Magnetic domain walls displacement : automotion vs. spin-transfer torque

The magnetization dynamics equation predicts that a domain wall that changes structure should undergo a displacement by itself - automotion - due to the relaxation of the linear momentum that is associated with the wall structure. We experimentally demonstrate this effect in soft nanostrips,transforming under spin transfer torque a metastable asymmetric transverse wall into a vortex wall. Displacements more than three times as large as under spin transfer torque only are measured for 1~ns pulses. The results are explained by analytical and numerical micromagnetics. Their relevance to domain wall motion under spin transfer torque is emphasized

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

Hybrid chiral domain walls and skyrmions in magnetic multilayers

Noncollinear spin textures in ferromagnetic ultrathin films are currently the subject of renewed interest since the discovery of the interfacial Dzyaloshinskii-Moriya interaction (DMI). This antisymmetric exchange interaction selects a given chirality for the spin textures and allows stabilising configurations with nontrivial topology. Moreover, it has many crucial consequences on the dynamical properties of these topological structures, including chiral domain walls (DWs) and magnetic skyrmions. In the recent years the study of noncollinear spin textures has been extended from single ultrathin layers to magnetic multilayers with broken inversion symmetry. This extension of the structures in the vertical dimension allows very efficient current-induced motion and room-temperature stability for both N\'eel DWs and skyrmions. Here we show how in such multilayered systems the interlayer interactions can actually lead to more complex, hybrid chiral magnetisation arrangements. The described thickness-dependent reorientation of DWs is experimentally confirmed by studying demagnetised multilayers through circular dichroism in x-ray resonant magnetic scattering. We also demonstrate a simple yet reliable method for determining the magnitude of the DMI from static domains measurements even in the presence of these hybrid chiral structures, by taking into account the actual profile of the DWs. The advent of these novel hybrid chiral textures has far-reaching implications on how to stabilise and manipulate DWs as well as skymionic structures in magnetic multilayers.Comment: 22 pages, 5 figure

Skyrmions in Magnetic Multilayers: Chirality, Electrical Detection and Current-induced Motion

Sub-100-nm skyrmions are stabilized in magnetic metallic multilayers and observed using transmission electron microscopy, magnetic force microscopy, scanning transmission X-ray microscopy and X-ray resonant magnetic scattering. All these advanced imaging techniques demonstrate the presence of 'pure' Neel skyrmion textures with a determined chirality. Combining these observations with electrical measurements allows us to demonstrate reproducible skyrmion nucleation using current pulses, and measure their contribution to the transverse resistivity to detect them electrically. Once nucleated, skyrmions can be moved using charge currents. We find predominantly a creep-like regime, characterized by disordered skyrmion motion, as observed by atomic force microscopy and scanning transmission X-ray microscopy. These observations are explained qualitatively and to some extent quantitatively by the presence of crystalline grains of about 20nm lateral size with a distribution of magnetic properties

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

Textures magnétiques et Parois de Domaines : les objets du transfert de spin

The objective of this PhD thesis is the study of magnetic domain walls, and more generally of magnetic textures, in nanostructures. This work is motivated by the coupling between electrons responsible for magnetism and those for electrical conduction, nowadays called spin transfer torque, enabling domain wall dynamics under spin current. Knowledge of domain walls' dynamic behaviour and static characteristics is thus required. The tool used all along this work has been the magnetic force microscopy. The main study of this work is the experimental demonstration of magnetic domain wall automotion in NiFe nanostrips. We studied this phenomenon in the case of a domain wall transformation and then analyzed its role for domain wall dynamics under short current pulses. The static properties of such structures prove equally important. They have been studied using anisotropic magnetoresistance measurement for domain walls under quasi-static rotating magnetic field. Finally, new material studies being one the main research direction for spin transfer torque issues, a third part has been devoted to study the complex magnetization distribution in NiPd nanostructures. For all the topics developed in this work, micromagnetic simulations have been carried out in order to deepen our understanding.Cette thèse a pour objectif l'étude des parois de domaines magnétiques, et plus généralement des textures magnétiques, dans des nanostructures. Ce travail est motivé par le couplage entre les électrons responsables du magnétisme et ceux de la conduction électrique, appelé transfert de spin, qui donne lieu à la dynamique de parois de domaines sous courant de spin. La connaissance du comportement dynamique et des caractéristiques statiques de telles structures est alors indispensable. L'outil principalement utilisé dans de ce travail est la microscopie à force magnétique, et ce présent dans l'ensemble des sujet abordés. L'étude principale de ce travail concerne la mise en évidence expérimentale de l'automouvement de paroi dans des nanopistes de NiFe. Nous avons montré ce phénomène dans le cas d'une transformation de parois puis analysé l'importance qu'il a pour la dynamique de parois dans le cas général aux impulsions courtes. Les propriétés statiques ont aussi été étudiées par des mesures de magnétorésistance anisotrope pour des parois soumises à un champ magnétique tournant quasi-statique. Finalement, L'aspect “nouveaux matériaux” étant un des grands axes de recherche concernant le transfert de spin, une troisième partie a été consacrée à l'étude de la distribution d'aimantation complexe des nanostructures de NiPd. Pour tous les sujets abordés dans ce travail, des calculs micromagnétiques ont été effectués pour appuyer notre compréhension

La lumière change de couleur pour révéler les ordres couplés de la matière

International audienceSonder la matière pour la comprendre est certainement l’un des concepts les plus anciens de la physique et de la science des matériaux. Quand les systèmes étudiés présentent des ordres complexes, l’optique non-linéaire, et en particulier la génération de seconde harmonique, est une approche de choix pour en révéler les propriétés remarquables telles que le couplage d’ordres ferroïques dans les matériaux dits « multiferroïques »

Sensing multiferroic states non-invasively using optical second harmonic generation

The current boost in the search for energy-efficient device paradigms motivates the integration of materials with coexisting and coupled electric and magnetic order parameters into application-relevant architectures. In the so-called multiferroic magnetoelectrics, the understanding of switching events, however, is most of the time obstructed by the complex physics involved and in the non-trivial domain and domain wall configurations. This perspective offers an insightful overview of the most recent progress in the non-invasive optical probe of technology-significant ferroelectricity and antiferromagnetic order: the optical second harmonic generation (SHG). Over the last decade, its use in materials science has evolved, and SHG now enables the monitoring of the emergence of polarization in thin films, even during the epitaxial deposition process. Its long working distance further expedites the probe of multiple order parameters in various environments and under multi-stimuli excitations. The potential for the probe of complex electric dipole textures, such as ferroelectric skyrmions and time-resolved measurements, using SHG-based approaches in the most recent materials systems will be discussed.ISSN:2770-299