| 1. |
- Gliech, Manuel, et al.
(författare)
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Solute Incorporation at Oxide-Oxide Interfaces Explains How Ternary Mixed-Metal Oxide Nanocrystals Support Element-Specific Anisotropic Growth
- 2020
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-301X .- 1616-3028. ; 30:10
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Tidskriftsartikel (refereegranskat)abstract
- Fundamental understanding of anisotropic growth in oxide nanocrystals is crucial to establish new synthesis strategies and to tailor the nanoscale electronic, magnetic, optical, and electrocatalytic properties of these particles. While several growth investigations of metal alloy nanoparticles have been reported, mechanistic studies on the growth of ternary oxide materials are still missing. This work constitutes the first study on the evolution of anisotropic growth of manganese–cobalt oxide nanoparticles by monitoring the elemental distribution and morphology during the particle evolution via scanning transmission electron microscopy–X‐ray spectroscopy. A new growth mechanism based on a “solution‐solid‐solid” pathway for mixed manganese cobalt oxides is revealed. In this mechanism, the MnO seed formation occurs in the first step, followed by the surface Co enrichment, which catalyzes the growth along the <100> directions in all the subsequent growth stages, creating rod, cross‐, and T‐shaped mixed metal oxides, which preferentially expose {100} facets. It is shown that the interrelation of both Mn and Co ions initializes the anisotropic growth and presents the range of validity of the proposed mechanism as well as the shape‐determining effect based on the metal‐to‐metal ratio.
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| 2. |
- Görlin, Mikaela, et al.
(författare)
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Formation of unexpectedly active Ni-Fe oxygen evolution electrocatalysts by physically mixing Ni and Fe oxyhydroxides
- 2019
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Ingår i: Chemical Communications. - : Royal Society of Chemistry (RSC). - 1359-7345 .- 1364-548X. ; 55:6, s. 818-821
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Tidskriftsartikel (refereegranskat)abstract
- We present an unusual, yet facile, strategy towards formation of physically mixed Ni-Fe(OxHy) oxygen evolution electrocatalysts. We use in situ X-ray absorption and UV-vis spectroscopy, and high-resolution imaging to demonstrate that physical contact between two inferior Ni(OH)(2) and Fe(OOH) catalysts self-assemble into atomically intermixed Ni-Fe catalysts with unexpectedly high activity.
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| 3. |
- Hoffmann, Markus, et al.
(författare)
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Skyrmions, Antiskyrmions and Bobbers : Novel particles in chiral magnets
- 2019
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Ingår i: The 10th International Conference on Metamaterials, Photonic Crystals and Plasmonics, META 2019. - : META Conference. ; , s. 677-678
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Konferensbidrag (refereegranskat)abstract
- Chiral magnets are an emerging class of topological matter harbouring localized and topologically protected vortexlike magnetic textures called skyrmions, which are currently under intense scrutiny as a new entity for information storage and processing. Chirality in magnetism is introduced through the chiral symmetry breaking Dzyaloshinskii-Moriya interaction, which arizes through the spin-orbit interaction in magnets with structural bulk and interface inversion asymmetry. We have shown that chiral magnets cannot only host skyrmions but also antiskyrmions and chiral bobbers as localized magnetization particles. We present a multiscale approach relating the electronic structure to an atomistic spin-lattice model and further to micromagnetic models by means of densityfunctional calculations, derive practical criteria for their occurrence and coexistence with skyrmions and minimizing the energy on a mesoscopic scale applying varying spinrelaxation methods. We conjecture on possible materials for antiskyrmions and give experimental evidence for bobbers. The technology potential of having two distinctly different stable particles will be discussed.
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| 4. |
- Jin, Chiming, et al.
(författare)
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Control of morphology and formation of highly geometrically confined magnetic skyrmions
- 2017
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Ingår i: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- The ability to controllably manipulate magnetic skyrmions, small magnetic whirls with particle-like properties, in nanostructured elements is a prerequisite for incorporating them into spintronic devices. Here, we use state-of-the-art electron holographic imaging to directly visualize the morphology and nucleation of magnetic skyrmions in a wedge-shaped FeGe nanostripe that has a width in the range of 45-150 nm. We find that geometrically-confined skyrmions are able to adopt a wide range of sizes and ellipticities in a nanostripe that are absent in both thin films and bulk materials and can be created from a helical magnetic state with a distorted edge twist in a simple and efficient manner. We perform a theoretical analysis based on a three-dimensional general model of isotropic chiral magnets to confirm our experimental results. The flexibility and ease of formation of geometrically confined magnetic skyrmions may help to optimize the design of skyrmion-based memory devices.
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| 5. |
- Josten, Elisabeth, et al.
(författare)
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Strong size selectivity in the self-assembly of rounded nanocubes into 3D mesocrystals
- 2020
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Ingår i: Nanoscale Horizons. - : Royal Society of Chemistry (RSC). - 2055-6764 .- 2055-6756. ; 5:7, s. 1065-1072
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Tidskriftsartikel (refereegranskat)abstract
- The self-assembly of nanoparticles into highly ordered crystals is largely influenced by variations in the size and shape of the constituent particles, with crystallization generally not observed if their polydispersity is too large. Here, we report on size selectivity in the self-assembly of rounded cubic maghemite nanoparticles into three-dimensional mesocrystals. Different X-ray scattering techniques are used to study and compare a nanoparticle dispersion that is used later for self-assembly, an ensemble of mesocrystals grown on a substrate, as well as an individual mesocrystal. The individual lm-sized mesocrystal is isolated using a focused-ion-beam-based technique and investigated by the diffraction of a micro-focused X-ray beam. Structural analysis reveals that individual mesocrystals have a drastically smaller size dispersity of nanoparticles than that in the initial dispersion, implying very strong size selectivity during self-assembly. The small size dispersity of the nanoparticles within individual mesocrystals is accompanied by a very narrow lattice parameter distribution. In contrast, the lattice parameter distribution within all mesocrystals of an ensemble is about four times wider than that of individual mesocrystals, indicating significant size fractionalization between mesocrystals during self-assembly. The small size dispersity within each mesocrystal has important implications for their physical properties.
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| 6. |
- Kovacs, Andras, et al.
(författare)
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Effect of post-growth annealing on secondary phase formation in low-temperature-grown Mndoped GaAs
- 2013
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 46:14, s. 7-145309
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Tidskriftsartikel (refereegranskat)abstract
- The microstructures of annealed GaAs layers containing 0.1%, 0.5% and 2% Mn are studied using aberration-corrected transmission electron microscopy (TEM). The layers were grown by molecular beam epitaxy at 270 ◦C. After heat treatment at 400, 560 and 630 ◦C, they are found to contain precipitate complexes of cubic or hexagonal (Mn, Ga) As, orthorhombic or rhombohedral As and voids. Information about the crystallographic structures and compositions of the phases is obtained using high-resolution TEM, scanning TEM and energy-dispersive x-ray spectroscopy. A phase diagram for secondary phase formation in annealed GaMnAs layers doped with low Mn concentrations is proposed.
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| 7. |
- Kovacs, Andras, et al.
(författare)
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Graphoepitaxy of High-Quality GaN Layers on Graphene/6H-SiC
- 2015
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Ingår i: ADVANCED MATERIALS INTERFACES. - : Wiley: 12 months. - 2196-7350. ; 2:2
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Tidskriftsartikel (refereegranskat)abstract
- The implementation of graphene layers in gallium nitride (GaN) heterostructure growth can solve self-heating problems in nitride-based high-power electronic and light-emitting optoelectronic devices. In the present study, high-quality GaN layers are grown on patterned graphene layers and 6H-SiC by metalorganic chemical vapor deposition. A periodic pattern of graphene layers is fabricated on 6H-SiC by using polymethyl methacrylate deposition and electron beam lithography, followed by etching using an Ar/O-2 gas atmosphere. Prior to GaN growth, an AlN buffer layer and an Al0.2Ga0.8N transition layer are deposited. The atomic structures of the interfaces between the 6H-SiC and graphene, as well as between the graphene and AlN, are studied using scanning transmission electron microscopy. Phase separation of the Al0.2Ga0.8N transition layer into an AlN and GaN superlattice is observed. Above the continuous graphene layers, polycrystalline defective GaN is rapidly overgrown by better quality single-crystalline GaN from the etched regions. The lateral overgrowth of GaN results in the presence of a low density of dislocations (approximate to 10(9) cm(-2)) and inversion domains and the formation of a smooth GaN surface.
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| 8. |
- Li, Zhuo, et al.
(författare)
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Atomic Structure and Electron Magnetic Circular Dichroism of Individual Rock Salt Structure Antiphase Boundaries in Spinel Ferrites
- 2021
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Ingår i: Advanced Functional Materials. - : John Wiley & Sons. - 1616-301X .- 1616-3028. ; 31:21
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Tidskriftsartikel (refereegranskat)abstract
- Spinel ferrites are an important class of materials, whose magnetic properties are of interest for industrial applications. The antiphase boundaries (APBs) that are commonly observed in spinel ferrite films can hinder their applications in spintronic devices and sensors, as a result of their influence on magnetic degradation and magnetoresistance of the materials. However, it is challenging to correlate magnetic properties with atomic structure in individual APBs due to the limited spatial resolution of most magnetic imaging techniques. Here, aberration-corrected scanning transmission electron microscopy and electron energy-loss magnetic chiral dichroism are used to measure the atomic structure and electron magnetic circular dichroism (EMCD) of a single APB in NiFe2O4 that takes the form of a rock salt structure interlayer and is associated with a crystal translation of (1/4)a[011]. First principles density functional theory calculations are used to confirm that this specific APB introduces antiferromagnetic coupling and a significant decrease in the magnitude of the magnetic moments, which is consistent with an observed decrease in EMCD signal at the APB. The results provide new insight into the physical origins of magnetic coupling at an individual defect on the atomic scale.
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| 9. |
- Rotunno, Enzo, et al.
(författare)
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Orbital angular momentum resolved electron magnetic chiral dichroism
- 2019
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 100:22
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Tidskriftsartikel (refereegranskat)abstract
- We propose to use the recently introduced orbital angular momentum spectrometer in a transmission electron microscope to perform electron magnetic chiral dichroism experiments, dispersing the inelastically scattered electrons from a magnetic material in both energy and angular momentum. The technique offers several advantages over previous formulations of electron magnetic chiral dichroism as it requires much simpler experimental conditions in terms of specimen orientation and thickness. A simulation algorithm, based on the multislice description of the beam propagation, is used to anticipate the advantages of the approach over current electron magnetic chiral dichroism implementations. Numerical calculations confirm an increased magnetic signal to noise ratio with in plane atomic resolution.
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| 10. |
- Sadowski, Janusz, et al.
(författare)
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GaAs-MnAs nanowires
- 2011
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Ingår i: Physica Status Solidi. B: Basic Research. - : Wiley. - 0370-1972. ; 248:7, s. 1576-1580
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Tidskriftsartikel (refereegranskat)abstract
- Different strategies for obtaining nanowires (NWs) with ferromagnetic properties using the molecular beam epitaxy (MBE) grown nanostructures combining GaAs and Mn were investigated. Four types of structures have been studied: (i) self-catalyzed GaAs: Mn NWs grown at low temperatures on GaAs(100) substrates; (ii) GaAs: Mn NWs grown at high temperatures on Si(100) substrates; (iii) GaAs-GaMnAs core-shell NW structures; (iv) GaAs-MnAs core-shell NW structures grown on Si(100). Structures of types (i), (iii), and (iv) exhibit ferromagnetic properties. Right: Scanning electron microscopy image of Mn doped GaAs NWs with Ga droplets at the tops, grown by MBE on oxidized Si(100) substrate in the autocatalytic growth mode. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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