arXiv subjects

Vincent Cros

Publications and source records attributed to Vincent Cros.

At least 73 records · Page 4Linked to original sources

arXiv2017

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.

cond-mat.mes-hall↗

arXiv2017

Current-driven skyrmion expulsion from magnetic nanostrips

We study the current-driven skyrmion expulsion from magnetic nanostrips using micromagnetic simulations and analytic calculations. We explore the threshold current density for the skyrmion expulsion, and show that this threshold is determined by the critical boundary force as well as the spin-torque parameters. We also find the dependence of the critical boundary force on the magnetic parameters; the critical boundary force decreases with increasing the exchange stiffness and perpendicular anisotropy constants, while it increases with increasing Dzyaloshinskii-Moriya interaction and saturation magnetization constants. Using a simple model describing the skyrmion and locally-tilted edge magnetization, we reveal the underlying physics of the dependence of the critical boundary force on the magnetic parameters based on the relation between the scaled Dzyaloshinskii-Moriya-interaction parameter and the critical boundary force. This work provides a fundamental understanding of the skyrmion expulsion and the interaction between the skymion and boundaries of devices and shows that the stability of the skyrmion in devices can be related to the scaled Dzyaloshinskii-Moriya-interaction parameter of magnetic materials.

cond-mat.mes-hall↗

arXiv2017

Chirality-mediated bistability and strong frequency downshifting of the gyrotropic resonance of a dynamically de-stiffened magnetic vortex

We demonstrate an enhanced, bidirectional, in-plane magnetic field tuning of the gyrotropic resonance frequency of a magnetic vortex within a disk by introducing a flat edge. When the core is in its vicinity, the flat edge locally reduces the core's directional dynamic stiffness for movement parallel to the edge. This strongly reduces the net dynamic core stiffness, leading to the gyrotropic frequency being significantly less than when the core is centered (or located near the round edge). This leads to the measurable range of gyrotropic frequencies being more than doubled and also results in a clear chirality-mediated bistability of the gyrotropic resonance frequency due to what is effectively a chirality-dependence of the core's confining potential.

cond-mat.mes-hall↗

arXiv2017

Neuromorphic computing with nanoscale spintronic oscillators

Neurons in the brain behave as non-linear oscillators, which develop rhythmic activity and interact to process information. Taking inspiration from this behavior to realize high density, low power neuromorphic computing will require huge numbers of nanoscale non-linear oscillators. Indeed, a simple estimation indicates that, in order to fit a hundred million oscillators organized in a two-dimensional array inside a chip the size of a thumb, their lateral dimensions must be smaller than one micrometer. However, despite multiple theoretical proposals, there is no proof of concept today of neuromorphic computing with nano-oscillators. Indeed, nanoscale devices tend to be noisy and to lack the stability required to process data in a reliable way. Here, we show experimentally that a nanoscale spintronic oscillator can achieve spoken digit recognition with accuracies similar to state of the art neural networks. We pinpoint the regime of magnetization dynamics leading to highest performance. These results, combined with the exceptional ability of these spintronic oscillators to interact together, their long lifetime, and low energy consumption, open the path to fast, parallel, on-chip computation based on networks of oscillators.

cs.ET↗

arXiv2017

Room-temperature current-induced generation and motion of sub-100nm skyrmions

Magnetic skyrmions are nanoscale windings of the spin configuration that hold great promise for technology due to their topology-related properties and extremely reduced sizes. After the recent observation at room temperature of sub-100 nm skyrmions stabilized by interfacial chiral interaction in magnetic multilayers, several pending questions remain to be solved, notably about the means to nucleate individual compact skyrmions or the exact nature of their motion. In this study, a method leading to the formation of magnetic skyrmions in a micrometer-sized nanotrack using homogeneous current injection is evidenced. Spin-transfer-induced motion of these small electricalcurrent-generated skyrmions is then demonstrated and the role of the out-of-plane magnetic field in the stabilization of the moving skyrmions is also analysed. The results of these experimental observations of spin torque induced motion are compared to micromagnetic simulations reproducing a granular type, non-uniform magnetic multilayer, in order to address the particularly important role of the magnetic inhomogeneities on the current-induced motion of sub-100 nm skyrmions, for which the material grains size is comparable to the skyrmion diameter.

cond-mat.mes-hall↗

arXiv2016

Controlling Dzyaloshinskii-Moriya Interaction via Chirality Dependent Layer Stacking, Insulator Capping and Electric Field

Using first-principle calculations, we demonstrate several approaches to manipulate Dzyaloshinskii-Moriya Interaction (DMI) in ultrathin magnetic films. First, we find that DMI is significantly enhanced when the ferromagnetic (FM) layer is sandwiched between nonmagnetic (NM) layers inducing additive DMI in NM/FM/NM structures. For instance, as Pt and Ir below Co induce DMI of opposite chirality, inserting Co between Pt (below) and Ir (above) in Ir/Co/Pt trilayers enhances the DMI of Co/Pt bilayers by 15\%. Furthermore, in case of Pb/Co/Pt trilayers (Ir/Fe/Co/Pt multilayers), DMI can be enhanced by 50\% (almost doubled) compared to Co/Pt bilayers reaching a very large DMI amplitude of 2.7 (3.2) meV/atom. Our second approach for enhancing DMI is to use oxide capping layer. We show that DMI is enhanced by 60\% in Oxide/Co/Pt structures compared to Co/Pt bilayers. Moreover, we unveiled the DMI mechanism at Oxide/Co inerface due to interfacial electric field effect, which is different to Fert-Levy DMI at FM/NM interfaces. Finally, we demonstrate that DMI amplitude can be modulated using an electric field with efficiency factor comparable to that of the electric field control of perpendicular magnetic anisotropy in transition metal/oxide interfaces. These approaches of DMI controlling pave the way for skyrmions and domain wall motion-based spintronic applications.

cond-mat.mtrl-sci↗

arXiv2016

Spin transfer driven resonant expulsion of a magnetic vortex core for efficient rf detector

Spin transfer magnetization dynamics have led to considerable advances in Spintronics, including opportunities for new nanoscale radiofrequency devices. Among the new functionalities is the radiofrequency(rf) detection using the spin diode rectification effect in spin torque nano-oscillators (STNOs). In this study, we focus on a new phenomenon, the resonant expulsion of a magnetic vortex in STNOs. This effect is observed when the excitation vortex radius, due to spin torques associated to rf currents, becomes larger than the actual radius of the STNO. This vortex expulsion is leading to a sharp variation of the voltage at the resonant frequency. Here we show that the detected frequency can be tuned by different parameters; furthermore, a simultaneous detection of different rf signals can be achieved by real time measurements with several STNOs having different diameters. This result constitutes a first proof-of-principle towards the development of a new kind of nanoscale rf threshold detector.

physics.ins-det↗

arXiv2016

Report on imaging of individual skyrmions of MML systems made by different techniques

Deliverable 1.1 is a report on imaging of individual skyrmions of MML systems made by different techniques. This deliverable falls within work package 1 (WP1:Static equilibrium properties of interface induced skyrmions in magnetic films and multilayers) of the MAGicSky research proposal whose objectives are: the growth of magnetic multilayer systems with large interfacial chiral interaction, the determination of the amplitude of the Dzyaloshinskii--Moriya interactions (DMi), the detailed structural and magnetic characterization of interfaces, the magnetic imaging of individual skyrmions and the thermal stability and interaction of skyrmions with intrinsic and/or artificial defects. It should be emphasized that imaging magnetic skyrmionic structures forms the basis for all the planned subsequent material optimization, electrical transport experiments and devices preparation in the MAGicSky research program. In particular, imaging is paramount as a technique since it enables us to unequivocally demonstrate chiral magnetic skyrmions by directly imaging i) the effects of engineering multilayer samples with large DMis and ii) the skyrmionic structure and properties. In order to get complementary physical insight on the properties of magnetic skyrmions and to corroborate our research results, we utilise a plethora of key state-of-the-art advanced magnetic imaging techniques, namely: i) Spin-polarised Scanning Tunnelling Microscopy (SP-STM), Scanning Transmission X-ray Microscopy (STXM) Lorentz Transmission Electron Microscopy (Lorentz TEM) as well as, Magnetic Force Microscopy (MFM). These imaging experiments can therefore stand as the base for all the other objectives in WP1 as well as WP2 that will study Dynamic properties: nucleation, propagation and detection of skyrmions. Imaging has been aimed to test and get feedback from the theoretical and numerical investigations of skyrmion systems of WP3. Finally, for WP4: Towards skyrmion based devices for information storage and processing, some of the techniques detailed here in D1.1 will enable us to image and test the first devices and their functionality. In short, the MAGicSky consortium research results in D1.1 constitute a major milestone that acts as a stepping-stone to every other experimental effort and we have achieved it. The results relevant to the D1.1 deliverable were presented in 5 publications and several invited talks at international workshop and conferences:

cond-mat.mtrl-sci↗

arXiv2016

Inside the perpendicular spin-torque memristor

Memristors are non-volatile nano-resistors. Their resistance can be tuned by applied currents or voltages and set to a large number of levels between two limit values. Thanks to these properties, memristors are ideal building blocks for a number of applications such as multilevel non-volatile memories and artificial nano-synapses, which are the focus of this work. A key point towards the development of large scale memristive neuromorphic hardware is to build these neural networks with a memristor technology compatible with the best candidates for the future mainstream non-volatile memories. Here we show the first experimental achievement of a memristor compatible with Spin-Torque Magnetic Random Access Memory. The resistive switching in our spin-torque memristor is linked to the displacement of a magnetic domain wall by spin-torques in a perpendicularly magnetized magnetic tunnel junction. We demonstrate that our magnetic synapse has a large number of intermediate resistance states, sufficient for neural computation. Moreover, we show that engineering the device geometry allows leveraging the most efficient spin torque to displace the magnetic domain wall at low current densities and thus to minimize the energy cost of our memristor. Our results pave the way for spin-torque based analog magnetic neural computation.

cond-mat.mes-hall↗

arXiv2016

Electrical measurement of magnetic-field-impeded polarity switching of a ferromagnetic vortex core

Vortex core polarity switching in NiFe disks has been evidenced using an all-electrical rectification scheme. Both simulation and experiments yield a consistent loss of the rectified signal when driving the core at high powers near its gyrotropic resonant frequency. The frequency range over which the loss occurs grows and shifts with increasing signal power, consistent with non-linear core dynamics and periodic switching of the core polarity induced by the core attaining its critical velocity. We demonstrate that core polarity switching can be impeded by displacing the core towards the disk's edge where an increased core stiffness reduces the maximum attainable core velocity.

cond-mat.mes-hall↗

arXiv2016

Twist in the bias-dependence of spin-torques in magnetic tunnel junctions

The spin-torque in magnetic tunnel junctions possesses two components that both depend on the applied voltage. Here, we develop a new method for the accurate extraction of this bias-dependence from experiments over large voltage ranges. We study several junctions with different magnetic layer structures of the top electrode. Our results obtained on junctions with symmetric CoFeB electrodes agree well with theoretical calculations. The bias-dependences of asymmetric samples, with top electrodes containing NiFe, however, are twisted compared to the quadratic form generally assumed. Our measurements reveal the complexity of spin-torque mechanisms at large bias.

cond-mat.mes-hall↗

arXiv2015

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.

cond-mat.mes-hall↗

arXiv2015

Increased magnetic damping of a single domain wall and adjacent magnetic domains detected by spin torque diode in a nanostripe

We use spin-torque resonance to probe simultaneously and separately the dynamics of a magnetic domain wall and of magnetic domains in a nanostripe magnetic tunnel junction. Thanks to the large associated resistance variations we are able to analyze quantitatively the resonant properties of these single nanoscale magnetic objects. In particular, we find that the magnetic damping of both domains and domain walls is doubled compared to the damping value of their host magnetic layer. We estimate the contributions to damping arising from dipolar couplings between the different layers in the junction and from the intralayer spin pumping effect. We find that they cannot explain the large damping enhancement that we observe. We conclude that the measured increased damping is intrinsic to large amplitudes excitations of spatially localized modes or solitons such as vibrating or propagating domain walls

cond-mat.mtrl-sci↗

arXiv2015

Generation of coherent spin-wave modes in Yttrium Iron Garnet microdiscs by spin-orbit torque

Spin-orbit effects [1-4] have the potential of radically changing the field of spintronics by allowing transfer of spin angular momentum to a whole new class of materials. In a seminal letter to Nature [5], Kajiwara et al. showed that by depositing Platinum (Pt, a normal metal) on top of a 1.3 $μ$m thick Yttrium Iron Garnet (YIG, a magnetic insulator), one could effectively transfer spin angular momentum through the interface between these two different materials. The outstanding feature was the detection of auto-oscillation of the YIG when enough dc current was passed in the Pt. This finding has created a great excitement in the community for two reasons: first, one could control electronically the damping of insulators, which can offer improved properties compared to metals, and here YIG has the lowest damping known in nature; second, the damping compensation could be achieved on very large objects, a particularly relevant point for the field of magnonics [6,7] whose aim is to use spin-waves as carriers of information. However, the degree of coherence of the observed auto-oscillations has not been addressed in ref. [5]. In this work, we emphasize the key role of quasi-degenerate spin-wave modes, which increase the threshold current. This requires to reduce both the thickness and lateral size in order to reach full damping compensation [8] , and we show clear evidence of coherent spin-orbit torque induced auto-oscillation in micron-sized YIG discs of thickness 20 nm.

cond-mat.mtrl-sci↗

arXiv2015

Skyrmions at room temperature : From magnetic thin films to magnetic multilayers

Facing the ever-growing demand for data storage will most probably require a new paradigm. Magnetic skyrmions are anticipated to solve this issue as they are arguably the smallest spin textures in magnetic thin films in nature. We designed cobalt-based multilayered thin films where the cobalt layer is sandwiched between two heavy metals providing additive interfacial Dzyaloshinskii-Moriya interactions, which reach about 2 mJ/m2 in the case of the Ir/Co/Pt multilayers. Using a magnetization-sensitive scanning x-ray transmission microscopy technique, we imaged magnetic bubble-like domains in these multilayers. The study of their behavir in magnetic field allows us to conclude that they are actually magnetic skyrmions stabilized by the Dzyaloshinsskii-Moriya interaction. This discoevry of stable skyrmions at room temperature in a technologically relevant material opens the way for device applications in a near future.

cond-mat.mtrl-sci↗

arXiv2015

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.

cond-mat.mes-hall↗

arXiv2014

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.

cond-mat.mes-hall↗

arXiv2014

Breathing modes of confined skyrmions in ultrathin magnetic dots

The dynamics of individual magnetic skyrmions confined in ultrathin film dots is studied theoretically. The systems considered are transition metal ferromagnets possessing perpendicular magnetic anisotropy and particular attention is given to the dynamic response of the skyrmions to perpendicular driving fields. By using micromagnetics simulations, it is shown that breathing modes can hybridize with geometrically-quantized spin wave eigenmodes of the circular dots considered, leading to distinct features in the power spectrum that differ to the behavior expected for uniformly magnetized systems. The field dependence of the breathing modes offers a direct means of detecting and characterizing such skyrmion states in experiment.

cond-mat.mtrl-sci↗