Histerese térmica de sistemas magnéticos nanoestruturados

We report a theoretical investigation of thermal hysteresis in magnetic nanoelements. Thermal hysteresis originates in the existence of meta-stable states in temperature intervals which may be tuned by small values of the external magnetic field, and are controlled by the systems geometric dimensions as well as the composition. Two systems have been investigated. The first system is a trilayer consisting of one antiferromagnetic MnF2 film, exchange coupled with two Fe lms. At low temperatures the ferromagnetic layers are oriented in opposite directions. By heating in the presence of an external magnetic field, the Zeeman energy induces a gradual orientation of the ferromagnets with the external field and the nucleation of spin- op-like states in the antiferromagnetic layer, leading eventually, in temperatures close to the Neel temperature, to full alignment of the ferromagnetic films and the formation of frustrated exchange bonds in the center of the antiferromagnetic layer. By cooling down to low temperatures, the system follows a different sequence of states, due to the anisotropy barriers of both materials. The width of the thermal hysteresis loop depends on the thicknesses of the FM and AFM layers as well as on the strength of the external field. The second system consists in Fe and Permalloy ferromagnetic nanoelements exchange coupled to a NiO uncompensated substrate. In this case the thermal hysteresis originates in the modifications of the intrinsic magnetic

Nucleação de vortices em nanoelementos elípticos

The present work reports a theoretical study of the vortex nucleation in elliptical nanoelements of iron with the dimensions of the principal and secondary axes in the range of hundreds of nanometers. It will be divided into three steps: first of all, a general panorama and a justification for the interest of the study of nanosystems and their applications. Second, a explanation about the computational simulations applied for the calculations of the remanent states after the saturation in a external field which value is grater then the exchange field of the material for nanoelements coupled or not to a antiferromagnetic substrate. Systems with that range of axes dimensions and height in the range of a few dozens of nanometers have a natural tendency to nucleate closed magnetic ux, like vortex. Third, we will emphasize the nucleation of double vortex, the main types and the dimensions in which they occur (phase diagram) and the factors that may in uence in the nucleation and control of the remanent state. We shown that we can control specially the distance between the vortex cores changing the value of the interface field. Finally, we present a expectative of continuity of this work with objectives and applications

Análise do Efeito Goos-Hanchen e Não-reciprocidade do fluxo de potência em antiferromagnéticos

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Frases magnéticas, polaritons magnéticos e modos magnetostáticos em filmes e cristais magnônicos de terras-raras

Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq

Frases magnéticas, polaritons magnéticos e modos magnetostáticos em filmes e cristais magnônicos de terras-raras

Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq

Efeito do campo elétrico em nanocones formados por aln e BN: um estudo por primeiros princípios

The development of computers and algorithms capable of making increasingly accurate and rapid calculations as well as the theoretic foundation provided by quantum mechanics has turned computer simulation into a valuable research tool. The importance of such a tool is due to its success in describing the physical and chemical properties of materials. One way of modifying the electronic properties of a given material is by applying an electric field. These effects are interesting in nanocones because their stability and geometric structure make them promising candidates for electron emission devices. In our study we calculated the first principles based on the density functional theory as implemented in the SIESTA code. We investigated aluminum nitride (AlN), boron nitride (BN) and carbon (C), subjected to external parallel electric field, perpendicular to their main axis. We discuss stability in terms of formation energy, using the chemical potential approach. We also analyze the electronic properties of these nanocones and show that in some cases the perpendicular electric field provokes a greater gap reduction when compared to the parallel field