191 research outputs found
Intercalating cobalt between graphene and iridium (111): a spatially-dependent kinetics from the edges
Using low-energy electron microscopy, we image in real time the intercalation
of a cobalt monolayer between graphene and the (111) surface of iridium. Our
measurements reveal that the edges of a graphene flake represent an energy
barrier to intercalation. Based on a simple description of the growth kinetics,
we estimate this energy barrier and find small, but substantial, local
variations. These local variations suggest a possible influence of the graphene
orientation with respect to its substrate and of the graphene edge termination
on the energy value of the barrier height. Besides, our measurements show that
intercalated cobalt is energetically more favorable than cobalt on bare
iridium, indicating a surfactant role of graphene
Nano-oscillator-based classification with a machine learning-compatible architecture
Pattern classification architectures leveraging the physics of coupled
nano-oscillators have been demonstrated as promising alternative computing
approaches, but lack effective learning algorithms. In this work, we propose a
nano-oscillator based classification architecture where the natural frequencies
of the oscillators are learned linear combinations of the inputs, and define an
offline learning algorithm based on gradient back-propagation. Our results show
significant classification improvements over a related approach with online
learning. We also compare our architecture with a standard neural network on a
simple machine learning case, which suggests that our approach is economical in
terms of numbers of adjustable parameters. The introduced architecture is also
compatible with existing nano-technologies: the architecture does not require
changes in the coupling between nano-oscillators, and it is tolerant to
oscillator phase noise
Improved spectral stability in spin transfer nano-oscillators: single vortex versus coupled vortices dynamics
We perform a comparative study of spin transfer induced excitation of the
gyrotropic motion of a vortex core with either uniform or vortex spin
polarizers. The microwave output voltage associated with the vortex dynamics,
detected in both cases, displays a strong reduction of phase fluctuations in
the case of the vortex polarizer, with a decrease of the peak linewidth by one
order of magnitude down to 200kHz at zero field. A thorough study of rf
emission features for the different accessible vortex configurations shows that
this improvement is related to the excitation of coupled vortex dynamics by
spin transfer torques
Dynamique par transfert de spin et synchronisation d'oscillateurs couplés à base de vortex magnétiques
Le sujet de cette thèse concerne la dynamique auto-entretenue excitée par transfert de spin de vortex couplés, dans des structures de type nano-piliers vannes de spin (Py/Cu/Py). Un premier objectif a été de comprendre les processus de transport polarisé en spin et de transfert de spin associés à des configurations d aimantation fortement non-homogènes. Cette étude a permis d identifier et ainsi de précisément contrôler les configurations magnétiques à base de vortex, et en particulier d observer l influence du transfert de spin sur les mécanismes de renversement du cœur de vortex. En combinant des calculs analytiques et des simulations micro-magnétiques, nous avons également pu déterminer les conditions sur les paramètres relatifs des deux vortex (chiralités et polarités) pour obtenir des oscillations gyrotropiques couplées auto-entretenues de deux vortex dans un pilier unique. Un cas très intéressant est prévu pour les piliers de plus grands diamètres (typiquement supérieurs à 200nm) pour lesquels le courant critique est réduit potentiellement à zéro. Les résultats expérimentaux confirment les prédictions sur l existence d une dynamique couplée de vortex, avec des largeurs de raies atteignant 200kHz, un record à champ nul (soit un facteur de qualité Q 5000, un ordre de grandeur plus grand que pour les auto-oscillations de vortex unique) et diminuant même jusqu à 50kHz sous champ extérieur. Un second objectif de ce travail a été l étude de la synchronisation de deux auto-oscillateurs à transfert de spin à base de vortex. Nous avons démontré que le verrouillage des phases par couplage dipolaire de deux oscillateurs identiques peut être théoriquement obtenu indépendamment des paramètres des deux vortex. Toutefois un couplage trois fois plus important est prévu dans le cas de vortex de polarités opposées. Du point de vue expérimental, des premiers résultats ont permis de démontrer une faculté de synchronisation de deux oscillateurs présentant un écart en fréquence atteignant jusqu'à 10% de leurs fréquences d'auto-oscillation. Ce travail de thèse, qui s inscrit dans l effort de recherche mené pour améliorer les performances rf des nano-oscillateurs à transfert de spin, a permis d illustrer que l excitation de modes d aimantations couplées est une voie à poursuivre dans le but d aboutir à des largeurs de raies de plus en plus faibles.My PhD work is dedicated to the spin transfer induced self-sustained dynamics of two coupled vortices, in nano-pillars spin-valves structures (Py/Cu/Py). A first objective was to understand the spin-polarized transport processes as well as spin transfer mechanisms associated to highly non-homogeneous magnetic configurations. This study allows me to identify and then precisely tune the vortex based magnetic configurations, and notably to observe the influence of spin transfer on reversal mechanisms of the vortex core. Combining analytical calculations and micro-magnetic simulations, we determine the conditions on relative parameters for the two vortices (chiralities and polarities) necessary to obtain self-sustained gyrotropic oscillations of the coupled vortices in a single pillar. A very interesting case is predicted for the pillars with larger diameters (typically over 200nm) for which the critical current is reduced to zero. The experimental results confirm the predictions that a coupled dynamics exists with linewidths as narrow as 200kHz, that is a record at zero field (corresponding to a quality factor Q 5000, an order of magnitude over the self-sustained oscillations of a single vortex), and even down to 50kHz under external field.A second objective was to investigate the synchronization of two vortex based spin transfer oscillators. We demonstrate theoretically that the phase locking through dipolar coupling of two identical oscillators can be achieved for any parameters of the two vortex. However, the coupling is three times stronger when vortices have opposite core polarities. From an experimental point of view, the synchronization capability for two oscillators having a frequency mismatch reaching up to 10 % of the auto-oscillation frequency has been demonstrated. This work, being part of the research effort made to improve the rf properties of spin transfer nano-oscillators emphasizes how the excitation of coupled magnetizations modes is important to reach lower and lower linewidths.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF
Nonlinear behavior and mode coupling in spin transfer nano-oscillators
By investigating thoroughly the tunable behavior of coupled modes, we
highlight how it provides new means to handle the properties of spin transfer
nano-oscillators. We first demonstrate that the main features of the microwave
signal associated with coupled vortex dynamics i.e. frequency, spectral
coherence, critical current, mode localization, depends drastically on the
relative vortex core polarities. Secondly we report a large reduction of the
nonlinear linewidth broadening obtained by changing the effective damping
through the control of the core configuration. Such a level of control on the
nonlinear behavior reinforces our choice to exploit the microwave properties of
collective modes for applications of spintronic devices in novel generation of
integrated telecommunication devices
Fragmentation of magnetism in artificial kagome dipolar spin ice
Geometrical frustration in magnetic materials often gives rise to exotic,
low-temperature states of matter, like the ones observed in spin ices. Here we
report the imaging of the magnetic states of a thermally-active artificial
magnetic ice that reveal the fingerprints of a spin fragmentation process. This
fragmentation corresponds to a splitting of the magnetic degree of freedom into
two channels and is evidenced in both real and reciprocal space. Furthermore,
the internal organization of both channels is interpreted within the framework
of a hybrid spin-charge model that directly emerges from the parent spin model
of the kagome dipolar spin ice. Our experimental and theoretical results
provide insights into the physics of frustrated magnets and deepen our
understanding of emergent fields through the use of tailor-made magnetism.Comment: 9 pages, 5 figures. Published version available on the Nat. Comm. web
site:
http://www.nature.com/ncomms/2016/160513/ncomms11446/full/ncomms11446.htm
Observation of Bloch-point domain walls in cylindrical magnetic nanowires
Topological protection is an elegant way of warranting the integrity of
quantum and nanosized systems. In magnetism one example is the Bloch-point, a
peculiar object implying the local vanishing of magnetization within a
ferromagnet. Its existence had been postulated and described theoretically
since several decades, however it has never been observed. We con rm
experimentally the existence of Bloch points, imaged within domain walls in
cylindrical magnetic nanowires, combining surface and transmission XMCD-PEEM
magnetic microscopy. This opens the way to the experimental search for peculiar
phenomena predicted during the motion of Bloch-point-based domain walls
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