Finite Element Formalism for Micromagnetism

The aim of this work is to present the details of the finite element approach we developed for solving the Landau-Lifschitz-Gilbert equations in order to be able to treat problems involving complex geometries. There are several possibilities to solve the complex Landau-Lifschitz-Gilbert equations numerically. Our method is based on a Galerkin-type finite element approach. We start with the dynamic Landau-Lifschitz-Gilbert equations, the associated boundary condition and the constraint on the magnetization norm. We derive the weak form required by the finite element method. This weak form is afterwards integrated on the domain of calculus. We compared the results obtained with our finite element approach with the ones obtained by a finite difference method. The results being in very good agreement, we can state that our approach is well adapted for 2D micromagnetic systems.Comment: Proceedings of conference EMF200

Fast computation of magnetostatic fields by Non-uniform Fast Fourier Transforms

The bottleneck of micromagnetic simulations is the computation of the long-ranged magnetostatic fields. This can be tackled on regular N-node grids with Fast Fourier Transforms in time N logN, whereas the geometrically more versatile finite element methods (FEM) are bounded to N^4/3 in the best case. We report the implementation of a Non-uniform Fast Fourier Transform algorithm which brings a N logN convergence to FEM, with no loss of accuracy in the results

Innovative Weak Formulation for The Landau-Lifshitz-Gilbert Equations

A non-conventional finite element formalism is proposed to solve the dynamic Landau-Lifshitz-Gilbert micromagnetic equations. Two bidimensional test problems are treated to estimate the validity and the accuracy of this finite element approachComment: 4 pages, proceedings for Intermag Madrid, May 2008 (oral contribution

Nouvelles formulations éléments finis pour le micromagnétisme et Déplacement de parois par courant polarisé en spin

The purpose of the work presented here is twofold. The first task was to provide a simulation tool based on the finite element method. Two finite element formulations were derived for the dynamic Landau-Lifshitz-Gilbert equation. The first is based on the resolution of the dynamic equation including the constraint on the magnetization, while the second uses test functions that belong to the tangent space of the magnetization. Only the second approach reproduced accurately the magnetization dynamics for several test cases. The second part of the manuscript concerns the study of magnetic domain wall displacement in systems with perpendicular magnetocrystalline anisotropy. This study required the insertion of two new terms in the Landau-Lifshitz-Gilbert equation. Both ideal and systems with different kind of defects were studied. Whenever possible, comparison with experimental results is carried out.Cette thèse comporte deux parties. L'objectif de la première partie était l'implémentation d'une méthode de résolution de l'équation dynamique de Landau-Lifshitz-Gilbert fondée sur l'approximation des éléments finis. Deux approches ont été présentées: la première reposant sur une formulation avec contrainte et la seconde mettant en œuvre des fonctions tests dans le plan tangent à l'aimantation. Seule la seconde approche reproduit la dynamique obtenue en différences finies sur des cas tests. Dans la seconde partie, le but était d'étudier le déplacement de parois de domaines magnétiques sous l'effet d'un champ magnétique ou d'un courant polarisé en spin dans des systèmes à anisotropie perpendiculaire. Il a été nécessaire d'introduire dans l'équation dynamique les termes dus au transfert de spin. Des systèmes idéaux et des systèmes avec différents types de défauts ont été étudiés. Les résultats numériques ont été comparés avec les données expérimentales disponibles

Nouvelles formulations éléments finis pour le micromagnétisme et déplacement de parois par courant polarisé en spin

Cette thèse comporte deux parties. L'objectif de la première partie était l'implémentation d'une méthode de résolution de l'équation dynamique de Landau-Lifshitz-Gilbert fondée sur l'approximation des éléments finis. Deux approches ont été présentées: la première reposant sur une formulation avec contrainte et la seconde mettant en œuvre des fonctions tests dans le plan tangent à l'aimantation. Seule la seconde approche reproduit la dynamique obtenue en différences finies sur des cas tests. Dans la seconde partie, le but était d'étudier le déplacement de parois de domaines magnétiques sous l'effet d'un champ magnétique ou d'un courant polarisé en spin dans des systèmes à anisotropie perpendiculaire. Il a été nécessaire d'introduire dans l'équation dynamique les termes dus au transfert de spin. Des systèmes idéaux et des systèmes avec différents types de défauts ont été étudiés. Les résultats numériques ont été comparés avec les données expérimentales disponibles.The purpose of the work presented here is twofold. The first task was to provide a simulation tool based on the finite element method. Two finite element formulations were derived for the dynamic Landau-Lifshitz-Gilbert equation. The first is based on the resolution of the dynamic equation including the constraint on the magnetization, while the second uses test functions that belong to the tangent space of the magnetization. Only the second approach reproduced accurately the magnetization dynamics for several test cases. The second part of the manuscript concerns the study of magnetic domain wall displacement in systems with perpendicular magnetocrystalline anisotropy. This study required the insertion of two new terms in the Landau-Lifshitz-Gilbert equation. Both ideal and systems with different kind of defects were studied. Whenever possible, comparison with experimental results is carried out.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Domain wall motion in ferromagnetic systems with perpendicular magnetization

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Transparent and Mechanically Resistant Silver-Nanowire-Based Low-Emissivity Coatings

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Effect of crystalline defects on domain wall motion under field and current in nanowires with perpendicular magnetization

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Design Optimization of Printed Multi-layered Electroactive Actuators Used for Steerable Guidewire in Micro-Invasive Surgery

International audienceTo treat cardiovascular diseases (i.e., a major cause of mortality after cancers), endovascular-technique-based guidewire has been employed for intra-arterial navigation. To date, most commercially available guidewires (e.g., Terumo, Abbott, Cordis, etc.) are non-steerable, which is poorly suited to the human arterial system with numerous bifurcations and angulations. To reach a target artery, surgeons frequently opt for several tools (guidewires with different size integrated into angulated catheters) that might provoke arterial complications such as perforation or dissection. Steerable guidewires would, therefore, be of high interest to reduce surgical morbidity and mortality for patients as well as to simplify procedure for surgeons, thereby saving time and health costs. Regarding these reasons, our research involves the development of a smart steerable guidewire using electroactive polymer (EAP) capable of bending when subjected to an input voltage. The actuation performance of the developed device is assessed through the curvature behavior (i.e., the displacement and the angle of the bending) of a cantilever beam structure, consisting of single- or multi-stack EAP printed on a substrate. Compared to the single-stack architecture, the multi-stack gives rise to a significant increase in curvature, even when subjected to a moderate control voltage. As suggested by the design framework, the intrinsic physical properties (dielectric, electrical, and mechanical) of the EAP layer, together with the nature and thickness of all materials (EAP and substrate), do have strong effect on the bending response of the device. The analyses propose a comprehensive guideline to optimize the actuator performance based on an adequate selection of the relevant materials and geometric parameters. An analytical model together with a finite element model (FEM) are investigated to validate the experimental tests. Finally, the design guideline leads to an innovative structure (composed of a 10-stack active layer screen-printed on a thin substrate) capable of generating a large range of bending angle (up to 190°) under an acceptable input level of 550 V, which perfectly matches the standard of medical tools used for cardiovascular surgery

Fast current-induced domain-wall motion controlled by the Rashba effect

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