935 research outputs found
Spin transfer switching of spin valve nanopillars using nanosecond pulsed currents
Spin valve nanopillars are reversed via the mechanism of spin momentum
transfer using current pulses applied perpendicular to the film plane of the
device. The applied pulses were varied in amplitude from 1.8 mA to 7.8 mA, and
varied in duration within the range of 100 ps to 200 ns. The probability of
device reversal is measured as a function of the pulse duration for each pulse
amplitude. The reciprocal pulse duration required for 95% reversal probability
is linearly related to the pulse current amplitude for currents exceeding 1.9
mA. For this device, 1.9 mA marks the crossover between dynamic reversal at
larger currents and reversal by thermal activation for smaller currents
Opportunities for emotion and mental health research in the resource-rationality framework
We discuss opportunities in applying the resource-rationality framework toward answering questions in emotion and mental health research. These opportunities rely on characterization of individual differences in cognitive strategies; an endeavor that may be at odds with the normative approach outlined in the target article. We consider ways individual differences might enter the framework and the translational opportunities offered by each
Flux flow of Abrikosov-Josephson vortices along grain boundaries in high-temperature superconductors
We show that low-angle grain boundaries (GB) in high-temperature
superconductors exhibit intermediate Abrikosov vortices with Josephson cores,
whose length along GB is smaller that the London penetration depth, but
larger than the coherence length. We found an exact solution for a periodic
vortex structure moving along GB in a magnetic field and calculated the
flux flow resistivity , and the nonlinear voltage-current
characteristics. The predicted dependence describes well our
experimental data on unirradiated and irradiated
bicrystals, from which the core size , and the intrinsic depairing
density on nanoscales of few GB dislocations were measured for the
first time. The observed temperature dependence of
indicates a significant order parameter suppression in current channels between
GB dislocation cores.Comment: 5 pages 5 figures. Phys. Rev. Lett. (accepted
Switching Distributions for Perpendicular Spin-Torque Devices within the Macrospin Approximation
We model "soft" error rates for writing (WSER) and for reading (RSER) for
perpendicular spin-torque memory devices by solving the Fokker-Planck equation
for the probability distribution of the angle that the free layer magnetization
makes with the normal to the plane of the film. We obtain: (1) an exact, closed
form, analytical expression for the zero-temperature switching time as a
function of initial angle; (2) an approximate analytical expression for the
exponential decay of the WSER as a function of the time the current is applied;
(3) comparison of the approximate analytical expression for the WSER to
numerical solutions of the Fokker-Planck equation; (4) an approximate
analytical expression for the linear increase in RSER with current applied for
reading; (5) comparison of the approximate analytical formula for the RSER to
the numerical solution of the Fokker-Planck equation; and (6) confirmation of
the accuracy of the Fokker-Planck solutions by comparison with results of
direct simulation using the single-macrospin Landau-Lifshitz-Gilbert (LLG)
equations with a random fluctuating field in the short-time regime for which
the latter is practical
Determining the minimum mass and cost of a magnetic refrigerator
An expression is determined for the mass of the magnet and magnetocaloric
material needed for a magnetic refrigerator and these are determined using
numerical modeling for both parallel plate and packed sphere bed regenerators
as function of temperature span and cooling power. As magnetocaloric material
Gd or a model material with a constant adiabatic temperature change,
representing a infinitely linearly graded refrigeration device, is used. For
the magnet a maximum figure of merit magnet or a Halbach cylinder is used. For
a cost of \$40 and \$20 per kg for the magnet and magnetocaloric material,
respectively, the cheapest 100 W parallel plate refrigerator with a temperature
span of 20 K using Gd and a Halbach magnet has 0.8 kg of magnet, 0.3 kg of Gd
and a cost of \$35. Using the constant material reduces this cost to \$25. A
packed sphere bed refrigerator with the constant material costs \$7. It is also
shown that increasing the operation frequency reduces the cost. Finally, the
lowest cost is also found as a function of the cost of the magnet and
magnetocaloric material.Comment: 12 pages, 10 figure
Adjusting magnetic nanostructures for high-performance magnetic sensors
The magnetic properties of the soft ferromagnetic layer in magnetic tunnel junctions are one of key factors to determine the performance of magnetoresistance sensors. We use a three-step orthogonal annealing procedure to modify the nanostructures of the free layer in the magnetic tunnel junction to control features such as magnetization reversal, coercivity, exchange field, and tunnel magnetoresistance ratio. We present a sensor with an improved sensitivity as high as 3944%/mT. This magnetic sensor only dissipates 200 lW of power while operating under an applied voltage of 1V
Large T1 contrast enhancement using superparamagnetic nanoparticles in ultra-low field MRI
Superparamagnetic iron oxide nanoparticles (SPIONs) are widely investigated and utilized as magnetic resonance imaging (MRI) contrast and therapy agents due to their large magnetic moments. Local field inhomogeneities caused by these high magnetic moments are used to generate T2 contrast in clinical high-field MRI, resulting in signal loss (darker contrast). Here we present strong T1 contrast enhancement (brighter contrast) from SPIONs (diameters from 11 nm to 22 nm) as observed in the ultra-low field (ULF) MRI at 0.13 mT. We have achieved a high longitudinal relaxivity for 18 nm SPION solutions, r1 = 615 s−1 mM−1, which is two orders of magnitude larger than typical commercial Gd-based T1 contrast agents operating at high fields (1.5 T and 3 T). The significantly enhanced r1 value at ultralow fields is attributed to the coupling of proton spins with SPION magnetic fluctuations (Brownian and Néel) associated with a low frequency peak in the imaginary part of AC susceptibility (χ”). SPION-based T1-weighted ULF MRI has the advantages of enhanced signal, shorter imaging times, and iron-oxidebased nontoxic biocompatible agents. This approach shows promise to become a functional imaging technique, similar to PET, where low spatial resolution is compensated for by important functional information
Electron correlation in two-photon double ionization of helium from attosecond to FEL pulses
We investigate the role of electron correlation in the two-photon double
ionization of helium for ultrashort XUV pulses with durations ranging from a
hundred attoseconds to a few femtoseconds. We perform time-dependent ab initio
calculations for pulses with mean frequencies in the so-called "sequential"
regime (photon energy above 54.4 eV). Electron correlation induced by the time
correlation between emission events manifests itself in the angular
distribution of the ejected electrons, which strongly depends on the energy
sharing between them. We show that for ultrashort pulses two-photon double
ionization probabilities scale non-uniformly with pulse duration depending on
the energy sharing between the electrons. Most interestingly we find evidence
for an interference between direct ("nonsequential") and indirect
("sequential") double photo-ionization with intermediate shake-up states, the
strength of which is controlled by the pulse duration. This observation may
provide a route toward measuring the pulse duration of FEL pulses.Comment: 9 pages, 6 figure
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