Finsler Geometry Modeling and Monte Carlo Study on Geometrically Confined Skyrmions in Nanodots

Using the Finsler geometry modeling (FG) technique without spontaneous magnetic anisotropy, we numerically study the stability and morphology of geometrically confined skyrmions experimentally observed in nanodots. We find a confinement effect that stabilizes skyrmions for a low external magnetic field without mechanical stresses by decreasing the diameter of the cylindrical lattice and strain effects that cause the sky and vortex to emerge under the zero magnetic field. Moreover, the obtained MC data on the morphological changes are also consistent with the reported experimental data.Comment: 4 pages, 9 figure

Schwinger boson theory of anisotropic ferromagnetic ultrathin films

Ferromagnetic thin films with magnetic single-ion anisotropies are studied within the framework of Schwinger bosonization of a quantum Heisenberg model. Two alternative bosonizations are discussed. We show that qualitatively correct results are obtained even at the mean-field level of the theory, similar to Schwinger boson results for other magnetic systems. In particular, the Mermin-Wagner theorem is satisfied: a spontaneous magnetization at finite temperatures is not found if the ground state of the anisotropic system exhibits a continuous degeneracy. We calculate the magnetization and effective anisotropies as functions of exchange interaction, magnetic anisotropies, external magnetic field, and temperature for arbitrary values of the spin quantum number. Magnetic reorientation transitions and effective anisotropies are discussed. The results obtained by Schwinger boson mean-field theory are compared with the many-body Green's function technique.Comment: 14 pages, including 7 EPS figures, minor changes, final version as publishe

Complexité, désordre et frustration

A paraître dans "Désordre et Complexité", Ed. J.C.S. Lévy, Editions Matériologiques (2017)In this paper, we show and discuss the coexistence of order and disorder at equilibrium in systems with competing interactions between particles. These particles can be electrons, atoms or molecules constituting materials, but they can be human individuals of a society. To simplify the presentation, we shall use a spin language borrowed from statistical physics. A spin represents an individual which can be a particle or a human being. The nature is rich of secrets and rules that a description using simple languages and tools cannot reveal. We have to recourse to sophisticated mathematics and theories of physics to discover a part of these mysteries. Once uncovered, a phenomenon can be explained by a simpler manner. This is what we try to do in this paper on recent phenomena discovered in frustrated systems and thin films such as partial disorder, reentrance and disorder lines by examining regions around the phase frontiers. A parallel description between physical phenomena and social issues is given.Dans cet article, j'aborde la question de la coexistence de l'ordre et du désordre due à la compétition des interactions entre les particules d'un système. Ces particules peuvent être des particules microscopiques constituantes de la matière telles que les électrons, les atomes ou molécules, mais elles peuvent aussi être des personnes humaines dans les sociétés. Pour simplifier la présentation, j'utilise les spins comme particules, mais le lecteur peut les transcrire en d'autres types d'individu. La nature est riche de secrets et de règles que les langages simples ne peuvent pas dévoiler. Des méthodes mathématiques et physiques permettent de découvrir une partie de ces mystères. Une fois dévoilé, un phénomène peut être expliqué d'une manière simple. C'est ce que je m'efforce de faire dans cet article sur des notions telles que le désordre partiel, la réentrance, les lignes de désordre, ... en examinant des régions autour des frontières des phases des systèmes frustrés et des films minces