The skyrmion switch: turning magnetic skyrmion bubbles on and off with an electric field

Nanoscale magnetic skyrmions are considered as potential information carriers for future spintronics memory and logic devices. Such applications will require the control of their local creation and annihilation, which involves so far solutions that are either energy consuming or difficult to integrate. Here we demonstrate the control of skyrmion bubbles nucleation and annihilation using electric field gating, an easily integrable and potentially energetically efficient solution. We present a detailed stability diagram of the skyrmion bubbles in a Pt/Co/oxide trilayer and show that their stability can be controlled via an applied electric field. An analytical bubble model, with the Dzyaloshinskii-Moriya interaction imbedded in the domain wall energy, account for the observed electrical skyrmion switching effect. This allows us to unveil the origin of the electrical control of skyrmions stability and to show that both magnetic dipolar interaction and the Dzyaloshinskii-Moriya interaction play an important role in the skyrmion bubble stabilization

Gate-Controlled Skyrmion Chirality

Magnetic skyrmions are localized chiral spin textures, which offer great promise to store and process information at the nanoscale. In the presence of asymmetric exchange interactions, their chirality, which governs their dynamics, is generally considered as an intrinsic parameter set during the sample deposition. In this work, we experimentally demonstrate that this key parameter can be controlled by a gate voltage. We observed that the current-induced skyrmion motion can be reversed by the application of a gate voltage. This local and dynamical reversal of the skyrmion chirality is due to a sign inversion of the interfacial Dzyaloshinskii-Moriya interaction that we attribute to ionic migration of oxygen under gate voltage. Micromagnetic simulations show that the chirality reversal is a continuous transformation, in which the skyrmion is conserved. This gate-controlled chirality provides a local and dynamical degree of freedom, yielding new functionalities to skyrmion-based logic devices.Comment: 4 figure

Ferromagnétisme à l’échelle nanométrique

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Barrier Breakdown Mechanisms in MgO-Based Magnetic Tunnel Junctions and Correlation With Low-Frequency Noise

International audienceAn investigation of barrier breakdown in MgO-based magnetic tunnel junctions (MTJs) submitted to pulsed electrical stress is presented. By studying the effect of delay between successive pulses, we observed that a very pronounced optimum in endurance of MTJs is obtained for an intermediate value of the delay between pulses corresponding to the characteristic time for a trapped electron in the barrier to escape from its trap. A charge trapping-detrapping model was proposed which consistently explains our experimental data. The delay between successive pulses affects the density of electrons trapped in the barrier. The average value in time and the time-modulation of the density of trapped charge give rise to distinct breakdown mechanisms. Our model allows evaluating the MTJ probability of breakdown for different applied pulse conditions. An expected endurance of the MTJs is then derived depending on the characteristics of the electrical stress in terms of delay, amplitude, unipolarity versus bipolarity

Static and dynamic properties of 1-kink skyrmion in Pt/Co/MgO trilayer

International audienceThe 1-kink skyrmion is a topological spin texture composed of a skyrmion having a chiral kink inside its Néel domain wall, this chiral kink being also called domain wall skyrmion. By micromagnetic simulations we have studied the stability of the 1-kink skyrmion as well as its spin-driven dynamics in Pt/Co/MgO. Such trilayer system allows simultaneously the stabilization of various spin textures including skyrmion (Sk, topological charge Q = 1), 1-kink skyrmion (1kSk, Q = 2), and domain wall skyrmion (DWSk, Q = 1). The 1-kink skyrmion is found to be robust against both thermal fluctuations and normally distributed anisotropy for different grainlike region sizes. Such magnetization pattern has a certain resilience to the variation of the Dzyaloshinskii-Moriya interaction and applied magnetic field. The static analysis is complemented with the investigation of the dynamic regime of motion under electrical current. In the limit of moderate injected current the skyrmion and the 1-kink skyrmion behave similarly. However, depending on the geometry of the injected current, the 1-kink skyrmion can be transformed into a single skyrmion or splits in two skyrmions, opening the path to conceive high density logical devices, enabling novel functionalities for logic operations

Mapping different skyrmion phases in double wedges of Ta/FeCoB/TaOx trilayers

International audienceSkyrmions are chiral magnetic textures that have immense potential for applications in spintronic devices.However, their formation is quite challenging and necessitates a subtle balance of the magnetic interactionsat play. Here, we study Ta/FeCoB/TaOx trilayer using crossed double wedges, i.e., thickness gradients ofFeCoB and of top Ta, which is subsequently oxidized, leading to an oxidation gradient. This enabled us toobserve micron-sized skyrmions in the vicinity of different transition regions of the sample: from perpendicularmagnetic anisotropy to paramagnetic phase and also from perpendicular to in-plane magnetic anisotropy.These observations can be explained by the isolated skyrmion model taking into account the different energycontributions at play, namely, anisotropy, exchange, Dzyaloshinskii-Moriya, dipolar, and Zeeman. We alsoqualitatively compare the current-induced motion of skyrmions obtained in different transition regions. Ourstudy not only provides an effective means to form skyrmions by tuning the interfacial magnetic propertiesbut also highlights the differences pertaining to the skyrmions observed in different transition zones, which isextremely crucial for any envisaged application

Electric field control of interfacial Dzyaloshinskii-Moriya interaction in Pt/Co/AlOx thin films

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Kinetics of Ion Migration in the Electric Field‐Driven Manipulation of Magnetic Anisotropy of Pt/Co/Oxide Multilayers

International audienceMagneto-ionics is a fast developing research field which opens the perspective of energy efficient magnetic devices, where the magnetization direction is controlled by an electric field which drives the migration of ionic species. In this work, the interfacial perpendicular magnetic anisotropy (PMA) of Pt/Co/oxide stacks covered by ZrO2, acting as a ionic conductor, is tuned by a gate voltage at room temperature. A large variation of the PMA is obtained by modifying the oxidation of the cobalt layer through the migration of oxygen ions: the easy magnetization axis can be switched reversibly from in-plane, with under-oxidized Co, to in-plane, with over-oxidized Co, passing through an out-of-plane magnetization state. The switching time between the different magnetic states is limited by the ion drift velocity. This depends exponentially on the gate voltage, and is varied by over five orders of magnitude, from several minutes to a few ms. The variation of the PMA versus time during the application of the gate voltage can be modeled with a parabolic variation of the PMA and an exponential decrease of the Co oxidation rate. The possibility to explain the observed effect with a simple theoretical model opens the possibility to engineer materials with optimized properties

Interfacial Dzyaloshinskii-Moriya Interaction, Perpendicular Magnetic Anisotropy and damping in CoFeB/oxide-based systems

International audiencePerpendicular magnetic anisotropy (PMA), spin pumping induced damping and interfacial Dzyaloshinskii-Moriya interaction (iDMI), which are spin-orbit coupling-related phenomena of utmost importance for applications, were experimentally investigated in as grown and 225°C annealed CoFeB/PtOx, CoFeB/TaOx and Ta/CoFeB/TaOx systems by means of vibrating sample magnetometry, microstrip ferromagnetic resonance and Brillouin light scattering techniques. By varying Co8Fe72B20 (CoFeB) thickness in the range 0.8-10 nm, the effect of Ta buffer layer on anisotropy and damping was first studied, where a large surface magnetic anisotropy (Ks=2.1±0.16 erg/cm2) was measured in the unbuffered CoFeB/TaOx(0.8nm) system most likely due to their higher roughness induced by the substrate. Ks degrades significantly for CoFeB film thickness below 2 nm where spontaneous perpendicular magnetization was found to be impossible without Ta buffer layer. PMA, iDMI and damping of as-deposited and 225°C annealed CoFeB(1.5 nm)/PtOx systems were measured as a function of PtOx thickness in the range 0.7-1.6 nm. Their strong dependence versus the PtOx thickness was attributed to the decrease of the magnetic dead layer as PtOx thickness increases. Linear dependence of damping versus PMA constant was obtained confirming their relation with the spin orbit coupling. Moreover, annealing increases PMA and the effective mixing conductance probably due to the enhancement of the CoFeB crystal structure and interfaces