Role of hexagonal boron nitride in protecting ferromagnetic nanostructures from oxidation

Ferromagnetic contacts are widely used to inject spin polarized currents into non-magnetic materials such as semiconductors or 2-dimensional materials like graphene. In these systems, oxidation of the ferromagnetic materials poses an intrinsic limitation on device performance. Here we investigate the role of ex-situ transferred chemical vapour deposited hexagonal boron nitride (hBN) as an oxidation barrier for nanostructured cobalt and permalloy electrodes. The chemical state of the ferromagnets was investigated using X-ray photoemission electron microscopy owing to its high sensitivity and lateral resolution. We have compared the oxide thickness formed on ferromagnetic nanostructures covered by hBN to uncovered reference structures. Our results show that hBN reduces the oxidation rate of ferromagnetic nanostructures suggesting that it could be used as an ultra-thin protection layer in future spintronic devices.Comment: 7 pages, 6 figure

Magnetic anisotropy modulation of magnetite in Fe3O4/BaTiO3(100) epitaxial structures

Temperature dependent magnetometry and transport measurements on epitaxial Fe3O4 films grown on BaTiO3(100) single crystals by molecular beam epitaxy show a series of discontinuities, that are due to changes in the magnetic anisotropy induced by strain in the different crystal phases of BaTiO3. The magnetite film is under tensile strain at room temperature, which is ascribed to the lattice expansion of BaTiO3 at the cubic to tetragonal transition, indicating that the magnetite film is relaxed at the growth temperature. From the magnetization versus temperature curves, the variation in the magnetic anisotropy is determined and compared with the magnetoelastic anisotropies. These results demonstrate the possibility of using the piezoelectric response of BaTiO3 to modulate the magnetic anisotropy of magnetite films.Comment: 4 pages, 4 figure

An application of semi-infinite programming to air pollution control

Environment issues are more then ever important in a modern society. Complying with stricter legal thresholds on pollution emissions raises an important economic issue. This talk presents some ideas in the use of optimization tools to help in the planning and control of non mobile pollution sources. We assume a Gaussian plume model where a plume rise and weather stabilities classes are considered. Three main semi-infinite programming formulations are described and numerical results are shown

Optimização e controlo da poluição atmosférica

Alguns problemas de controlo da poluição atmosférica podem ser formulados como problemas de programação semi-infinita (PSI). Estas formulações, das quais descrevemos três abordagens, permitem que as instalações fabris cumpram a legislação da poluição atmosférica enquanto que o impacto económico é minimizado. As mesmas ferramentas da programação matemática podem também ser usadas pelas autoridades competentes no sentido de verificar que os limites impostos por lei são cumpridos, através do planeamento da localização dos postos de amostragem/controlo. A primeira das formulações consiste em optimizar um determinado objectivo enquanto que o nível de poluição atmosférica é mantido abaixo de um valor de referência. A segunda consiste no cálculo da poluição atmosférica máxima atingida numa determinada região e a terceira considera um problemas de redução da poluição. Estas formulações permitem obter os melhores parâmetros de controlo e as posições onde os valores máximos da poluição são atingidos, posições essas que correspondem ao melhor posicionamento dos postos de amostragem/controlo. As abordagens propostas são ilustradas com quatro problemas académicos. As ferramentas actualmente existentes apoiam a PSI desde a modelação do problema até à sua resolução. A linguagem de modelação (SIP)AMPL foi usada para codificar os problemas propostos e o solver NSIPS foi empregue na resolução dos mesmos

Semi-infinite air pollution control problems

Semi-infinite programming (SIP) problems arise in many engineering areas. Robot trajectory planning and optimal signal sets are two fine examples. Air pollution abatement problems, which are linear SIP problems, were proposed in the seventies by Gustafson and Kortanek [Analytical properties of some multiple-source urban diffusion models, Environment and Planning 4, pp. 31- 41, 1972]. Recent available tools for non-linear SIP allow the formulation of more general air control problems, namely the optimum stack design. In the air control SIP problem an objective function is to be optimised (minimum stack height or minimum cleaning costs), where the air pollution, at ground level, is kept bellow a given threshold. A Gaussian model is used to provide estimates of air pollution in a region where mean weather conditions are assumed. In this talk, we present three formulated air pollution control problems coded in the (SIP)AMPL modelling language and numerical results obtained with the discretization method of the NSIPS solver

Optimal control of fed-batch processes with particle swarm optimization

Optimal control problems appear in several engineering fields. These problems are often described by sets of nonlinear differential and algebraic equations, usually subject to constraints in the state and control variables. Some bioprocess optimal control problems are revisited and a numerical approach to its solution is introduced. The numerical procedure used to solve the problems takes advantage of the well know modeling AMPL language, providing an external dynamic library that solve the nonlinear differential equations. The optimal control problem as generally presented belongs to the class of semi-infinite programming (SIP) problems. A transformation of the SIP problem results in a nonlinear optimization problem (NLP) that can be address by off-the-shelf optimization software. The NLP formulation results in nondifferentiable optimization problems were the global solution is mostly desirable. We apply a particle swarm optimization strategy implemented in the MLOCPSOA [13] solver. Particle swarm optimization (PSO) is a stochastic technique that mimics the social behavior of a swarm

Field-dependent anisotropic magnetoresistance and planar Hall effect in epitaxial magnetite thin films

A systematic study of the temperature and magnetic field dependence of the longitudinal and transverse resistivities of epitaxial thin films of magnetite (Fe3O4) is reported. The anisotropic magnetoresistance (AMR) and the planar Hall effect (PHE) are sensitive to the in-plane orientation of current and magnetization with respect to crystal axes in a way consistent with the cubic symmetry of the system. We also show that the AMR exhibit sign reversal as a function of temperature, and that it shows significant field dependence without saturation up to 9 T. Our results provide a unified description of the anisotropic magnetoresistance effects in epitaxial magnetite films and illustrate the need for a full determination of the resistivity tensor in crystalline systems