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

Ensemble inequivalence: A formal approach

Ensemble inequivalence has been observed in several systems. In particular it has been recently shown that negative specific heat can arise in the microcanonical ensemble in the thermodynamic limit for systems with long-range interactions. We display a connection between such behaviour and a mean-field like structure of the partition function. Since short-range models cannot display this kind of behaviour, this strongly suggests that such systems are necessarily non-mean field in the sense indicated here. We further show that a broad class of systems with non-integrable interactions are indeed of mean-field type in the sense specified, so that they are expected to display ensemble inequivalence as well as the peculiar behaviour described above in the microcanonical ensemble.Comment: 4 pages, no figures, given at the NEXT2001 conference on non-extensive thermodynamic

PROJECTING WORLD FOOD DEMAND USING ALTERNATIVE DEMAND SYSTEMS

Computable General Equilibrium (CGE) models are increasingly being used to project world food markets in order to support forward-looking policy analysis. Such projections hinge critically on the underlying functional form for representing consumer demand. Simple functional forms can lead to unrealistic projections by failing to capture changes in income elasticities of demand. We adopt as our benchmark the recently introduced AIDADS demand system and compare it with several alternaive demand systems currently in widespread use in CGE models. This comparison is conducted in the context of projections for disaggregated global food demand using a global CGE model. We find that AIDADS represents a substantial improvement, particularly for the rapidly growing developing countries. For these economies, the most widely used demand systems tend to over-predict future food demands, and hence overestimate future production and import requirements for agricultural products.food demand, agricultural trade, functional form, demand system, CGE modeling, Demand and Price Analysis, D12, C68, F17, 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

Recalling in a State of Natural or Experimental Depression

In three experiments we attempted to extend the cognitive-effort account of depressive deficits in memory to naturally depressed college students. This account maintains that depression reduces attentional resources, thereby impairing performance on demanding tasks, and has received support through experimental inductions of depressed moods. Nondepressed, naturally depressed, and (in Experiment 2) experimentally depressed college students performed unannounced tests of free recall following learning tasks with two levels of difficulty and (in Experiment 2) two degrees of structure. In Experiments 1 and 2 we measured cognitive effort on those tasks via latencies on a secondary task. Latencies and subsequent recall increased with the structure and difficulty of the learning task for nondepressed and naturally depressed subjects, but these effects were reduced or absent for experimentally depressed subjects. When the secondary task was omitted (Experiment 3), naturally depressed students still recalled without a deficit. We discuss possible differences associated with the two types of depression and implications for the cognitive-effort account

Depressive Deficits in Memory: Focusing Attention Improves Subsequent Recall

58 Ss (aged up to 55 yrs) diagnosed as depressed, recovered from depression, or without a history of depression performed an unintentional learning task, followed by tests of free and forced recall. In the learning task, Ss decided whether a series of nouns sensibly completed corresponding sentence frames that varied in decision difficulty. For half of the Ss, the focus of attention was unconstrained by the demands of this task. The others, however, were required to repeat the targeted noun at the end of the trial as a means of focusing their attention on the task. Depressed Ss in the unfocused condition subsequently recalled fewer words than did both control groups, but this deficit disappeared in the focused condition. These results suggest that depression might not fundamentally impair the resources required for good performance on such tasks. The results\u27 relevance to resource-allocation, initiative, and inhibition accounts of depressive deficits in memory is discussed

Three-dimensional magnetic flux-closure patterns in mesoscopic Fe islands

We have investigated three-dimensional magnetization structures in numerous mesoscopic Fe/Mo(110) islands by means of x-ray magnetic circular dichroism combined with photoemission electron microscopy (XMCD-PEEM). The particles are epitaxial islands with an elongated hexagonal shape with length of up to 2.5 micrometer and thickness of up to 250 nm. The XMCD-PEEM studies reveal asymmetric magnetization distributions at the surface of these particles. Micromagnetic simulations are in excellent agreement with the observed magnetic structures and provide information on the internal structure of the magnetization which is not accessible in the experiment. It is shown that the magnetization is influenced mostly by the particle size and thickness rather than by the details of its shape. Hence, these hexagonal samples can be regarded as model systems for the study of the magnetization in thick, mesoscopic ferromagnets.Comment: 12 pages, 11 figure

Projecting World Food Demand: A Comparison of Alternative Demand Systems

Projections of world food demands hinge critically on the underlying functional form used to predict future demands. Simple functional forms can lead to unrealistic projections by failing to capture changes in income elasticities of demand as consumer becomes wealthier. This paper compares several demand systems in the projection of disaggregated food demand across a wide range of countries with different income levels using a global general equilibrium model. We find that the recently introduced AIDADS system represents a substantial improvement over existing demand systems currently in use in CGE modeling. In particular, our projection results show that for relatively poor regions experiencing rapid income growth, the widely used LES and CDE demand systems tend to over-predict growth in consumer demand, and hence import and output requirements for food products and under-predict that for non-food products, compared to the AIDADS system. On the other hand, for high-income regions with modest income growth, the choice of functional form is less critical.food demand, agricultural trade, functional form, demand system, CGE modeling, Demand and Price Analysis,