The skyrmion-bubble transition in a ferromagnetic thin film

Magnetic skyrmions and bubbles, observed in ferromagnetic thin films with perpendicular magnetic anisotropy, are topological solitons which differ by their characteristic size and the balance in the energies at the origin of their stabilisation. However, these two spin textures have the same topology and a continuous transformation between them is allowed. In the present work, we derive an analytical model to explore the skyrmion-bubble transition. We evidence a region in the parameter space where both topological soliton solutions coexist and close to which transformations between skyrmion and bubbles are observed as a function of the magnetic field. Above a critical point, at which the energy barrier separating both solutions vanishes, only one topological soliton solution remains, which size can be continuously tuned from micrometer to nanometer with applied magnetic field

Broadband Setup for Magnetic-Field-Induced Domain Wall Motion in Cylindrical Nanowires

In order to improve the precision of domain wall dynamics measurements, we develop a coplanar waveguide-based setup where the domain wall motion should be triggered by pulses of magnetic field. The latter are produced by the Oersted field of the waveguide as a current pulse travels toward its termination, where it is dissipated. Our objective is to eliminate a source of bias in domain wall speed estimation while optimizing the field amplitude. Here, we present implementations of this concept for magnetic force microscopy (MFM) and synchrotron-based investigation

Motion of magnetic domain walls in engineered cylindrical nanowires

Le sujet de cette thèse est l'observation de parois de domaines ferromagnétiques dans des nanofils cylindriques, ainsi que leur dynamique sous champs magnétiques appliqués. Ces nanostructures ont été électrodéposées par mes collègues dans des membranes d'alumine nanoporeuse servant de gabarits à géométrie adaptable. Les matériaux sont des alliages magnétiques doux de FeNi ou CoNi ; les diamètres vont de 150 nm à 250-300 nm, avec une longueur typique de 30 µm.Mon travail a d'abord englobé des développements instrumentaux de porte-échantillons et d'électronique haute fréquence visant au déplacement de parois sous champ. J'ai étudié cette dernière grâce au dichroïsme magnétique circulaire des rayons X couplé à la microscopie électronique de photoémission (XMCD-PEEM), en géométrie dite de transmission ou d'ombre. Cette technique synchrotron permet le suivi de la configuration interne de paroi avant et après déplacement ; en raison de la forte reproductibilité requise par le XMCD-PEEM résolu en temps, la dynamique en temps réel est pour le moment inaccessible.La réponse des parois de domaines ferromagnétiques à un champ magnétique est notoirement caractérisée par leur mobilité, c'est-à-dire le rapport de la vitesse atteinte sur le champ. Dans les nanofils cylindriques, un ingrédient nouveau apparaît dans le cas d'un type de paroi absent dans les bandes plates : la paroi à point de Bloch (Bloch point wall, BPW). Non seulement cette paroi comporte une singularité micromagnétique, c'est-à-dire un point où l'aimantation disparaît (le point de Bloch), mais elle possède également un degré de liberté discret représentant le sens d'enroulement d'aimantation autour de l'axe du fil. Il a été prédit que le déplacement de BPW sous champ suffisamment intense résulte en la sélection de l'un des deux seuls sens possibles. En d'autres termes, un des deux enroulements devient instable. Dans cette thèse, je rapporte l'observation expérimentale de cette sélection dans une majorité de déplacements de BPW.Il n'a pas été possible de mener des mesures de mobilité, néanmoins, mes expériences ont mis en évidence des transformations jusqu'ici non prévues en simulation entre types de parois. La BPW contenant un défaut topologique (le point de Bloch lui-même), ce comportement inattendu remet en question la protection topologique parfois attribuée aux textures topologiquement non-triviales. Bien que rappelant la conversion entre parois transverse et vortex dans les bandes, ces transformations dans les nanofils cylindriques impliquent des configurations micromagnétiques topologiquement non-équivalentes, par contraste avec les parois des bandes sus-mentionnées. De plus, la toute relative stabilité observée des types de parois suggère la prudence dans l'interprétation de future mesures de mobilité dans de tels systèmes dès lors que la configuration interne de paroi n'est pas résolue.En-dehors de tels échantillons électrodéposés, j'ai également étudié un nanofil cœur-coquille crû verticalement par des collègues. Cette nanostructure cylindrique créée par dépôt induit par faisceau d'électron focalisé (FEBID) possèdait un cœur nanocristallin de cobalt et une coquille de platine. Sa configuration magnétique a été également étudiée par XMCD-PEEM en transmission. Contrairement au fils précédemment mentionnés et posés sur leur substrat sur toute leur longueur, cet échantillon cœur-coquille était vertical et sans modulations de diamètre. En revanche, la géométrie coudée du fil a été conçue pour favoriser le piégeage de parois. Dans cette configuration innovante d'imagerie, le défi a été de remonter autant que possible à l'état magnétique du fil ; il m'a été possible de démontrer la présence d'au moins une paroi de domaine.The thesis is concerned with the observation of ferromagnetic domain walls in cylindrical nanowires, as well as their dynamics under applied magnetic fields. These nanostructures were electrodeposited by colleagues of mine into nanoporous alumina templates with a tailored pore geometry. The materials are soft FeNi or CoNi alloys; the diameters range from 150 nm to 250-300 nm, with a typical length of 30 µm.My work first comprised experimental developments of sample holders and high-frequency electronics towards field-induced domain wall motion. The latter I investigated with X-ray Magnetic Circular Dichroism coupled to transmission PhotoEmission Electron Microscopy (XMCD-PEEM). This synchrotron-based technique allows to monitor the internal domain wall configuration before and after displacement; due to the stringent requirements of time-resolved XMCD-PEEM experiments in terms of reproducibility, the real-time dynamics is out of reach as of yet.The response of ferromagnetic domain walls to applied magnetic fields is notably characterized by their mobility, i.e. the ratio of attained velocity to field strength. In cylindrical nanowires, a novel ingredient emerges in the case of one domain wall type that is absent in flat strips: the Bloch point domain wall. Not only does this domain wall host a micromagnetic singularity, that is to say a point where magnetization vanishes (the Bloch point), but it also possesses a discrete degree of freedom representing the sense of magnetization winding around the nanowire axis. It has been predicted that Bloch point wall motion under sufficiently high fields leads to this degree of freedom selecting one of its only two possible values. In other words, one winding becomes unstable. I report in this thesis experimental evidence of such a selection in a majority of Bloch point wall motion events.Although mobility measurements could not be carried out, my experiments have furthermore evidenced transformations between domain wall types that had not been predicted in simulations. Since the Bloch point wall contains a topological defect (the Bloch point itself), this unexpected behaviour questions the sometimes argued protection attributed to topologically non-trivial textures. While reminiscent of the well-known conversion between transverse and vortex walls in strips, these transformations in cylindrical nanowires involve topologically non-equivalent micromagnetic configurations, in contrast with the aforementioned transverse and vortex walls. Moreover, the observed only relative stability of domain wall types suggests caution in the interpretation of future mobility measurements in such systems, if the internal wall configuration cannot be resolved.Aside from such electrodeposited samples, I have also studied an upright core-shell nanowire grown by colleagues with Focused-Electron-Beam-Induced Deposition. This nanostructure featured a nanocrystalline cobalt core and a platinum shell. Its magnetic configuration was investigated with transmission XMCD-PEEM as well. Contrary to the aforementioned horizontally-lying wires, the core-shell sample was vertical with no diameter modulations. On the other hand, the geometry featured bends engineered to favour domain wall pinning. In this novel imaging configuration, the challenge was to recover as much of the nanowire's magnetic state as possible. I was able to demonstrate the presence of at least one domain wall

Mouvement de parois de domaines magnétiques dans des nanofils cylindriques modulés

The thesis is concerned with the observation of ferromagnetic domain walls in cylindrical nanowires, as well as their dynamics under applied magnetic fields. These nanostructures were electrodeposited by colleagues of mine into nanoporous alumina templates with a tailored pore geometry. The materials are soft FeNi or CoNi alloys; the diameters range from 150 nm to 250-300 nm, with a typical length of 30 µm.My work first comprised experimental developments of sample holders and high-frequency electronics towards field-induced domain wall motion. The latter I investigated with X-ray Magnetic Circular Dichroism coupled to transmission PhotoEmission Electron Microscopy (XMCD-PEEM). This synchrotron-based technique allows to monitor the internal domain wall configuration before and after displacement; due to the stringent requirements of time-resolved XMCD-PEEM experiments in terms of reproducibility, the real-time dynamics is out of reach as of yet.The response of ferromagnetic domain walls to applied magnetic fields is notably characterized by their mobility, i.e. the ratio of attained velocity to field strength. In cylindrical nanowires, a novel ingredient emerges in the case of one domain wall type that is absent in flat strips: the Bloch point domain wall. Not only does this domain wall host a micromagnetic singularity, that is to say a point where magnetization vanishes (the Bloch point), but it also possesses a discrete degree of freedom representing the sense of magnetization winding around the nanowire axis. It has been predicted that Bloch point wall motion under sufficiently high fields leads to this degree of freedom selecting one of its only two possible values. In other words, one winding becomes unstable. I report in this thesis experimental evidence of such a selection in a majority of Bloch point wall motion events.Although mobility measurements could not be carried out, my experiments have furthermore evidenced transformations between domain wall types that had not been predicted in simulations. Since the Bloch point wall contains a topological defect (the Bloch point itself), this unexpected behaviour questions the sometimes argued protection attributed to topologically non-trivial textures. While reminiscent of the well-known conversion between transverse and vortex walls in strips, these transformations in cylindrical nanowires involve topologically non-equivalent micromagnetic configurations, in contrast with the aforementioned transverse and vortex walls. Moreover, the observed only relative stability of domain wall types suggests caution in the interpretation of future mobility measurements in such systems, if the internal wall configuration cannot be resolved.Aside from such electrodeposited samples, I have also studied an upright core-shell nanowire grown by colleagues with Focused-Electron-Beam-Induced Deposition. This nanostructure featured a nanocrystalline cobalt core and a platinum shell. Its magnetic configuration was investigated with transmission XMCD-PEEM as well. Contrary to the aforementioned horizontally-lying wires, the core-shell sample was vertical with no diameter modulations. On the other hand, the geometry featured bends engineered to favour domain wall pinning. In this novel imaging configuration, the challenge was to recover as much of the nanowire's magnetic state as possible. I was able to demonstrate the presence of at least one domain wall.Le sujet de cette thèse est l'observation de parois de domaines ferromagnétiques dans des nanofils cylindriques, ainsi que leur dynamique sous champs magnétiques appliqués. Ces nanostructures ont été électrodéposées par mes collègues dans des membranes d'alumine nanoporeuse servant de gabarits à géométrie adaptable. Les matériaux sont des alliages magnétiques doux de FeNi ou CoNi ; les diamètres vont de 150 nm à 250-300 nm, avec une longueur typique de 30 µm.Mon travail a d'abord englobé des développements instrumentaux de porte-échantillons et d'électronique haute fréquence visant au déplacement de parois sous champ. J'ai étudié cette dernière grâce au dichroïsme magnétique circulaire des rayons X couplé à la microscopie électronique de photoémission (XMCD-PEEM), en géométrie dite de transmission ou d'ombre. Cette technique synchrotron permet le suivi de la configuration interne de paroi avant et après déplacement ; en raison de la forte reproductibilité requise par le XMCD-PEEM résolu en temps, la dynamique en temps réel est pour le moment inaccessible.La réponse des parois de domaines ferromagnétiques à un champ magnétique est notoirement caractérisée par leur mobilité, c'est-à-dire le rapport de la vitesse atteinte sur le champ. Dans les nanofils cylindriques, un ingrédient nouveau apparaît dans le cas d'un type de paroi absent dans les bandes plates : la paroi à point de Bloch (Bloch point wall, BPW). Non seulement cette paroi comporte une singularité micromagnétique, c'est-à-dire un point où l'aimantation disparaît (le point de Bloch), mais elle possède également un degré de liberté discret représentant le sens d'enroulement d'aimantation autour de l'axe du fil. Il a été prédit que le déplacement de BPW sous champ suffisamment intense résulte en la sélection de l'un des deux seuls sens possibles. En d'autres termes, un des deux enroulements devient instable. Dans cette thèse, je rapporte l'observation expérimentale de cette sélection dans une majorité de déplacements de BPW.Il n'a pas été possible de mener des mesures de mobilité, néanmoins, mes expériences ont mis en évidence des transformations jusqu'ici non prévues en simulation entre types de parois. La BPW contenant un défaut topologique (le point de Bloch lui-même), ce comportement inattendu remet en question la protection topologique parfois attribuée aux textures topologiquement non-triviales. Bien que rappelant la conversion entre parois transverse et vortex dans les bandes, ces transformations dans les nanofils cylindriques impliquent des configurations micromagnétiques topologiquement non-équivalentes, par contraste avec les parois des bandes sus-mentionnées. De plus, la toute relative stabilité observée des types de parois suggère la prudence dans l'interprétation de future mesures de mobilité dans de tels systèmes dès lors que la configuration interne de paroi n'est pas résolue.En-dehors de tels échantillons électrodéposés, j'ai également étudié un nanofil cœur-coquille crû verticalement par des collègues. Cette nanostructure cylindrique créée par dépôt induit par faisceau d'électron focalisé (FEBID) possèdait un cœur nanocristallin de cobalt et une coquille de platine. Sa configuration magnétique a été également étudiée par XMCD-PEEM en transmission. Contrairement au fils précédemment mentionnés et posés sur leur substrat sur toute leur longueur, cet échantillon cœur-coquille était vertical et sans modulations de diamètre. En revanche, la géométrie coudée du fil a été conçue pour favoriser le piégeage de parois. Dans cette configuration innovante d'imagerie, le défi a été de remonter autant que possible à l'état magnétique du fil ; il m'a été possible de démontrer la présence d'au moins une paroi de domaine

Multicomponent wavefield characterization with a novel scanning laser interferometer

The in-plane component of the wavefield provides valuable information about media properties from seismology to nondestructive testing. A new compact scanning laser ultrasonic interferometer collects light scattered away from the angle of incidence to provide the absolute ultrasonic displacement for both the out-of-plane and an in-plane components. This new system is tested by measuring the radial and vertical polarization of a Rayleigh wave in an aluminum half-space. The estimated amplitude ratio of the horizontal and vertical displacement agrees well with the theoretical value. The phase difference exhibits a small bias between the two components due to a slightly different frequency response between the two processing channels of the prototype electronic circuitry. © 2010 American Institute of Physics

Multicomponent Wavefield Characterization with a Novel Scanning Laser Interferometer

The in-plane component of the wavefield provides valuable information about media properties from seismology to nondestructive testing. A new compact scanning laser ultrasonic interferometer collects light scattered away from the angle of incidence to provide the absolute ultrasonic displacement for both the out-of-plane and an in-plane components. This new system is tested by measuring the radial and vertical polarization of a Rayleigh wave in an aluminum half-space. The estimated amplitude ratio of the horizontal and vertical displacement agrees well with the theoretical value. The phase difference exhibits a small bias between the two components due to a slightly different frequency response between the two processing channels of the prototype electronic circuitry

Small-angle X-ray resonant magnetic scattering at the Co M2,3_{2,3} and L3_3 edges observed with photoemission electron microscopy

13 pages, 11 figuresX-ray magnetic circular dichroism is an efficient contrast mechanism allowing for a direct sensitivity to magnetization. Combined with an imaging technique such as photoemission electron microscopy, it has been successfully applied to high-resolution investigations of ferromagnetic thin films but also of three-dimensional systems thanks to the transmission-type contrast in their shadow. Our focus in this work is the wave-optics scattering pattern that can be observed near such a shadow's rim. Taking advantage of non-uniform magnetic states present in near-micron-size Co$_{1-x}$Gd$_x$ beads, we first show how X-ray resonant magnetic scattering affects the Fresnel diffraction at the Co L$_3$ edge. In order to confirm this observation, we then turn to the Co M$_{2,3}$ edges. There, we measure magnetic scattering patterns with a significantly increased spatial extent (due to the larger wavelength), despite the signal's weakness. The patterns' origin is supported by a comparison between our experimental data and a simple analytical model, then numerical simulations

The skyrmion-bubble transition in a ferromagnetic thin film

Magnetic skyrmions and bubbles, observed in ferromagnetic thin films with perpendicular magnetic anisotropy, are topological solitons which differ by their characteristic size and the balance in the energies at the origin of their stabilisation. However, these two spin textures have the same topology and a continuous transformation between them is allowed. In the present work, we derive an analytical model to explore the skyrmion-bubble transition. We evidence a region in the parameter space where both topological soliton solutions coexist and close to which transformations between skyrmion and bubbles are observed as a function of the magnetic field. Above a critical point, at which the energy barrier separating both solutions vanishes, only one topological soliton solution remains, which size can be continuously tuned from micrometer to nanometer with applied magnetic field

Coherent diffraction imaging at space-group forbidden reflections

International audienceOn one hand, coherent diffraction imaging (CDI) in Bragg geometry has emerged as a unique 3D microscopy of nanocrystals thanksto 3rd generation synchrotron sources. Away from absorption edges and at space-group allowed reflections, it provides not only theelectronic density, but also, encoded in the phase, the atomic displacement field with respect to the mean lattice, which in turn revealscrystal strain, defects and domains [1–3]. On the other hand, some crystal structures have crystallographic reflections which areforbidden by the space-group symmetry but can nevertheless be observed at a suitable X-ray absorption edge, due to the anisotropy ofthe tensor of scattering (ATS) [4]. They are several orders of magnitude weaker than allowed reflections, but the absence of Thomsonscattering allows the observation of various electronic phenomena related to electronic orders (magnetic, charge, orbital), static anddynamic atomic displacements

Imaging magnetic flux-closure domains and domain walls in electroless-deposited CoNiB nanotubes

Magnetic nanotubes are predicted to host mag- netic domains and domain walls with a topology different from that for at elements[1, 2]. Com- pared to cylindrical nanowires, reports on mag- netism of isolated metallic nanotubes are scarce and so far no material has given rise to well- defined magnetic domains. Here we report the fabrication of high-aspect ratio CoNiB nanotubes by electroless plating inside a porous template. Through imaging, we evidenced multiple mag- netic domains and domain walls in these nano- tubes. Surprisingly, magnetization in the do- mains is orthoradial (azimuthal, vortex-like), a situation not anticipated by theory for long nano- tubes. The material is therefore technologically appealing for a dense 3D magnetic device such as the racetrack memory[3] (based on shifting mag- netic walls), as ux-closure domains should efi- ciently prevent cross-talk related to internal di- polar fields. Further, we show tuning of a growth- induced anisotropy and thus of the magnetic state of the tube by annealing