Adatoms in graphene as a source of current polarization: Role of the local magnetic moment

We theoretically investigate spin-resolved currents flowing in large-area graphene, with and without defects, doped with single atoms of noble metals (Cu, Ag and Au) and 3d-transition metals (Mn,Fe,Co and Ni). We show that the presence of a local magnetic moment is a necessary but not sufficient condition to have a non zero current polarization. An essential requirement is the presence of spin-split localized levels near the Fermi energy that strongly hybridize with the graphene pi-bands. We also show that a gate potential can be used to tune the energy of these localized levels, leading to an external way to control the degree of spin-polarized current without the application of a magnetic field.Comment: 7 pages, 6 figure

Magnetic ordering in GaAlAs:Mn double well structure

The magnetic order in the diluted magnetic semiconductor barrier of double AlAs/GaAs: Mn quantum well structures is investigated by Monte Carlo simulations. A confinement adapted RKKY mechanism is implemented for indirect exchange between Mn ions mediated by holes. It is shown that, depending on the barrier width and the hole concentration a ferromagnetic or a spin-glass order can be established.Comment: 3 figure

Spin-polarized transport in ferromagnetic multilayered semiconductor nanostructures

The occurrence of inhomogeneous spin-density distribution in multilayered ferromagnetic diluted magnetic semiconductor nanostructures leads to strong dependence of the spin-polarized transport properties on these systems. The spin-dependent mobility, conductivity and resistivity in (Ga,Mn)As/GaAs,(Ga,Mn)N/GaN, and (Si,Mn)/Si multilayers are calculated as a function of temperature, scaled by the average magnetization of the diluted magnetic semiconductor layers. An increase of the resistivity near the transition temperature is obtained. We observed that the spin-polarized transport properties changes strongly among the three materials.Comment: 3 pages, 4 figure

On the nature of the spin-polarized hole states in a quasi-two-dimensional GaMnAs ferromagnetic layer

A self-consistent calculation of the density of states and the spectral density function is performed in a two-dimensional spin-polarized hole system based on a multiple-scattering approximation. Using parameters corresponding to GaMnAs thin layers, a wide range of Mn concentrations and hole densities have been explored to understand the nature, localized or extended, of the spin-polarized holes at the Fermi level for several values of the average magnetization of the Mn ystem. We show that, for a certain interval of Mn and hole densities, an increase on the magnetic order of the Mn ions come together with a change of the nature of the states at the Fermi level. This fact provides a delocalization of spin-polarized extended states anti-aligned to the average Mn magnetization, and a higher spin-polarization of the hole gas. These results are consistent with the occurrence of ferromagnetism with relatively high transition temperatures observed in some thin film samples and multilayered structures of this material.Comment: 3 page

Modelo conceptual da ocorrência hidromineral do Gerês : fundamentos sobre a delimitação da área de recarga do sistema hidrotermal

A ocorrência hidromineral do Gerês localiza-se na freguesia de Vilar da Veiga, concelho de Terras de Bouro, distrito de Braga. Está associada ao acidente Gerês – Lobios, a porção intermédia de uma megaestrutura que se prolonga desde Lugo (Espanha) até Vila da Feira (Portugal), seguindo uma orientação geral N17ºE. O acidente Gerês – Lobios originou um vale de factura profundamente encaixado, que recorta o maciço granítico da Serra do Gerês. É na vertente oriental deste vale que se situam as emergências termais, aproveitando fracturas paralelas e transversais à falha principal. Tratando-se de um fluido de origem meteórica, a conceptualização do funcionamento hidrogeológico do sistema hidrotermal do Gerês pressupõe, à partida, a identificação e a delimitação da respectiva área de recarga. Com base no gradiente isotópico altimétrico regional, estima-se que a altitude média de recarga do sistema hidrotermal é de 1300 m, admitindo-se que as áreas situadas a altitudes superiores a 1100 m podem estar envolvidas na recarga do sistema. Estas áreas localizam-se predominantemente no sector Este da vale de fractura associado do acidente Gerês – Lobios. No entanto, as assinaturas isotópicas de águas superficiais e subterrâneas de áreas situadas a cotas inferiores sugerem que a área de recarga é de âmbito mais alargado, incluindo o vale de fractura do Rio Gerês. Assim, com base em critérios de natureza isotópica, geomorfológica e estrutural, admite-se que a área de recarga do sistema hidrotermal está circunscrita às bacias hidrográficas do Rio Homem e do Rio Gerês, a montante da Portela do Homem e das emergências termais, respectivamente