23 research outputs found
Mécanismes de recuit dans le silicium implanté par faisceau d’ion caractérisés par nanocalorimétrie
Nous présenterons le procédé de fabrication, la caractérisation, ainsi qu’un modèle numérique permettant l’optimisation d’un nouveau dispositif permettant d’effectuer des mesures de nanocalorimétrie sur un échantillon de silicium monocristallin. Ce dernier possède entre autre des propriétés thermiques nous permettant d’effectuer des mesures à des températures supérieures à 900 C, avec une résolution meilleure que 16 C. Ceci nous a permis d’étudier la dynamique des défauts induits par implantation ionique dans le silicium monocristallin. Deux comportements différents sont observés dans la germination de la phase amorphe induite par implantation à 10 et 80 keV. Ces résultats ont été confrontés à des simulations Monte-Carlo basées sur le modèle des paires lacunesinterstitiels. La comparaison entre les simulations et les mesures expérimentales ont montré que ce modèle est incomplet car il ne reproduit qualitativement que certaines caractéristiques observées expérimentalement. Des mesures réalisées à partir de -110 C dans le silicium monocristallin et amorphisé implanté avec des ions légers, ont mis en évidence des différences claires entre la relaxation dans le silicium amorphe et le recuit des défauts dans le silicium monocristallin. Deux processus à des énergies d’activation de 0.48 et 0.6 eV ont été observés pour les implantations réalisées dans le silicium monocristallin tandis qu’un relâchement de chaleur uniforme ne révélant qu’un spectre continu d’énergie d’activation a été observé dans le silicium amorphe.We present the fabrication process, characterization and numerical model allowing the optimization of a new device that allows us to perform nanocalorimetry measurements on a silicon single crystals. The thermal properties of this device allows us to perform measurements at temperature higher than 900 C with a resolution better than 16 C. The device is used to study the ion implantation induced defect dynamic in monocrystalline silicon. Two different behaviours regarding the nucleation of the amorphous phase are observed at 10 and 80 keV. These results are confronted to Monte Carlo simulations based on the interstitial vacancy pair model. The comparison between simulations and measurements show that the model is incomplete as it reproduces only qualitatively some features of the experimental observations. Measurements performed from -110 C in monocrystalline and amorphized silicon implanted with light ions revealed clear differences between structural relaxation in amorphous silicon and defect annealing in monocrystalline silicon. Two processes with activation energies of 0.48 and 0.6 eV are observed after implantation performed in monocrystalline silicon while a uniform heat release associated with a continuous spectrum in terms of activation energy is observed in amorphous silicon
Electrically tunable multi-terminal SQUID-on-tip
We present a new nanoscale superconducting quantum interference device
(SQUID) whose interference pattern can be shifted electrically in-situ. The
device consists of a nanoscale four-terminal/four-junction SQUID fabricated at
the apex of a sharp pipette using a self-aligned three-step deposition of Pb.
In contrast to conventional two-terminal/two-junction SQUIDs that display
optimal sensitivity when flux biased to about a quarter of the flux quantum,
the additional terminals and junctions allow optimal sensitivity at arbitrary
applied flux, thus eliminating the magnetic field "blind spots". We demonstrate
spin sensitivity of 5 to 8 over a continuous field
range of 0 to 0.5 T, with promising applications for nanoscale scanning
magnetic imaging
Emergent nanoscale superparamagnetism at oxide interfaces
Atomically sharp oxide heterostructures exhibit a range of novel physical
phenomena that do not occur in the parent bulk compounds. The most prominent
example is the appearance of highly conducting and superconducting states at
the interface between the band insulators LaAlO3 and SrTiO3. Here we report a
new emergent phenomenon at the LaMnO3/SrTiO3 interface in which an
antiferromagnetic insulator abruptly transforms into a magnetic state that
exhibits unexpected nanoscale superparamagnetic dynamics. Upon increasing the
thickness of LaMnO3 above five unit cells, our scanning nanoSQUID-on-tip
microscopy shows spontaneous formation of isolated magnetic islands of 10 to 50
nm diameter, which display random moment reversals by thermal activation or in
response to an in-plane magnetic field. Our charge reconstruction model of the
polar LaMnO3/SrTiO3 heterostructure describes the sharp emergence of
thermodynamic phase separation leading to nucleation of metallic ferromagnetic
islands in an insulating antiferromagnetic matrix. The model further suggests
that the nearby superparamagnetic-ferromagnetic transition can be gate tuned,
holding potential for applications in magnetic storage and spintronics
Replenish and relax: explaining logarithmic annealing in disordered materials
Fatigue and aging of materials are, in large part, determined by the
evolution of the atomic-scale structure in response to strains and
perturbations. This coupling between microscopic structure and long time scales
remains one of the main challenges in materials study. Focusing on a model
system, ion-damaged crystalline silicon, we combine nanocalorimetric
experiments with an off-lattice kinetic Monte Carlo simulation to identify the
atomistic mechanisms responsible for the structural relaxation over long time
scales. We relate the logarithmic relaxation, observed in a number of systems,
with heat-release measurements. The microscopic mechanism associated with
logarithmic relaxation can be described as a two-step replenish and relax
process. As the system relaxes, it reaches deeper energy states with
logarithmically growing barriers that need to be unlocked to replenish the
heat-releasing events leading to lower energy configurations
Direct Observation of a Superconducting Vortex Diode
The interplay between magnetism and superconductivity can lead to
unconventional proximity and Josephson effects. A related phenomenon that has
recently attracted considerable attention is the superconducting diode effect,
in which a non-reciprocal critical current emerges. Although superconducting
diodes based on superconducting/ferromagnetic (S/F) bilayers were demonstrated
more than a decade ago, the precise underlying mechanism remains unclear. While
not formally linked to this effect, the Fulde-Ferrell-Larkin-Ovchinikov (FFLO)
state is a plausible mechanism, due to the 2-fold rotational symmetry breaking
caused by the finite center-of-mass-momentum of the Cooper pairs. Here, we
directly observe, for the first time, a tunable superconducting vortex diode in
Nb/EuS (S/F) bilayers. Based on our nanoscale SQUID-on-tip (SOT) microscope and
supported by in-situ transport measurements, we propose a theoretical model
that captures our key results. Thus, we determine the origin for the vortex
diode effect, which builds a foundation for new device concepts
Probing dynamics and pinning of single vortices in superconductors at nanometer scales
The dynamics of quantized magnetic vortices and their pinning by materials
defects determine electromagnetic properties of superconductors, particularly
their ability to carry non-dissipative currents. Despite recent advances in the
understanding of the complex physics of vortex matter, the behavior of vortices
driven by current through a multi-scale potential of the actual materials
defects is still not well understood, mostly due to the scarcity of appropriate
experimental tools capable of tracing vortex trajectories on nanometer scales.
Using a novel scanning superconducting quantum interference microscope we
report here an investigation of controlled dynamics of vortices in lead films
with sub-Angstrom spatial resolution and unprecedented sensitivity. We
measured, for the first time, the fundamental dependence of the elementary
pinning force of multiple defects on the vortex displacement, revealing a far
more complex behavior than has previously been recognized, including striking
spring softening and broken-spring depinning, as well as spontaneous hysteretic
switching between cellular vortex trajectories. Our results indicate the
importance of thermal fluctuations even at 4.2 K and of the vital role of
ripples in the pinning potential, giving new insights into the mechanisms of
magnetic relaxation and electromagnetic response of superconductors.Comment: 15 pages and 5 figures (main text) + 15 pages and 11 figures
(supplementary material
Hidden spin-texture at topological domain walls drive exchange bias in a Weyl semimetal
Exchange bias is a phenomenon critical to solid-state technologies that
require spin valves or non-volatile magnetic memory. The phenomenon is usually
studied in the context of magnetic interfaces between antiferromagnets and
ferromagnets, where the exchange field of the former acts as a means to pin the
polarization of the latter. In the present study, we report an unusual instance
of this phenomenon in the topological Weyl semimetal Co3Sn2S2, where the
magnetic interfaces associated with domain walls suffice to bias the entire
ferromagnetic bulk. Remarkably, our data suggests the presence of a hidden
order parameter whose behavior can be independently tuned by applied magnetic
fields. For micron-size samples, the domain walls are absent, and the exchange
bias vanishes, suggesting the boundaries are a source of pinned uncompensated
moment arising from the hidden order. The novelty of this mechanism suggests
exciting opportunities lie ahead for the application of topological materials
in spintronic technologies.Comment: Main text: 11 pages, 4 figures. Supplementary information: 7 pages, 6
figures. Supplementary videos:
Interior and edge magnetization in thin exfoliated CrGeTe3 films
CrGeTe3 (CGT) is a semiconducting vdW ferromagnet shown to possess magnetism
down to a two-layer thick sample. Although CGT is one of the leading candidates
for spintronics devices, a comprehensive analysis of CGT thickness dependent
magnetization is currently lacking. In this work, we employ scanning
SQUID-on-tip (SOT) microscopy to resolve the magnetic properties of exfoliated
CGT flakes at 4.2 K. Combining transport measurements of CGT/NbSe2 samples with
SOT images, we present the magnetic texture and hysteretic magnetism of CGT,
thereby matching the global behavior of CGT to the domain structure extracted
from local SOT magnetic imaging. Using this method, we provide a thickness
dependent magnetization state diagram of bare CGT films. No zero-field magnetic
memory was found for films thicker than 10 nm and hard ferromagnetism was found
below that critical thickness. Using scanning SOT microscopy, we identify a
unique edge magnetism, contrasting the results attained in the CGT interior.Comment: Main text: 15 pages, 5 figures. Supplementary information: 9 pages,
10 figures. Supplementary videos: