Interlayer exchange coupling in (Ga,Mn)As based multilayers

Exhibiting antiferromagnetic interlayer coupling in dilute magnetic semiconductor multilayers is essential for the realisation of magnetoresistances analogous to giant magnetoresistance in metallic multilayer structures. In this work we use a mean-field theory of carrier induced ferromagnetism to explore possible (Ga,Mn)As based multilayer structures that might yield antiferromagnetic coupling.Comment: 4 pages, 2 figures. To be published in physica status solidi c as the proceedings of the PASPS IV conferenc

Metastable states influence on the magnetic behavior of the triangular lattice: Application to the spin-chain compound Ca3Co2O6

It is known that the spin-chain compound Ca3Co2O6 exhibits very interesting plateaus in the magnetization as a function of the magnetic field at low temperatures. The origin of them is still controversial. In this paper we study the thermal behavior of this compound with a single-flip Monte Carlo simulation on a triangular lattice and demonstrate the decisive influence of metastable states in the splitting of the ferrimagnetic 1/3 plateau below 10 K. We consider the [Co2O6]n chains as giant magnetic moments described by large Ising spins on planar clusters with open boundary conditions. With this simple frozen-moment model we obtain stepped magnetization curves which agree quite well with the experimental results for different sweeping rates. We describe particularly the out-of-equilibrium states that split the low-temperature 1/3 plateau into three steps. They relax thermally to the 1/3 plateau, which has long-range order at the equilibrium. Such states are further analyzed with snapshots unveiling a domain-wall structure that is responsible for the observed behavior of the 1/3 plateau. A comparison is also given of the exact results in small triangular clusters with our Monte Carlo results, providing further support for our thermal description of this compound.Comment: 8 pages, 11 figures, submitted to PR

Influence of disorder on the perpendicular magnetoresistance of magnetic multilayers

The effect of disorder on the perpendicular magnetoresistance of magnetic multilayers is investigated theoretically. Various kinds of disorder are considered: (i) interface substitutional disorder and (ii) bulk disorder in the various layers and in the leads. The calculations are based upon the non-equilibrium Green's function formalism, together with the recursion method for calculating the real-space Green's function.Comment: RevTeX, 3 pages, 3 eps figures included; minor changes in v

Hot Electron Magnetotransport in a Spin-Valve Transistor at Finite temperatures

The hot electron magnetotransport in a spin-valve transistor has been theoretically explored at finite temperatures. We have explored the parallel and anti-parallel collector current changing the relative spin orientation of the ferromagnetic layers at finite temperatures. In this model calculations, hot electron energy redistribution due to spatial inhomogeneity of Schottky barrier heights and hot electron spin polarization in the ferromagnetic layer at finite temperatures have been taken into account. The results of this model calculations accord with the experimental data semi-quantitative manner. We therefore suggest that both effects remarked above should be taken into account substantially when one explores the hot electron magnetotransport in a spin-valve system transistor at finite temperatures.Comment: p pages, 3 figure

Spin transport in disordered single-wall carbon nanotubes contacted to ferromagnetic leads

Recent conductance measurements on multi-wall carbon nanotubes (CNTs) reveal an effective behavior similar to disordered single-wall CNTs. This is due to the fact that electric current flows essentially through the outermost shell and is strongly influenced by inhomogeneous electrostatic potential coming from the inner tubes. Here, we present theoretical studies of spin-dependent transport through disorder-free double-wall CNTs as well as single-wall CNTs with Anderson-type disorder. The CNTs are end-contacted to ferromagnetic electrodes modelled as fcc (111) surfaces. Our results shed additional light on the giant magnetoresistance effect in CNTs. Some reported results concern realistically long CNTs, up to several hundred nanometers.Comment: 9 pages, 5 figures, presented at the European Conference PHYSICS OF MAGNETISM 2005, Poznan, Polan

Influence of sample width on the magnetoresistance and planar Hall effect of Co/Cu multilayers

We have studied Co/Cu multilayers to understand the effect of the sample’s width on their magnetoresistance (MR). By keeping constant both the length and the thickness and exploring the widths within the range of usual magnetic domain sizes in those nanostructures, we were able to observe effects on the MR curve, as well as infer linked changes in the magnetization process. Associating MR and Planar Hall Effect (PHE) measurements, coupled to an analysis of the MR plots’ symmetry, we were able to establish that, apart from the expected antiferromagnetic coupling, reducing the width forces the magnetization to stay aligned to the current channel, thus inducing more symmetric, closer to model Giant Magnetoresistance (GMR). Also, the sample edges might contribute by adding extra coupling through magnetostatic interaction. The added effects result in a counter-intuitive trend that goes from the near ideal wide samples through intermediate sizes with fairly abrupt changes in MR, and finally to the closer to bell-shaped ideal GMR at narrow widths

Using a TV Set to Show Electron Diffraction

We show the feasibility of constructing a simple set-up to demonstrate the diffraction of electrons starting from a standard TV set. Two concentric rings around a central maxima appears on the screen of the modified TV set corresponding to atomic planes (1, 0, 0) and (1, 1, 0) of a hexagonal structure of graphite sample. A reasonable Planck’s constant value is determined through the slope of a plot of λd2 vs 1/V where we used the average Bragg wavelength for both diffraction rings for each value of accelerating voltage. Since wave particle behavior of matter is shown, we believe this simple set-up is useful for introductory physics courses at high school and undergraduate levels.We show the feasibility of constructing a simple set-up to demonstrate the diffraction of electrons starting from a standard TV set. Two concentric rings around a central maxima appears on the screen of the modified TV set corresponding to atomic planes (1, 0, 0) and (1, 1, 0) of a hexagonal structure of graphite sample. A reasonable Planck’s constant value is determined through the slope of a plot of λd2 vs 1/V where we used the average Bragg wavelength for both diffraction rings for each value of accelerating voltage. Since wave particle behavior of matter is shown, we believe this simple set-up is useful for introductory physics courses at high school and undergraduate levels

Giant magnetoresistance and extraordinary magnetoresistance in inhomogeneous semiconducting DyNiBi

The semiconducting half-Heulser compound DyNiBi shows a negative giant magnetoresistance (GMR) below 200 K. Except for a weak deviation, this magnetoresistance scales roughly with the square of the magnetization in the paramagnetic state, and is related to the metal-insulator transition. At low temperature, a positive magnetoresistance is found, which can be suppressed by high fields. The magnitude of the positive magnetoresistance changes slightly with the amount of impurity phase.Comment: 3 pages, 3 figure

Os Flintstones e o preconceito na escola

Em nosso país, que é tido pelo imaginário mundial e por si mesmo, como um paraíso de mestiçagem, a hipocrisia do não questionamento suficiente no que tange às diferenças que são muitas, tem permitido a manutenção e o agravamento do silêncio que fala a favor do preconceito. A não pluralidade na educação é apenas uma das formas de preservação do grande tapete da democracia racial que esconde a falta de democracia real. Este artigo, ao passear por entre os conceitos de alteridade e preconceito, pretende convidar o leitor à reflexão sobre a prática da negação do preconceito no âmbito da escola e suas conseqüências funestas para a manutenção do mesmo no tecido social. Com o intuito de ilustrar as considerações acerca do processo de varredura para debaixo do tapete utilizado na maioria dos espaços de socialização em nossa sociedade e não menos notadamente nas escolas, busca, mediante entrevista com alguns professores de uma escola pública do Estado do Paraná, dados para melhor diagnosticar a atitude da escola bem como a dos professores no que tange ao reconhecimento da existência à profilaxia e/ou o tratamento do preconceito. As análises indicam que a escola, mesmo a que se pretende voltada à convivência entre os diferentes, enfrenta, além dos inimigos já conhecidos, a grande e aparentemente intransponível barreira da negação que, deliberadamente ou não, promove a manutenção do estado de preconceito bem como da geração de indivíduos preconceituosos. The Flintstones and prejudice in school Abstract In our country, that is seen by the global imaginary and by itself as a paradise of the crossing of races, the hypocrisy of the not enough questioning about the differences, that are a few, has allowed for the maintenance and the aggravating of the silence that speaks in favour of prejudice. The lack of plurality in education is only one of the forms of preservation of the big carpet of the racial democracy that hides the lack of real democracy. This paper, while going through the concepts of alterity and prejudice, intends to invite the reader to reflect about the practice of the prejudice negation in the school space and its regrettable consequences to its maintenance in the social web. Aiming at illustrating the considerations about the process of sweeping to under the carpet using most of the socialising spaces in our society and, not less notably, in the schools, searches, by means of some interviews with teachers of a Public School of Paraná State, data to allow for a better diagnostic of the school attitude, as well as of the teachers in respect to the acknowledge of the existence, to the profilaxes and/or the treatment of the prejudice. The analyses indicate that the school, even when it is intended to be orientated to the coexistence of the different, faces, apart from the enemies already known, the big and apparently non transposed barrier of the negation that, deliberately or not, promotes the maintenance of the state of prejudice as well as the generation of prejudiced individuals

A discovery that changed the way we think or how science changes humanity

A partir da observação das mudanças introduzidas pela descoberta científica (no caso, a magnetorresistência gigante, de que participamos), tentamos retraçar os desenvolvimentos tecnológicos dos últimos anos. Mesmo sem apelar para conhecimento específico profundo, podemos mostrar como aconteceu a mudança em nossos hábitos por conta da mudança tecnológica e associar como se muda a humanidade a partir da ciência bem feita.Starting from the observation of the changes introduced by scientific discovery (specifically, giant magnetoresistance, that we participated), we try to retrace the technological development of late. Even without using advanced specific knowledge of the field, we can show how the changes in our everyday life happened as a result of technological revolution and understand how humanity is changed by good science