Electron Microscopy Investigation On Magnetic Circular Dichroism Of Amorphous Materials And Topological Defect-induced Vortex Beams

Electron energy-loss magnetic chiral dichroism(EMCD)is a novel technique that allows the quantitative and element-specific determination of magnetic information.However,constrained by a predefined diffraction geometry applied in regular EMCD experiments,it has not yet been feasible to obtain EMCD signals from amorphous materials due to the lack of long range ordering.Here we propose a protocol for EMCD detection in amorphous materials utilizing a single-crystalline overlayer as an EMCD two-beam splitter.The experiment is performed on the amorphous FeOx thin film grown on an Y2O3 stabilized ZrO2 single-crystalline substrate.Both experimental results and theoretical calculations demonstrate significant EMCD signals of amorphous materials.On the other hand,as a new route to realize the atomic scale resolution of EMCD measurements,electron vortex beams carrying quantized orbital angular momentum are regarded to be a probe for magnetism in materials.To achieve this,the electron vortex beam should be isolated with sufficient small beam size.Here we demonstrate the generation of electron vortex beams making use of a 2? phase-winding topological defect—edge dislocation in NiO—with separation angles of about 12 mrad.As the electron probe scans through the edge dislocation,the Bragg eclipse evolutions are observed in experimental and simulated results.Based on the redundant diffraction measurement,we apply the ptychographic iterative algorithm to solve for the phase of Bragg eclipse and present exit-wave reconstructions with none overlapping disks.Moreover,we also investigate the dislocation pipe diffusion of Mn during the annealing of 0.2%C-5%Mn steel using transmission electron microscopy.The line scan of Mn distribution demonstrated a high Mn concentration in austenite and dislocations,indicating the undergoing dislocation pipe diffusion of Mn at the early stage of annealing besides the ?/? interface diffusion.The in-situ observation at high temperature revealed that due to Ostwald ripening large cementite precipitate grew while small cementite precipitate dissolved via Mn diffusion along dislocations.