Roman Gröger

Electron-beam irradiation of ultrasmall CeO ₂ nanoparticles induces surface oxygen ejection and generates Ce ³⁺ defects, enabling structure engineering and property adjustment.

The process by which magnetic moments switch directions is crucial for understanding the performance of magnetic storage materials and in biomedical applications such as magnetic particle imaging and magnetic hyperthermia. Here, we utilize the geodesic nudged elastic band method to identify the atomic-level minimum energy path for a field-free reorientation of magnetic moments in bulk magnetite and single-domain cuboidal magnetite nanoparticles terminated by { 100 } surfaces. We show that this magnetization reversal involves three successive elementary rotations of magnetic moments in distinct { 110 } planes. For iron-rich terminated nanoparticles, this energy barrier depends on the degree of surface spin anisotropy. However, no such effect was observed for oxygen-rich terminations. The former effect is particularly pronounced in nanoparticles with large surface-to-volume ratio and diminishes as particle size increases.

We recently developed an experimental device that allows us to observe the slip traces under stress at the atomic scale. Here, we report experimental results obtained at the latter scale on Nb single crystals making it possible to observe dislocation dipoles (DD), which are evidenced by two slip traces formed by emerging moving dislocations of opposite Burgers vectors ending very close to each other. The geometry and stability of the DD were fully characterized in the framework of linear anisotropic elasticity theory and by atomistic simulations. This allows us to calculate a local opposite stress impeding dislocation motion of the dislocations of the dipole.

The spontaneous wüstite-to-magnetite phase transition in 23 nm nanocubes was investigated by time-resolved XRD. By using three coupled reaction–diffusion master equations, we created a prediction model capable of showing good agreement with the experimental data.

Epitaxial growth of wurtzite AlN films on Si {111} results in 19% lattice misfit, which gives rise to a large density of threading dislocations with different recombination rates of electron–hole pairs. Here, we investigate types and distributions of threading dislocations of the MOVPE-grown 200 nm AlN/Si {111} film, whereby the dislocations are visualized using the technique of wet chemical etching. Atomic force microscopy suggests the existence of four different types of etch pits without any topological differences. Cross-sectional transmission electron microscope studies on etched samples are employed to associate the types of dislocations with the shapes of their etch pits. The recombination activity of individual dislocations was quantified by measuring the electron beam induced current and by correlative measurement of topography, secondary electron imaging, and the electron beam absorbed current. The strongest recombination activity was obtained for the m + c-type (mixed), c-type (screw), and a + c-type (mixed) threading dislocations, whereas the a-type (edge) threading dislocations were nearly recombination-inactive.

Many transition metals crystalizing in the body-centered cubic (bcc) structure exhibit anomalous slip on low-stressed { 110 } planes at low homologous temperatures, which cannot be reconciled with the Schmid law. Specifically, for uniaxial loading in the center of the [ 001 ] − [ 011 ] − [ 1 ˉ 11 ] stereographic triangle, this is manifested by 1 / 2 [ 111 ] and 1 / 2 [ 1 1 ˉ 1 ˉ ] screw dislocations moving on low-stressed ( 0 1 ˉ 1 ) planes. While the anomalous slip is often attributed to non-planar cores of 1 / 2 ⟨ 111 ⟩ screw dislocations or to the tendency for their networks to glide easily, it remains unclear why it dominates the plastic deformation in some bcc metals, whereas it is weak or even absent in others. Using molecular statics simulations at 0 K, we demonstrate that the anomalous slip in bcc metals is intimately linked with the stability of ⟨ 100 ⟩ screw junctions between two intersecting 1 / 2 ⟨ 111 ⟩ screw dislocations under stress (for example, 1 / 2 [ 111 ] and 1 / 2 [ 1 1 ˉ 1 ˉ ] screws giving rise to the [100] junction). Our atomic-level studies show that in nearly all bcc metals of the 5th and 6th groups these junctions cannot be broken by the applied stress and the three dislocations can only move on the common { 110 } plane (in the above example on the ( 0 1 ˉ 1 ) plane). On the other hand, these junctions are found to be unstable in alkali metals, tantalum, and iron, where the application of stress results in unzipping of the two dislocations and their further glide on the planes predicted for isolated dislocations. These results also suggest that the experimentally observed increased propensity for the anomalous slip in further stages of plastic deformation may be explained by reduced curvatures of 1 / 2 ⟨ 111 ⟩ screw dislocations in dense networks.