A model for vortex formation in magnetic nanodots

We use Monte Carlo simulation to study the vortex nucleation on magnetic nanodots at low temperature. In our simulations, we have considered a simple microscopic two-dimensional anisotropic Heisenberg model with term to describe the anisotropy due to the presence of the nanodot edge. We have considered the thickness of the edge, which was not considered in previous works, introducing a term that controls the energy associated to the edge. Our results clearly show that the thickness of the edge has a considerable influence in the vortex nucleation on magnetic nanodots. We have obtained the hysteresis curve for several values of the surface anisotropy and skin depth parameter ($\xi$). The results are in excellent agreement with experimental data

Separation of particles leading to decay and unlimited growth of energy in a driven stadium-like billiard

A competition between decay and growth of energy in a time-dependent stadium billiard is discussed giving emphasis in the decay of energy mechanism. A critical resonance velocity is identified for causing of separation between ensembles of high and low energy and a statistical investigation is made using ensembles of initial conditions both above and below the resonance velocity. For high initial velocity, Fermi acceleration is inherent in the system. However for low initial velocity, the resonance allies with stickiness hold the particles in a regular or quasi-regular regime near the fixed points, preventing them from exhibiting Fermi acceleration. Also, a transport analysis along the velocity axis is discussed to quantify the competition of growth and decay of energy and making use distributions of histograms of frequency, and we set that the causes of the decay of energy are due to the capture of the orbits by the resonant fixed points

Estrategias cognitivas desarrolladas por estudiantes de noveno grado para la soluci?n de problemas de combinatoria simple

65 P?ginasEsta investigaci?n tuvo como poblaci?n de estudio estudiantes de grado noveno, estudiantes que se encuentran en un rango de edad entre 13 y 15 a?os, quienes hasta el d?a de hoy no han recibido instrucci?n alguna sobre conceptos relacionados con el an?lisis combinatorio. Nuestra investigaci?n identificar? y caracterizar? las diferentes estrategias cognitivas que utilizan los estudiantes del grado 9 cuando resuelven problemas simples de combinatoria, apoyados exclusivamente en su intuici?n y su capacidad cognitiva, evidenciando c?mo los adolescentes descubren procedimientos sistem?ticos de enumeraci?n y recuentos combinatorios de manera espont?nea y sin ayuda de la instrucci?n. La investigaci?n se apoya en trabajos realizados por Carmen Batanero sobre educaci?n Estad?stica, la clasificaci?n planteada por Dubois, trabajos sobre el desarrollo cognitivo de Piaget y la Teor?a de los Campos Conceptuales de Vergnaud.ABSTRACT This research was to study population ninth year students, students who are at an age range between 13 and 15 years, who until today have not received any instruction on concepts related to combinatorial analysis. Our research will identify and characterize the different cognitive strategies used by students in grade 9 when solving simple combinatorial problems, supported solely on intuition and cognition, demonstrating how teens discover systematic procedures and combinatorial enumeration counts spontaneously without aid instruction. The research is based on trabajaos made by Carmen Batanero on education statistics, silver classification Dubois, work on the cognitive development of Piaget and theory of conceptual fields of Vergnaud.INTRODUCCI?N 14 1. PLANTEAMIENTO DEL PROBLEMA 17 1.1 ANTECEDENTES. 17 1.1.1 Razonamiento Combinatorio en Alumnos de Secundaria. . 17 1.1.2 Razonamiento Combinatorio en Estudiantes con Preparaci?n Matem?tica Avanzada18 1.1.3 Modelos Combinatorios Impl?citos y Resoluci?n de Problemas en Clase de cuarto a?o. 18 1.1.4 Conteo. Una Propuesta Did?ctica y su An?lisis. 19 1.2 PREGUNTA GENERADORA 20 2. OBJETIVOS 21 2.1 OBJETIVO GENERAL 21 2.2 OBJETIVOS ESPEC?FICOS 21 3. MARCO DE REFERENCIA22 3.1 MARCO TE?RICO 22 3.1.1 La teor?a de los campos conceptuales. 22 3.1.2 Forma operatoria y forma predicativa del conocimiento 24 3.1.3 Actividad 24 3.1.4 Jean Piaget. 26 3.1.5 Efraim Fischbein. 27 4. METODOLOG?A 31 4.1 ESTRATEGIAS UTILIZADAS EN LA RESOLUCI?N DE PROBLEMAS 31 4.1.1 Descripci?n del cuestionario 31 4.2 ESTRATEGIAS GENERALES EMPLEADAS POR LOS ALUMNOS 32 4.2.1 Problema 1 33 4.2.2 Problema 2 35 4.2.3 Problema 3 37 4.2.4 Problema 4 38 4.2.5 Problema 5. 40 4.2.6 Problema 6 42 4.2.7 Problema 7. 44 4.2.8 Problema 8 46 5. CONCLUSIONES 49 REFERENCIAS 52 ANEXOS 5

Breaking down the Fermi acceleration with inelastic collisions

The phenomenon of Fermi acceleration is addressed for a dissipative bouncing ball model with external stochastic perturbation. It is shown that the introduction of energy dissipation (inelastic collisions of the particle with the moving wall) is a sufficient condition to break down the process of Fermi acceleration. The phase transition from bounded to unbounded energy growth in the limit of vanishing dissipation is characterized.Comment: A complete list of my papers can be found in: http://www.rc.unesp.br/igce/demac/denis

Vortex behavior near a spin vacancy in 2D XY-magnets

The dynamical behavior of anisotropic two dimensional Heisenberg models is still a matter of controversy. The existence of a central peak at all temperatures and a rich structure of magnon peaks are not yet understood. It seems that the central peaks are related, in some way, to structures like vortices. In order to contribute to the discussion of the dynamical behavior of the model we use Monte Carlo and spin dynamics simulations as well analytical calculations to study the behavior of vortices in the presence of nonmagnetic impurities. Our simulations show that vortices are attracted and trapped by the impurities. Using this result we show that if we suppose that vortices are not very much disturbed by the presence of the impurities, then they work as an attractive potential to the vortices explaining the observed behavior in our simulations.Comment: 4 pages, 6 figure