| 1. |
- Castellanos-Reyes, José Ángel, et al.
(author)
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Unveiling the impact of temperature on magnon diffuse scattering detection in the transmission electron microscope
- 2023
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 108:13
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Journal article (peer-reviewed)abstract
- Magnon diffuse scattering (MDS) signals could, in principle, be studied with high spatial resolution in scanning transmission electron microscopy (STEM), thanks to recent technological progress in electron energy-loss spectroscopy. However, detecting MDS signals in STEM is technically challenging due to their overlap with the much stronger thermal diffuse scattering (TDS) signals. In bcc Fe at 300 K, MDS signals greater than or comparable to TDS signals have been predicted to occur under the central Bragg disk, well into a currently inaccessible energy-loss region. Therefore, to successfully detect MDS in STEM, it is necessary to identify conditions in which TDS and MDS signals can be distinguished from one another in regions outside the central Bragg disk. Temperature may be a key factor due to the distinct thermal signatures of magnon and phonon signals. In this work, we present a study on the effects of temperature on MDS and TDS in bcc Fe-considering a detector outside the central Bragg disk and a fixed convergent electron probe-using the frozen phonon and frozen magnon multislice methods. Our study reveals that neglecting the effects of atomic vibrations causes the MDS signal to grow approximately linearly up to the Curie temperature of Fe, after which it exhibits less variation. The MDS signal displays an alternating behavior due to dynamical diffraction, instead of increasing monotonically as a function of thickness. The inclusion of the effects of atomic vibrations through a complex atomic electrostatic potential causes the linear growth of the MDS signal to change to a nonlinear behavior that exhibits a predominant peak for a sample of thickness 16.072 nm at 1100 K. In contrast, the TDS signal grows more linearly than the MDS signal through the studied temperature range but still exhibits appreciable dynamical diffraction effects. An analysis of the signal-to-noise ratio (SNR) shows that the MDS signal can be a statistically significant contribution to the total scattering intensity under realizable measurement conditions and feasible acquisition times. For example, our study found that a SNR of 3 can be achieved with a beam current of 1 nA in less than 30 min for the 16.072-nm-thick bcc Fe sample at 1100 K.
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| 2. |
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| 3. |
- Idrobo, Juan Carlos, et al.
(author)
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Detecting magnetic ordering with atomic size electron probes
- 2016
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In: ADVANCED STRUCTURAL AND CHEMICAL IMAGING. - : SPRINGER HEIDELBERG. - 2198-0926. ; 2
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Journal article (peer-reviewed)abstract
- Although magnetism originates at the atomic scale, the existing spectroscopic techniques sensitive to magnetic signals only produce spectra with spatial resolution on a larger scale. However, recently, it has been theoretically argued that atomic size electron probes with customized phase distributions can detect magnetic circular dichroism. Here, we report a direct experimental real-space detection of magnetic circular dichroism in aberration-corrected scanning transmission electron microscopy (STEM). Using an atomic size-aberrated electron probe with a customized phase distribution, we reveal the checkerboard antiferromagnetic ordering of Mn moments in LaMnAsO by observing a dichroic signal in the Mn L-edge. The novel experimental setup presented here, which can easily be implemented in aberration-corrected STEM, opens new paths for probing dichroic signals in materials with unprecedented spatial resolution.
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| 4. |
- Krivanek, Ondrej L., et al.
(author)
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Toward Single Mode, Atomic Size Electron Vortex Beams
- 2014
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In: Microscopy and Microanalysis. - 1431-9276 .- 1435-8115. ; 20:3, s. 832-836
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Journal article (peer-reviewed)abstract
- We propose a practical method of producing a single mode electron vortex beam suitable for use in a scanning transmission electron microscope (STEM). The method involves using a holographic "fork" aperture to produce a row of beams of different orbital angular momenta, as is now well established, magnifying the row so that neighboring beams are separated by about 1 mu m, selecting the desired beam with a narrow slit, and demagnifying the selected beam down to 1-2 angstrom in size. We show that the method can be implemented by adding two condenser lenses plus a selection slit to a straight-column cold-field emission STEM. It can also be carried out in an existing instrument, the monochromated Nion high-energy-resolution monochromated electron energy-loss spectroscopy-STEM, by using its monochromator in a novel way. We estimate that atom-sized vortex beams with <= 20 pA of current should be attainable at 100-200 keV in either instrument.
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| 5. |
- Krizek, Filip, et al.
(author)
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Atomically sharp domain walls in an antiferromagnet
- 2022
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In: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 8:13
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Journal article (peer-reviewed)abstract
- The interest in understanding scaling limits of magnetic textures such as domain walls spans the entire field of magnetism from its physical fundamentals to applications in information technologies. Here, we explore antiferromagnetic CuMnAs in which imaging by x-ray photoemission reveals the presence of magnetic textures down to nanoscale, reaching the detection limit of this established microscopy in antiferromagnets. We achieve atomic resolution by using differential phase-contrast imaging within aberration-corrected scanning transmission electron microscopy. We identify abrupt domain walls in the antiferromagnetic film corresponding to the Néel order reversal between two neighboring atomic planes. Our work stimulates research of magnetic textures at the ultimate atomic scale and sheds light on electrical and ultrafast optical antiferromagnetic devices with magnetic field–insensitive neuromorphic functionalities.
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| 6. |
- Lyon, Keenan, et al.
(author)
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Theory of magnon diffuse scattering in scanning transmission electron microscopy
- 2021
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:21
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Journal article (peer-reviewed)abstract
- We present a theory and a simulation of diffuse scattering due to the excitation of magnons in scanning transmission electron microscopy. The calculations indicate that magnons can present atomic contrast when detected by electron energy-loss spectroscopy using atomic-size electron beams. The results presented here indicate that the intensity of the magnon diffuse scattering in bcc iron at 300 K is 4 orders of magnitude weaker than the intensity of thermal diffuse scattering arising from atomic vibrations.
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| 7. |
- Negi, Devendra Singh, et al.
(author)
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Probing the localization of magnetic dichroism by atomic-size astigmatic and vortex electron beams
- 2018
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In: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 8
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Journal article (peer-reviewed)abstract
- We report localization of a magnetic dichroic signal on atomic columns in electron magnetic circular dichroism ( EMCD), probed by beam distorted by four-fold astigmatism and electron vortex beam. With astigmatic probe, magnetic signal to noise ratio can be enhanced by blocking the intensity from the central part of probe. However, the simulations show that for atomic resolution magnetic measurements, vortex beam is a more effective probe, with much higher magnetic signal to noise ratio. For all considered beam shapes, the optimal SNR constrains the signal detection at low collection angles of approximately 6-8 mrad. Irrespective of the material thickness, the magnetic signal remains strongly localized within the probed atomic column with vortex beam, whereas for astigmatic probes, the magnetic signal originates mostly from the nearest neighbor atomic columns. Due to excellent signal localization at probing individual atomic columns, vortex beams are predicted to be a strong candidate for studying the crystal site specific magnetic properties, magnetic properties at interfaces, or magnetism arising from individual atomic impurities.
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| 8. |
- Negi, Devendra Singh, et al.
(author)
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Proposal for a three-dimensional magnetic measurement method with nanometer-scale depth resolution
- 2018
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In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 98:17
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Journal article (peer-reviewed)abstract
- We propose a magnetic measurement method based on combining depth sectioning and electron magnetic circular dichroism in scanning transmission electron microscopy. Electron vortex beams with large convergence angles, as those achievable in current state-of-the-art aberration correctors, could produce atomic lateral resolution and depth resolution below 2 nm.
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| 9. |
- Negi, Devendra Singh, et al.
(author)
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Prospect for detecting magnetism of a single impurity atom using electron magnetic chiral dichroism
- 2019
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In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 100:10
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Journal article (peer-reviewed)abstract
- Dopants, even single atoms, can influence the electrical and magnetic properties of materials. Here we demonstrate the opportunity for detecting the magnetic response of an embedded magnetic impurity in a nonmagnetic host material. We combine a depth sectioning approach with electron magnetic circular dichroism in scanning transmission electron microscopy to compute the depth-resolved magnetic inelastic-scattering cross section of single Co impurity buried in the host crystal of GaAs. Our calculations suggest that the magnetic dichroic signal intensity is sensitive to the depth and lateral position of the electron probe relative to the magnetic impurity. Additionally, a more precise dichroic signal localization can be achieved via choosing higher-collection-angle (beta) apertures. Quantitative evaluation of the inelastic-scattering cross section and signal-to-noise ratio indicates that the magnetic signal from a single Co atom is on the verge of being detectable with today's state-of-the-art instrumentation.
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| 10. |
- Rusz, Ján, et al.
(author)
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Aberrated electron probes for magnetic spectroscopy with atomic resolution : Theory and practical aspects
- 2016
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In: Physical review B. - 2469-9950. ; 93:10
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Journal article (peer-reviewed)abstract
- It was recently proposed that electron magnetic circular dichroism can be measured in scanning transmission electron microscopy with atomic resolution by tuning the phase distribution of an electron beam. Here, we describe the theoretical and practical aspects for the detection of out-of-plane and in-plane magnetization utilizing atomic size electron probes. We present the calculated optimized astigmatic probes and discuss how to achieve them experimentally.
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| 11. |
- Rusz, Jan, et al.
(author)
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Achieving Atomic Resolution Magnetic Dichroism by Controlling the Phase Symmetry of an Electron Probe
- 2014
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 113:14, s. 145501-
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Journal article (peer-reviewed)abstract
- The calculations presented here reveal that an electron probe carrying orbital angular momentum is just a particular case of a wider class of electron beams that can be used to measure electron magnetic circular dichroism (EMCD) with atomic resolution. It is possible to obtain an EMCD signal with atomic resolution by simply breaking the symmetry of the electron probe phase distribution using the aberration-corrected optics of a scanning transmission electron microscope. The required phase distribution of the probe depends on the magnetic symmetry and crystal structure of the sample. The calculations indicate that EMCD signals utilizing the phase of the electron probe are as strong as those obtained by nanodiffraction methods.
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| 12. |
- Rusz, Jan, et al.
(author)
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Vorticity in electron beams : definition, properties, and its relationship with magnetism
- 2016
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In: Physical Review B. - 2469-9950 .- 2469-9969. ; 94:14
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Journal article (peer-reviewed)abstract
- Vorticity is a concept well established in fluid dynamics to describe the local tendency of a fluid to rotate. Here, we explore the vorticity of electron waves and show that it can be used to qualitatively estimate the strength of an electron magnetic circular dichroism (EMCD) signal, without resorting to expensive inelastic electron scattering calculations. We discuss the properties of vorticity, its relationship with orbital angular momentum, and how it can be used to investigate the characteristics of electron beams.
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| 13. |
- Spiegelberg, Jakob, et al.
(author)
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Local low rank denoising for enhanced atomic resolution imaging
- 2018
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In: Ultramicroscopy. - : Elsevier. - 0304-3991 .- 1879-2723. ; 187, s. 34-42
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Journal article (peer-reviewed)abstract
- Atomic resolution imaging and spectroscopy suffers from inherently low signal to noise ratios often prohibiting the interpretation of single pixels or spectra. We introduce local low rank (LLR) denoising as tool for efficient noise removal in scanning transmission electron microscopy (STEM) images and electron energy-loss (EEL) spectrum images. LLR denoising utilizes tensor decomposition techniques, in particular the multilinear singular value decomposition (MLSVD), to achieve a denoising in a general setting largely independent of the signal features and data dimension, by assuming that the signal of interest is of low rank in segments of appropriately chosen size. When applied to STEM images of graphene, LLR denoising suppresses statistical noise while retaining fine image features such as scan row-wise distortions, possibly related to rippling of the graphene sheet and consequent motion of atoms. When applied to EEL spectra, LLR denoising reveals fine structures distinguishing different lattice sites in the spinel system CoFe2O4.
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| 14. |
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