466 research outputs found

    Controllable switching of vortex chirality in magnetic nanodisks by a field pulse

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    We propose a way of fast switching the chirality in a magnetic nanodisk by applying a field pulse. To break the symmetry with respect to clockwise or counterclockwise chirality a mask is added by which an inhomogeneous field influences the vortex state of a nanodisk. Using numerical spin--lattice simulations we demonstrate that chirality can be controllably switched by a field pulse, whose intensity is above some critical value. A mathematical definition for the chirality of an arbitrary shaped particle is proposed.Comment: REVTeX, 4 pages, 3 figure

    Simulation Studies of Nanomagnet-Based Architecture

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    We report a simulation study on interacting ensembles of Co nanomagnets that can perform basic logic operations and propagate logic signals, where the state variable is the magnetization direction. Dipole field coupling between individual nanomagnets drives the logic functionality of the ensemble and coordinated arrangements of the nanomagnets allow for the logic signal to propagate in a predictable way. Problems with the integrity of the logic signal arising from instabilities in the constituent magnetizations are solved by introducing a biaxial anisotropy term to the Gibbs magnetic free energy of each nanomagnet. The enhanced stability allows for more complex components of a logic architecture capable of random combinatorial logic, including horizontal wires, vertical wires, junctions, fanout nodes, and a novel universal logic gate. Our simulations define the focus of scaling trends in nanomagnet-based logic and provide estimates of the energy dissipation and time per nanomagnet reversal

    Role of micronutrients in HIV infection

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    More than 60% of the estimated 40 million persons with HIV/AIDS worldwide live in sub-Saharan Africa, where poverty, social insecurity, food shortages and malnutrition are major problems.1 In children under the age of 5 years, who live in developing countries, malnutrition has been associated with 50% of the 10.8 million deaths mainly caused by neonatal disorders, diarrhoea, pneumonia, malaria and HIV/AIDS.2 Likewise micronutrient deficiencies are widespread and are associated with increased morbidity and mortality particularly in relation to infectious diseases.3 This review focuss on the interaction between micronutrients and HIV/AIDS and discusses recent research findings that may have important public health implications in terms of the case management of persons with HIV/AIDS Southern African Journal of HIV Medicine Vol. 6 (2) 2005: pp. 18-2

    Vortex motion in a finite-size easy-plane ferromagnet and application to a nanodot

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    We study the motion of a non-planar vortex in a circular easy-plane ferromagnet, which imitates a magnetic nanodot. Analysis was done using numerical simulations and a new collective variable theory which includes the coupling of Goldstone-like mode with the vortex center. Without magnetic field the vortex follows a spiral orbit which we calculate. When a rotating in-plane magnetic field is included, the vortex tends to a stable limit cycle which exists in a significant range of field amplitude B and frequency ω\omega for a given system size L. For a fixed ω\omega, the radius R of the orbital motion is proportional to L while the orbital frequency Ω\Omega varies as 1/L and is significantly smaller than ω\omega. Since the limit cycle is caused by the interplay between the magnetization and the vortex motion, the internal mode is essential in the collective variable theory which then gives the correct estimate and dependency for the orbit radius R∼BL/ωR\sim B L/\omega. Using this simple theory we indicate how an ac magnetic field can be used to control vortices observed in real magnetic nanodots.Comment: 15 pages (RevTeX), 14 figures (eps

    Geometric Aspects of the Dipolar Interaction in Lattices of Small Particles

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    The hysteresis curves of systems composed of small interacting magnetic particles, regularly placed on stacked layers, are obtained with Monte Carlo simulations. The remanence as a function of temperature, in interacting systems, presents a peak that separates two different magnetic states. At low temperatures, small values of remanence are a consequence of antiferromagnetic order due to the dipolar interaction. At higher values of temperature the increase of the component normal to the lattice plane is responsible for the small values of remanence. The effect of the number of layers, coordination number and distance between particles are investigated.Comment: 5 pages, 7 figure

    Effective anisotropy of thin nanomagnets: beyond the surface anisotropy approach

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    We study the effective anisotropy induced in thin nanomagnets by the nonlocal demagnetization field (dipole-dipole interaction). Assuming a magnetization independent of the thickness coordinate, we reduce the energy to an inhomogeneneous onsite anisotropy. Vortex solutions exist and are ground states for this model. We illustrate our approach for a disk and a square geometry. In particular, we obtain good agreement between spin-lattice simulations with this effective anisotropy and micromagnetic simulations.Comment: ReVTeX, 14 pages, 6 figure

    String Method for the Study of Rare Events

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    We present a new and efficient method for computing the transition pathways, free energy barriers, and transition rates in complex systems with relatively smooth energy landscapes. The method proceeds by evolving strings, i.e. smooth curves with intrinsic parametrization whose dynamics takes them to the most probable transition path between two metastable regions in the configuration space. Free energy barriers and transition rates can then be determined by standard umbrella sampling technique around the string. Applications to Lennard-Jones cluster rearrangement and thermally induced switching of a magnetic film are presented.Comment: 4 pages, 4 figure
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