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Supplementary material: Machine learning based classification of vector field configurations
No full textThe data set contains the supplementary material to support the paper: Machine learning based classification of vector field configurations. The dataset contains simulation files, scripts for data generation and a notebook which shows the steps undertaken to perform the study. The study shows how to cluster magnetisation vector fields into meaningful classes, based on their magnetisation configuration, using an unsupervised machine learning approach.</span
Machine learning based classification of vector field configurations
Get PDFMagnetic materials at the nanoscale are important for science and technology. A key aspect for their research and advancement is the understanding of the emerging magnetization vector field configurations within samples and devices. A systematic parameter space exploration—varying for example material parameters, temperature, or sample geometry—leads to the creation of many thousands of field configurations that need to be sighted and classified. This task is usually carried out manually, for example by looking at a visual representation of the field configurations. We report that it is possible to automate this process using an unsupervised machine learning algorithm, greatly reducing the human effort. We use a combination of convolutional auto-encoder and density-based spatial clustering of applications with noise (DBSCAN) algorithm. To evaluate the method, we create the magnetic phase diagram of a FeGe disc as a function of changing external magnetic field using computer simulation to generate the configurations. We find that the classification algorithm is accurate, fast, requires little human intervention, and compares well against the published results in the literature on the same material geometry and range of external fields. Our study shows that machine learning can be a powerful tool in the research of magnetic materials by automating the classification of magnetization field configurations
Controlling stable Bloch points with electric currents
Get PDFThe Bloch point is a point singularity in the magnetisation configuration, where the magnetisation vanishes. It can exist as an equilibrium configuration and plays an important role in many magnetisation reversal processes. In the present work, we focus on manipulating Bloch points in a system that can host stable Bloch points—a two-layer FeGe nanostrip with opposite chirality of the two layers. We drive Bloch points using spin-transfer torques and find that Bloch points can move collectively without any Hall effect and report that Bloch points are repelled from the sample boundaries and each other. We study pinning of Bloch points at wedge-shaped constrictions (notches) in the nanostrip and demonstrate that arrays of Bloch points can be moved past a series of notches in a controlled manner by applying consecutive current pulses of different strength. Finally, we simulate a T-shaped geometry and demonstrate that a Bloch point can be moved along different paths by applying current between suitable strip ends
Numerical simulation projects in micromagnetics with Jupyter
Get PDFWe report a case study where an existing materials science course was modified to include numerical simulation projects on the micromagnetic behavior of materials. The Ubermag micromagnetic simulation software package is used in order to solve problems computationally. The simulation software is controlled through the Python code in Jupyter notebooks. Our experience is that the self-paced problem-solving nature of the project work can facilitate a better in-depth exploration of the course contents. We discuss which aspects of the Ubermag and the project Jupyter ecosystem have been beneficial for the students' learning experience and which could be transferred to similar teaching activities in other subject areas
Simulation de skyrmions tridimensionnels dans géométries confinées
No full textIn non-centrosymmetric magnetic material, the Dzyaloshinskii-Moriya Interaction favors the formation of chiral magnetization structures such as helical states and skyrmions. In this thesis, the effect of three-dimensional (3D) geometric confinement on such chiral structures is investigated with finite-element micromagnetic simulations. As a first example, an exhaustive study discusses the magnetization states forming in FeGe nanospheres as a function of the particle size and the applied field. A surprisingly large multiplicity of possible magnetization states is found, which could be a useful feature for multi-state memory devices. The effect of 3D confinement is then studied in a different context, demonstrating that circular pockets in extended films can act as pinning sites for skyrmions. Finally, it is shown that geometric confinement effects can also be used to guide electrically driven skyrmions in shift-register type devices, thereby preventing unwanted lateral deflections.Dans les matériaux magnétiques non centrosymétriques, l'interaction Dzyaloshinskii-Moriya favorise les structures d'aimantation chirales telles que les états hélicoïdaux et les skyrmions. Cette thèse étudie l’effet du confinement géométrique tridimensionnel (3D) sur de telles structures en utilisant des simulations micromagnétiques par éléments finis. Une étude exhaustive examine les états d’aimantation se formant dans les nanosphères de FeGe en fonction de leur taille et du champ appliqué. On trouve une grande multiplicité d'états d’aimantation, ce qui pourrait être utile pour les dispositifs de mémoire multi-états. Il est ensuite démontré que des poches circulaires dans des couches étendues peuvent, grâce à l'effet du confinement 3D, agir comme sites de piégeage pour les skyrmions. Enfin, il est montré que des effets de confinement peuvent guider des skyrmions déplacés électriquement dans des dispositifs de type registre à décalage, évitant ainsi des déflexions Indésirables
Three-Dimensional Chiral Magnetization Structures in FeGe Nanospheres
No full text9 pages, 9 figuresInternational audienceSkyrmions, spin spirals, and other chiral magnetization structures developing in materials with intrinsic Dzyaloshinsky-Moriya Interaction display unique properties that have been the subject of intense research in thin-film geometries. Here we study the formation of three-dimensional chiral magnetization structures in FeGe nanospheres by means of micromagnetic finite-element simulations. In spite of the deep sub-micron particle size, we find a surprisingly large number of distinct equilibrium states, namely, helical, meron, skyrmion, chiral-bobber and quasi-saturation state. The distribution of these states is summarized in a phase diagram displaying the ground state as a function of the external field and particle radius. This unusual multiplicity of possible magnetization states in individual nanoparticles could be a useful feature for multi-state memory devices. We also show that the magneto-dipolar interaction is almost negligible in these systems, which suggests that the particles could be arranged at high density without experiencing unwanted coupling
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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