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Since the early 1960's, the discovery of Dzyaloshinskii-Moriya interaction (DMI) helped to explain the physical mechanisms behind certain magnetic phenomena, such as net moment in antiferromagnets, or enhanced anisotropy field from heavy metals impurity in dilute Cu:Mn alloy. Since the researchers unveil the key role that DMI plays in stabilizing chiral Neel type magnetic domain wall and magnetic skyrmions, the studies on DMI have received growing interest. Governed by spin-orbit coupling (SOC) and various types of inversion symmetry breaking (ISB) in magnetic systems, DMI drives the forming of distinct morphologies of magnetic skyrmions. Our aim is to briefly introduce the research history of DMI and its significance in the field of modern spintronics., Published in J. Phys. Soc. Jpn

The Dzyaloshinskii-Moriya interaction (DMI) and perpendicular magnetic anisotropy (PMA) were measured on four series of Co films (1-2.2 nm thick) grown on Pt or Au and covered with h-BN or Cu. Clean h-BN/Co interfaces were obtained by exfoliating h-BN and transferring it onto the Co film in situ in the ultra-high-vacuum evaporation chamber. By comparing h-BN and Cu-covered samples, the DMI induced by the Co/h-BN interface was extracted and found to be comparable in strength to that of the Pt/Co interface, one of the largest known values. The strong observed DMI despite the weak spin-orbit interaction in h-BN supports a Rashba-like origin in agreement with recent theoretical results. Upon combination of it with Pt/Co in Pt/Co/h-BN heterostructures, even stronger PMA and DMI are found which stabilizes skyrmions at room temperature and a low magnetic field.

International audience; The actual mechanisms occurring at interfaces underlying the Dzyaloshinskii-Moriya interaction (DMI) remain a question in nanomagnetism. In this study, we investigate the origin of the interfacial DMI, aiming at estimating how independent the DMI contributions of the two interfaces of a FM layer are and what their relative weight in the effective DMI amplitude is. To this aim, we explore the correlation between the effective interfacial DMI and the metal properties, namely, atomic number, electronegativity, and the work function of the metal M. A clear linear relationship is found between the interfacial DMI and the work function difference at the Co/M interface. This result is strong evidence of the independent DMI contributions of the two interfaces for the chosen Co thickness(1 nm). It also suggests that the Co/Cu interface bears no interfacial DMI. These findings can guide the optimization of the magnetic properties of future devices.

Magnetic skyrmions are topologically stable spin swirling particle like entities which are appealing for next generation spintronic devices. The expected low critical current density for the motion of skyrmions makes them potential candidates for future energy efficient electronic devices. Several heavy metal/ferromagnetic (HM/FM) systems have been explored in the past decade to achieve faster skyrmion velocity at low current densities. In this context, we have studied Pt/CoFeB/MgO heterostructures in which skyrmions have been stabilized at room temperature (RT). It has been observed that the shape of the skyrmions are perturbed even by the small stray field arising from low moment magnetic tips while performing the magnetic force microscopy (MFM), indicating presence of low pinning landscape in the samples. This hypothesis is indeed confirmed by the low threshold current density to drive the skyrmions in our sample, at velocities of few ∼10 m s−1.

The domain wall dynamics driven by an out of plane magnetic field was measured for a series of magnetic trilayers with different strengths of the interfacial Dzyaloshinskii-Moriya interaction (DMI). The features of the field-driven domain wall velocity curves strongly depend on the amplitude of the HD field stabilising chiral N\'eel walls. The measured Walker velocity, which in systems with large DMI is maintained after the Walker field giving rise to a velocity plateau up to the Slonczewski field HS, can be easily related to the DMI strength. Yet, when the DMI field HD and the domain wall demagnetising field HDW have comparable values, a careful analysis needs to be done in order to evaluate the impact of the DMI on the domain wall velocity. By means of a one-dimensional model and 2D simulations, we successfully extend this method to explain experimental results to cases where HD and HDW are comparable., Comment: 9 pages, 3 Figures

We demonstrate that the effects of spin-orbit coupling and inversion asymmetry exist in a single $\mathrm{Gd}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Co}$ ferrimagnetic layer, even without a heavy-metal interface. We use electric transport measurements to quantify the spin-orbit torques. We measure the Dzyaloshinskii-Moriya interaction using the Brillouin light-scattering measurement technique, and we observe the resulting chiral magnetic textures using x-ray photoemission electron microscopy. We attribute these effects to a composition variation along the thickness that we observe by scanning transmission electron microscopy. We show that these effects can be optimized by varying the $\mathrm{Gd}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Co}$ thickness or in combination with interfacial effects.

The effect of damping-like spin-orbit torque (DL SOT) on magnetic domain walls (DWs) in in-plane magnetised soft tracks is studied analytically and with micromagnetic simulations. We find that DL SOT drives vortex DWs, whereas transverse DWs, the other typical DW structure in soft tracks, propagate only if the Dzyaloshinskii-Moriya interaction (DMI) is present. The SOT drive can add to, and be more efficient than, spin-transfer torque (STT), and so may greatly benefit applications that require in-plane DWs. Our analysis based on the Thiele equation shows that the driving force arises from a cycloidal distortion of the DW structure caused by geometrical confinement or DMI. This distortion is higher, and the SOT more efficient, in narrower, thinner tracks. These results show that the effects of SOT cannot be understood by simply considering the effective field at the center of the structure, an ill-founded but often-used estimation. We also show that the relative magnitude of STT and DL SOT can be determined by comparing the motion of different vortex DW structures in the same track., Accepted for publication in Physical Review B Rapid Communications

The in-plane current profile within multilayers of generic structure Ta/Pt/(CoNi)/Pt/Ta is investigated. A large set of samples where the thickness of each layer is systematically varied was grown, followed by the measurement of the sheet resistance of each sample. The data are analyzed by a series of increasingly elaborate models, from a macroscopic engineering approach to mesoscopic transport theory. Non-negligible variations of the estimated repartition of current between the layers are found. The importance of having additional structural data is highlighted., Comment: 10 pages, 7 figures

International audience; Static and dynamic magnetic properties of Zn0.4Fe2.6O4 nanoparticles synthesized by thermal decomposition have been investigated. Two ranges of diameter have been used: small particles (diameter about 6.2 nm) and larger ones (diameter about 22.4 nm). The nanoparticle microstructure has been characterized by transmission electron microscopy. The temperature dependence of the zero-field dynamic permeability for both nanoparticle sizes has been studied, revealing a superparamagnetic state for the small ones. Effects of the nanoparticle size on the dynamic permeability have been studied, and linked to the superparamagnetic state. A dynamic susceptibility model has been found to reproduce the experimental behavior as well as its temperature dependence.

Spin-orbit coupling in inversion-asymmetric magnetic crystals and structures has emerged as a powerful tool to generate complex magnetic textures, interconvert charge and spin under applied current, and control magnetization dynamics. Current-induced spin-orbit torques mediate the transfer of angular momentum from the lattice to the spin system, leading to sustained magnetic oscillations or switching of ferromagnetic as well as antiferromagnetic structures. The manipulation of magnetic order, domain walls and skyrmions by spin-orbit torques provides evidence of the microscopic interactions between charge and spin in a variety of materials and opens novel strategies to design spintronic devices with potentially high impact in data storage, nonvolatile logic, and magnonic applications. This paper reviews recent progress in the field of spin-orbitronics, focusing on theoretical models, material properties, and experimental results obtained on bulk noncentrosymmetric conductors and multilayer heterostructures, including metals, semiconductors, and topological insulator systems. Relevant aspects for improving the understanding and optimizing the efficiency of nonequilibrium spin-orbit phenomena in future nanoscale devices are also discussed., Comment: 90 pages, 50 figures

We demonstrate room-temperature stabilization of dipolar magnetic skyrmions with diameters in the range of $100$ nm in a single ultrathin layer of the Heusler alloy Co$_2$FeAl (CFA) under moderate magnetic fields. Current-induced skyrmion dynamics in microwires is studied with a scanning Nitrogen-Vacancy magnetometer operating in the photoluminescence quenching mode. We first demonstrate skyrmion nucleation by spin-orbit torque and show that its efficiency can be significantly improved using tilted magnetic fields, an effect which is not specific to Heusler alloys and could be advantageous for future skyrmion-based devices. We then show that current-induced skyrmion motion remains limited by strong pinning effects, even though CFA is a magnetic material with a low magnetic damping parameter., 5 pages, 4 figures

Extended numerical simulations enable to ascertain the diffusive behavior at finite temperatures of chiral walls and skyrmions in ultra-thin model Co layers exhibiting symmetric - Heisenberg - as well as antisymmetric - Dzyaloshinskii-Moriya - exchange interactions. The Brownian motion of walls and skyrmions is shown to obey markedly different diffusion laws as a function of the damping parameter. Topology related skyrmion diffusion suppression with vanishing damping parameter, albeit already documented, is shown to be restricted to ultra-small skyrmion sizes or, equivalently, to ultra-low damping coefficients, possibly hampering observation.

International audience; Magnetic domain walls are objects whose dynamics is inseparably connected to their structure. In this work we investigate magnetic bilayers, which are engineered such that a coupled pair of domain walls, one in each layer, is stabilized by a cooperation of Dzyaloshinskii-Moriya interaction and flux-closing mechanism. The dipolar field mediating the interaction between the two domain walls, links not only their position but also their structure. We show that this link has a direct impact on their magnetic field induced dynamics. We demonstrate that in such a system the coupling leads to an increased domain wall velocity with respect to single domain walls. Since the domain wall dynamics is observed in a precessional regime, the dynamics involves the synchronization between the two walls, to preserve the flux closure during motion. Properties of these coupled oscillating walls can be tuned by an additional in-plane magnetic field enabling a rich variety of states, from perfect synchronization to complete detuning.

The Dzyaloshinskii-Moriya interaction has been shown to stabilise Nèel domain walls in magnetic thin films, allowing high domain wall velocities driven by spin current effects. The interfacial Dzyaloshinskii-Moriya interaction (IDMI) occurs at the interface between ferromagnetic and heavy metal layers with strong spin-orbit coupling, but details of the interaction remain to be understood and the role of proximity induced magnetism (PIM) in the heavy metal is unknown. Here IDMI and PIM are reported in Pt determined as a function of Au and Ir spacer layers in Pt/Co/Au,Ir/Pt. Both interactions are found to be sensitive to sub-nanometre changes in the spacer thickness, correlating over sub-monolayer spacer thicknesses, but not for thicker spacers where IDMI continues to change even after PIM is lost.

Nitrogen-vacancy magnetic microscopy is employed in quenching mode as a non-invasive, high resolution tool to investigate the morphology of isolated skyrmions in ultrathin magnetic films. The skyrmion size and shape are found to be strongly affected by local pinning effects and magnetic field history. Micromagnetic simulations including static disorder, based on a physical model of grain-to-grain thickness variations, reproduce all experimental observations and reveal the key role of disorder and magnetic history in the stabilization of skyrmions in ultrathin magnetic films. This work opens the way to an in-depth understanding of skyrmion dynamics in real, disordered media., 9 pages, 8 figures, including supplementary informations

Brillouin light spectroscopy is a powerful and robust technique for measuring the interfacial Dzyaloshinskii-Moriya interaction in thin films with broken inversion symmetry. Here we show that the magnon visibility, i.e. the intensity of the inelastically scattered light, strongly depends on the thickness of the dielectric seed material - SiO$_2$. By using both, analytical thin-film optics and numerical calculations, we reproduce the experimental data. We therefore provide a guideline for the maximization of the signal by adapting the substrate properties to the geometry of the measurement. Such a boost-up of the signal eases the magnon visualization in ultrathin magnetic films, speeds-up the measurement and increases the reliability of the data.

International audience; The combined effect of magnetic field and current on domain wall motion is investigated in epitaxial [Co/Ni] microwires. Both thermally activated and flow regimes are found to be strongly affected by current. All experimental data can be understood by taking into account both adiabatic and nonadiabatic components of the spin transfer torque, the parameters of which are extracted. In the precessional flow regime, it is shown that the domain wall can move in the electron flow direction against a strong applied field, as previously observed. In addition, for a large range of applied magnetic field and injected current, a stochastic domain wall displacement after each pulse is observed. Two-dimensional micromagnetic simulations, including some disorder, show a random fluctuation of the domain wall position that qualitatively matches the experimental results.

The Dzyaloshinskii-Moriya Interaction (DMI) between spins is induced by spin-orbit coupling in magnetic materials lacking inversion symmetry. DMI is recognized to play a crucial role at the interface between ferromagnetic (FM) and heavy nonmagnetic (NM) metals to create topological textures called magnetic skyrmions which are very attractive for ultra-dense information storage and spintronic devices. DMI also plays an essential role for fast domain wall (DW) dynamics driven by spin-orbit torques. Here, we present first principles calculations which clarify the main features and microscopic mechanisms of DMI in Co/Pt bilayers. DMI is found to be predominantly located at the interfacial Co layer, originating from spin-orbit energy provided by the adjacent NM layer. Furthermore, no direct correlation is found between DMI and proximity induced magnetism in Pt. These results clarify underlying mechanisms of DMI at FM/NM bilayers and should help optimizing material combinations for skyrmion- and DW-based storage and memory devices., 16 pages, 4 figures

We employ a scanning nitrogen-vacancy-center microscope to perform stray field imaging of bubble magnetic domains in a perpendicularly magnetized Pt/Co/AlOx trilayer with 6 A of Co. The stray field created by the domain walls is quantitatively mapped with few-nanometer spatial resolution, with a probe-sample distance of about 100 nm. As an example of application, we show that it should be possible to determine the Bloch or Neel nature of the domain walls, which is of crucial importance to the understanding of current-controlled domain wall motion.

We show that the Dzyaloshinskii-Moriya interaction (DMI) can lead to a tilting of the domain wall (DW) surface in perpendicularly magnetized magnetic nanotracks when DW dynamics is driven by an easy axis magnetic field or a spin polarized current. The DW tilting affects the DW dynamics for large DMI and the tilting relaxation time can be very large as it scales with the square of the track width. The results are well explained by an analytical model based on a Lagrangian approach where the DMI and the DW tilting are included. We propose a simple way to estimate the DMI in a magnetic multilayers by measuring the dependence of the DW tilt angle on a transverse static magnetic field. Our results shed light on the current induced DW tilting observed recently in Co/Ni multilayers with inversion asymmetry, and further support the presence of DMI in these systems., 12 pages, 3 figures, 1 Supplementary Materials

We explore a new type of domain wall structure in ultrathin films with perpendicular anisotropy, that is influenced by the Dzyaloshinskii-Moriya interaction due to the adjacent layers. This study is performed by numerical and analytical micromagnetics. We show that these walls can behave like Neel walls with very high stability, moving in stationary conditions at large velocities under large fields. We discuss the relevance of such walls, that we propose to call Dzyaloshinskii domain walls, for current-driven domain wall motion under the spin Hall effect., 7 pages, 3 figures. Accepted for publication in Europhysics Letters

We demonstrate quantitative magnetic field mapping with nanoscale resolution, by applying a lock-in technique on the electron spin resonance frequency of a single nitrogen-vacancy defect placed at the apex of an atomic force microscope tip. In addition, we report an all-optical magnetic imaging technique which is sensitive to large off-axis magnetic fields, thus extending the operation range of diamond-based magnetometry. Both techniques are illustrated by using a magnetic hard disk as a test sample. Owing to the non-perturbing and quantitative nature of the magnetic probe, this work should open up numerous perspectives in nanomagnetism and spintronics.

International audience; We have investigated experimentally the field-driven propagation of domain walls (DWs) in perpendicularly magnetized ferromagnetic semiconductor layers. The results were then compared with the historical one-dimensional (1D) DW propagation model widely used in spintronics studies of magnetic nanostructures. Anomalous velocity peaks, not predicted by the 1D model, were observed experimentally. Using micromagnetic simulations we show indeed that, in the particular regime of layer thickness (h) of the order of the exchange length, velocity peaks appear in the precessional regime, their shape and position shifting with h. Analyses of the simulations show a distinct correlation between the curvature of the DW and the twist of the magnetization vector within it and the velocity peak. Associating a phenomenological description of this twist with a four-coordinate DW propagation model, we show that the velocity peaks result from the torque exerted by the stray field created by the domains on the twisted magnetization. The position of the peaks is well predicted from the DW's first flexural mode frequency and depends strongly on the layer thickness. Comparison of the proposed model with data obtained on GaMnAs and GaMnAsP shows that the anomalies observed close to Walker breakdown are indeed induced by the flexion resonance of the domain wall.

We have observed peculiar magnetization textures in Ni${}_{80}$Pd${}_{20}$ nanostrips using three different imaging techniques: magnetic force microscopy, photoemission electron microscopy under polarized x-ray absorption, and scanning electron microscopy with polarization analysis. The appearance of diamondlike domains with strong lateral charges and of weak-stripe structures reveals the presence of both a transverse and a perpendicular anisotropy in these nanostructures. The anisotropy is seen to reinforce as temperature decreases, as testified by observations performed at 150 K. A thermal stress model with relaxation is proposed to account for these observations. Elastic calculations coupled to micromagnetic simulations support qualitatively this model.

We study the magnetic domain wall structure in magnetic bilayers (two ultrathin ferromagnetic layers separated by a non magnetic spacer) with perpendicular magnetization. Combining magnetic force and ballistic electron emission microscopies, we are able to reveal the details of the magnetic structure of the wall with a high spatial accuracy. In these layers, we show that the classical Bloch wall observed in single layers transforms into superposed N\'eel walls due to the magnetic coupling between the ferromagnetic layers. Quantitative agreement with micromagnetic calculations is achieved., Comment: Author adresses AB, SR, JM and AT: Laboratoire de Physique des Solides, CNRS, Universit\'e Paris Sud, UMR 8502, 91405 Orsay Cedex, France ML : Laboratoire PMTM, Institut Galil\'ee, CNRS, Universit\'e Paris-13, UPR 9001, 93430 Villetaneuse, France

International audience; The effect of electrical current pulses on magnetic domain walls is studied in a nanometer-scale magnetic track defined in a He +-irradiated Pt/Co0.5 nm/Pt film with out-of-plane anisotropy. Current pulses in a wide range of intensities and durations are shown to result either in track demagnetization, or in polarity-independent domain-wall propagation, due to Joule heating during the current pulse. None of the expected spin transfer effects is shown to occur. To explain this, the spin-dependent current-density distribution in the stack is evaluated, in the frame of an adapted Fuchs-Sondheimer model. The spin-polarized current density in the cobalt layer is shown to be unexpectedly low as compared to the charge current in the whole stack, which causes Joule heating. This is explained by a low electron transmission at Co/Pt interfaces, as is deduced from resistance measurements on a series of samples with different cobalt thicknesses. This leads us to emphasize that the balance between spin and total charge current densities should be carefully considered when addressing spin transfer torque effects.

The magnetization process in microfabricated NiFe wires was observed using a Kerr microscope. Magnetic wires were made from a 20-nm-thick NiFe film by using lift-off techniques. Their width Wand length L were designed as W= 0.5, 1.0 and 2.0 /Lm and L= 50/Lm, respectively. One end of the wire was connected to a square shaped head with a side of 2 W, which is designed to act as a domain wall source. In each wire, necks with different width of 0.2W, 0.6W, and 0.8W were introduced as artificial pinning sites of a domain wall. By using an oil-immersion lens (NA = 1.3) and a Hg lamp, magnetization reversals in very narrow wires, as narrow as 0.5 /Lm, were clearly observed. It is confirmed that domain wall penetration, pinning, depinning, and also the direction of wall motion are controllable using square shaped head and necks with optimized width.

Magnetic and magnetostrictive hysteresis loops of TbCo/CoFe multilayers under field applied along the hard magnetization axis are studied using vectorial magnetization measurements, optical deflectometry and magneto optical Kerr microscopy. Even a very small angle misalignment between hard axis and magnetic field direction is shown to drastically change the shape of magnetization and magnetostrictive torsion hysteresis loops. Two kinds of magnetic domains are revealed during the magnetization: big regions with opposite rotation of spontaneous magnetization vector and spontaneous magnetic domains which appear in a narrow field interval and provide an inversion of this rotation. We show that the details of the hysteresis loops of our exchange-coupled films can be described using the classical model of homogeneous magnetization rotation of single uniaxial films and the configuration of observed domains. The understanding of these features is crucial for applications (for MEMS or microactuators) which benefit from the greatly enhanced sensitivity near the point of magnetic saturation at the transverse applied field., 10 pages, 11 figures

In order to explain recent experiments reporting a motion of magnetic domain walls (DW) in nanowires carrying a current, we propose a modification of the spin transfer torque term in the Landau-Lifchitz-Gilbert equation. We show that it explains, with reasonable parameters, the measured DW velocities as well as the variation of DW propagation field under current. We also introduce coercivity by considering rough wires. This leads to a finite DW propagation field and finite threshold current for DW propagation, hence we conclude that threshold currents are extrinsic. Some possible models that support this new term are discussed., 5 pages, 3 figures, submitted to Europhys. Lett, revised version

By combining a finite element tip model and numerical simulations of the tip–sample interaction, it is shown that magnetic force microscopy images of patterned soft elements may be quantitatively compared to experiments, even when performed at low lift heights, while preserving physically realistic tip characteristics. The analysis framework relies on variational principles. Assuming magnetically hard tips, the model is both exact and numerically more accurate than hitherto achieved., Partial support from the SUBMAGDEV Training and Mobility of Researchers European Program is gratefully acknowledged.

International audience; The first three-dimensional switching field measurements of an individual nanometer-sized magnetic particle are presented. The angular dependence of the switching field can be described by the model of uniform rotation of magnetization. However, the anisotropy function proposed by Stoner and Wohlfarth appears to be simplistic. In order to reproduce the measured switching fields, one has to take into account higher order anisotropy terms. These terms are very important to understand dynamical magnetization reversal properties in the classical and the quantum regimes.

Recent developments in the field of nanotechnology allowed the access to adequate length scale necesary to closely investigate spins and opened large prospects of using electrons spin degree of freedom in new generation electronic devices. This have lead to the development of a vibrant field dubbed spintronics.Here, we present experiments that combine two very promising materials: namely cardon nanotubes and palladium-nickel (PdNi), with the purpose to manipulate the electronic spin both in the classical and in the quantum regime. We implement a quantum dot connected to two non-collinear ferromagnetic leads that acts as a spin-valve device. The versatility of carbon nanotubes to fabricate quantum dots when connected to PdNi electrodes via tunneling barriers is combined with the particular transversal anisotropy of the PdNi when shaped in nanometric stripes.For devices exploiting actively the electronic spin, however control over classical or quantum spin rotations has still to be achieved. A detailed understanding of the magnetic characteristics of PdxNi 100-x alloy is crucial both for understanding the switching characteristics of such the spin-valve device and for optimizing its electronic properties. We present a magnetic study of Pd20Ni80 and Pd90Ni10 nanostripes by means of extraordinary Hall effect measurements, at low temperature, for various dimensions, thicknesses and capping films. In the case of Pd20Ni80, this experiment is a first at low temperature.The CNT-based device proposed here was tested both in linear and nonlinear transportregimes. While the linear spin dependent transport displays the usual signatures of electronicconfinement, the finite bias magnetoresistance displays an impressive magnetoresistance antisymmetric reversal in contrast with the linear regime. This effect can only be understood if electronic interactions are considered. It is accompanied by a linear dispersion of the zeromagnetoresistance point in the bias-field plane. Simulations based on a proposed model confirm a current induced spin precession, electrically tunable due to the quantum nature ofthe device.; Les développements récents des nanotechnologies ont permis d’accéder à des dimensions qui permettent d’’étudier les spins des électrons. Ceci ouvre la voie à l’utilisation du degré de liberté du spin des électrons dans des dispositifs électroniques de nouvelle génération. C’est l’origine d’un nouveau domaine de recherche prometteur baptisé spintronique.Dans ce travail, on présente des expériences dans le domaine de la spintronique en utilisant deux matériaux très prometteurs : les nanotubes de carbone (CNT) et le palladiumnickel (PdNi), un ferromagnet versatile dans le but de manipuler le spin électronique dans les deux régimes, classiques et quantiques. Une compréhension détaillée des caractéristiques magnétiques de PdxNi 100-x devient cruciale à la fois pour comprendre les caractéristiques de basculement d’un tel dispositif mais aussi pour optimiser ses propriétés électroniques.Une étude sur des structures micrométrique et nanométrique en PdNi a été menée grâce à des mesures de l’effet Hall extraordinaires (EHE sur des croix lithographiées). Les analyses montrent que la géométrie, l’épaisseur, la composition chimique ainsi que la couche de recouvrement, ont tous une influence sur l’aimantation des électrodes de PdNi, en particulier celles de taille nanométrique. Cela est dû à la relaxation des contraintes sur les bords qui devient important pour les dispositifs de petites dimensions.On met en place un point quantique connecté `a deux contacts ferromagnétiques non colinéaires (source et drain), évaporées sur le CNT; le nanotube est connecté à une tension de grille pendant qu’une source de tension source-drain est utilisée pour varier le potentiel chimiques des ferromagnets. Les électrodes sont conçues pour former un angle téta = Pi/2. On attend alors un comportement similaire à celui d’une vanne de spin, donc un effet fini de magnétorésistance à effet tunnel est à prévoir.Des mesures de transport de spin ont révélé un régime de blocage de Coulomb, confirmé par la spectroscopie de transport. Les régimes linéaires et non-linéaires ont été également testés.En régime linéaire, les résultats montrent un signal de TMR lorsqu’il est placé dans un champ qui est balayé, un comportement typique pour un dispositif vanne de spin. Dans le régime non linéaire, ont été obtenues des variations du signal d’hystérésis lorsque la polarisation change de signe. De plus, la TMR affiche un comportement presque antisymétrique avec la conductance. Les mesures réalisées pour différentes valeurs de la tension grille et celle source drain prouvent la non trivialité de ce comportement. Cette variation antisymétrique, qui a la même symétrie que le courant, indique un courant de spin induite par un phénomène de précession. Des simulations théoriques appuient également cette hypothèse: une combinaison de phénomènes d’accumulation de spin (induite par la polarisation en spin du courant) et des phénomènes de relaxation de spin (qui agissent contre la première catégorie) déterminent une précession du spin lors de son passage dans le nanotube.

The domain wall dynamics is used in many spintronic devices based on the uniaxial ferromagnetic wires to transport and store information. Therefore, the domain wall velocity is one of the main parameters that determine the operation speed of these devices. Recently, a big attention is being paid to amorphous glass-coated microwires due to the very high domain wall velocities that reach up to 20 km/s. In this work, the fast domain wall propagation in amorphous glass-coated microwires was found in the presence of two main factors: (i) relatively low magnetic anisotropy, (ii) complex geometry of magnetic anisotropies given by internal distribution of mechanical stresses. The domain wall dynamics was examined in amorphous glass-coated microwires of reduced diameter down to 1 μm. It was shown, that the domain wall dynamics in these wires is the same as in wires of bigger diameter. It proves that the high domain wall velocities in microwires are not the effect of microwire diameter value. The direct observation of the surface domain wall structure by use of MOKE microscope confirmed that the domain wall is inclined relatively to the main axis. A new method for magneto-optical observation of the samples with cylindrical geometry was proposed. The inclined structure of the domain wall was found to be partially responsible for the high apparent domain wall velocity measured by the Sixtus-Tonks method in microwires.; La dynamique des paroi de domaine est utilisée dans de nombreux dispositifs de spintronique basés sur des micro et nanofils magnétiques pour la transmission et le stockage de l'information. La vitesse de la paroi de domaine est donc un des paramètres qui déterminent la vitesse de fonctionnement de ces dispositifs. Actuellement, un accent considérable est mis sur la compréhension de l'origine des grandes vitesses parois de domaines dans les microfils, qui peuvent atteindre 20 km/s. Dans ce travail, des fortes vitesses de parois ont été trouvées en présence de deux principaux facteurs: (i) une valeur relativement faible de l'anisotropie magnétique, et (ii) une distribution complexe de l'anisotropie magnétique due aux contraintes internes. En outre, la dynamique d'une paroi de domaine a été étudiée pour les échantillons à diamètre réduit, jusqu'à 1 μm. Il a été démontré que la dynamique d'une paroi de domaine est la même que dans les échantillons plus épais, ce qui confirme que les vitesses élevées ne sont pas seulement liées à la taille des microfils. L'observation directe de la structure de surface des parois de domaines par microscopie MOKE confirmé la forme de la paroi de domaine inclinée par rapport à l'axe du fil. Une nouvelle méthode a été proposée pour effectuer des observations sur des échantillons cylindriques. La structure inclinée de la paroi de domaine est jugée en partie responsable des valeurs élevées de vitesse apparente des parois de domaines mesurées par la méthode Sixte-Tonks dans ces microfils.; Dynamika doménovej steny sa používa v mnohých spintronických zariadeniach na báze tenkých magnetických drôtoch na prenos a uchovávanie informácie. Rýchlosť doménovej steny je preto jedným z parametrov, ktoré určujú operačnú rýchlosť týchto zariadení. V súčasnosti je kladený značný dôraz na pochopenie pôvodu veľkých rýchlostí doménovej steny v mikrodrôtoch, kde rýchlosti dosahujú až 20 km/s. Veľké rýchlosti doménovej steny v mikrodrôtoch boli v tejto práci nájdené v prítomnosti dvoch faktorov: (i) relatívne nízka hodnota magnetickej anizotropie a (ii) zložitá distribúcia magnetických anizotropií daných vnútornými pnutiami. Navyše, dynamika doménovej steny bola študovaná aj na vzorkách s redukovaným priemerom až do 1 μm. Bolo ukázané, že dynamika doménovej steny je v týchto drôtoch rovnaká ako je tomu v hrubších vzorkách, čo potvrdzuje, že vysoké rýchlosti nie sú len efektom rozmeru amorfných, sklom potiahnutých mikrodrôtoch. Priame pozorovania povrchovej štruktúry doménovej steny pomocou MOKE mikroskopu potvrdili naklonený tvar doménovej steny vzhľadom na os drôtu. Bola navrhnutá nová metóda na vykonávanie magneto-optických pozorovaní valcových vzoriek. Naklonený tvar doménovej steny bol nájdený ako jeden z faktorov umožňujúcich zdanlivo veľké rýchlosti doménovej steny meraných Sixtusovou-Tonkosvou metódou na mikrodrôtoch.

The domain wall dynamics is used in many spintronic devices based on the uniaxial ferromagnetic wires to transport and store information. Therefore, the domain wall velocity is one of the main parameters that determine the operation speed of these devices. Recently, a big attention is being paid to amorphous glass-coated microwires due to the very high domain wall velocities that reach up to 20 km/s. In this work, the fast domain wall propagation in amorphous glass-coated microwires was found in the presence of two main factors: (i) relatively low magnetic anisotropy, (ii) complex geometry of magnetic anisotropies given by internal distribution of mechanical stresses. The domain wall dynamics was examined in amorphous glass-coated microwires of reduced diameter down to 1 μm. It was shown, that the domain wall dynamics in these wires is the same as in wires of bigger diameter. It proves that the high domain wall velocities in microwires are not the effect of microwire diameter value. The direct observation of the surface domain wall structure by use of MOKE microscope confirmed that the domain wall is inclined relatively to the main axis. A new method for magneto-optical observation of the samples with cylindrical geometry was proposed. The inclined structure of the domain wall was found to be partially responsible for the high apparent domain wall velocity measured by the Sixtus-Tonks method in microwires., La dynamique des paroi de domaine est utilisée dans de nombreux dispositifs de spintronique basés sur des micro et nanofils magnétiques pour la transmission et le stockage de l'information. La vitesse de la paroi de domaine est donc un des paramètres qui déterminent la vitesse de fonctionnement de ces dispositifs. Actuellement, un accent considérable est mis sur la compréhension de l'origine des grandes vitesses parois de domaines dans les microfils, qui peuvent atteindre 20 km/s. Dans ce travail, des fortes vitesses de parois ont été trouvées en présence de deux principaux facteurs: (i) une valeur relativement faible de l'anisotropie magnétique, et (ii) une distribution complexe de l'anisotropie magnétique due aux contraintes internes. En outre, la dynamique d'une paroi de domaine a été étudiée pour les échantillons à diamètre réduit, jusqu'à 1 μm. Il a été démontré que la dynamique d'une paroi de domaine est la même que dans les échantillons plus épais, ce qui confirme que les vitesses élevées ne sont pas seulement liées à la taille des microfils. L'observation directe de la structure de surface des parois de domaines par microscopie MOKE confirmé la forme de la paroi de domaine inclinée par rapport à l'axe du fil. Une nouvelle méthode a été proposée pour effectuer des observations sur des échantillons cylindriques. La structure inclinée de la paroi de domaine est jugée en partie responsable des valeurs élevées de vitesse apparente des parois de domaines mesurées par la méthode Sixte-Tonks dans ces microfils., Dynamika doménovej steny sa používa v mnohých spintronických zariadeniach na báze tenkých magnetických drôtoch na prenos a uchovávanie informácie. Rýchlosť doménovej steny je preto jedným z parametrov, ktoré určujú operačnú rýchlosť týchto zariadení. V súčasnosti je kladený značný dôraz na pochopenie pôvodu veľkých rýchlostí doménovej steny v mikrodrôtoch, kde rýchlosti dosahujú až 20 km/s. Veľké rýchlosti doménovej steny v mikrodrôtoch boli v tejto práci nájdené v prítomnosti dvoch faktorov: (i) relatívne nízka hodnota magnetickej anizotropie a (ii) zložitá distribúcia magnetických anizotropií daných vnútornými pnutiami. Navyše, dynamika doménovej steny bola študovaná aj na vzorkách s redukovaným priemerom až do 1 μm. Bolo ukázané, že dynamika doménovej steny je v týchto drôtoch rovnaká ako je tomu v hrubších vzorkách, čo potvrdzuje, že vysoké rýchlosti nie sú len efektom rozmeru amorfných, sklom potiahnutých mikrodrôtoch. Priame pozorovania povrchovej štruktúry doménovej steny pomocou MOKE mikroskopu potvrdili naklonený tvar doménovej steny vzhľadom na os drôtu. Bola navrhnutá nová metóda na vykonávanie magneto-optických pozorovaní valcových vzoriek. Naklonený tvar doménovej steny bol nájdený ako jeden z faktorov umožňujúcich zdanlivo veľké rýchlosti doménovej steny meraných Sixtusovou-Tonkosvou metódou na mikrodrôtoch.

Le 20 Janvier 2009; In this work, we experimentally study spin-dependent hot electron transport through metallic multilayers (ML), containing single magnetic layers or "spin-valve" (SV) trilayers. For this purpose, we have set up a ballistic electron emission microscope (BEEM), a three terminal extension of scanning tunnelling microscopy on metal/semiconductor structures. The implementation of the BEEM requirements into the sample fabrication is described in detail. Using BEEM, the hot electron transmission through the ML's was systematically measured in the energy range 1-2 eV above the Fermi level. By varying the magnetic layer thickness, the spin-dependent hot electron attenuation lengths were deduced. For the materials studied (Co and NiFe), they were compared to calculations and other determinations in the literature. For sub-monolayer thickness, a non uniform morphology was observed, with large transmission variations over subnanometric distances. This effect is not yet fully understood. In the imaging mode, the magnetic configurations of SV?s were studied under field, focussing on 360° domain walls in Co layers. The effects of the applied field intensity and direction on the DW structure were studied. The results were compared quantitatively to micromagnetic calculations, with an excellent agreement. From this, it can be shown that the BEEM magnetic resolution is better than 50 nm.; Nous étudions expérimentalement le transport dépendant du spin d'électrons chauds dans des multicouches (MC) magnétiques, qui contiennent des couches uniques magnétiques, ou des tricouches de type "vannes de spin" (VS). Pour cela, nous avons mis en oeuvre la microscopie à émission d'électrons balistiques (BEEM), une extension à trois contacts de la microscopie à effet tunnel sur des structures métal/semiconducteur. La méthode mise au point pour satisfaire les nombreuses contraintes imposées par le BEEM sur les échantillons est décrite en détail. La transmission des électrons chauds dans des MC a été systématiquement mesurée dans la gamme d'énergie 1-2 eV au dessus du niveau de Fermi. De l'étude en fonction des épaisseurs des couches magnétiques nous avons déduit les longueurs d'atténuation des électrons chauds en fonction du spin et de l'énergie. Ces mesures, sur le cobalt et l'alliage doux NiFe, sont comparées à des calculs et résultats expérimentaux de la littérature. Pour des épaisseurs inférieures à la monocouche atomique, une organisation spatialement hétérogene a été observée, avec un effet très important sur la transmission BEEM, variant sur une échelle subnanométrique. En mode imagerie, nous avons étudié les configurations magnétiques de VS, en particulier des parois à 360° dans des couches de cobalt. Les effets de l'intensité et la direction du champ appliqué sur la structure de ces parois ont été observés. Ces résultats ont été comparés quantitativement à des calculs micromagnétiques, avec un accord excellent. Ceci a permis de montrer que la résolution magnétique du BEEM est meilleure que 50 nm.

In this work, we experimentally study spin-dependent hot electron transport through metallic multilayers (ML), containing single magnetic layers or "spin-valve" (SV) trilayers. For this purpose, we have set up a ballistic electron emission microscope (BEEM), a three terminal extension of scanning tunnelling microscopy on metal/semiconductor structures. The implementation of the BEEM requirements into the sample fabrication is described in detail. Using BEEM, the hot electron transmission through the ML's was systematically measured in the energy range 1-2 eV above the Fermi level. By varying the magnetic layer thickness, the spin-dependent hot electron attenuation lengths were deduced. For the materials studied (Co and NiFe), they were compared to calculations and other determinations in the literature. For sub-monolayer thickness, a non uniform morphology was observed, with large transmission variations over subnanometric distances. This effect is not yet fully understood. In the imaging mode, the magnetic configurations of SV?s were studied under field, focussing on 360° domain walls in Co layers. The effects of the applied field intensity and direction on the DW structure were studied. The results were compared quantitatively to micromagnetic calculations, with an excellent agreement. From this, it can be shown that the BEEM magnetic resolution is better than 50 nm., Nous étudions expérimentalement le transport dépendant du spin d'électrons chauds dans des multicouches (MC) magnétiques, qui contiennent des couches uniques magnétiques, ou des tricouches de type "vannes de spin" (VS). Pour cela, nous avons mis en oeuvre la microscopie à émission d'électrons balistiques (BEEM), une extension à trois contacts de la microscopie à effet tunnel sur des structures métal/semiconducteur. La méthode mise au point pour satisfaire les nombreuses contraintes imposées par le BEEM sur les échantillons est décrite en détail. La transmission des électrons chauds dans des MC a été systématiquement mesurée dans la gamme d'énergie 1-2 eV au dessus du niveau de Fermi. De l'étude en fonction des épaisseurs des couches magnétiques nous avons déduit les longueurs d'atténuation des électrons chauds en fonction du spin et de l'énergie. Ces mesures, sur le cobalt et l'alliage doux NiFe, sont comparées à des calculs et résultats expérimentaux de la littérature. Pour des épaisseurs inférieures à la monocouche atomique, une organisation spatialement hétérogene a été observée, avec un effet très important sur la transmission BEEM, variant sur une échelle subnanométrique. En mode imagerie, nous avons étudié les configurations magnétiques de VS, en particulier des parois à 360° dans des couches de cobalt. Les effets de l'intensité et la direction du champ appliqué sur la structure de ces parois ont été observés. Ces résultats ont été comparés quantitativement à des calculs micromagnétiques, avec un accord excellent. Ceci a permis de montrer que la résolution magnétique du BEEM est meilleure que 50 nm.