147 research outputs found
Damage in graphene due to electronic excitation induced by highly charged ions
Graphene is expected to be rather insensitive to ionizing particle radiation.
We demonstrate that single layers of exfoliated graphene sustain significant
damage from irradiation with slow highly charged ions. We have investigated the
ion induced changes of graphene after irradiation with highly charged ions of
different charge states (q = 28-42) and kinetic energies E_kin = 150-450 keV.
Atomic force microscopy images reveal that the ion induced defects are not
topographic in nature but are related to a significant change in friction. To
create these defects, a minimum charge state is needed. In addition to this
threshold behaviour, the required minimum charge state as well as the defect
diameter show a strong dependency on the kinetic energy of the projectiles.
From the linear dependency of the defect diameter on the projectile velocity we
infer that electronic excitations triggered by the incoming ion in the
above-surface phase play a dominant role for this unexpected defect creation in
graphene
Domain walls in (Ga,Mn)As diluted magnetic semiconductor
We report experimental and theoretical studies of magnetic domain walls in an
in-plane magnetized (Ga,Mn)As dilute moment ferromagnetic semiconductor. Our
high-resolution electron holography technique provides direct images of domain
wall magnetization profiles. The experiments are interpreted based on
microscopic calculations of the micromagnetic parameters and
Landau-Lifshitz-Gilbert simulations. We find that the competition of uniaxial
and biaxial magnetocrystalline anisotropies in the film is directly reflected
in orientation dependent wall widths, ranging from approximately 40 nm to 120
nm. The domain walls are of the N\'eel type and evolve from near-
walls at low-temperatures to large angle [10]-oriented walls and small
angle [110]-oriented walls at higher temperatures.Comment: 5 pages, 4 figure
On the interpretation of spin-polarized electron energy loss spectra
We study the origin of the structure in the spin-polarized electron energy
loss spectroscopy (SPEELS) spectra of ferromagnetic crystals. Our study is
based on a 3d tight-binding Fe model, with constant onsite Coulomb repulsion U
between electrons of opposite spin. We find it is not the total density of
Stoner states as a function of energy loss which determines the response of the
system in the Stoner region, as usually thought, but the densities of Stoner
states for only a few interband transitions. Which transitions are important
depends ultimately on how strongly umklapp processes couple the corresponding
bands. This allows us to show, in particular, that the Stoner peak in SPEELS
spectra does not necessarily indicate the value of the exchange splitting
energy. Thus, the common assumption that this peak allows us to estimate the
magnetic moment through its correlation with exchange splitting should be
reconsidered, both in bulk and surface studies. Furthermore, we are able to
show that the above mechanism is one of the main causes for the typical
broadness of experimental spectra. Finally, our model predicts that optical
spin waves should be excited in SPEELS experiments.Comment: 11 pages, 7 eps figures, REVTeX fil
Eindrapportage van het onderzoeksprogramma Welzijn gezelschapsdieren
Dit eindrapport beschrijft de resultaten van het onderzoeksprogramma Welzijn Gezelschapsdieren. Hoofdstuk 1 geeft in het kort de achterliggende redenen voor en de totstandkoming van het programma weer. De hoofdstukken 2 tot en met 7 geven een overzicht van opzet, verloop en bevindingen van de zes afzonderlijke projecten. In hoofdstuk 8 wordt het geheel kort geëvalueerd
Proposal for a standard problem for micromagnetic simulations including spin-transfer torque
The spin-transfer torque between itinerant electrons and the magnetization in a ferromagnet is of fundamental interest for the applied physics community. To investigate the spin-transfer torque, powerful simulation tools are mandatory. We propose a micromagnetic standard problem includingthe spin-transfer torque that can be used for the validation and falsication of micromagnetic simulation tools. The work is based on the micromagnetic model extended by the spin-transfer torque in continuously varying magnetizations as proposed by Zhang and Li. The standard problem geometry is a permalloy cuboid of 100 nm edge length and 10 nm thickness, which contains a Landau pattern with a vortex in the center of the structure. A spin-polarized dc current density of 1012 A/m2 flows laterally through the cuboid and moves the vortex core to a new steady-state position. We show that the new vortex-core position is a sensitive measure for the correctness of micromagnetic simulatorsthat include the spin-transfer torque. The suitability of the proposed problem as a standard problem is tested by numerical results from four different finite-difference and finite-element-based simulation tools
Surface structure of epitaxial Gd(0001) films on W(110) studied by quantitative LEED analysis
The surface structure of thick (400 Å) Gd(0001) films, epitaxially grown on W(110), is investigated by low-energy electron-diffraction (LEED) IV measurements in combination with dynamical LEED calculations. A first-layer contraction of 2.4% and a second-layer spacing expansion of 1% is found. These findings are in good agreement with literature values determined for the (0001) surface of bulk Gd crystals. No significant difference in the LEED IV data is found between films grown at room temperature and films grown at elevated temperatures.published_or_final_versio
Plateau voor vleesvarkens
De varkenshouderij in Nederland heeft te maken met het Varkensbesluit 1998, waarin een aantal welzijnsnormen zijn opgenomen. Een van de normen is dat de minimum beschikbare vloeroppervlakte per varken (85-110 kg) is vergroot van 0,7 m2 tot 1,0 m2 ten opzichte van het Varkensbesluit 199
HICS: Highly charged ion collisions with surfaces
The layout of a new instrument designed to study the interaction of highly
charged ions with surfaces, which consists of an ion source, a beamline
including charge separation and a target chamber, is presented here. By varying
the charge state and impact velocity of the projectiles separately, the
dissipation of potential and kinetic energy at or below the surface can be
studied independently. The target chamber offers the use of tunable
metal-insulator-metal devices as detectors for internal electronic excitation,
a timeof-flight system to study the impact induced particle emission and the
possibility to transfer samples in situ to a UHV scanning probe microscope.
Samples and detectors can be prepared in situ as well. As a first example data
on graphene layers on SrTiO3 which have been irradiated with Xe36+ are
presented.
Key words: highly charged ions, sputtering, AFM, grapheneComment: 4 pages, 4 figures, conference proceeding to 17th Internat. Workshop
for Ion Surf. Collision
Adsorption of CO on a Platinum (111) surface - a study within a four-component relativistic density functional approach
We report on results of a theoretical study of the adsorption process of a
single carbon oxide molecule on a Platinum (111) surface. A four-component
relativistic density functional method was applied to account for a proper
description of the strong relativistic effects. A limited number of atoms in
the framework of a cluster approach is used to describe the surface. Different
adsorption sites are investigated. We found that CO is preferably adsorbed at
the top position.Comment: 23 Pages with 4 figure
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