Neuromorphic weighted sum with magnetic skyrmions

Integrating magnetic skyrmion properties into neuromorphic computing promises advancements in hardware efficiency and computational power. However, a scalable implementation of the weighted sum of neuron signals, a core operation in neural networks, has yet to be demonstrated. In this study, we exploit the non-volatile and particle-like characteristics of magnetic skyrmions, akin to synaptic vesicles and neurotransmitters, to perform this weighted sum operation in a compact, biologically-inspired manner. To this aim, skyrmions are electrically generated in numbers proportional to the input with an efficiency given by a non-volatile weight. These chiral particles are then directed using localized current injections to a location where their presence is quantified through non-perturbative electrical measurements. Our experimental demonstration, currently with two inputs, can be scaled to accommodate multiple inputs and outputs using a crossbar array design, potentially nearing the energy efficiency observed in biological systems.Comment: 12 pages, 5 figure

Driving skyrmions in flow regime in synthetic ferrimagnets

Despite significant advances in the last decade regarding the room temperature stabilization of skyrmions or their current induced dynamics, the impact of local material inhomogeneities still remains an important issue that impedes to reach the regime of steady state motion of these spin textures. Here, we study the spin-torque driven motion of skyrmions in synthetic ferrimagnetic multilayers with the aim of achieving high mobility and reduced skyrmion Hall effect. We consider Pt|Co|Tb multilayers of various thicknesses with antiferromagnetic coupling between the Co and Tb magnetization. The increase of Tb thickness in the multilayers allows to reduce the total magnetic moment and increases the spin-orbit torques allowing to reach velocities up to 400 m.s-1 for skyrmions with diameters of about 160 nm. We demonstrate that due to reduced skyrmion Hall effect, combined with the edge repulsion of the magnetic track making the skyrmions moving along the track without any transverse deflection. Further, by comparing the field-induced domain wall motion and current-induced skyrmion motion, we demonstrate that the skyrmions at the largest current densities present all the characteristics of a dynamical flow regime.Comment: 14 pages, 4 figure

Ultrafast time-evolution of chiral N\'eel magnetic domain walls probed by circular dichroism in x-ray resonant magnetic scattering

Non-collinear spin textures in ferromagnetic ultrathin films are attracting a renewed interest fueled by possible fine engineering of several magnetic interactions, notably the interfacial Dzyaloshinskii-Moriya interaction. This allows the stabilization of complex chiral spin textures such as chiral magnetic domain walls (DWs), spin spirals, and magnetic skyrmions. We report here on the ultrafast behavior of chiral DWs after optical pumping in perpendicularly magnetized asymmetric multilayers, probed using time-resolved circular dichroism in x-ray resonant magnetic scattering (CD-XRMS). We observe a picosecond transient reduction of the CD-XRMS, which is attributed to the spin current-induced coherent and incoherent torques within the continuously dependent spin texture of the DWs. We argue that a specific demagnetization of the inner structure of the DW induces a flow of hot spins from the interior of the neighboring magnetic domains. We identify this time-varying change of the DW textures shortly after the laser pulse as a distortion of the homochiral N'eel shape toward a transient mixed Bloch-N\'eel-Bloch textures along a direction transverse to the DW. Our study highlights how time-resolved CD-XRMS can be a unique tool for studying the time evolution in other systems showing a non-collinear electric/magnetic ordering such as skyrmion lattices, conical/helical phases, as well as the recently observed antiskyrmion lattices, in metallic or insulating materials

Skyrmions magnétiques dans des multicouches : contrôle de leur nucléation et déplacement induits par courant électrique

Les skyrmions magnétiques sont des textures de spin stabilisées grâce à l'interaction Dzyaloshinskii-Moriya. Cette interaction chirale favorise un enroulement particulier de l'aimantation et leur confère des propriétés intéressantes. Ils se comportent par exemple comme des particules et possèdent une topologie différente de l'état d'aimantation uniforme. Depuis les premières observations en 2011, les skyrmions ont fait l'objet d'études approfondies en raison de leur potentiel pour des applications futures, telles que la "racetrack memory". Cette thèse se concentre sur les skyrmions Néel, un type particulier de skyrmions, qui peuvent être stabilisés dans des multicouches magnétiques. En commençant par l'étude des différentes interactions magnétiques, il a été possible de définir des épaisseurs critiques qui régissent leur formation et de proposer un empilement optimisé pour les observer expérimentalement. Nous avons ensuite étudié leur nucléation, induite par impulsions de courant électrique dans divers dispositifs, par exemple grâce à l'insertion de constrictions sur les bords des pistes. Le cas des skyrmions dans des multicouches formant un antiferromagnétique synthétique, soit deux skyrmions de polarité opposée couplés ensemble, est également abordé par le biais d'une ingénierie des différentes épaisseurs des matériaux conduisant à leur stabilisation dans des multicouches à anisotropie variable. Dans le but de les intégrer dans des technologies futures, leur dynamique induite par courant électrique a aussi été étudiée avec l'optimisation des couples de spin afin d'améliorer leur mobilité. Cette étude a conduit à des systèmes dans lesquels les skyrmions sont plus rapides que les parois de domaine, ce qui est intéressant dans le but d'explorer comment ces deux objets interagissent l'un avec l'autre. Ce résultat est notamment attribué à la présence d'un fort couple field-like. Enfin, la question de leur fiabilité est abordée avec l'observation de leur déplacement sur plusieurs dizaines de micromètres et avec une discussion sur l'utilisation des mécanismes de répulsions skyrmion-skyrmion et skyrmion-paroi (de domaines) pour améliorer leur dynamique. Le déplacement des skyrmions le long de domaines magnétiques ouvre notamment de nouvelles possibilités concernant le contrôle de leur trajectoire et pourrait conduire à de nouvelles perspectives concernant les dispositifs visant à utiliser les skyrmions pour faire des portes logiques.Magnetic skyrmions are localized spin textures which stabilize thanks to the Dzyaloshinskii-Moriya interaction. This chiral interaction promotes a twisting of the magnetization, which grants skyrmions interesting properties. For example, they behave as particles and are topologically different from the uniform magnetization state. Since their first observations in 2011, skyrmions have been extensively studied because of their promise for future applications e.g. racetrack memory, neuromorphic computer, etc. This thesis is focused on Néel skyrmions, a peculiar type of skyrmions, which can be stabilized in various magnetic multilayers. Starting from the investigation of the different magnetic interactions present in such systems, it has been possible to define critical thicknesses which governs the formation of chiral textures and to propose optimized stacking to investigate experimentally their nucleation in thin films. This analysis is followed by their current induced formation in patterned devices, an important milestone for the potential applications mentioned before. As an example, through the insertion of notches at the edges of devices, a deterministic control of the nucleation events is achieved. The case of antiferromagnetic skyrmions, which are basically two skyrmions with opposite polarity coupled together, is also addressed through an engineering of the different materials thicknesses leading to their stabilization in multilayers with varying anisotropies along the vertical axis. With the goal to implement them in actual technologies, their current induced dynamics have been investigated through an optimization of the spin orbit torques, to enhance the skyrmion's mobility, for example through the use of Pt (below the magnetic layer) and Ta (above the magnetic layer). This study also leads to systems in which skyrmions are faster than domain walls, which is interesting to explore how both textures interact one with the other. This result is attributed to the presence of an unexpected Rashba mechanism at the Co|Al interface, which increases the field like component of the torque and slows down the domain walls. Finally, the question of their reliability is discussed with observations of their motion on tens of micrometres distances and with a presentation of the use of skyrmion-skyrmion and skyrmion-domains repulsions to improve their dynamics. This last result, the motion of skyrmions along magnetic domains, opens new possibilities concerning the control of their trajectory and could lead to new prospects for skyrmion logic devices

Skyrmions magnétiques dans des multicouches : contrôle de leur nucléation et déplacement induits par courant électrique

Magnetic skyrmions are localized spin textures which stabilize thanks to the Dzyaloshinskii-Moriya interaction. This chiral interaction promotes a twisting of the magnetization, which grants skyrmions interesting properties. For example, they behave as particles and are topologically different from the uniform magnetization state. Since their first observations in 2011, skyrmions have been extensively studied because of their promise for future applications e.g. racetrack memory, neuromorphic computer, etc. This thesis is focused on Néel skyrmions, a peculiar type of skyrmions, which can be stabilized in various magnetic multilayers. Starting from the investigation of the different magnetic interactions present in such systems, it has been possible to define critical thicknesses which governs the formation of chiral textures and to propose optimized stacking to investigate experimentally their nucleation in thin films. This analysis is followed by their current induced formation in patterned devices, an important milestone for the potential applications mentioned before. As an example, through the insertion of notches at the edges of devices, a deterministic control of the nucleation events is achieved. The case of antiferromagnetic skyrmions, which are basically two skyrmions with opposite polarity coupled together, is also addressed through an engineering of the different materials thicknesses leading to their stabilization in multilayers with varying anisotropies along the vertical axis. With the goal to implement them in actual technologies, their current induced dynamics have been investigated through an optimization of the spin orbit torques, to enhance the skyrmion's mobility, for example through the use of Pt (below the magnetic layer) and Ta (above the magnetic layer). This study also leads to systems in which skyrmions are faster than domain walls, which is interesting to explore how both textures interact one with the other. This result is attributed to the presence of an unexpected Rashba mechanism at the Co|Al interface, which increases the field like component of the torque and slows down the domain walls. Finally, the question of their reliability is discussed with observations of their motion on tens of micrometres distances and with a presentation of the use of skyrmion-skyrmion and skyrmion-domains repulsions to improve their dynamics. This last result, the motion of skyrmions along magnetic domains, opens new possibilities concerning the control of their trajectory and could lead to new prospects for skyrmion logic devices.Les skyrmions magnétiques sont des textures de spin stabilisées grâce à l'interaction Dzyaloshinskii-Moriya. Cette interaction chirale favorise un enroulement particulier de l'aimantation et leur confère des propriétés intéressantes. Ils se comportent par exemple comme des particules et possèdent une topologie différente de l'état d'aimantation uniforme. Depuis les premières observations en 2011, les skyrmions ont fait l'objet d'études approfondies en raison de leur potentiel pour des applications futures, telles que la "racetrack memory". Cette thèse se concentre sur les skyrmions Néel, un type particulier de skyrmions, qui peuvent être stabilisés dans des multicouches magnétiques. En commençant par l'étude des différentes interactions magnétiques, il a été possible de définir des épaisseurs critiques qui régissent leur formation et de proposer un empilement optimisé pour les observer expérimentalement. Nous avons ensuite étudié leur nucléation, induite par impulsions de courant électrique dans divers dispositifs, par exemple grâce à l'insertion de constrictions sur les bords des pistes. Le cas des skyrmions dans des multicouches formant un antiferromagnétique synthétique, soit deux skyrmions de polarité opposée couplés ensemble, est également abordé par le biais d'une ingénierie des différentes épaisseurs des matériaux conduisant à leur stabilisation dans des multicouches à anisotropie variable. Dans le but de les intégrer dans des technologies futures, leur dynamique induite par courant électrique a aussi été étudiée avec l'optimisation des couples de spin afin d'améliorer leur mobilité. Cette étude a conduit à des systèmes dans lesquels les skyrmions sont plus rapides que les parois de domaine, ce qui est intéressant dans le but d'explorer comment ces deux objets interagissent l'un avec l'autre. Ce résultat est notamment attribué à la présence d'un fort couple field-like. Enfin, la question de leur fiabilité est abordée avec l'observation de leur déplacement sur plusieurs dizaines de micromètres et avec une discussion sur l'utilisation des mécanismes de répulsions skyrmion-skyrmion et skyrmion-paroi (de domaines) pour améliorer leur dynamique. Le déplacement des skyrmions le long de domaines magnétiques ouvre notamment de nouvelles possibilités concernant le contrôle de leur trajectoire et pourrait conduire à de nouvelles perspectives concernant les dispositifs visant à utiliser les skyrmions pour faire des portes logiques

General treatment of off-specular resonant soft x-ray magnetic scattering using the distorted-wave Born approximation: Numerical algorithm and experimental studies with hybrid chiral domain structures

International audienc

Densely packed skyrmions stabilized at zero magnetic field by indirect exchange coupling in multilayers

Room-temperature stabilization of skyrmions in magnetic multilayered systems results from a fine balance between several magnetic interactions, namely, symmetric and antisymmetric exchange, dipolar interaction and perpendicular magnetic anisotropy as well as, in most cases, Zeeman through an applied external field. Such field-driven stabilization approach is, however, not compatible with most of the anticipated skyrmion based applications, e.g., skyrmion memories and logic or neuromorphic computing, which motivates a reduction or a cancellation of field requirements. Here, we present a method to stabilize at room-temperature and zero-field, a densely packed skyrmion phase in ferromagnetic multilayers with moderate number of repetitions. To this aim, we finely tune the multilayer parameters to stabilize a dense skyrmion phase. Then, relying on the interlayer electronic coupling to an adjacent bias magnetic layer with strong perpendicular magnetic anisotropy and uniform magnetization, we demonstrate the stabilization of sub-60 nm diameter skyrmions at zero-field with adjustable skyrmion density.ISSN:2166-532

Chiral spin spiral in synthetic antiferromagnets probed by circular dichroism in x-ray resonant magnetic scattering

International audienc

Interfacial potential gradient modulates Dzyaloshinskii-Moriya interaction in Pt/Co/metal multilayers

International audienceThe 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

Spatial extent of the Dzyaloshinskii-Moriya interaction at metallic interfaces

We experimentally investigate the range of the Dzyaloshinskii-Moriya interaction (DMI) occurring at magnetic interfaces within metallic heterostructures. To this aim we perform Brillouin light scattering spectroscopy on a set of Co-based, asymmetric metallic heterostructures, incorporating atomically thin continuous films obtained by room-temperature sputtering, and of identical orientation and quality. We thus get access to the intrinsic dependence of the interfacial DMI and other magnetic interactions on the thickness of the nonmagnetic layer adjacent to Co, which is chosen among Pt, Ru, and Au. Notably, we observe that a robust DMI is already generated by as few as two atomic planes of Pt and that interfacial DMI can be efficiently suppressed by a dusting of Ru equivalent to a single atomic plane coverage. These results point directly towards a mechanism where DMI is generated within the two first atomic planes away from the interface, in agreement with first-principles calculations. This locally generated DMI is, however, likely to be modulated by more distant atoms in the case of strain effects. The short-range aspect of the interfacial DMI opens up the synthesis of dense magnetic multilayers, allowing for a strong interfacial DMI even with very thin layers, which can be further tuned by strain engineering