163 research outputs found
Tuning magnetic chirality by dipolar interactions
Chiral magnetism has gained enormous interest in recent years because of the
anticipated wealth of applications in nanoelectronics. The demonstrated
stabilization of chiral magnetic domain walls and skyrmions has been attributed
to the actively investigated Dzyaloshinskii-Moriya interaction. Recently,
however, predictions were made that suggest dipolar interactions can also
stabilize chiral domain walls and skyrmions, but direct experimental evidence
has been lacking. Here we show that dipolar interactions can indeed stabilize
chiral domain walls by directly imaging the magnetic domain walls using
scanning electron microscopy with polarization analysis. We further show that
the competition between the Dzyaloshinskii-Moriya and dipolar interactions can
reverse the domain-wall chirality. Finally, we suggest that this competition
can be tailored by a Ruderman-Kittel-Kasuya-Yosida interaction. Our work
therefore reveals that dipolar interactions play a key role in the
stabilization of chiral spin textures. This insight will open up new routes
towards balancing interactions for the stabilization of chiral magnetism
Origin of the time dependence of wet oxidation of AlGaAs
The time-dependence of the wet oxidation of high-Al-content AlGaAs can be either linear, indicating reaction-rate limitation, or parabolic, indicating diffusion-limited rates. The transition from linear to parabolic time dependence can be explained by the increased rate of the formation of intermediate As{sub 2}O{sub 3} vs. its reduction to elemental As. A steadily increasing thickness of the As{sub 2}O{sub 3}-containing region at the oxidation front will shift the process from the linear to the parabolic regime. This shift from reaction-rate-limited (linear) to diffusion-limited (parabolic) time dependence is favored by increasing temperature or increasing Al mole fraction
Differences between students and physicians in their entitlement towards procedural skills education â a needs assessment of skills training in internal medicine
Objective: Procedural skills education has a high priority for medical students. However, it is not clear what kind of skills they consider important and whether their perception differs from the physiciansâ view
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Contributions of inelastically scattered electrons to defect images.
We have made measurements of the contribution of inelastically scattered electrons to images of dislocations in Ni{sub 3}Ga and nanometer-sized defects in ion-irradiated Au under weak-beam dark-field diffraction conditions [1]. The purpose is to determine the conditions for data acquisition required to eventually make detailed and quantitative comparisons to simulations of images for various defect models, thus determining defect structure, composition, and local strain field. Such image simulations usually consider only elastically scattered electrons, and thus it is important to understand and possibly eliminate the contribution of inelastically scattered electrons to the experimental images for quantitative comparisons with image simulations. Experimental data have been acquired with either JEOL 2010F or 3000F microscopes, both equipped with Gatan Imaging Filter electron spectrometers. Samples examined in the 2010F were Au, ion-irradiated to low dose (10{sup 11}Kr ions at 1 MeV energy) to form individual defects (1-10nm sized Frank dislocation loops and partial stacking fault tetrahedra). Samples examined in the 3000F were Ni{sub 3}Ga with long dislocation defects. Imaging conditions included weak-beam dark-field with deviation parameter generally > 0.2 nm{sup -1}. Energy filter slit width was set to 10 eV and centered on the zero loss peak in both instruments to obtain images produced by elastically scattered electrons. In the Au experiment an energy window was also set to image the 10-40 eV loss range to investigate the contribution to the defect images of plasmon-loss electrons. Electron intensity in defect images and backgrounds was measured by summing pixel values in appropriately sized rectangles using Digital Micrograph (Gatan) software
Magnetic chirality controlled by the interlayer exchange interaction
Chiral magnetism, wherein there is a preferred sense of rotation of the
magnetization, has become a key aspect for future spintronic applications. It
determines the chiral nature of magnetic textures, such as skyrmions, domain
walls or spin spirals, and a specific magnetic chirality is often required for
spintronic applications. Current research focuses on identifying and
controlling the interactions that define the magnetic chirality. The influence
of the interfacial Dzyaloshinskii-Moriya interaction (iDMI) and, recently, the
dipolar interactions have previously been reported. Here, we experimentally
demonstrate that an indirect interlayer exchange interaction can be used as an
additional tool to effectively manipulate the magnetic chirality. We image the
chirality of magnetic domain walls in a coupled bilayer system using scanning
electron microscopy with polarization analysis (SEMPA). Upon increasing the
interlayer exchange coupling, we induce a transition of the magnetic chirality
from clockwise rotating N\'eel walls to degenerate Bloch-N\'eel domain walls
and we confirm our findings with micromagnetic simulations. In multi-layered
systems relevant for skyrmion research a uniform magnetic chirality across the
magnetic layers is often desired. Additional simulations show that this can be
achieved for reduced iDMI values when exploiting the interlayer exchange
interaction. This work opens up new ways to control and tailor the magnetic
chirality by the interlayer exchange interaction.Comment: Ms was off by a factor
Metal Core Bonding Motifs of Monodisperse Icosahedral Au13 and Larger Au Monolayer-Protected Clusters As Revealed by X-ray Absorption Spectroscopy and Transmission Electron Microscopy
The atomic metal core structures of the subnanometer clusters Au13[PPh3]4[S(CH2)11CH3]2Cl2 (1) and Au13[PPh3]4[S(CH2)11CH3]4 (2) were characterized using advanced methods of electron microscopy and X-ray absorption spectroscopy. The number of gold atoms in the cores of these two clusters was determined quantitatively using high-angle annular dark field scanning transmission electron microscopy. Multiple-scattering-path analyses of extended X-ray absorption fine structure (EXAFS) spectra suggest that the Au metal cores of each of these complexes adopt an icosahedral structure with a relaxation of the icosahedral strain. Data from microscopy and spectroscopy studies extended to larger thiolate-protected gold clusters showing a broader distribution in nanoparticle core sizes (183 ± 116 Au atoms) reveal a bulklike fcc structure. These results further support a model for the monolayer-protected clusters (MPCs) in which the thiolate ligands bond preferentially at 3-fold atomic sites on the nanoparticle surface, establishing an average composition for the MPC of Au180[S(CH2)11CH3]40. Results from EXAFS measurements of a gold(I) dodecanethiolate polymer are presented that offer an alternative explanation for observations in previous reports that were interpreted as indicating Au MPC structures consisting of a Au core, Au2S shell, and thiolate monolayer
Stress-driven instability in growing multilayer films
We investigate the stress-driven morphological instability of epitaxially
growing multilayer films, which are coherent and dislocation-free. We construct
a direct elastic analysis, from which we determine the elastic state of the
system recursively in terms of that of the old states of the buried layers. In
turn, we use the result for the elastic state to derive the morphological
evolution equation of surface profile to first order of perturbations, with the
solution explicitly expressed by the growth conditions and material parameters
of all the deposited layers. We apply these results to two kinds of multilayer
structures. One is the alternating tensile/compressive multilayer structure,
for which we determine the effective stability properties, including the effect
of varying surface mobility in different layers, its interplay with the global
misfit of the multilayer film, and the influence of asymmetric structure of
compressive and tensile layers on the system stability. The nature of the
asymmetry properties found in stability diagrams is in agreement with
experimental observations. The other multilayer structure that we study is one
composed of stacked strained/spacer layers. We also calculate the kinetic
critical thickness for the onset of morphological instability and obtain its
reduction and saturation as number of deposited layers increases, which is
consistent with recent experimental results. Compared to the single-layer film
growth, the behavior of kinetic critical thickness shows deviations for upper
strained layers.Comment: 27 pages, 11 figures; Phys. Rev. B, in pres
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