Brownian motion of magnetic domain walls and skyrmions, and their diffusion constants

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

Domain wall propagation by spin-orbit torques in in-plane magnetized systems

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.Comment: Accepted for publication in Physical Review B Rapid Communication

Velocity enhancement by synchronization of magnetic domain walls

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

Anatomy of Dzyaloshinskii-Moriya Interaction at Co/Pt Interfaces

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.Comment: 16 pages, 4 figure

Evidence of large Dzyaloshinskii Moriya interaction at the cobalt/hexagonal boron nitride interface

Hexagonal boron nitride (h-BN) is used in 2D van der Waals heterostructures as a neutral material, often with a passivation role. However, here we show that h-BN deposited on ultra-thin Co film induces large Dzyaloshinskii Moriya interaction (DMI) and perpendicular magnetic anisotropy (PMA). Clean Co/h-BN were obtained by combining ultra-high vacuum growth and mechanical exfoliation. DMI and PMA are measured using Brillouin light scattering spectroscopy on series of samples of varying Co thickness grown on Pt or Au and covered either with h-BN or Cu. By comparing the h-BN-covered samples with their corresponding control Cu-covered samples, the effect of the Co/h-BN interface is extracted. This reveals that the Co/h-BN interface induces a large PMA and DMI comparable in strength to the largest known effects, such as those of the Pt/Co interface. This effect can be combined to that of Pt/Co in Pt/Co/h-BN films, to produce an even larger PMA and DMI. This enables the stabilisation of skyrmions in the Pt/Co/h-BN system, as was observed at room temperature with low magnetic fields. Our findings further expand the applications of h-BN, paving the way for using h-BN as functional material in the field of spintronics despite its weak spin-orbit interaction

Chirality-induced asymmetric magnetic nucleation in Pt/Co/AlOx ultrathin microstructures

The nucleation of reversed magnetic domains in Pt/Co/AlO$_{x}$ microstructures with perpendicular anisotropy was studied experimentally in the presence of an in-plane magnetic field. For large enough in-plane field, nucleation was observed preferentially at an edge of the sample normal to this field. The position at which nucleation takes place was observed to depend in a chiral way on the initial magnetization and applied field directions. An explanation of these results is proposed, based on the existence of a sizable Dzyaloshinskii-Moriya interaction in this sample. Another consequence of this interaction is that the energy of domain walls can become negative for in-plane fields smaller than the effective anisotropy field.Comment: Published version, Physical Review Letters 113, 047203 (2014

Imaging the Quantum Capacitance of Strained MoS2 Monolayers by Electrostatic Force Microscopy

We implemented radio frequency-assisted electrostatic force microscopy (RF-EFM) to investigate the electric field response of biaxially strained molybdenum disulfide (MoS2) monolayers (MLs) in the form of mesoscopic bubbles, produced via hydrogen (H)-ion irradiation of the bulk crystal. MoS2 ML, a semiconducting transition metal dichalcogenide, has recently attracted significant attention due to its promising optoelectronic properties, further tunable by strain. Here, we take advantage of the RF excitation to distinguish the intrinsic quantum capacitance of the strained ML from that due to atomic scale defects, presumably sulfur vacancies or H-passivated sulfur vacancies. In fact, at frequencies fRF larger than the inverse defect trapping time, the defect contribution to the total capacitance and to transport is negligible. Using RF-EFM at fRF = 300 MHz, we visualize simultaneously the bubble topography and its quantum capacitance. Our finite-frequency capacitance imaging technique is non-invasive and nanoscale, and can contribute to the investigation of time and spatial-dependent phenomena, such as the electron compressibility in quantum materials, which are difficult to measure by other methods

Croissance et magnétisme de nanostructures organisées sur surfaces cristallines

Jury de thèse : Stéphane Andrieu, Rapporteur, Olivier Fruchart, Rapporteur, Jacques Ferré, Andrés Saúl, Didier Schmaus, Sylvie RoussetIn this work, we investigate the growth of magnetic nanodots arrays, in order to study their magnetic properties. The dots are grown on Au(788), a spontaneously nanostructured template, which enables to obtain an nanodots array with a long range order and a narrow size distribution. We first present our study of the nucleation and growth mechanism on an array of traps. We link the energetic parameters with the temperature range for the organized growth. Then, we study the growth of cobalt and iron nanodots on Au(788). Using a variable temperature Scanning Tunnelling Microscope, we perform an exhaustive growth study versus the substrate temperature. The comparison with multi-scaled calculations (Molecular Dynamics, Kinetic Monte Carlo) enables us to determine the atomistic mechanisms, which leads to the organized growth. In a second part, we focus on the magnetic properties of our nanostructures, and particularly on the link between dots morphology and magnetic anisotropy energy (MAE). We first investigate the limits of the macrospin model using micromagnetic simulations. Then, we experimentally extract the MAE distribution from the hysteresis loops, measured at different temperatures. The comparison with the size distribution shows a non trivial relation between size and MAE.Dans ce travail de thèse, nous nous intéressons à l'élaboration de réseaux de nanoplots magnétiques, afin d'en étudier les propriétés magnétiques. Les nanoplots sont élaborés sur Au(788), une surface spontanément pré-structurée et qui permet la croissance de réseaux avec un ordre à grande distance et une distribution de taille étroite. La première partie de ce travail consiste à étudier les mécanismes qui conduisent à la croissance organisée sur un réseau de pièges ponctuels. Nous établissons le lien entre la plage de température qui permet d'observer la croissance organisée et les paramètres énergétiques du problème. Ensuite, grâce à des études exhaustives de la croissance en fonction de la température de substrat, réalisées à l'aide d'un Microscope à Effet Tunnel à température variable, nous étudions la croissance du cobalt et du fer sur Au(788). La comparaison avec des simulations multi-échelles (Dynamique Moléculaire, Monte Carlo cinétique) nous permet alors de déterminer les mécanismes atomiques à l'origine de l'organisation. La deuxième partie de ce travail est consacrée à l'étude du magnétisme des nanostructures que nous savons former, et plus particulièrement à la détermination du lien entre la morphologie des plots et leur énergie d'anisotropie magnétique (MAE). Après une étude sur les limites du modèle du macromoment et du retournement cohérent de l'aimantation habituellement utilisé pour les nanostructures, nous nous intéressons aux propriétés magnétiques de nanoplots contenant une centaine d'atomes de cobalt. Grâce à la mesure de cycles d'hystérésis à différentes températures, nous déduisons la distribution de MAE dans l'échantillon. Celle-ci est large, contrairement à des résultats précédents sur des îlots plus gros. La comparaison avec la distribution de taille des îlots implique une relation non triviale entre la taille des îlots et leur MAE