Dynamics of bistable N\'eel domain walls under spin-orbit torque

N\'eel magnetic domain walls that are stabilized by achiral energy terms instead of the usual Dzyaloshinskii-Moriya interaction will be bistable, with the two possible chiral forms being degenerate. Here we focus on the theoretical study of the spin-orbit torque driven dynamics of such bistable N\'eel domain walls. We find that, for a given domain wall, two propagation directions along a nanowire are possible, depending on its initial state. These dynamics also exhibit complex dependence on the spin-orbit torque magnitude, leading to important transient regimes. Finally, a few ways are proposed for controlled or random reversal of the domain wall propagation direction. A robust analytical model which handles all the observed behaviors of such domain walls is developed and validated by comparing with numerical simulations. The obtained new dynamics open the way for new uses of domain walls in information storage and processing devices

Domain wall dynamics in antiferromagnetically-coupled double-lattice systems

In ferromagnetic materials, the rich dynamics of magnetic domain walls (DWs) under magnetic field or current have been successfully described using the well-known q-{\phi} analytical model. We demonstrate here that this simple unidimensional model holds for multiple-sublattice materials such as ferrimagnetic alloys or synthetic antiferromagnets (SAF) by using effective parameters, and is in excellent agreement with double-lattice micromagnetic simulations. We obtain analytical laws for the DW velocity and internal precession angle as a function of net magnetisation for different driving forces (magnetic field, spin transfer and spin-orbit torques) and different propagation regimes in ferrimagnetic alloys and SAFs. The model predicts that several distinctive dynamical features occur near or at the magnetic and the angular compensation points when the net magnetization or the net angular momentum of the system vanishes, and we discuss the experimental observations that have been reported for some of them. Using a higher degree-of-freedom analytical model that accounts for inter-sublattice distortions, we give analytical expressions for these distortions that agree with the micromagnetic simulations. This model shows that the DW velocity and precession rate are independent of the strength of the inter-sublattice exchange coupling, and justifies the use of the simpler effective parameters model

Terahertz wave generation via optical rectification from multiferroic BiFeO3

We detected broadband coherent terahertz (THz) emission from multiferroic BiFeO3 after illuminating a high-quality bulk single ferroelectric domain crystal with a ~100 fs optical pulse. The dependence of the emitted THz waveform on the energy and polarization of the optical pulse is consistent with the optical rectification mechanism of THz emission. The THz emission provides a sensitive probe of the electric polarization state of BiFeO3, enabling applications in ferroelectric memories and ferroelectric domain imaging. We also report room-temperature THz optical constants of BiFeO3.Comment: accepted for publication in Applied Physics Letter

Dynamique de paroi de domaine sous courant de spin dans des alliages ferrimagnétiques

Despite the large success of spintronics, several questions remain concerning the improvement of efficiency and speed of the magnetization manipulation by electrical current. Those issues can be addressed through the study of new exotic materials that mix different magnetic sub-lattices. Rare earth-transition metal ferrimagnetic alloys are composed of two different magnetic sub-lattices that are antiferromagneticaly coupled. Specifically, two interesting configurations can emerge called the magnetic and the angular compensation points at which the alloy’s net magnetization or net angular momentum independently vanishes. In these configurations, ferrimagnets seem to present new and very convenient properties which makes them promising for both fundamental and technological point of view. In this thesis, these materials were experimentally and theoretically studied through the prism of magnetic domain wall dynamics driven by spin-currents.Ferrimagnetic alloys (such as TbFeCo or GdFeCo) were grown in thin films by co-evaporation. Their structural and magnetic properties were studied by combining magnetization, electrical and optical methods which have revealed their spintronic value. Imaging techniques showed a perpendicularly magnetized domain organization separated by easily handled domain walls. These statics properties studies also showed a chemical depth gradient which induces surface-like effects in the bulk region of films such as DMI à définir.The domain wall dynamics driven by spin current were investigated in two studies revealing very high efficiency and speed of their electrical manipulation. First, the efficiency of the current manipulation via spin-transfer torque was measured by studying the domain wall motion under combined effects of field and current in the creep regime. Secondly, the domain wall dynamics driven by spin-orbit torque was fully characterized using in-plane fields. This measurement revealed a singular dynamic of the domain wall at the angular compensation point which is the direct signature of the precession-free reversal of the magnetization.Finally, an effective theoretical model of both the static and dynamic properties of ferrimagnets was developed. It allows the description of all the observed experimental results. Using this formalism, we analytically and numerically studied the domain wall dynamics driven by field or spin-currents thus revealing new propagation regimes such as precession-free dynamics or the vanishing of transient motions.Malgré les grands succès de la spintronique de ces dernières années, plusieurs questions demeurent quant à l'efficacité et la rapidité de la manipulation électrique de l’aimantation. Ces problèmes semblent pouvoir être résolus en considérant des nouveaux matériaux plus exotiques mélangeant différents sous-réseaux magnétiques. Les alliages ferrimagnétiques de type terres rares-métaux de transitions sont composés de deux populations magnétiques couplées antiferromagnétiquement. Dans ces matériaux, deux configurations particulièrement intéressantes se distinguent : les points de compensation magnétique et angulaire auxquels l'aimantation ou le moment angulaire totale de l'alliage s’annulent. Dans ces configurations, ces matériaux ferrimagnétiques présentent de nouvelles propriétés très intéressantes tant sur le plan fondamental que technologique. Dans cette thèse, la dynamique d’aimantation dans ces matériaux a été étudié expérimentalement et théoriquement à travers la dynamique de paroi de domaine magnétique par application de courants de spin.Les alliages ferrimagnétiques (comme le TbFeCo ou le GdFeCo) ont été déposés en couche mince par co-évaporation et étudiés en combinant plusieurs méthodes : magnétiques, électriques et optiques ce qui révéla leur grand intérêt spintronic. Des techniques d'imagerie ont montré une organisation en domaines magnétiques, séparés par des parois facilement manipulables. Cette étude des propriétés statiques a également montré l’existence d’un gradient chimique en épaisseur induisant des effets habituellement surfaciques dans la zone centrale de films comme le DMI.La dynamique de paroi sous courant de spin (par couple de transfert de spin et spin-orbite) a été étudiée dans deux études qui ont mis en évidence l'efficacité et la rapidité du contrôle électrique de l’aimantation. L’une d’elles a également révélé une dynamique particulière qui est la signature directe d’un retournement magnétique sans précession à la compensation angulaire.Enfin, un modèle théorique effectif des propriétés statique et dynamique des alliages ferrimagnétiques a été développé et a révélé de nouveaux modes de propagation de paroi comme le retournement sans précession ou la disparition des régimes transitoires

Dataset Associated with Breathing Modes of Skyrmion Strings in a Synthetic Antiferromagnet

Dataset associated with publication on skyrmion breathing modes in a synthetic ntiferromagnetic multilayer. All data was generated using the micromagnetic solver mumax3, studying the static behaviour of skyrmions as a function of applied magnetic field, and interlayer antiferromagnetic coupling strength. Also studied are the breathing modes of these skyrmions as a function of field, coupling strength structural disorder and temperature

Quantitative analysis of spin wave dynamics in ferrimagnets across compensation points

International audienceRare-earth transition-metal ferrimagnets have two strongly coupled sublattices of distinct chemical nature, which give rise to complex and fast dynamics of great interest to spintronics. However, the dynamics of ferrimagnets remains less understood than ferromagnets. We measure the spin wave (SW) spectra of a GdFeCo film by Brillouin light scattering spectroscopy (BLS) across its compensation temperatures-temperatures at which either the sublattices' magnetizations or their angular moments cancel out, mimicking an antiferromagnet. We find two SW modes per wave vector with complex thermal dependencies, which cross at a field-dependent temperature. We develop an analytical model based on two sublattices corresponding to the rare earth and the transition metal, which reproduces quantitatively the SW spectra and their evolution with temperature and field. This validates the proposed energy and dynamical model of the ferrimagnet, and demonstrates the usefulness of BLS in the study of this promising class of materials

Size-dependent mobility of skyrmions beyond pinning in ferrimagnetic GdCo thin films

Supplemental information available from the final journalMagnetic skyrmions are swirling magnetic textures that can be efficiently driven with spin-orbit torques with a deflected trajectory. However, pinning slows skyrmions down and alters their trajectory, which prevents a quantitative comparison to analytical models. Here, we study skyrmions driven by spin-orbit torques at room temperature in ferrimagnetic GdCo thin films, an amorphous material with low pinning. Above a sharp current depinning threshold, we observe a clearly linear velocity increase with current that extrapolates to zero and a constant deflection angle, reaching high velocities up to 200 m/s. The mobility increases and the depinning threshold current decreases with the skyrmion diameter, which we vary using an external magnetic field. An analytical model based on the Thiele equation quantitatively reproduces these findings with a single fitting parameter. This validates the linear flow regime description and shows, in particular, the important role of skyrmion size in its dynamics

Size-dependent mobility of skyrmions beyond pinning in ferrimagnetic GdCo thin films

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Spin-Orbit Coupling in Single-Layer Ferrimagnets: Direct Observation of Spin-Orbit Torques and Chiral Spin Textures

International audienceWe demonstrate that the effects of spin-orbit coupling and inversion asymmetry exist in a single Gd-Fe-Co ferrimagnetic layer, even without a heavy-metal interface. We use electric transport mea- surements 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 vari- ation along the thickness that we observe by scanning transmission electron microscopy. We show that these effects can be optimized by varying the Gd-Fe-Co thickness or in combination with interfacial effects