Research On The Electron Magnetic Chiral Dichroism Technique In The Transmission Electron Microscope

One of the most challenging issues when characterizing magnetic materials in the transmission electron microscope is to obtain quantitative magnetic parameters at the nanometer scale.Electron magnetic chiral dichroism is a new technique that allows the local magnetic properties of materials to be quantitatively measured with close-to-atomic spatial resolution and element specificity in the TEM,following the Lorentz microscopy and off-axis electron holography.However,there are still many problems to be addressed and much room to be developed before it becomes a more general and powerful technique for magnetic characterization in the TEM.In this dissertation,a general way to achieve the quantitative measurement of magnetic parameters are firstly developed.Y₃Fe₅O₁₂2 is used to demonstrate the general method and the magnetic parameters with element-specific and site-specific are obtained.Three main parts are involved:the way of seeking diffraction conditions required for quantitative measurement is established to break through the limitation of site-specific EMCD technique;the up-down asymmetry in the present three-beam case is pointed out to guide the experimental design for intrinsic EMCD signals;the simulations are conducted to optimize the detector positions to improve the signal-noise-ratio.Furthermore,the diffraction geometry is extended to the zone axis with higher symmetry.The EMCD signals for Y₃Fe₅O₁₂ are experimentally detected with the assistant of theoretical simulations under the zone axis diffraction condition.The in-plane EMCD technique is developed under a new specific diffraction geometry,compared to the traditional out-of-plane EMCD technique,making it possible to measure the intrinsic magnetic properties.The experimental results recorded from a Co nanoplate under the Lorentz mode are consistent with the simulations,demonstrating that an EMCD signal originating from in-plane magnetization can be detected successfully.In addition,by combining the converged beam electron diffraction with EMCD technique,the magnetic measurement with high-spatial resolution is applied the Y₃Fe₅O₁₂-Pt interface.Along with the analysis of atomic structure,electron structure and chemical composition by some other advanced techniques in the TEM,the origin of disorder layer at the interface is revealed and the decreased efficiency of spin current transport is attributed to the deteriorated magnetic properties.At last,the other two magnetic characterization techniques,Lorentz microscopy and off-axis electron holography,are demonstrated to study the evolution of magnetic edge-states in geometrically confined FeGe skyrmion nanostripe.The magnetic vectors of the edge-states are quantitatively measured to explain the underlying mechanism of nucleation and alinilation of skyrmion at the edge of nanostripe under the stimuli of spin polarization current.These three different techniques are together compared,and the possible obstacles and corresponding solutions are addressed during their combination for magnetic characterization.