Current-induced magnetic vortex core switching in a Permalloy nanodisk

We report on the switching of a magnetic vortex core in a sub-micron Permalloy disk, induced by a short current pulse applied in the film plane. Micromagnetic simulations including the adiabatic and non-adiabatic spin-torque terms are used to investigate the current-driven magnetization dynamics. We predict that a core reversal can be triggered by current bursts a tenth of a nanosecond long. The vortex core reversal process is found to be the same as when an external field pulse is applied. The control of a vortex core's orientation using current pulses introduces the technologically relevant possibility to address individual nanomagnets within dense arrays.Comment: 3 pages, 3 figure

Potential Implications of a Special Safeguard Mechanism in the WTO: the Case of Wheat

The Special Safeguard Mechanism (SSM) was a key issue in the July 2008 failure to reach agreement in the WTO negotiations under the Doha Development Agenda. It includes both price (P-SSM) and quantity-triggered measures (Q-SSM). This paper uses a stochastic simulation model of the world wheat market to investigate the effects of policy makers implementing policies based on the SSM rules. As expected, implementation of the Q-SSM is found to reduce imports, raise domestic prices, and boost mean domestic production in the SSM regions. However, rather than insulating countries that use it from price volatility, it would actually increase domestic price volatility in developing countries, largely by restricting imports when domestic output is low and prices high. We estimate that implementation of the Q-SSM would shrink average wheat imports by nearly 50% in some regions, with world wheat trade falling by 4.7%. The P-SSM is discriminatory against low price, developing country exporters and tends to contribute to additional producer price instability.Safeguard, SSM, WTO, volatility, wheat, food security, Agricultural and Food Policy, International Development, Q1, Q17, Q18,

Crosstalk between nanotube devices: contact and channel effects

At reduced dimensionality, Coulomb interactions play a crucial role in determining device properties. While such interactions within the same carbon nanotube have been shown to have unexpected properties, device integration and multi-nanotube devices require the consideration of inter-nanotube interactions. We present calculations of the characteristics of planar carbon nanotube transistors including interactions between semiconducting nanotubes and between semiconducting and metallic nanotubes. The results indicate that inter-tube interactions affect both the channel behavior and the contacts. For long channel devices, a separation of the order of the gate oxide thickness is necessary to eliminate inter-nanotube effects. Because of an exponential dependence of this length scale on dielectric constant, very high device densities are possible by using high-k dielectrics and embedded contacts

Spin-Transfer Torque Induced Vortex Dynamics in Fe/Ag/Fe Nanopillars

We report experimental and analytical work on spin-transfer torque induced vortex dynamics in metallic nanopillars with in-plane magnetized layers. We study nanopillars with a diameter of 150 nm, containing two Fe layers with a thickness of 15 nm and 30 nm respectively, separated by a 6 nm Ag spacer. The sample geometry is such that it allows for the formation of magnetic vortices in the Fe disks. As confirmed by micromagnetic simulations, we are able to prepare states where one magnetic layer is homogeneously magnetized while the other contains a vortex. We experimentally show that in this configuration spin-transfer torque can excite vortex dynamics and analyze their dependence on a magnetic field applied in the sample plane. The center of gyration is continuously dislocated from the disk center, and the potential changes its shape with field strength. The latter is reflected in the field dependence of the excitation frequency. In the second part we propose a novel mechanism for the excitation of the gyrotropic mode in nanopillars with a perfectly homogeneously magnetized in-plane polarizing layer. We analytically show that in this configuration the vortex can absorb energy from the spin-polarized electric current if the angular spin-transfer efficiency function is asymmetric. This effect is supported by micromagnetic simulations.Comment: The article has been sent to J. Phys. D. Submitted on August 9, 2010. (7 pages and 4 figures.

Depressive Deficits in Forgetting

The aim of this study was to investigate whether difficulties in forgetting (like difficulties in remembering) are associated with depressive states. First, dysphoric and nondysphoric students learned 40 word pairs, each consisting of a positive or negative adjective and a neutral noun (target). Next, the students practiced responding with some targets and suppressing others, when given the adjective as cue, for a varied number of repetitions. On the final test, they were told to disregard the prior instruction to suppress and to recall the target associated with every cue. Compared with nondysphoric students, dysphoric students recalled similar percentages of targets from sets assigned for response practice but higher percentages from sets assigned for suppression practice. The degree of forgetting showed some mood-congruent tendencies and was significantly correlated with self-report measures of rumination and unwanted thoughts

Collisionless energy absorption in the short-pulse intense laser-cluster interaction

In a previous Letter [Phys. Rev. Lett. 96, 123401 (2006)] we have shown by means of three-dimensional particle-in-cell simulations and a simple rigid-sphere model that nonlinear resonance absorption is the dominant collisionless absorption mechanism in the intense, short-pulse laser cluster interaction. In this paper we present a more detailed account of the matter. In particular we show that the absorption efficiency is almost independent of the laser polarization. In the rigid-sphere model, the absorbed energy increases by many orders of magnitude at a certain threshold laser intensity. The particle-in-cell results display maximum fractional absorption around the same intensity. We calculate the threshold intensity and show that it is underestimated by the common over-barrier ionization estimate.Comment: 12 pages, 13 figures, RevTeX

Calculations of three-dimensional magnetic normal modes in mesoscopic permalloy prisms with vortex structure

Static flux-closure structures in three-dimensional (3D) mesoscopic ferromagnets are known to differ quite significantly from their 2D counterparts. How these differences reflect in the dynamic properties of the magnetization is, to date, an open question. Micromagnetic simulations are employed to study the normal modes of magnetic oscillations in thick (60-80 nm) rectangular Permalloy prisms with 3D Landau-type flux-closure domain structure. Various magnetic normal modes are excited by a short field pulse and extracted using methods based on Fourier analysis. In particular, well-defined modes in the range of a few GHz are identified as oscillations of vortices, domain walls, and as excitations localized in the corners. The asymmetric Bloch wall in the center of the 3D Landau structure wall is a genuinely three-dimensional feature and thus gives rise to effects which were not reported in previous studies on 2D systems. It is argued that experimental evidence of these findings can be obtained

Ultrafast dynamics of electrons in image-potential states on clean and Xe-covered Cu(111)

Lifetimes of electrons in the n=1 and n=2 image states on Cu(111) are studied with femtosecond time-resolved photoemission. Adsorption of one monolayer of Xe results in a pronounced increase of the image-state lifetime, which for the n=1 state changes from 18±5 fs at clean Cu(111) to 75±15 fs at the Xe-covered surface. The slower relaxation rate induced by the Xe layer is attributed to a reduced overlap of the image-state wave function with bulk states. A density-matrix calculation reveals the importance of dephasing in the excitation process

Ultrafast dynamics of a magnetic antivortex - Micromagnetic simulations

The antivortex is a fundamental magnetization structure which is the topological counterpart of the well-known magnetic vortex. We study here the ultrafast dynamic behavior of an isolated antivortex in a patterned Permalloy thin-film element. Using micromagnetic simulations we predict that the antivortex response to an ultrashort external field pulse is characterized by the production of a new antivortex as well as of a temporary vortex, followed by an annihilation process. These processes are complementary to the recently reported response of a vortex and, like for the vortex, lead to the reversal of the orientation of the antivortex core region. In addition to its fundamental interest, this dynamic magnetization process could be used for the generation and propagation of spin waves for novel logical circuits.Comment: 4 pages, 4 figures. To be published in Physical Review B (R