we have discussed the atomic plane resolved beam shift EMCD signals in both perfectly ordered Sr₂FeReO₆ and Sr₂FeReO₆ with antiphase boundaries. For Sr₂FeReO₆ including antiphase boundaries, we first carried out chemical and structure analysis through EELS mapping, statistics analysis of high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) imaging, and image simulation. Density functional theory calculations have been carried out on the Sr₂FeReO₆ with antiphase boundaries confirming an antiferromagnetic coupling at the antiphase boundary. After determining the crystal structure and magnetic configuration, we optimized the illumination parameters for the beam shift EMCD and systematically simulated the atomic plane resolved beam shift EMCD in Sr₂FeReO₆ and Sr₂FeReO₆ including antiphase boundaries. The strength of magnetic signals is highly dependent on the crystal structure and variations in thickness. Beam shift EMCD signals of antiphase boundaries have also been systematically analyzed.

To summarize, we have utilized spatially resolved EMCD to study magnetically coupled interfaces and defects in various material systems. We have used both parallel and convergent illumination modes to achieve high spatial resolution. Our results suggest that spatially resolved EMCD is a versatile method to uncover the relationship between atomic structure and local magnetic properties. This method is of significant importance in understanding the origins of magnetic coupling at interfaces and defects.

Date of Award	23 Apr 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Xiaoyan ZHONG (Supervisor) & Fu-Rong CHEN (Co-supervisor)