Off-axis electron holography for the quantitative study of magnetic properties of nanostructures: from single nano-magnets to complex devices

International audienceOff-axis electron holography for the quantitative study of magnetic properties of nanostructures. From single nano-magnets to complex devices 1. Spin configurations in size controlled single Fe nanomagnets 2. In situ electron holography of the dynamic field emanating from a HDD writer => 0D => 3

200 keV cold field emission source using carbon cone nanotip: Application to scanning transmission electron microscopy

International audienceWe report the use of a pyrolytic carbon cone nanotip as field emission cathode inside a modern 200 kV dedicated scanning transmission electron microscope. We show an unprecedented improvement in the probe current stability while maintaining all the fundamental properties of a cold field emission source such as a small angular current density together with a high brightness. We have also studied the influence of the low extraction voltage, as enabled by the nanosized apex of the cones, on the electron optics properties of the source that prevent the formation of a virtual beam cross-over of the gun. We have addressed this resolution-limiting issue by coming up with a new electron optical source design

In situ electron holography of the dynamic magnetic field emanating from a hard-disk drive writer

International audienceThe proliferation of mobile devices in society accessing data via the “cloud” is imposing a dramatic increase in the amount of information to be stored on hard disk drives (HDD) used in servers. Forecasts are that areal densities will need to increase by as much as 35% compound per annum and by 2,020 cloud storage capacity will be around 7 zettabytes corresponding to areal densities of 2 Tb/in2. This requires increased performance from the magnetic pole of the electromagnetic writer in the read/write head in the HDD. Current state-of-art writing is undertaken by morphologically complex magnetic pole of sub 100 nm dimensions, in an environment of engineered magnetic shields and it needs to deliver strong directional magnetic field to areas on the recording media around 50 nm × 13 nm. This points to the need for a method to perform direct quantitative measurements of the magnetic field generated by the write pole at the nanometer scale. Here we report on the complete in situ quantitative mapping of the magnetic field generated by a functioning write pole in operation using electron holography. The results point the way towards a new nanoscale magnetic field source to further develop in situ transmission electron microscopy

High magnetic field spin-valley-split Shubnikov-de Haas oscillations in a WSe2 monolayer

International audienceWe study Shubnikov-de Haas oscillations in a p-type WSe2 monolayer under very high magnetic field. The oscillation pattern is complex due to a large spin and valley splitting, in the non-fully-resolved Landau level regime. Our experimental data can be reproduced with a model in which the main parameter is the ratio between the Zeeman energy and the cyclotron energy. The model takes into account the Landau levels from both valleys with the same Gaussian broadening, which allows to predict the relative amplitude of the resistance oscillation originating from each valley. The Zeeman energy is found to be several times larger than the cyclotron energy. It translates into a large and increasing effective Landé factor as the hole density decreases, in the continuity of the values reported in the literature at lower carrier density

Self-organization mechanisms in a Fe-Au film: from isolated core-shell to multicore nanoparticles

International audienceMany nanotechnological applications necessitate a high density of nanoparticles (NPs), making NP morphology control highly challenging. In this work, the morphology of bimetallic NPs formed by magnetron sputtering deposition of a Fe(3 nm)-Au(2 nm) bilayer film on an amorphous silica substrate is analyzed using high-angle dark-field scanning transmission electron microscopy (HAADF-STEM). While all the NPs adopt a Fe-Au core-shell chemical order, they can be sorted into three different types. Isolated NPs, displaying either a highly symmetric centered core geometry (CC-type) or an asymmetrical off-centered core geometry (OC-type), are observed despite the rather large metallic volume. The majority of the NPs however displays a multicore geometry, with Fe cores in the 10-12 nm range, larger than the NPs observed in a pure Fe (3nm) film grown under identical conditions. The driving forces leading to the different morphologies are discussed together with the role played by the environnement

Dynamical effects in strain measurements by dark-field electron holography

cited By 6International audienceHere, we study the effect of dynamic scattering on the projected geometric phase and strain maps reconstructed using dark-field electron holography (DFEH) for non-uniformly strained crystals. The investigated structure consists of a {SiGe/Si} superlattice grown on a (001)-Si substrate. The three-dimensional strain field within the thin TEM lamella is modelled by the finite element method. The observed projected strain is simulated in two ways by multiplying the strain at each depth in the crystal by a weighting function determined from a recently developed analytical two-beam dynamical theory, and by simply taking the average value. We demonstrate that the experimental results need to be understood in terms of the dynamical theory and good agreement is found between the experimental and simulated results. Discrepancies do remain for certain cases and are likely to be from an imprecision in the actual two-beam diffraction conditions, notably the deviation parameter, and points to limitations in the 2-beam approximation. Finally, a route towards a 3D reconstruction of strain fields is proposed

Synthesis and (some) applications of carbon-nanotube-supported pyrolytic carbon nanocones

Graphene-based cones with nanosized apex can be obtained by means of high temperature pyrolytic carbon deposition process using methane and hydrogen as gaseous feedstock and single carbon nanotubes as deposition substrates. Aside the cones, micrometer-sized carbon beads or fibre segments are deposited meanwhile which are a key morphological component for allowing handling and mounting the carbon cones and then using them for various applications. Based on both the literature dealing with pyrolytic carbon deposition processes and experimental observations, a peculiar deposition mechanism is proposed, involving the transient formation of pitch-like liquid phase droplets which deposit onto the individual carbon nanotubes. In this picture, it is believed that a key parameter is the ratio between the droplet and the nanotube diameters, respectively. The cone concentric texture and perfect nanotexture are shown by high resolution transmission electron microscopy, which allows interesting mechanical and conducting properties to be predicted. Correspondingly, applications of the carbon nanocones as electron emitters for cold-field electron sources on the one hand, and as probes for various modes of near-field microscopy on the other hand, have been tested

Epitaxial BaTiO3 on SiGe: nanoscale characterization of the film and its interface with the semiconductor

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Epitaxial BaTiO3 on SiGe: nanoscale characterization of the film and its interface with the semiconductor

International audienc