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Amaral, Marco Antônio. "Dinâmica de skyrmions e cristais de skyrmions auto-organizados." Universidade Federal de Viçosa, 2013. http://www.locus.ufv.br/handle/123456789/9561.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, CAPES, Brasil.
Tais redes são uma boa descrição de materiais reais quase bidimensionais, pois estes, com frequência, possuem defeitos e impurezas, bem como fortes efeitos de borda, impedindo que um modelo infinito e contínuo os descreva bem. Foram analisados Skyrmions em redes livres de impurezas bem como sua interação com defeitos magnéticos, campos magnéticos externos e outras excitações do mesmo tipo na rede. Em especial foi analisada também a formação de cristais de skyrmions devido a interação entre excitações e sua consequente auto-organização. O estudo destes foi feito através de métodos computacionais de Dinâmica de Spins utilizando-se integradores do tipo Preditor-Corretor. Os resultados obtidos mostram que o modelo proposto se ajusta aos modelos teóricos contínuos no limite termodinâmico. Ainda assim, foram encontrados vários fenômenos novos para redes pequenas em que a dimensionalidade reduzida e discreteza do sistema pode gerar novas interações não previstas por modelos analíticos. Entre tais fenômenos podemos incluir a aniquilação de Skyrmions via impurezas, movimentação de skyrmions por campos aplicados e energias de interação inter-skyrmions. Tais simulações são de grande importância visto que recentemente foi demonstrado [1–4] que redes hexagonais de Skyrmions podem se formar em filmes finos e podem ser detectadas experimentalmente [5–11]. Considerando a área de memórias magnéticas de alta densidade, quasi-partı́culas estáveis como skyrmions seriam de grande aplicação prática [12].
Such lattices are a good approximation to real low dimensional magnetic materials because those, often, have impurities, discrete defects and strong border effects. Therefore, an infinite and continuous model would not be able to precisely describe such real magnetic materials. We have analised skyrmions inside defect-free lattices as well as magnetical impurities interactions, external aplied magnetic fields and interactions with other excitations of the same kind. Especifically we studied the formation of skyrmions crystals due to skyrmion interaction, and the eventual auto-organization of these lattices. Such study was taken by computational methods using spin dynamics. To this end, a Preditor-Corretor integrador was used. The obtained results shows that the proposed model adjusts to analytical theoretical models in the thermodynamics limit. Yet we observed various phenomena that weren’t present in the continuous theory. Skyrmions annihilation by magnetic defects, skyrmion motion due to magnetic fields and skyrmion-skyrmion interaction are some of those. These simulations are of great importance noticing that it was directly observed [1–4] recently that hexagonal skyrmion lattices can be found in thin films and be experimentally detected [5–11]. Considering the high-density magnetic memories area stable skyrmionic excitations would be of great interest [12].
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Wood, Stephen William. "Skyrmions and nuclei." Thesis, University of Cambridge, 2010. https://www.repository.cam.ac.uk/handle/1810/252183.
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Krusch, Steffen. "Structure of skyrmions." Thesis, University of Cambridge, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.621100.
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Silva-Lobo, Jorge Ivan. "Lattices of generalized Skyrmions." Thesis, Durham University, 2011. http://etheses.dur.ac.uk/3228/.
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Generalized Skyrme systems are those which include both the Skyrme and the Skyrme-Faddeev models through an interpolating parameter \alpha \in [0,1] the former corresponds to \alpha=0 and the latter to \alpha=1. Our numerical and analytical investigations centre around the \alpha=0 Skyrme crystal, its deformations, and its behaviour and symmetries as a function of \alpha, called the generalized Skyrme crystal. We show that a double square lattice emerges when the Skyrme crystal is deformed in a certain limit; we compare its energy with the one corresponding to a double hexagonal lattice and show that it has a lower energy-per-charge than its hexagonal counterpart. On the other hand, vortex-like structures with two 1-vortices (vortices of order 1) and two 1-antivortices, denoted V+AV+V+AV, appear when the Skyrme crystal is deformed in a different limit, as well as when the generalized Skyrme crystal is taken close to the Skyrme-Faddeev limit. This leads us to the study of generalized V+AV and V+AV+V+AV configurations, as a function of \alpha. We show that when these configurations are stacked in the axial direction, they exhibit some winding and linking properties as they are taken close to the Skyrme-Faddeev limit, where the V+AV+V+AV configurations appear to be more stable than their V+AV counterparts. Finally, the study of such configurations led to the discovery of two crystalline solutions whose properties are investigated in some detail: a 2-vortex/2-antivortex pair, denoted 2V+2AV, and a “multi-sheet” solution, both of which have a lower energy-per-charge than the V+AV+V+AV solution, in the Skyrme-Faddeev limit.
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Jennings, Paul Robert. "Knots and planar Skyrmions." Thesis, Durham University, 2015. http://etheses.dur.ac.uk/11161/.
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In this thesis the research presented relates to topological solitons in (2+1) and (3+1)-dimensional Skyrme theories. Solutions in these theories have topologically invariant quantities which results in stable solutions which are topologically distinct from a vacuum. In Chapter 2 we discuss the broken baby Skyrme model, a theory which breaks symmetry to the dihedral group D_N. It has been shown that the unit soliton solution of the theory is formed of N distinct peaks, called partons. The multi-soliton solutions have already been numerically simulated for N = 3 and were found to be related to polyiamonds. We extend this for higher values of N and demonstrate that a polyform structure continues. We discuss our numerical simulations studying the dynamics of this model and show that the time dependent behaviour of solutions in the model can be understood by considering the interactions of individual pairs of partons. Results of these dynamics are then compared with those of the standard baby Skyrme model. Recently it has been demonstrated that Skyrmions of a fixed size are able to exist in theories without a Skyrme term so long as the Skyrmion is located on a domain wall. In Chapter 3 we present a (2+1)-dimensional O(3) sigma model, with a potential term of a particular form, in which such Skyrmions exist. We numerically compute domain wall Skyrmions of this type. We also investigate Skyrmion dynamics so that we can study Skyrmion stability and the scattering of multi-Skyrmions. We consider scattering events in which Skyrmions remain on the same domain wall and find they are effectively one-dimensional. At low speeds these scatterings are well-approximated by kinks in the integrable sine-Gordon model. We also present more exotic fully two-dimensional scatterings in which Skyrmions initially on different domain walls emerge on the same domain wall. The Skyrme-Faddeev model is a (3+1)-dimensional non-linear field theory that has topological soliton solutions, called hopfions. Solutions of this theory are unusual in that that they are string-like and take the form of knots and links. Solutions found to date take the form of torus knots and links of these. In Chapter 4 we show results which address the question of whether any non-torus knot hopfions exist. We present a construction of fields which are knotted in the form of cable knots to which an energy minimisation scheme can be applied. We find static hopfions of the theory which do not have the form of torus knots, but instead take the form of cable and hyperbolic knots. In Chapter 5 we consider an approximation to the Skyrme-Faddeev model in which the soliton is modelled by elastic rods. We use this as a mechanism to study examples of particular knots to attempt to gain an understanding of why such knots have not been found in the Skyrme-Faddeev model. The aim of this study is to focus the search for appropriate rational maps which can then be applied in the Skyrme-Faddeev model. The material presented in this thesis relates to two published papers and corresponding to Chapters 2 and 3 respectively, which were done as part of a collaboration. In this thesis my own results are presented. Chapter 4 concerns material which relates to the preprint which is all my own work. Chapter 5 discusses my own ongoing work.
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Leblond, Frédéric. "Quantification rigide de skyrmions déformés." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape15/PQDD_0006/MQ33694.pdf.
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Lau, Pak Hang. "Construction and quantisation of skyrmions." Thesis, University of Cambridge, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.709100.
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Halcrow, Christopher James. "Skyrmions : beyond rigid body quantisation." Thesis, University of Cambridge, 2017. https://www.repository.cam.ac.uk/handle/1810/269290.
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In the Skyrme model, nuclei are described as topological solitons known as Skyrmions. To make contact with nuclear data one must quantise these Skyrmions; most calculations to date have used rigid body quantisation, where the Skyrmions are allowed to rotate but remain rigid. The method reproduces some experimental results for light nuclei but there are some contradictions with data. In this thesis we study a more sophisticated quantisation scheme where the Skyrmions may deform, called vibrational quantisation, in the hope of fixing some of these problems. Vibrational quantisation is applied to the dodecahedral $B=7$ Skyrmion, which models Lithium-$7$. Using rigid body quantisation, the Skyrme model predicts a spin $\frac{7}{2}$ ground state while in reality the Lithium-$7$ nucleus ground state has spin $\frac{3}{2}$. We show that a quantisation which includes a $5$-dimensional vibrational manifold of deformed Skyrme configurations remedies this problem, giving the correct ground state spin. Further, the model leads to a robust prediction that the ground state of the nucleus has a larger root mean square matter radius than the second quantum state, in contrast with standard nuclear models. We consider the vibrational modes of the tetrahedral $B=16$ Skyrmion, to describe Oxygen-$16$. Motivated by Skyrme dynamics, a special $2$-dimensional submanifold of configurations is constructed. We study the manifold in detail by modelling it as a $6$-punctured sphere with constant negative curvature. The Schr\"odinger equation is solved on the sphere and the results give an excellent fit to the experimental energy spectrum. The model describes an energy splitting between certain states with equal spins but opposite parities, which is hard to explain in other models. We also find the first ever isospin $0$, spin-parity $0^-$ state in the Skyrme model. A method to calculate electromagnetic transition rates between states is formulated and then applied to our system. By considering a special type of Skyrme configuration, where a single Skyrmion orbits a large core, we show that the Skyrme model can reproduce a classical spin-orbit force due to the structure of the Skyrme fields. We quantise this model to try and find out if the classical picture holds quantum mechanically.
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Lian, Yunlong. "Skyrmions in quantum Hall systems." Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLS308/document.
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Dans cette thèse, j’étudie les skyrmions dans le ferromagnétique SU(4) d’effet Hall quantique. Les skyrmions sont des textures localisées dans les systèmes ferromagnétiques. La monocouche de graphène dans un fort champ magnétique peut être considérée comme un ferromagnétique avec le spin électronique et le pseudospin de vallée de Dirac. Les niveaux de Landau associés à des spins et des vallées différentes sont proches en l’énergie et forment des groupes bien séparés. Dans un groupe, l’interaction de Coulomb montre forme invariance de SU(4). Le modèle de skyrmions utilisé dans cette thèse est une théorie de champ classique et statique obtenue à partir du principe variationnel. Le modèle comporte des paramètres phénoménologiques, qui dépendent des substrats et d’autres paramètres expérimentaux. Sur la base de l’analyse de symétrie, nous proposons un ansatz pour les skyrmions au quart de remplissage et à la moitié du remplissage du niveau de Landau N = 0 de la monocouche de graphène. La minimisation de l’énergie du skyrmion unique est ensuite effectuée pour déterminer les paramètres dans l’ansatz de skyrmion ansatz, ce qui entraîne différents types de skyrmions spin-valley aux deux facteurs de remplissage. Des grands skyrmions sont identifiés dans certaines gammes des paramètres phénoménologiques, où l’arrière-plan ferromagnétique du skyrmion subit une transition de phase. Les ondes de spin-vallée monomode sont également analysées pour caractériser le ferromagnétique SU(4) d’effet Hall quantique. Un exemple particulier montre l’instabilité de l’état fondamental ferromagnétiqueThis thesis studies skyrmions in the SU(4) quantum Hall ferromagnet. Skyrmions are localized textures in ferromagnetic systems. The graphene monolayer in a strong magnetic field can be viewed as a ferromagnet with electron spin and Dirac-valley pseudospin – Landau levels with different spin and valley are close in energy and form well-separated groups. Within one group, the Coulomb interaction has a manifest SU(4)-invariant form. The model of skyrmions used in this thesis is a classical, static field theory obtained from the variational principle. The model has phenomenological parameters, which depend on substrates and other experimental settings. Based on symmetry analysis, I propose the ansatz for skyrmions at quarter-filling and halffilling of the N = 0 Landau level in graphene monolayer. Energy minimization of single skyrmions is then performed to determine the parameters in the skyrmion ansatz, resulting in different types of spin-valley skyrmions at both filling factors. Large skyrmions are identified in certain ranges of the phenomenological parameters, where the ferromagnetic background of the skyrmion undergoes a phase transition. Single-mode spin-valley waves are also analyzed to characterize the SU(4) quantum Hall ferromagnet. A particular example shows instability of the ferromagnetic ground state
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Chess, Jordan J. "Mapping Topological Magnetization and Magnetic Skyrmions." Thesis, University of Oregon, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10684160.
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A 2014 study by the US Department of Energy conducted at Lawrence Berkeley National Laboratory estimated that U.S. data centers consumed 70 billion kWh of electricity. This represents about 1.8% of the total U.S. electricity consumption. Putting this in perspective 70 billion kWh of electricity is the equivalent of roughly 8 big nuclear reactors, or around double the nation's solar panel output. Developing new memory technologies capable of reducing this power consumption would be greatly beneficial as our demand for connectivity increases in the future. One newly emerging candidate for an information carrier in low power memory devices is the magnetic skyrmion. This magnetic texture is characterized by its specific non-trivial topology, giving it particle-like characteristics. Recent experimental work has shown that these skyrmions can be stabilized at room temperature and moved with extremely low electrical current densities. This rapidly developing field requires new measurement techniques capable of determining the topology of these textures at greater speed than previous approaches. In this dissertation, I give a brief introduction to the magnetic structures found in Fe/Gd multilayered systems. I then present newly developed techniques that streamline the analysis of Lorentz Transmission Electron Microscopy (LTEM) data. These techniques are then applied to further the understanding of the magnetic properties of these Fe/Gd based multilayered systems.
This dissertation includes previously published and unpublished co-authored material.
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Green, Andrew G. "Skyrmions in the quantum Hall effect." Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.361993.
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Chess, Jordan. "Mapping topological magnetization and magnetic skyrmions." Thesis, University of Oregon, 2018. http://hdl.handle.net/1794/23188.
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A 2014 study by the US Department of Energy conducted at Lawrence Berkeley National Laboratory estimated that U.S. data centers consumed 70 billion kWh of electricity[1]. This represents about 1.8% of the total U.S. electricity consumption. Putting this in perspective 70 billion kWh of electricity is the equivalent of roughly 8 big nuclear reactors, or around double the nation's solar panel output[2]. Developing new memory technologies capable of reducing this power consumption would be greatly beneficial as our demand for connectivity increases in the future. One newly emerging candidate for an information carrier in low power memory devices is the magnetic skyrmion. This magnetic texture is characterized by its specific non-trivial topology, giving it particle-like characteristics.
Recent experimental work has shown that these skyrmions can be stabilized at room temperature and moved with extremely low electrical current densities. This rapidly developing field requires new measurement techniques capable of determining the topology of these textures at greater speed than previous approaches. In this dissertation, I give a brief introduction to the magnetic structures found in Fe/Gd multilayered systems. I then present newly developed techniques that streamline the analysis of Lorentz Transmission Electron Microscopy (LTEM) data. These techniques are then applied to further the understanding of the magnetic properties of these Fe/Gd based multilayered systems.
This dissertation includes previously published and unpublished co-authored material.
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Manko, Olga Vladimirovna. "New approaches to the quantization of Skyrmions." Thesis, University of Cambridge, 2008. https://www.repository.cam.ac.uk/handle/1810/252080.
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Gouverneur, Yves. "Phase de Berry et quantification de skyrmions." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape15/PQDD_0002/MQ33663.pdf.
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Zhang, Shilei. "Chiral and topological nature of magnetic skyrmions." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:11306f2a-77e6-4f65-a3dd-3b1c2365ea32.
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This work focuses on characterising the chiral and topological nature of magnetic skyrmions in noncentrosymmetric helimagnets. In these materials, the skyrmion lattice phase appears as a long-range-ordered, close-packed lattice of nearly millimetre-level correlation length, while the size of a single skyrmion is 3-100 nm. This is a very challenging range of lengthscales (spanning 5 orders of magnitude from tens of nm to mm) for magnetic characterisation techniques. As a result, only three methods have been proven to be applicable for characterising certain aspects of the magnetic information: neutron diffraction, electron microscopy, and magnetic force microscopy. Nevertheless, none of them reveals the complete information about this fascinating magnetically ordered state. On the largest scale, the skyrmions form a three-dimensional lattice. The lateral structure and the depth profile are of importance for understanding the system. On the mesoscopic scale, the rigid skyrmion lattice can break up into domains, with the domain size about tens to hundreds of micrometers. The information of the domain shape, distribution, and the domain boundary is of great importance for a magnetic system. On the smallest scale, a single skyrmion has an extremely fine structure that is described by the topological winding number, helicity angle, and polarity. These pieces of information reveal the underlying physics of the system, and are currently the focus of spintronics applications. However, so far, there is no experimental technique that allows one to quantitatively study these fine structures. It has to be emphasised that the word 'quantitative' here means that no speculations have to be made and no theoretical modelling is required to assist the data interpretation -- what has been measured must be straightforward, and give a unique and unambiguous answer. Motivated by these questions, we developed soft x-ray scattering techniques that allow us to acquire much deeper microscopic information of the magnetic skyrmions -- reaching far beyond what has been possible so far. We will show that by using only one technique, all the information about the magnetic structure (spanning 5 orders of magnitude in length) can be accurately measured. The thesis is structured as follows: The key development is the Dichroism Extinction Rule, which is summarised in Chapter 6, and quintessentially summarises the thesis. In Chapter 1, the well-established theory for skyrmions is introduced, reconstructing the picture from single skyrmions to the skyrmion crystal. A few comments about the current characterisation techniques will be given. In Chapter 2, we will start with the largest lengthscale, the long-range-ordered skyrmion lattice phase. This is an intensely studied phase, mostly using neutron diffraction, and we will show that this piece of information can be equivalently (or actually even better) obtained using resonant x-ray diffraction. The theoretical foundation of this technique is also given. In Chapter 3, we will demonstrate imaging technique with which we were able to effectively map the skyrmion domains. The measurements also suggest a way to control the formation of skyrmion domains, which might be the key for enabling skyrmion-based device applications. Chapters 4 and 5 present the highlights of this work, in which we will show that using the dichroism extinction rule, the topological winding number and the skyrmion helicity angle can be unambiguously determined. In this sense, this technique is capable of accurately measuring the internal structure of single skyrmions.
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Dwyer, Daniel A. (Daniel Andrew) 1976. "Collision induced decay of metastable baby skyrmions." Thesis, Massachusetts Institute of Technology, 2000. http://hdl.handle.net/1721.1/8722.
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Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2000.Includes bibliographical references (p. 89-90).
Many extensions of the standard model predict heavy metastable particles which may be modeled as solitons (skyrmions of the Higgs field), relating their particle number to a winding number. Previous work has shown that the electroweak interactions admit processes in which these solitons decay, violating standard model baryon number. We motivate the hypothesis that baryon-number-violating decay is a generic outcome of collisions between these heavy particles. We do so by exploring a 2+ 1 dimensional theory which also possesses metastable skyrmions. We use relaxation techniques to determine the size, shape and energy of static solitons in their ground state. These solitons could decay by quantum mechanical tunneling. Classically, they are metastable: only a finite excitation energy is required to induce their decay. We attempt to induce soliton decay in a classical simulation by colliding pairs of solitons. We analyze the collision of solitons with varying inherent stabilities and varying incident velocities and orientations. Our results suggest that winding-number violating decay is a generic outcome of collisions. All that is required is sufficient (not necessarily very large) incident velocity; no fine-tuning of initial conditions is required.
by Daniel D. Dwyer.
S.B.
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Mashaal, Maurice. "Contribution à l'étude du modèle des skyrmions." Paris 11, 1988. http://www.theses.fr/1988PA112062.
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Certains aspects phénoménologiques du modèle des skyrmions sont étudiés. On présente d'abord une brève introduction à la chromodynamique quantique et sa limite quand le nombre de couleurs est grand, qui est la motivation moderne pour le modèle de Skyrme. Dans cette limite, les baryons seraient des solitons associés à des lagrangiens effectifs de mésons. Le modèle de base, étudié par Adkins, Nappi et Witten, est présenté en détail. Ensuite on envisage l'amélioration du modèle en construisant des lagrangiens effectifs par développement en puissances de dérivées du champ du pion, conformément à l'esprit de l'algèbre des courants. Avec les paramètres mésoniques fixés par l'expérience, les masses des baryons obtenues sont trop élevées. Des lagrangiens effectifs incorporant explicitement d'autres mésons sont alors considérés, sans que la conclusion essentielle soit modifiée: les masses prédites pour les baryons sont trop élevées, bien que les autres propriétés statiques soient en accord raisonnable avec l'expérience.
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Meng, Keng-Yuan. "Magnetic Skyrmions in Oxide Thin Film Heterostructures." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1562856036665345.
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Rowland, James R. IV. "Theoretical Investigations of Skyrmions in Chiral Magnets." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1565920470204146.
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Mashaal, Maurice. "Contribution à l'étude du modèle des skyrmions." Grenoble 2 : ANRT, 1988. http://catalogue.bnf.fr/ark:/12148/cb376158887.
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Luckock, H. C. "Strings, p-branes and Skyrmions in curved space." Thesis, University of Newcastle Upon Tyne, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.384005.
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BORGES, Damares Santos Silva. "Defeitos em matéria condensada: de twistons a skyrmions." Universidade Federal de Campina Grande, 2018. http://dspace.sti.ufcg.edu.br:8080/jspui/handle/riufcg/1577.
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Capes
Os defeitos topológicos são caracterizados como soluções estáveis de equações de movimento em uma ou mais dimensões espaciais e desempenham papel importante na ciência não-linear. Neste trabalho de dissertação, damos ênfase a defeitos em (1+1) e (2+1) dimensões espaço-temporais. No primeiro caso, abordamos configurações conhecidas como twistons (soluções topológicas tipo kink) presentes em cristais de polietileno. Nessa primeira abordagem, revisitamos trabalhos anteriores e, a partir do método de extensão, construímos novas famílias de potenciais que descrevem bem sistemas desse tipo. Apresentamos soluções topológicas analíticas e que não possuem problemas de degenerescência infinita. No segundo caso, estudamos estruturas conhecidas como skyrmions com base na sua descrição em materiais magnéticos, em que são denotados como configurações da magnetização em nanoescala e topologicamente estáveis.Recorremos novamente ao método de extensão e apresentamos um potencial, função de dois campos escalares acoplados, a partir do qual conseguimos modelar essas estruturas magnéticas. Além disso, o novo modelo de dois campos tem soluções analíticas conhecidas, permitindo análises interessantes como a determinação de uma quantidade topológica conservada, estudo das diferentes configurações da magnetização e cálculo do raio médio de matéria.
Topological defects are characterized as stable equation of motion solutions in one or more spatial dimensions and play an important role in nonlinear science. In this study, space-time (1 + 1) and (2 + 1) dimension defects are emphasized. In the first case, configurations known as twistons (kink-like topological solutions) present in polyethylene crystals are assessed. In this first approach, previous works were reviewed and new families of potentials that adequately describe these types of systems were constructed from the extension method, presenting analytical topological solutions that do not display infinite degeneracy problems. In the second case, structures known as skyrmions were studied based on their description in magnetic materials,where they are denoted as topologically stable nanoscale magnetization configurations. The extension method was applied and a potential from which such magnetic structures can be modelled, function of two coupled scalar fields was presented. In addition, the new two-field model possesses known analytical solutions, allowing for interesting analyses, such as the determination of a conserved topological quantity, the study of the different magnetization configurations and calculation of mean matter radius.
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Afghah, Seyedeh Sajedeh. "MODELING SKYRMIONS, DEFECT TEXTURES, AND ELECTRICAL SWITCHINGIN LIQUID CRYSTALS." Kent State University / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=kent1532952208004472.
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Srivastava, Abhay Kant [Verfasser], Stuart S. P. [Gutachter] Parkin, Ingrid [Gutachter] Mertig, and Christian [Gutachter] Back. "Investigations of anti-skyrmions and Néel skyrmions using Lorentz Transmission Electron Microscopy / Abhay Kant Srivastava ; Gutachter: Stuart S. P. Parkin, Ingrid Mertig, Christian Back." Halle (Saale) : Universitäts- und Landesbibliothek Sachsen-Anhalt, 2021. http://d-nb.info/122730174X/34.
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Lin, Wen-Tsan. "Vibrational modes of massive Skyrmions within the rational map Ansatz." Thesis, Durham University, 2007. http://etheses.dur.ac.uk/2486/.
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In this thesis, we study the vibrational modes of some Skyrmion solutions within the rational map approximation. When the Skyrme field is perturbed around a static solution, the radial and angular vibrations of the field are decoupled; this gives us two sets of eigenvalue equations that provide us the vibrational modes of the solutions when solved. Using the symmetry properties of the solutions of the equation of motion of the model, the conjugate relations among these solutions can be identified. With a physical contraint on the dispersion relation between the mass and the vibration frequency, we obtain an upper bound for the critical pion mass. Finally, we compare between our results with some numerical results obtained by Houghton et al.
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Luo, Wenchen. "Skyrmions dans la simple couche et la bicouche de graphène." Mémoire, Université de Sherbrooke, 2009. http://savoirs.usherbrooke.ca/handle/11143/4826.
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Graphene, a monolayer of graphite, has been isolated in 2004 by Novoselov et al. [1]. Since then, it has been the subject of intense study both theoretically and experimentally. In this thesis, we study the ground states of the two-dimensional electron gas (2DEG) in graphene and bilayer graphene in a quantizing magnetic field. We first study the liquid phases at filling factors [nu] = 1, 2, 3, 4 including spins and valley pseudo-spins as well as the meron crystal phases with spin and valley pseudospin textures around filling factor [nu] = 1. We then look for single-particle excitations in a bilayer graphene system at the filling factor [nu] = -3. We find that charged pseudo-spin textures do not exist in such a system. The lowest charged excitations are the quasi-hole and quasi-electron states. We conclude that topological excitations do not play a significant role in the conductivity of the bilayer at [nu] = -3.
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Bouard, Chloé. "Elaboration et caractérisation de systèmes magnétiques pour l'observation de skyrmions." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY084/document.
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Les nouvelles technologies numériques sont très avides en capacité de stockage, ainsi qu’en efficacité (rapidité et bas coût énergétique) de transport d’information. Les dispositifs d’aujourd’hui atteignant leurs limites, la recherche de nouvelles solutions de stockage est primordiale.L’utilisation de parois magnétiques comme brique élémentaire au codage de l’information a été proposée il y a quelques années, dans l’optique de réaliser un enregistrement tridimensionnel et ainsi d’augmenter considérablement les capacités de stockage.Depuis, un nouvel objet magnétique plus robuste et moins sensible aux perturbations extérieures a été découvert : le skyrmion. Il est récemment devenu un sujet d’étude très actif et a été observé expérimentalement dans deux types de systèmes. Le premier est basé sur les matériaux hélimagnétiques, dont la structure cristalline est non centrosymétrique. Le FeGe est l’un d’eux, avec la température de transition magnétique observée la plus élevée. Le skyrmion a également été observé à l’interface entre un métal lourd à fort couplage spin-orbite et un matériau ferromagnétique. En particulier, les systèmes de multicouches à interfaces non symétriques ont montré leur fort potentiel pour la manipulation de skyrmions à température ambiante.Les deux principales difficultés rencontrées aujourd’hui sont la réalisation de systèmes avec des techniques d’élaboration intégrables à des dispositifs industriels, ainsi que la détection fiable de la présence de skyrmions. Cette thèse est donc séparée en deux problématiques, appliquées aux deux types de systèmes. Un protocole de croissance de couches minces de FeGe hélimagnétique par pulvérisation cathodique a été développé en s’appuyant principalement sur des caractérisations structurales par diffraction de rayons X. L’élaboration de multicouches à interfaces non symétriques du type [métal lourd 1/matériau ferromagnétique/métal lourd 2]n a également été étudiée. Ces deux systèmes ont ensuite été caractérisés magnétiquement par diverses techniques basées sur de l’imagerie magnétique, des mesures de magnéto-transport et de la spectroscopie magnétiqueNew technology needs huge storage capacity together with high speed and low-cost transport of information. Current devices meeting their limits, research on new storage solutions is needed.One of them, proposed a few years ago, consists in using magnetic domain walls. Aligning them in nanowires and using the thickness of the layers could enable the realization of a tridimensional recording device and then improve the storage capacity.A new object more robust and less sensitive to perturbations has been discovered since. Skyrmion is now widely studied. It has been experimentally observed in two kinds of systems. The first one is helimagnet, with non centrosymmetric crystal structure. FeGe is one of them, with the highest critical temperature observed yet. Skyrmion has been observed as well at the interface between a heavy metal with strong spin-orbit coupling and a ferromagnet. In particular, multilayers with non-symmetric interfaces are very promising systems for manipulation of skyrmions at room temperature.Nevertheless, the elaboration of systems for industrial devices and reliable detection of skyrmions is still challenging. These two problematics are explored in this thesis, applied to two different systems. A protocol to grow helimagnetic FeGe thin films was first established, thanks to structural characterization mainly based on X-ray diffraction. Growth of multilayers with non symmetrical interfaces [heavy metal 1/ferromagnet/heavy metal 2]n was studied as well. These systems were then magnetically characterized, using numerous techniques such as magnetic imaging, magneto transport measurements and magnetic spectroscopy
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Bamler, Robert Verfasser], Achim [Gutachter] Rosch, and Alexander [Gutachter] [Altland. "Phase-Space Berry Phases in Chiral Magnets: Skyrmion Charge, Hall Effect, and Dynamics of Magnetic Skyrmions / Robert Bamler. Gutachter: Achim Rosch ; Alexander Altland." Köln : Universitäts- und Stadtbibliothek Köln, 2016. http://d-nb.info/1113178728/34.
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Srivastava, Titiksha. "Ingénierie et contrôle dynamique des propriétés interfaciales dans les films ultra-minces pour ajuster les textures de spin magnétique." Thesis, Université Grenoble Alpes (ComUE), 2019. http://www.theses.fr/2019GREAY001/document.
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Le contrôle du magnétisme aux interfaces s’est avéré essentiel pour la spintronique et ses applications, en particulier celles basées sur des structures chirales de spin appelées skyrmions magnétiques. Ces skyrmions, décrits comme des solitons magnétiques, sont de potentiels vecteurs d’information. Dans des empilements ultraminces du type métal lourd / ferromagnétique / isolant, les skyrmions sont stabilisés par une interaction d’échange antisymétrique d’interface appelée interaction de Dzyaloshinskii-Moriya (DMI); celle-ci entre en compétition avec d’autres interactions telles que l’interaction d’échange symétrique ou l’anisotropie magnétique. Afin de contrôler ces skyrmions, les propriétés magnétiques aux interfaces doivent être ajustées finement et modulées par une excitation extérieure. Le champ électrique s’est avéré être un outil efficace pour manipuler ces propriétés d’interface. Il a notamment été montré dans un certain nombre d’études depuis 2009 qu’une différence de potentiel permet de modifier localement et de manière dynamique des propriétés telles que l’anisotropie magnéto-cristalline ou l’aimantation à saturation. Cependant, cet effet sur DMI, qu’il est crucial d’intégrer pour les systèmes avec skyrmions, n’avait pas été observé pour les films ultraminces.Cette thèse présente tout d'abord une optimisation des systèmes tricouches de type métal lourd/ferromagnétique/oxide dans lesquels peuvent exister des skyrmions. J’ai plus particulièrement étudié le système Ta/FeCoB/TaOx qui permet de énucléer des skyrmions en présence d’un faible champ magnétique appliqué perpendiculairement au plan des couches. Une étude approfondie en fonction de l’épaisseur de FeCoB et de l’état d’oxydation du TaOx a notamment été menée, permettant ainsi d’identifier les différentes zones présentant des skyrmions. D’autre part, le résultat majeur de cette thèse est la démonstration de la modulation de DMI par un champ électrique dans une tricouche Ta/FeCoB/TaOx. Des mesures de spectroscopie Brillouin sous champ électrique ont montré une très grande variation allant jusqu’à 130%. Puis, des observations complémentaires en microscopie à effet Kerr magnéto-optique ont permis de mesurer simultanément une variation monotone de DMI et de la taille des skyrmions en fonction du champ électrique avec une efficacité sans précédent. Puisque le champ électrique agit principalement sur l’interface entre le matériau ferromagnétique et l’oxyde (FeCoB/TaOx), cette étude indique l’existence d’une DMI de type Rashba, expliquant la forte sensibilité au champ électrique. Ces observations montrent également qu’un renversement du signe de l’IDM est possible, qui conduirait à une inversion de la chiralité des skyrmions. Cette manipulation dynamique de DMI permettrait de conférer un degré de contrôle supplémentaire pour le développement de mémoires ou de dispositifs logiques ou neuromorphiques à base de skyrmionsControl of interfacial magnetism has emerged to be of paramount importance for spintronics applications specially involving chiral magnetic structures called skyrmions. Skyrmions are envisaged to be the future information carriers owing to their solitonic properties. In heavy metal/ ferromagnet/ insulator heterostructures, skyrmions are stabilized by interfacial Dzyaloshinskii-Moriya interaction which is an antisymmetric exchange and competes with other interactions like symmetric exchange and magnetic anisotropy. In order to tune skyrmions, the interfacial magnetic properties need to be modulated. One of the energy efficient tools to maneuver interfacial magnetism is electric field effect. Voltage gating has been shown, in a number of studies since 2009, to locally and dynamically tune magnetic properties like interface anisotropy and saturation magnetization. However, its effect on interfacial Dzyaloshinskii-Moriya Interaction (DMI), which is crucial for the stability of magnetic skyrmions, has been challenging to achieve and has not been reported yet for ultrathin films.This thesis demonstrates an optimization of trilayer systems consisting of a heavy metal/ ferromagnet/ oxide where skyrmions can be stabilized. In particular, I focussed on the Ta/FeCoB/TaOx system to nucleate skyrmions in the presence of very small out of plane magnetic field. Further, the different skyrmionic zones as a function of the FeCoB thickness and TaOx oxidation state are studied. We then show electric field induced modulation of interfacial DMI which forms the most important result of this thesis. We demonstrate 130% variation of DMI with electric field in Ta/FeCoB/TaOx trilayers through Brillouin Light Spectroscopy (BLS). Using polar Magneto-Optical-Kerr-Effect microscopy, we further show a monotonic variation of DMI and skyrmionic bubble size with electric field, with an unprecedented efficiency. Since the electric field acts mainly on the FeCoB/TaOx interface, this study also points at the existence of the Rashba DMI explaining its high sensitivity to an applied voltage. We anticipate through our observations that a sign reversal of DMI with electric field is possible, leading to a chirality switch. This dynamic manipulation of DMI establishes an additional degree of control to engineer programmable skyrmion based memory, logic or neuromorphic devices
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Gladikowski, Jens. "Solitons in low-dimensional sigma models." Thesis, Durham University, 1997. http://etheses.dur.ac.uk/5077/.
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The aim of this thesis is to study topological soliton solutions in classical field theories, called sigma models, on a three-dimensional space. In chapter 1 we review the general field-theoretical framework of classical soliton solutions and exemplify it on the main features of the 0(3) σ-model and the Abehan Higgs model in (2+1) dimensions. In chapter 2 a U(l)-gauged 0(3) σ-model is discussed, where the behaviour of the gauge field is determined by a Chern-Simons term in the action. We find numerical solutions for radially symmetric fields and discuss those of degree one and two. They carry a non-vanishing angular momentum and can be interpreted as classical anyons. A similar model is studied in chapter 3. Here the potential is of Higgs-type and chosen to produce a Bogomol'nyi model where the energy is bounded from below by a linear combination of the topological degree of the matter fields and the local U(l)-charge. Depending on internal parameters, the solutions are solitons or vortices. We study them numerically and prove for a certain range of the matter field's vacuum value that there cannot be a 1-soliton.In chapter 4 we discuss a modified 0(3) σ-model in (3+0) dimensions. The topological stability of the solitons is here imphed by the degree of the map S(^3) → S(^2), which provides a lower boundon the potential energy of the configuration. Numerical solutions are obtained for configurations of azimuthal symmetry and the spectrum of slowly rotating solitons is approximated. Chapter 5 deals with a theory where the fields are maps IR(^2+1) → CP(^2). The Lagrangian includes a potential and a fourth-order term in the field-gradient. We find a family of static analytic solutions of degree one and study the 2-soIiton configuration numerically by using a gradient-flow equation on the moduli space of solutions. We conclude this thesis with a brief summary and give an outlook to open questions.
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Szyndel, Matthew Dennis Edward. "The RP² sigma and easy plane baby Skyrme models." Thesis, Durham University, 2000. http://etheses.dur.ac.uk/4518/.
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This thesis examines the behaviour of two new models exhibiting topological solitons. This analysis is predominantly numerical, but a limited collective coordinate approach is attempted where appropriate. In chapter 1 we review the field of solitons. In particular the nature of topological solitons and their associated mathematical formalism are explained. A number of models admitting solitons are defined. In chapter 2 we look at the numerical methods necessary to solve the time evolution of topological solitons in the S(^2) sigma model and the baby Skyrme model. We also examine methods for finding static solutions of the equations of motion of such models. In chapter 3 we define the RP(^2) sigma and baby Skyrme models. We examine the behaviour of these models and find them to be identical to their (S^2) counterparts for most field configurations. The topological reason for this is explained. The existence of a topological object called a defect is noted and the behaviour of solitons in the presence of a defect is examined. A collective coordinate approach is used to examine the behaviour of solitons in the presence of a defect. In chapter 4 the easy plane baby Skyrme model is defined. An ansatz for the static skyrmions is proposed and its energy found to be accurate to 1.2% for the 1-skyrmion and about 0.5% for 2 to 4-skyrmions. These skyrmions are composed of two quasi- independent soliton like objects which we name "half lumps". These objects may not exist alone. The scattering properties of these objects are examined numerically. The behaviour of these scattering processes are explained in terms of the fields and potential energy of their intermediate states in the simulation. In chapter 5 we summarise our work and propose future work in this field.
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Juge, Roméo. "Nucléation et dynamique de skyrmions magnétiques dans des films ultra-minces." Thesis, Université Grenoble Alpes, 2020. http://www.theses.fr/2020GRALY005.
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Les skyrmions magnétiques sont des enroulements chiraux de l'aimantation que l'on peut visualiser comme de minuscules domaines magnétiques circulaires délimités par des parois de domaine chirales. En raison de leur taille potentiellement nanométrique et car on leur prédit un déplacement efficace sous courant, les skyrmions magnétiques sont devenus des candidats prometteurs pour transporter l'information dans des mémoires et des dispositifs logiques non-volatiles et à forte densité de stockage. Ils ont récemment été observés à température ambiante dans des empilements du type métal lourd/ferromagnétique/non-magnétique, franchissant une étape importante en vue de développer des dispositifs utilisant des skyrmions. Suivant ces premières observations, l'objectif de cette thèse est d'étudier certaines propriétés clés des skyrmions que sont leur nucléation, leur stabilité ainsi que leur déplacement sous courant, toutes à température ambiante. Les premiers résultats présentés portent sur la nucléation de skyrmions dans des films ultra-minces (pour une épaisseur de ferromagnétique de l'ordre de 1 nm) réalisée par ingénierie des propriétés magnétiques aux interfaces ainsi que de la géométrie des échantillons. La nucléation de skyrmions dans des films ultra-minces étendus, en géométrie confinée, dans des films polarisés par échange inter-couche puis dans des motifs définis par irradiation d'ions est présentée. La deuxième partie de ce travail concerne l'étude de la dynamique des skyrmions magnétiques sous courant. Dans un film ultra-mince de composition Pt/Co/MgO, on mesure des vitesses atteignant 100 m/s pour des tailles de skyrmions de l'ordre de 100 nm. Cette étude met également en lumière l'effet Hall de skyrmion, effet signature de leur topologie décrivant la déviation de la trajectoire d'un skyrmion par rapport à celle dictée par le courant. Nous trouvons que cette déviation dépend nettement de la vitesse des skyrmions, contrairement à ce que prédisent les modèles existants. En combinant modèle analytique et simulations micromagnétiques et en s'appuyant sur une caractérisation poussée des propriétés du film, nous trouvons que cette dépendance avec la vitesse peut être entièrement attribuée à des effets de piégeage qui entravent le déplacement des skyrmions. Enfin, la dernière partie de ce travail porte sur l'étude expérimentale de multi-couches antiferromagnétiques synthétiques, caractérisées par un moment magnétique net nul. Dans ces systèmes, on s'attend à ce que le déplacement des skyrmions s'effectue dans la direction du courant, c'est-à-dire sans effet Hall de skyrmion, une condition requise pour les applications. L'optimisation de multi-couches spécifiques permet l'observation en microscopie à rayons X de skyrmions antiferromagnétiques synthétiques de l'ordre de 100 nm. Puisque leur nucléation est délicate, un dispositif est par la suite conçu pour injecter localement du courant électrique à travers des pointes. Ce dispositif permet de créer et de supprimer des skyrmions de manière contrôlée en utilisant uniquement du courant, rapprochant ces skyrmions un peu plus des applicationsMagnetic skyrmions are chiral magnetisation windings that can be pictured as minuscule circular magnetic domains bounded by chiral domain walls. Owing to their potential nanometre size and predicted efficient current-driven motion, magnetic skyrmions hold great promise as future information carriers in high-density non-volatile memory and logic applications. Their recent observation at room temperature in material stacks consisting of heavy metal/ferromagnet/non-magnet has lifted an important bottleneck towards the practical realisation of skyrmion-based devices. Following these early observations, the objective of this work is to tackle certain key attributes of magnetic skyrmions that are their nucleation, stability and current-driven motion, all at room temperature. The first results presented in this thesis deal with the stabilisation and nucleation of skyrmions in ultra-thin films (for a ferromagnetic thickness around 1 nm) by engineering of the interfacial magnetic properties and geometries. The nucleation of skyrmions in extended films, confined geometries, exchange-biased films and ion-irradiated films are presented. The second part of this work concerns the current-driven dynamics of magnetic skyrmions. In an ultra-thin Pt/Co/MgO film, we measure velocities up to 100 m/s for skyrmion sizes in the range of 100 nm. This study further highlights the skyrmion Hall effect, a hallmark of the skyrmion topology, which describes the deflection of a skyrmion trajectory from that imposed by the current. The angle of deflection is found to be dependent on the skyrmion velocity, in contrast with existing models. Combining analytical modelling and micromagnetic simulations, based on a thorough characterisation of the film properties, we find that this dependence on the velocity can be entirely attributed to pinning effects hindering the skyrmion motion. Finally, in the last part of this work, we investigate experimentally synthetic antiferromagnetic multi-layers with vanishing magnetic moment. In such systems, magnetic skyrmions are expected to be driven along the current direction without skyrmion Hall effect, a prerequisite for applications. By optimising specific multilayers and using X-ray microscopy, we observe synthetic antiferromagnetic skyrmions at room temperature with sizes in the range of 100 nm. As their nucleation is somewhat challenging, a device is then designed to locally inject current through patterned tips. This allows to create and delete skyrmions in a controlled fashion using solely current, bringing them one step closer to applications
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Lemesh, Ivan. "Static and dynamic properties of magnetic Skyrmions in engineered multilayer films." Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/122179.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2019Cataloged from PDF version of thesis.
Includes bibliographical references (pages 205-219).
Magnetic textures known as skyrmions promise new breakthroughs in memory, logic, and neuromorphic applications. Skyrmions have been found in a variety of material systems, yet there existed no experimental evidence of a material that could simultaneously host them at room temperature and also allow for their reproducible current-induced nucleation and motion. One main goal of this thesis is to fill this gap and demonstrate all the aforementioned properties in the introduced here [Pt/CoFeB/MgO]₁₅ thin film heterostructures, consisting of a perpendicularly magnetized ferromagnetic layer (M), a heavy metal (H), and a symmetry-breaking spacer layer (S). Here, I developed, fabricated, and characterized the [Pt/CoFeB/MgO]₁₅ multilayers with an extremely low density of pinning centers, which enable not only a fully reproducible skyrmion motion but also a clean study of the skyrmion nucleation process. By using X-ray microscopy, I performed the imaging of various magnetic textures in these multilayers and studied their current-induced generation and motion as a function of applied field and temperature. Finally, another goal of this work is to establish a direct link between the properties of these [H/M/S][subscript N]-type materials and the structure of magnetic textures that they can host. The energetics of such systems is understood very poorly due to the very complex multilayer stray fields and up until now, most of their analysis involved the exclusive use of micromagnetic simulations. Here, I develop an alternative theoretical approach by calculating all the stray field interactions analytically, which enables the prediction of the exact structure and dynamics of magnetic domain walls, domains, and skyrmions. Thesis
"Support of the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under Award No. DE-SC0012371, and of the DARPA TEE program"--page 7.
by Ivan Lemesh.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Materials Science and Engineering
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Cova, Ramón José Cova. "CP¹ model on a sphere and on a torus." Thesis, Durham University, 1997. http://etheses.dur.ac.uk/4851/.
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The work in this thesis is concerned with the numerical study of some stability and scattering properties of two CP¹ models in three dimensional space-time: The non-linear 0(3) model and its modified Skyrme version. Chapter 3 focuses principally on the Skyrme model on compactified plane, the topological sphere. Such model is obtained by supplementing the ordinary 0(3) lagrangian with both a Skyrme term and a potential term which, in the present work, has a rather general form. Under the numerical simulation the skyrmions behave stably and scatter either back-to-back or at 90 to the initial direction of motion, depending on the initial velocity. In the 0(3) limit the solitons are no longer stable and scatter at 90 irrespective of the speed. In the fourth chapter the 0(3) model is studied on a flat torus. Its solitons exhibit the usual instability but can be stabilised by the sole addition of a Skyrme term to the lagrangian. Scattering at right angles is observed in all cases considered, including skyrmions colliding at speeds that would bounce them back were they evolving in compactified plane. The periodic 0(3) model has no analytic solutions of degree one, so when a field configuration that resembles a single soliton is numerically evolved, it shrinks to become infinitely thin. Interestingly, such ansatz may be regarded as a soliton of unit topological charge in the context of the periodic skyrmion model. Chapter 5 closes with a summary and suggestions for future research.
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King, Christopher. "B=4N nuclei in the Skyrme model." Thesis, University of Cambridge, 2019. https://www.repository.cam.ac.uk/handle/1810/287465.
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The Skyrme model enables us to approximate nuclei via topological solitons known as Skyrmions. The B = 4 Skyrmion is of particular importance as its symmetry and stability means that multiple B = 4 Skyrmions can combine with each other to form larger B = 4N Skyrmions. In this thesis we investigate the properties of these B = 4N Skyrmions and compare them with results found in the wider nuclear physics community. We go beyond rigid body quantization and develop a formalism of using vibrational quantisation to generate the energy spectrum of the Oxygen−16 nucleus. The Oxygen−16 nucleus is treated as an arrangement of four B = 4 Skyrmions, whose dynamics enable us to create a 2−dimensional manifold of B = 16 configurations. We solve the Schrödinger equation on this manifold and discover new states previously not found in the B = 16 sector of the Skyrme model. We compare these states with those found experimentally and find that there is a excellent it to the energy spectrum. In order to apply vibrational quantization to a wider range of nuclei we create a novel approximation for Skyrmions and the interactions between them. By generating Skyrmions with Gaussian sources we find analytic expressions for the pion fields and interaction energies of Skyrmions, with particular focus on the B = 1 and B = 4 Skyrmions, and show how this could be applied to vibrational quantization and the clustering of B = 4 Skyrmions. B = 4N nuclei are the only nuclei with zero spin and isospin, which means that their electric charge density is proportional to their baryon density. This simplification makes these nuclei particularly susceptible to investigation via electron scattering. We develop a classical averaging method to calculate the Patterson function and the form factor for a B = 4N nucleus and make comparisons with experimental data. We also discover a way of using the baryon density directly to approximate the locations of zeroes and stationary points of the form factor.
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Büttner, Felix [Verfasser]. "Topological mass of magnetic Skyrmions probed by ultrafast dynamic imaging / Felix Büttner." Mainz : Universitätsbibliothek Mainz, 2013. http://d-nb.info/1046249096/34.
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Legrand, William. "Crafting magnetic skyrmions at room temperature : size, stability and dynamics in multilayers." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS066/document.
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Les skyrmions magnétiques sont des enroulements bidimensionnels et nanométriques de la configuration de spin, pouvant être stabilisés dans certains matériaux magnétiques soumis à l’interaction d’échange antisymétrique Dzyaloshinskii-Moriya. Ils présentent une topologie non-triviale et s’annoncent peut-être comme étant les plus petites configurations magnétiques pouvant être réalisées. Très récemment, des skyrmions magnétiques ont pu être stabilisés à température ambiante grâce à la conception de multicouches magnétiques brisant la symétrie d’inversion selon la direction verticale. Suite à cette avancée, l’objectif central de cette thèse est la compréhension et la maîtrise des multiples propriétés physiques des skyrmions hébergés dans ces systèmes multicouches. Pour aborder cet objectif, un modèle original est décrit puis employé, permettant la prédiction des profils adoptés par les skyrmions multicouches. Ce modèle numérique est très générique, n’utilisant que la symétrie cylindrique des skyrmions afin de simplifier la détermination des interactions magnétostatiques. Ce modèle est ensuite étendu afin de pouvoir approximer la stabilité thermique des skyrmions, ce qui constitue un élément clé dans leur obtention expérimentale. Une seconde dimension de ce travail consiste en l’étude expérimentale de la manipulation électrique des skyrmions multicouches, démontrant la possibilité de trois fonctionnalités centrales que sont leur nucléation par courants locaux, leur déplacement sous courant de spin et leur détection électrique individuelle par tension transverse. Le troisième aspect de ma thèse est l’étude des propriétés physiques influençant le déplacement des skyrmions dans les multicouches magnétiques. Un comportement d’ancrage sur des défauts est mis en évidence expérimentalement et est analysé à l’aide d’une modélisation micromagnétique. Un des résultats importants de ce travail est aussi la prédiction d’une chiralité hybride dans les configurations magnétiques de certaines multicouches, qui est ensuite démontrée expérimentalement par des mesures au synchrotron. Les conséquences attendues de cette chiralité hybride sur le déplacement des skyrmions sont étudiées pour permettre l’optimisation des multicouches, aboutissant à l’observation expérimentale de la propagation de skyrmions de 50 nm de rayon à des vitesses atteignant environ 40 m/s. La dernière partie de cette thèse vise à mettre à profit ces avancées théoriques et expérimentales afin de parvenir à réduire la taille des skyrmions à température ambiante. Après avoir analysé l’impact des interactions dipolaires sur la stabilité des skyrmions, il est entrepris d’optimiser les matériaux et la périodicité des couches. Je m’intéresse aussi à la conception expérimentale de textures magnétiques dont l’aimantation est compensée au sein de structures multicouches appelées antiferromagnétiques synthétiques, dont je montre qu’elles peuvent héberger des skyrmions antiferromagnétiques à température ambiante. Ce résultat final ouvre de nouvelles perspectives vers l’obtention de skyrmions à la fois mesurant moins de 10 nm et très mobiles, qui pourraient être utilisés dans la conception de composants de calcul et de stockage d’information plus compacts et plus efficacesMagnetic skyrmions are nanoscale two-dimensional windings in the spin configuration of some magnetic materials subject to the Dzyaloshinskii-Moriya antisymmetric exchange interaction. They feature a non-trivial topology and show promise to be the smallest achievable magnetic textures. Very recently, magnetic skyrmions have been successfully stabilised up to room temperature by leveraging on the design of magnetic multilayer systems breaking the vertical inversion symmetry. Following up on this achievement, the main objective of this thesis is the understanding and the control of the various physical properties of skyrmions hosted by such multilayer systems. As a first approach to this objective, an original model allowing to predict the profiles adopted by multilayer skyrmions is described and then employed. This numerical model is very generic, as it exploits only the cylindrical symmetry of multilayer skyrmions, in order to determine the magnetostatic interactions with less effort. This model is further extended in order to approximate the thermal stability of multilayer skyrmions, which is key to their experimental realisation. The next aspect of this thesis consists in the experimental study of the electrical manipulation of multilayer skyrmions, demonstrating three main functionalities that are nucleation by local currents, displacement under spin currents and individual detection by transverse voltage. The third aspect of my thesis is the study of the physical properties influencing the current-induced motion of skyrmions in magnetic multilayers. A pinning behaviour is evidenced experimentally and analysed relying on micromagnetic modelling. One of the important results of this work is also the prediction of hybrid chirality for some multilayer magnetic configurations, which is then demonstrated experimentally using a synchrotron technique. The impact of hybrid chirality on current-induced skyrmion motion is discussed and leads to the optimisation of the multilayer design, resulting in the experimental observation of motion for skyrmions below 50 nm in radius at velocities reaching around 40 m/s. The last part of this thesis aims at leveraging on these theoretical and experimental advances in order to reduce the size of skyrmions at room temperature. After the analysis of the impact of dipolar interactions on skyrmion stability, the engineering of the materials and of the layers periodicity is attempted. I also investigate experimentally the conception of magnetic textures with compensated magnetization in multilayer structures known as synthetic antiferromagnets, and show that they can host antiferromagnetic skyrmions at room temperature. This last result opens up new prospects for achieving room-temperature skyrmions combining size in the single-digit nm range and high mobility, potentially allowing applications towards energy-efficient computation and storage devices with a very dense integration
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Sassi, Yanis. "Magnetic skyrmions in multilayers : control of their current induced nucleation and motion." Electronic Thesis or Diss., université Paris-Saclay, 2023. https://tel.archives-ouvertes.fr/tel-04136857.
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Les skyrmions magnétiques sont des textures de spin stabilisées grâce à l'interaction Dzyaloshinskii-Moriya. Cette interaction chirale favorise un enroulement particulier de l'aimantation et leur confère des propriétés intéressantes. Ils se comportent par exemple comme des particules et possèdent une topologie différente de l'état d'aimantation uniforme. Depuis les premières observations en 2011, les skyrmions ont fait l'objet d'études approfondies en raison de leur potentiel pour des applications futures, telles que la "racetrack memory". Cette thèse se concentre sur les skyrmions Néel, un type particulier de skyrmions, qui peuvent être stabilisés dans des multicouches magnétiques. En commençant par l'étude des différentes interactions magnétiques, il a été possible de définir des épaisseurs critiques qui régissent leur formation et de proposer un empilement optimisé pour les observer expérimentalement. Nous avons ensuite étudié leur nucléation, induite par impulsions de courant électrique dans divers dispositifs, par exemple grâce à l'insertion de constrictions sur les bords des pistes. Le cas des skyrmions dans des multicouches formant un antiferromagnétique synthétique, soit deux skyrmions de polarité opposée couplés ensemble, est également abordé par le biais d'une ingénierie des différentes épaisseurs des matériaux conduisant à leur stabilisation dans des multicouches à anisotropie variable. Dans le but de les intégrer dans des technologies futures, leur dynamique induite par courant électrique a aussi été étudiée avec l'optimisation des couples de spin afin d'améliorer leur mobilité. Cette étude a conduit à des systèmes dans lesquels les skyrmions sont plus rapides que les parois de domaine, ce qui est intéressant dans le but d'explorer comment ces deux objets interagissent l'un avec l'autre. Ce résultat est notamment attribué à la présence d'un fort couple field-like. Enfin, la question de leur fiabilité est abordée avec l'observation de leur déplacement sur plusieurs dizaines de micromètres et avec une discussion sur l'utilisation des mécanismes de répulsions skyrmion-skyrmion et skyrmion-paroi (de domaines) pour améliorer leur dynamique. Le déplacement des skyrmions le long de domaines magnétiques ouvre notamment de nouvelles possibilités concernant le contrôle de leur trajectoire et pourrait conduire à de nouvelles perspectives concernant les dispositifs visant à utiliser les skyrmions pour faire des portes logiquesMagnetic skyrmions are localized spin textures which stabilize thanks to the Dzyaloshinskii-Moriya interaction. This chiral interaction promotes a twisting of the magnetization, which grants skyrmions interesting properties. For example, they behave as particles and are topologically different from the uniform magnetization state. Since their first observations in 2011, skyrmions have been extensively studied because of their promise for future applications e.g. racetrack memory, neuromorphic computer, etc. This thesis is focused on Néel skyrmions, a peculiar type of skyrmions, which can be stabilized in various magnetic multilayers. Starting from the investigation of the different magnetic interactions present in such systems, it has been possible to define critical thicknesses which governs the formation of chiral textures and to propose optimized stacking to investigate experimentally their nucleation in thin films. This analysis is followed by their current induced formation in patterned devices, an important milestone for the potential applications mentioned before. As an example, through the insertion of notches at the edges of devices, a deterministic control of the nucleation events is achieved. The case of antiferromagnetic skyrmions, which are basically two skyrmions with opposite polarity coupled together, is also addressed through an engineering of the different materials thicknesses leading to their stabilization in multilayers with varying anisotropies along the vertical axis. With the goal to implement them in actual technologies, their current induced dynamics have been investigated through an optimization of the spin orbit torques, to enhance the skyrmion's mobility, for example through the use of Pt (below the magnetic layer) and Ta (above the magnetic layer). This study also leads to systems in which skyrmions are faster than domain walls, which is interesting to explore how both textures interact one with the other. This result is attributed to the presence of an unexpected Rashba mechanism at the Co|Al interface, which increases the field like component of the torque and slows down the domain walls. Finally, the question of their reliability is discussed with observations of their motion on tens of micrometres distances and with a presentation of the use of skyrmion-skyrmion and skyrmion-domains repulsions to improve their dynamics. This last result, the motion of skyrmions along magnetic domains, opens new possibilities concerning the control of their trajectory and could lead to new prospects for skyrmion logic devices
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Battistel, Orildo Luis. "Fenomenologia hadrônica no modelo de Skyrme." Universidade de São Paulo, 1995. http://www.teses.usp.br/teses/disponiveis/43/43132/tde-03102012-153429/.
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Neste trabalho estudamos aspecto estruturais de modelos onde os bárions são tratados como sólitons quirais, tais como o proposto pó Skyrme e variantes, contendo um termo estabilizador de sexta ordem, proporcional à corrente bariônica. Modelos deste tipo têm sido bastante estudados na literatura e suas predições para as propriedades estásticas do núcleon, sistematicamente, se mantêm por volta de 2/3 dos respectivos valores experimentais. Em geral, uma dada versão do modelo envolve apenas dois ou três parâmetros, mas pode dar origem a mais de uma dezena de previsões, e esta proliferação de números torna difícil a sua avaliação. Por isso, neste trabalho investigamos os vínculos estruturais ou numéricos entre as previsões do modelo, de modo a tornar mais objetiva a sua comparação com a experimentação. Todos os modelos considerados têm a mesma parte de longo alcance. Assim, a busca de padrões é feita considerando-se tanto versões diferentes das lagrangianas de curto alcance, vários valores para os parâmetros, e campos do píon que se transformam segundo representações não usuais de SU(2) X SU(2). Dessas várias possibilidades emerge um número muito grande de resultados numéricos que, depois de organizados, mostram regularidade. Dessas regularidades decorre uma proposta para a análise dos dados experimentais. Finalmente, este trabalho também inclui um cálculo alternativo do fator de forma píon-núcleon, a partir da interação NN do modelo de Skyrme.In this work we study structural features of models where baryons are treated as chiral solitons, such as that proposed by Skyrme and variants, containing a sixth order stabilizing term, proportional to the baryonic current. Models of this kind have been widely considered in the literature and their predictions for nucleon static properties are systematically around 2/3 of the corresponding experimental values. In general a given version of the model contains only two or three parameters, but may hield more than ten observable predictions. This makes its assessment rather difficult. Therefore in this work we investigate structural and numerical constraints between the predictions of the model, so as to render the comparision with experimental more objective. All the models considered here contain the same long range parl. Hence the search of patterns is clone by considering different versions for the short range lagrangian. Various values for the free parameters and pion fields which transform according to unusual representations of SU(2) x SU(2) . A rather large amount of numerical results emerge form these various possibilities, which display regularities after being organized. These patterns motivate a proposal for the analysis of experimental information. Finally, this work also includes an altenative calculation for the pion-nucleon form factor, starting from the NN interaction in the Skyrme model.
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Collins, Jonathan. "The scattering of baby Skyrmions off potential obstructions, in a Landau-Lifshitz model." Thesis, Durham University, 2009. http://etheses.dur.ac.uk/2074/.
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The dynamics of baby skyrmions of the (2+1) new baby Skyrme model, in a Landau - Lifshitz model, was studied in the presence of various potential obstructions of varying geometries. The potential obstructions were created by introducing a localised inhomogeneity in the new baby Skyrme model's potential coefficient. The size and shape of the potential obstruction was varied and two systems were investigated, namely the symmetric and asymmetric systems. In the symmetric system the trajectory of the baby skyrmions, as they traverse the barrier, was deformed from the normal circular trajectory, during which time the skyrmions sped up. For critical values of the barrier height, the baby skyrmions no longer formed a bound state and were free to separate. In the case of a potential hole, the baby skyrmions no longer formed a bound state and moved asymptotically along the edge of the hole. In the asymmetric barrier system the baby skyrmions behaved the same as the skyrmions of the symmetric obstructions. Away from the obstruction the baby skyrmions orbited the boundary of the system. In the potential hole system the bound skyrmions moved along the edge of the hole. For critical values of the hole depth, the bound state between the skyrmions was broken, resulting in one of the skyrmions remaining stationary and the other traversing the edge of the hole. During our investigations into this system it was found that the definition of the angular momentum must be modified to ensure overall conservation. It was shown how these modifications arise and how they are calculated.
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Harbour, Louis. "Multi-skyrmions quasi-BPS et noyaux atomiques : énergie de Coulomb et configurations pleines." Thesis, Université Laval, 2012. http://www.theses.ulaval.ca/2012/29536/29536.pdf.
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Garnier, Maxime. "Les textures magnétiques pour la supraconductivité topologique Topological superconductivity with deformable magnetic skyrmions." Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPASP027.
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Dans le cadre des recherches sur la supraconductivité topologique et le fermion de Majorana, on s'intéresse à différents aspects de systèmes composés d'une texture magnétique d'une part et d'un supraconducteur conventionnel d'autre part. En particulier, en deux dimensions d'espace, on analyse la situation où les électrons du supraconducteur sont couplés à un skyrmion magnétique, une texture au cœur de nombreux développements récents en spintronique. Dans la lignée de travaux récents, on montre que cette texture permet de générer une phase topologique tout en affinant la compréhension de ses caractéristiques et en analysant sa robustesse à certaines déformations géométriques. Ces résultats suggèrent de renforcer la compréhension de ces systèmes en analysant la question des effets orbitaux induits par le champ magnétique créé par le skyrmion. Même si la question initiale de la possible nucléation de vortex supraconducteur est restée sans réponse, on a confirmé la stabilité de la phase topologique aux effets orbitaux. Dans cette perspective, on s'intéresse finalement aux conséquences des excitations magnétiques sur les propriétés des excitations topologiques pour des systèmes magnétiques plus simples que les skyrmionsIn the context of the search for topological superconductivity and the Majorana fermion, we analyze several aspects of magnetic textures coupled to conventional superconductors. In particular, we focus on the case where the texture is a magnetic skyrmion which is by itself much studied in the field of spintronics. In the spirit of recent work, we show that this texture can generate a topological superconducting phase. We further investigate its properties and more particularly its stability with respect to geometrical deformations. These results call for a deeper understanding of the physics of these systems. To that end, we next focus on the orbital effects induced by the skyrmion-generated magnetic field. Despite the fact that the original question of vortex nucleation remains unanswered, we show that the topological phase is stable upon coupling to the skymion's magnetic vector potential. Motivated by these results, we finally investigate the effects of magnetic excitations of simpler magnetic systems on the topological excitations
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Ahmed, Adam Saied. "Skyrmions and Novel Spin Textures in FeGe Thin Films and Artificial B20 Heterostructures." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492686407034025.
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Schroers, Bernd Johannes. "Quantised soliton interactions." Thesis, University of Cambridge, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.260578.
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Bailly-Reyre, Aurélien. "Spins mobiles sur réseau comme modèle pour cristaux liquides et excitations topologiques et skyrmions." Thesis, Cergy-Pontoise, 2018. http://www.theses.fr/2018CERG0956/document.
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Dans cette thèse, nous nous sommes intéressés initialement aux transitions de phase qui ont lieu dans les cristaux liquides (CL), d'un point de vue théorique et numérique. En effet, les résultats présentés ici découlent de simulations numériques Monte Carlo (MC) et de développements analytiques basés sur des modèles de physique statistique et de matière condensée. Une forte analogie existe entre les systèmes de spins et les CL.Par exemples, ces derniers présentent des phases où les molécules sont toutes alignées dans le même sens comparables aux spins dans les matériaux ferromagnétiques. D'autres phases, dites cholesteriques, sont caractérisées par un arrangement moléculaire ressemblant beaucoup aux structures helimagnétiques. Mais les CL étant un état de la matière intermédiaire, situé entre le solide et le liquide, il est nécessaire de tenir compte des mouvements des molécules dans les modèles et d'adapter en conséquence l'algorithme de MC utilisé.Après une courte introduction sur les CL et leurs applications dans le premier chapitre, le second chapitre est longuement dédié aux méthodes MC et à l'adaptation de l'algorithme de Metropolis-Hastings afin d'introduire la mobilité des molécules.Le chapitre III est en quelque sorte un cas test pour simuler les CL. On considère un ensemble de molécules sur réseau et dont le nombre est inférieur au nombre de sites du réseau. L'interaction entre spins de plus proches voisins est de type Potts. L'état de plus basse énergie correspond au cas où tous les spins sont tassés au fond de la cuve.Ce système est d'abord traité par une étude de champ moyen dont les résultats sont confirmés par les simulations. Il apparaît que les couches de surface subissent une fusion et que le cœur du solide restant subit une transition du premier ordre.Le quatrième chapitre est consacré à des structures topologiques particulières que sont les skyrmions et des structures en bandes que l'on peut trouver dans les CL. A l'aide d'interactions Dzyaloszhinski-Moria (DM) D en présence d'une interaction d'échange J dans des films minces, nous étudions dans un premier temps les excitations des ondes de spin, également appelées magnons, qui sont le résultat d'une excitation collective de spins. Grâce aux fonctions de Green, nous calculons le spectre d'ondes de spin permettant ainsi de déterminer les propriétés à T = 0 et à température finie. Dans un deuxième temps, nous appliquons un champ magnétique H orthogonal au film mince faisant apparaître un cristal de skyrmions. En utilisant des simulations MC, nous montrons des vortex pour lesquels chaque centre peut être considéré comme le nœud d'une super-structure. Nous parlons alors de cristal de skyrmions. Selon les valeurs de D/H les simulations peuvent montrer également une structure semblable à celle que l'on trouve dans certains CL.Le chapitre suivant est consacré à l'étude de la dynamique conduisant à la formation des phases nématique et smectique à l'aide d'un modèle mobile de Potts. Nous observons ici les mécanismes qui se produisent pour former un cristal liquide nématique ou smectique lors du refroidissement à partir d'une phase isotrope. Le choix des interactions est crucial pour modéliser ces deux phases.Dans le chapitre VI, nous traitons de l'interaction dipolaire dans les nano dots avec un modèle de spin d'Heisenberg. La première partie du chapitre est consacrée à l'état fondamental présentant un vortex autour du centre du dott. Les spins sont coplanaires au plan du dot sauf à proximité du centre du dot où ils ont une composante z non nulle. Nous étudions ensuite l'effet de la température et la fusion du dot. La température de fusion du dot ne dépend pas de la taille du système, ce qui est très différent de ce qui se passe dans le cas des spins localisés. Ce chapitre n'est pas directement lié aux CL, mais est le premier pas dans la construction d'un modèle plus complet pour décrire le mécanisme conduisant aux phases cholestériquesIn this thesis, we are initially interested in the phase transitions that take place in liquid crystals (LC), from a theoretical and numerical point of view. Indeed, the results presented here are derived from Monte Carlo (MC) simulations and analytical developments based on statistical physics and condensed matter models. A strong analogy exists between spin systems and LC. For example, the latter have phases where the molecules are all aligned in the same direction (orientational order) comparable to spins in ferromagnetic materials. Other phases, called cholesteric, are characterized by a molecular arrangement very similar to the helimagnetic structures. But LC being an intermediate state of matter, between the solid and the liquid phase, it is necessary to take into account the motions of the molecules in the models and to adapt accordingly the MC algorithm.After a short and general introduction on LC and their applications in the first chapter, the second chapter is devoted to MC methods and the adaptation of the Metropolis-Hastings algorithm in order to introduce the mobility of molecules in our systems.Chapter III is a test case to simulate LC. We consider a set of molecules on a lattice. The number of molecules is smaller than the number of the lattice sites to allow for a molecule mobility between sites. The interaction between nearest neighbouring spins is supposed to be a Potts model. The lowest energy state corresponds to the case where all the spins are packed at the bottom of the tank. This solid ground state becomes a liquid at high temperatures.This system is first treated with a mean-field analysis whose results are confirmed by the MC simulations. It appears that the surface layers undergo a melting and that the core of the remaining solid undergoes a first-order phase transition.The following chapter is devoted to particularly topological structures which are skyrmions and stripe structures. These structures are often observed in LC. We use a Dzyaloshinski-Moria (DM) interaction of strength D in addition to an exchange interaction J to study properties of thin films. In a first part of the chapter, we study the spin-wave excitations, also termed magnons, that are the result of a collective excitation of spins. Using the Green's function, we calculate the spin-wave spectrum which is used next to determine properties at T=0 and at finite temperatures. In the second part of the chapter, we apply a magnetic field H orthogonal to the thin film making appear a crystal of skyrmions. Using MC simulations, we show that skyrmions arranged on a super-structure of a triangular geometry. Depending on the value of D/H, these simulations also show a labyrinth-like structure very close to the filament-shaped structures found in certain LC.The next chapter is devoted to the study of the dynamics leading to the formation of the nematic and smectic phases using a mobile Potts model. We observe here how the nematic and smectic LC are dynamically formed upon cooling from the isotropic phase. The choice of the interactions is crucial to model these two phases.In the chapter VI, we deal with the dipolar interaction in nanodots using the Heisenberg spin model. The first part of the chapter is devoted to the determination of the ground state exhibiting a vortex around the center of the dot. The spins lie in the xy plane at the border of the dot but go out of the xy plane at the dot center to give rise to a non-zero z component. We then study the effect of the temperature and the melting of the dot. The melting temperature of the dot do not depend on the size of the system. This is very different with the case of localised spins where the transition temperature increases with increasing the film thickness. This chapter is not directly related to LC. It was the first step towards a more complicated model describing the mechanism leading to cholesteric LC phases
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Camosi, Lorenzo. "Solitons magnétiques topologiques dans des couches minces epitaxiées à symétrie réduite." Thesis, Université Grenoble Alpes (ComUE), 2018. http://www.theses.fr/2018GREAY012/document.
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Dans cette thèse, j'ai étudié la relation entre la symétrie cristalline, la symétrie des interactions magnétiques et des soliton topologiques dans des couches minces magnétiques épitaxiées. Le cas particulier de couches avec une symétrie C2v a été considéré. Ces couches ont un intérêt particulier par leurs propriétés anisotropes qui permettent une stabilisation de solitons magnétiques avec différentes symétries et nombres topologiques. J'ai utilisé des approches théoriques et expérimentaux pour étudier ce phénomène :Approche micromagnétique :La relation entre les formulations atomistes et micromagnétiques des interactions magnétiques a été étudiée en fonction de la symétrie cristalline. Ceci a permis d'expliquer la présence des interactions anisotropes et d'étudier leur effet sur la configuration des solitons magnétiques 1D et 2D.La discussion commence par le plus simple soliton 1D, la paroi des domaines, et pas par pas des nouvelles interactions et symétries sont ajoutées afin de caractériser les conditions de stabilité et les propriétés des solitons 2D, les skyrmion et anti-skyrmions.Notre méthode a permis d'étudier les solitons topologiques 2D sur une large gamme de paramètres, et de construire un diagramme de phase en fonction de l'interaction Dzyaloshinskii-Moriya (DMI) et du champ magnétique appliqué. Trois types de solitons topologiques 2D ont été identifiées (skyrmions, bulles skyrmioniques et skyrmions supercritiques) en fonction de leur taille et leur réponse à un champ magnétique externe.On a aussi montré qu'une inversion du signe de la DMI selon deux directions perpendiculaires permet la stabilisation d'anti-skyrmions. Un modèle micromagnétique a été développé pour étudier la différence de configuration et d'énergie entre skyrmions et anti-skyrmions. On montre que l'interaction dipolaire rompt la symétrie circulaire de l'anti-skyrmion et le rend plus stable que le skyrmion.Approche expérimentale :J'ai préparé différentes couches magnétiques épitaxiées de symétrie C2v. Pour chaque système, je décris les paramètres de croissance et la symétrie cristalline, suivi par les résultats des caractérisations magnétiques et finalement les résultats de microscopie magnétique.J'ai étudié la symétrie et l'intensité de la DMI dans une tricouche Au/Co/W à aimantation perpendiculaire. La DMI dans ce système induit une chiralité horaire de la modulation de spin avec une forte anisotropie de l'intensité de la DMI, venant de la symétrie C2v. Des skyrmions dans ce système devraient avoir une forme elliptique. Nous avons stabilisé des skyrmions dans des films continus et dans des nanostructures. Leur configuration magnétique a été étudiée par XMCD-PEEM et MFM, mais sans observer des propriétés anisotropes.Pour augmenter l'effet des interactions anisotropes sur la configuration des skyrmions, j'ai développé le système W/Co/Au-Pt(solution solide). Des études par microscopie ont montré la stabilisation des bandes magnétiques parallèles à l'axe facile dans le plan dans ce système. Des études par microscopie Kerr ont montré que l'origine de cette configuration en bandes parallèles est une forte anisotropie de la dynamique du mouvement des parois.Des mesures MFM en champ magnétique statique ont été effectuées afin de confiner des bulles skyrmioniques elliptiques, mais la sensibilité de ces mesures à des couches ultrafines a été insuffisante pour caractériser leurs propriétés anisotropes.Des mesures XMCD-PEEM ont permis d'observer la structure interne de parois selon l'axe planaire difficile du système. Ces mesures mettent en évidence un composant Néel de la paroi.Finalement, j'ai préparé et étudié un système W/Fe/Co/Au avec le but de stabiliser des anti-skyrmions. Cependant, le système n'a pas montré l'aimantation hors-du-plan qui est nécessaire pour stabiliser ces solitons. Ce signifie que l'anisotropie planaire de l'interface W/Fe domine l'anisotropie perpendiculaire de l'interface Co/AuIn this thesis I studied the relationship between the crystal symmetry, the symmetry of the magnetic interactions and topological solitons in epitaxial magnetic thin films. The case of thin films with C2v symmetry has been considered. These systems are particularly interesting for the anisotropic properties that allow stabilising magnetic solitons with different symmetries and topology. I used theoretical and experimental approaches to investigate this phenomenon:Micromagnetic approach:The relationship between the atomistic and the micromagnetic formulations of magnetic interactions was studied as a function of the crystal symmetry.This allowed to explain the presence of anisotropicinteractions and study their effect on the configurations of 1D and 2D magnetic solitons. The discussion starts from the simplest 1D soliton, the domain wall, and step-by-step new interactions and symmetries are added in order to characterize the stability conditions and the properties of 2D solitons, skyrmions and anti-skyrmions. Our method allowed to study 2D topological solitons over a wide range of parameters and build a phase diagram as a function of the Dzyaloshinskii-Moriya interaction (DMI) strength and magnetic field intensity. This allowed us to distinguish three kinds of 2D topological solitons (skyrmions, skyrmionic bubbles and supercritical skyrmions) as a function of their size and response to an external magnetic field. We show that an inversion of DMI strength along perpendicular directions allows the stabilisation of anti-skyrmions. A micromagnetic model is developed to study the configuration and energy differences between skyrmions and anti-skyrmions. This shows that the dipolar interaction breaks the circular symmetry of the antiskyrmion and makes it more stable than the skyrmion.Experimental approach:Epitaxial magnetic systems with C2v symmetry have been grown. For each system I describe the growth parameters and crystal symmetry, followed by the results of the magnetic characterisation and finally the results from the magnetic microscopy measurements.I have investigated the DMI symmetry and strength in an out-of-plane magnetised epitaxial Au/Co/W trilayer. The DMI in this system promotes a clockwise chirality of the spin modulation with a strong anisotropy in the DMI strength. This anisotropy arises from the C2v symmetry of the Co/W stack.Skyrmions in this system should have an elliptical shape. We stabilised skyrmions in continuous films and in nanopatterned structures. Their magnetic configurations have been displayed with different microscopic techniques, XMCD-PEEM and MFM, without identifying anisotropic properties.We designed the W/Co/Au-Pt (solid solution) system to increase the effect of the anisotropic interactions on the skyrmion configuration. Microscopy studies in naturally demagnetised areas show that stripe domains parallel to the in-plane easy axis are stable in this system. The configuration with a larger periodicity has been found even for thinner Co layer after demagnetisation with a magnetic field. Kerr microscopy studies of the DW dynamics allowed to evidence the origin of this magnetic configuration, which arises from a strong anisotropy in the DW motion.MFM measurements with the application of a static magnetic field have been performed in order to confine elliptical skyrmionic bubbles but the reduced sensitivity of this technique to thin magnetic systems did not allow to display and characterise them. XMCD-PEEM measurements allowed to display the internal structure of the DWs along the in-plane hard axis of the system. They show the presence of a Néel DW component. Finally I have grown and studied a W/Fe/Co/Au system where anti-skyrmions may in principle be stabilised. However, the system did not show the out-of-plane magnetisation which is fundamental for the stabilisation of skyrmions. This means that the W/Fe in-plane anisotropy dominates the Co/Au out-of-plane anisotropy
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Gross, Isabell. "Exploring non-collinear spin structures in thin magnetic films with Nitrogen-Vacancy Scanning magnetometry." Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLN064/document.
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Les films magnétiques ultra-minces font partie intégrante des technologies d'aujourd'hui, comme l'illustre leur omniprésence dans de nombreuses applications courantes telles que les disques durs. A cause de leurs dimensions réduites, les propriétés magnétiques spécifiques à ces échelles conduisent à la formation de structures de spin exotiques et de taille nanométrique. Pour explorer ces matériaux en détail, nous utilisons un magnétomètre à balayage développé dans notre laboratoire et qui est basé sur un défaut de spin unique dans le diamant. Ce capteur non-invasif peut mesurer à l'échelle nanométrique à la fois le champ magnétique et la topographie, et fonctionne aux conditions ambiantes. En développant une méthode d'évaluation originale du champ magnétique, nous déterminons la structure interne de parois de domaines ferromagnétiques et quantifions la force de l'interaction Dzyaloshinskii-Moriya dans des hétérostructures à couches minces. Ensuite, nous mettons en évidence le rôle clé du désordre et de l'histoire magnétique sur la stabilisation des skyrmions dans un échantillon de bicouche magnétique. Enfin, nous visualisons dans l'espace réel une spirale de spin de 70 nm de période dans le matériau multiferroïque BiFeO3 et nous manipulons sa direction de propagation avec des champs électriques. Les connaissances tirées de ces études aideront à exploiter au maximum les capacités des matériaux magnétiques à couche ultra-mince et à les mettre en œuvre dans de nouveaux dispositifs de spintroniqueThin film magnetic materials are an integral part of today’s technology and widespread applications like the magnetic hard drive disk mirror their potential. Due to their reduced dimensions, size-specific magnetic properties induce the formation of nanoscale, exotic spin structures. To explore such materials in detail, we utilize a home-built nitrogen vacancy scanning magnetometer, based on a single defect in diamond. This non-perturbative probe combines nanoscale magnetic field- and spatial resolution and works under ambient conditions. We develop a new way to determine the inner structure of magnetic domain walls and quantify the strength of the Dzyaloshinskii-Moriya interaction in thin film heterostructures. We reveal the key role of disorder and magnetic history on the stabilization of skyrmions in a magnetic bilayer sample. Finally, we reveal the 70nm-pitch spin spiral in the multiferroic bismuth ferrite in real space and manipulate its propagation direction with electric fields. The insight gained from these studies will help to exploit the full capacity of thin film magnetic materials for spintronic application
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Romming, Niklas [Verfasser], and Roland [Akademischer Betreuer] Wiesendanger. "Discovery and Manipulation of Individual Skyrmions in Ultrathin Magnetic Films / Niklas Romming ; Betreuer: Roland Wiesendanger." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2018. http://d-nb.info/1150401516/34.
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Müller, Jan [Verfasser], Achim [Gutachter] Rosch, Simon [Gutachter] Trebst, and Jairo [Gutachter] Sinova. "Magnetic Skyrmions and Topological Domain Walls / Jan Müller ; Gutachter: Achim Rosch, Simon Trebst, Jairo Sinova." Köln : Universitäts- und Stadtbibliothek Köln, 2018. http://d-nb.info/1155921429/34.
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Bourassa, Jérôme. "Étude de cristal de skyrmions CP(3) dans un double puits quantique en champ magnétique." Mémoire, Université de Sherbrooke, 2006. http://savoirs.usherbrooke.ca/handle/11143/4675.
Full textAbstract:
De récentes études expérimentales de temps de relaxation de spins nucléaires (T[indice inférieur 1]) dans les puits d'une hétérostructure à double puits quantique (DPQ) [1, 2] montrent une décroissance rapide de T[indice inférieur 1] en remplissage près de l'état à effet Hall quantique à [nu] = 1. Il a également été observé qu'une diminution rapide et continue de T[indice inférieur 1] survient lorsqu'un déséquilibre de charge est induit par un champ électrique [3]. Contrairement à la supposition générale, ces mesures suggèrent que le gaz d'électrons bidimensionnel (GE2D) n'est pas complètement polarisé en spin et que les excitations de plus basse énergie à [nu] = 1 renversent le pseudospin et le spin électronique. Bien que l'approximation du gradient SU(4) indique la présence de telles excitations [4], aucune étude microscopique d'un DPQ dans des conditions réalistes, incluant à la fois les degrés de liberté de spin et de pseudospin, n'avait encore été réalisée jusqu'à maintenant. Dans ce mémoire, nous élaborons uni modèle microscopique dans l'approximation de Hartree-Fock nous permettant de déterminer les paramètres d'ordre cristallins de différentes phases du GE2D dans un double puits quantique sous champ magnétique transverse. Nous effectuons tout d'abord une étude de l'état fondamental à [nu] = 1 à cohérence spontanée dans le cas général où un champ électrique créé un biais influençant la densité de charge contenue dans chacun des puits. Nous présentons ensuite une étude de l'énergie des différentes phases du GE2D à [nu] < 1 où nous montrons qu'un cristal de skyrmions CP[indice supérieur 3], excitation ayant des textures de spin et de pseudospin entrelacées, permet de minimiser l'énergie lorsque l'énergie tunnel ([delta][indice inférieur SAS]) est très faible. Nous montrons comment des excitations de spin peuvent minimiser l'énergie lorsque l'énergie tunnel est inférieure à l'énergie Zeeman ([delta][indice inférieur Z]), contrairement à la condition [delta][indice inférieur SAS] > [delta][indice inférieur Z] utilisée par les modèles en gradient. Nous montrons que la présence de skyrmions CP[indice supérieur 3] permet un passage continu entre le biméron du DPQ à l'équilibre et le skyrmion de spin du puits simple à fort biais et que la dépolarisation en spin induite par ces skyrmions CP[indice supérieur 3] permet d'expliquer le comportement de T[indice inférieur 1] observé expérimentalement.