128 research outputs found
Dans un laboratoire de nanosciences
An english version of this article is available (http://arxiv.org/abs/1105.5566) and will be published in the C.R. Physique in a dossier devoted to the merits and risks of Nanotechnologies.La fabrication, l'observation et la manipulation d'objets très petits est un tour de force, mais ces objets, susceptibles de s'infiltrer partout sans être perçus, peuvent susciter la méfiance. Pour mieux apprécier la situation, nous décrivons l'activité d'un institut de recherche spécialisé dans les nanosciences, certaines des méthodes qui y sont utilisées, l'esprit de ses chercheurs et leur attitude vis à vis des risques. An english version of this article is available (http://arxiv.org/abs/1105.5566) and will be published in the C.R. Physique in a dossier devoted to the merits and risks of Nanotechnologies
Epitaxial graphene prepared by chemical vapor deposition on single crystal thin iridium films on sapphire
Uniform single layer graphene was grown on single-crystal Ir films a few
nanometers thick which were prepared by pulsed laser deposition on sapphire
wafers. These graphene layers have a single crystallographic orientation and a
very low density of defects, as shown by diffraction, scanning tunnelling
microscopy, and Raman spectroscopy. Their structural quality is as high as that
of graphene produced on Ir bulk single crystals, i.e. much higher than on metal
thin films used so far.Comment: To appear in Appl. Phys. Let
In a nanoscience lab
The production, observation and manipulation of very small objects is a tour
de force, but these objects, which could infiltrate anywhere without being
seen, may arouse suspicion. To assess the situation at best, we describe the
activity of a nanoscience research institution, some of the methods used there,
the spirit of its researchers and their attitude towards risk.Comment: article accept\'e pour publication dans les C.R. Physique. El\'ement
d'un dossier 'Nanosciences et nanotechnologies: esp\'erances et
inqui\'etudes". D'autres articles de ce dossier, ainsi que la version
fran\c{c}aise de cet articles, seront \'egalement soumis \`a HA
The use of Lorentz microscopy for the determination of magnetic reversal mechanism of exchange-biased Co30Fe70/NiMn bilayer
Lorentz transmission electron microscopy (LTEM) combined with in-situ
magnetizing experiments is a powerful tool for the investigation of the
magnetization of the reversal process at the micron scale. We have implemented
this tool on a conventional transmission electron microscope (TEM) to study the
exchange anisotropy of a polycrystalline Co35Fe65/NiMn bilayer.
Semi-quantitative maps of the magnetic induction were obtained at different
field values by the differential phase contrast (DPC) technique adapted for a
TEM (SIDPC). The hysteresis loop of the bilayer has been calculated from the
relative intensity of magnetic maps. The curve shows the appearance of an
exchange-bias field reveals with two distinct reversal modes of the
magnetization: the first path corresponds to a reversal by wall propagation
when the applied field is parallel to the anisotropy direction whereas the
second is a reversal by coherent rotation of magnetic moments when the field is
applied antiparallel to unidirectional anisotropy direction
Dimensionality cross-over in magnetism: from domain walls (2D) to vortices (1D)
Dimensionality cross-over is a classical topic in physics. Surprisingly it
has not been searched in micromagnetism, which deals with objects such as
domain walls (2D) and vortices (1D). We predict by simulation a second-order
transition between these two objects, with the wall length as the Landau
parameter. This was conrmed experimentally based on micron-sized ux-closure
dots
Asymmetric hysteresis of N\'eel caps in flux-closure magnetic dots
We investigated with XMCD-PEEM magnetic imaging the magnetization reversal
processes of N\'eel caps inside Bloch walls in self-assembled, micron-sized
Fe(110) dots with flux-closure magnetic state. In most cases the
magnetic-dependent processes are symmetric in field, as expected. However, some
dots show pronounced asymmetric behaviors. Micromagnetic simulations suggest
that the geometrical features (and their asymmetry) of the dots strongly affect
the switching mechanism of the N\'eel caps.Comment: Proceeding for MMM-Intermag 2010 (Washington
Structure and magnetism of self-organized Ge(1-x)Mn(x) nano-columns
We report on the structural and magnetic properties of thin Ge(1-x)Mn(x)films
grown by molecular beam epitaxy (MBE) on Ge(001) substrates at temperatures
(Tg) ranging from 80deg C to 200deg C, with average Mn contents between 1 % and
11 %. Their crystalline structure, morphology and composition have been
investigated by transmission electron microscopy (TEM), electron energy loss
spectroscopy and x-ray diffraction. In the whole range of growth temperatures
and Mn concentrations, we observed the formation of manganese rich
nanostructures embedded in a nearly pure germanium matrix. Growth temperature
mostly determines the structural properties of Mn-rich nanostructures. For low
growth temperatures (below 120deg C), we evidenced a two-dimensional spinodal
decomposition resulting in the formation of vertical one-dimensional
nanostructures (nanocolumns). Moreover we show in this paper the influence of
growth parameters (Tg and Mn content) on this decomposition i.e. on nanocolumns
size and density. For temperatures higher than 180deg C, we observed the
formation of Ge3Mn5 clusters. For intermediate growth temperatures nanocolumns
and nanoclusters coexist. Combining high resolution TEM and superconducting
quantum interference device magnetometry, we could evidence at least four
different magnetic phases in Ge(1-x)Mn(x) films: (i) paramagnetic diluted Mn
atoms in the germanium matrix, (ii) superparamagnetic and ferromagnetic low-Tc
nanocolumns (120 K 400 K) and
(iv) Ge3Mn5 clusters.Comment: 10 pages 2 colonnes revTex formatte
Strain and correlation of self-organized Ge_(1-x)Mn_x nanocolumns embedded in Ge (001)
We report on the structural properties of Ge_(1-x)Mn_x layers grown by
molecular beam epitaxy. In these layers, nanocolumns with a high Mn content are
embedded in an almost-pure Ge matrix. We have used grazing-incidence X-ray
scattering, atomic force and transmission electron microscopy to study the
structural properties of the columns. We demonstrate how the elastic
deformation of the matrix (as calculated using atomistic simulations) around
the columns, as well as the average inter-column distance can account for the
shape of the diffusion around Bragg peaks.Comment: 9 pages, 7 figure
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