246 research outputs found
Advances in the Physics of Magnetic Skyrmions and Perspective for Technology
Magnetic skyrmions are small swirling topological defects in the
magnetization texture stabilized by the protection due to their topology. In
most cases they are induced by chiral interactions between atomic spins
existing in non-centrosymmetric magnetic compounds or in thin films in which
inversion symmetry is broken by the presence of an interface. The skyrmions can
be extremely small with diameters in the nanometer range and, importantly, they
behave as particles that can be moved, created or annihilated, making them
suitable for abacus-type applications in information storage, logic or
neuro-inspired technologies. Up to the last years skyrmions were observed only
at low temperature (and in most cases under large applied fields) but important
efforts of research has been recently devoted to find thin film and
multilayered structures in which skyrmions are stabilized above room
temperature and manipulated by current. This article focuses on these recent
advances on the route to devices prototypes.Comment: Published online 13 June 2017 : 17 pages, 8 figures and 2 boxe
Electrical signature of individual magnetic skyrmions in multilayered systems
Magnetic skyrmions are topologically protected whirling spin textures that
can be stabilized in magnetic materials in which a chiral interaction is
present. Their limited size together with their robustness against the external
perturbations promote them as the ultimate magnetic storage bit in a novel
generation of memory and logic devices. Despite many examples of the signature
of magnetic skyrmions in the electrical signal, only low temperature
measurements, mainly in magnetic materials with B20 crystal structure, have
demonstrated the skyrmions contribution to the electrical transport properties.
Using the combination of Magnetic Force Microscopy (MFM) and Hall resistivity
measurements, we demonstrate the electrical detection of sub-100 nm skyrmions
in multilayered thin film at room temperature (RT). We furthermore analyse the
room temperature Hall signal of a single skyrmion which contribution is mainly
dominated by anomalous Hall effect.Comment: 13 pages, 4 figure
Resonant translational, breathing and twisting modes of pinned transverse magnetic domain walls
We study translational, breathing and twisting resonant modes of transverse
magnetic domain walls pinned at notches in ferromagnetic nanostrips. We
demonstrate that a mode's sensitivity to notches depends strongly on the
characteristics of that particular resonance. For example, the frequencies of
modes involving lateral motion of the wall are the ones which are most
sensitive to changes in the notch intrusion depth (especially at the narrower,
more strongly confined end of the domain wall). In contrast, the breathing
mode, whose dynamics are concentrated away from the notches is relatively
insensitive to changes in the notches' sizes. We also demonstrate a sharp drop
in the translational mode's frequency towards zero when approaching depinning
which is found, using a harmonic oscillator model, to be consistent with a
reduction in the local slope of the notch-induced confining potential at its
edge.Comment: 11 pages, 10 figures, additional data and analysi
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
- …