30,068 research outputs found
Spin-torque driven magnetic vortex self-oscillations in perpendicular magnetic fields
We have employed complete micromagnetic simulations to analyze dc current
driven self-oscillations of a vortex core in a spin-valve nanopillar in a
perpendicular field by including the coupled effect of the spin-torque and the
magnetostatic field computed self-consistently for the entire spin-valve. The
vortex in the thicker nanomagnet moves along a quasi-elliptical trajectory that
expands with applied current, resulting in blue-shifting of the frequency,
while the magnetization of the thinner nanomagnet is non-uniform due to the
bias current. The simulations explain the experimental magnetoresistance-field
hysteresis loop and yield good agreement with the measured frequency vs.
current behavior of this spin-torque vortex oscillator.Comment: 10 pages, 3 figures, to be appear on AP
Micromagnetic simulations of persistent oscillatory modes excited by spin-polarized current in nanoscale exchange-biased spin valves
We perform 3D micromagnetic simulations of current-driven magnetization
dynamics in nanoscale exchange biased spin-valves that take account of (i) back
action of spin-transfer torque on the pinned layer, (ii) non-linear damping and
(iii) random thermal torques. Our simulations demonstrate that all these
factors significantly impact the current-driven dynamics and lead to a better
agreement between theoretical predictions and experimental results. In
particular, we observe that, at a non-zero temperature and a sub-critical
current, the magnetization dynamics exhibits nonstationary behaviour in which
two independent persistent oscillatory modes are excited which compete for the
angular momentum supplied by spin-polarized current. Our results show that this
multi-mode behaviour can be induced by combined action of thermal and spin
transfer torques.Comment: 7pages, 2 figures, submitted JAP via MMM 200
Macroclumping as solution of the discrepancy between H{\alpha} and P v mass loss diagnostics for O-type stars
Recent studies of O-type stars demonstrated that discrepant mass-loss rates
are obtained when different diagnostic methods are employed - fitting the
unsaturated UV resonance lines (e.g. P v) gives drastically lower values than
obtained from the H{\alpha} emission. Wind clumping may be the main cause for
this discrepancy. In a previous paper, we have presented 3-D Monte-Carlo
calculations for the formation of scattering lines in a clumped stellar wind.
In the present paper we select five O-type supergiants (from O4 to O7) and test
whether the reported discrepancies can be resolved this way. In the first step,
the analyses start with simulating the observed spectra with Potsdam Wolf-Rayet
(PoWR) non-LTE model atmospheres. The mass-loss rates are adjusted to fit best
to the observed H{\alpha} emission lines. For the unsaturated UV resonance
lines (i.e. P v) we then apply our 3-D Monte-Carlo code, which can account for
wind clumps of any optical depths, a non-void inter-clump medium, and a
velocity dispersion inside the clumps. The ionization stratifications and
underlying photospheric spectra are adopted from the PoWR models. From fitting
the observed resonance line profiles, the properties of the wind clumps are
constrained. Our results show that with the mass-loss rates that fit H{\alpha}
(and other Balmer and He II lines), the UV resonance lines (especially the
unsaturated doublet of P v) can also be reproduced without problem when
macroclumping is taken into account. There is no need to artificially reduce
the mass-loss rates, nor to assume a sub-solar phosphorus abundance or an
extremely high clumping factor, contrary to what was claimed by other authors.
These consistent mass-loss rates are lower by a factor of 1.3 to 2.6, compared
to the mass-loss rate recipe from Vink et al. Macroclumping resolves the
previously reported discrepancy between H{\alpha} and P v mass-loss
diagnostics.Comment: 18 pages, 14 figures, 5 tables, accepted for publication in
Astrononomy & Astrophysic
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The , , , , and as dynamically generated states from vector meson - vector meson interaction
We report on some recent developments in understanding the nature of the
low-lying mesonic resonances , , ,
, and . In particular we show that these five
resonances can be dynamically generated from vector meson--vector meson
interaction in a coupled-channel unitary approach, which utilizes the
phenomenologically very successful hidden-gauge Lagrangians to produce the
interaction kernel between two vector mesons, which is then unitarized by the
Bethe-Salpeter-equation method. The data on the strong decay branching ratios,
total decay widths, and radiative decay widths of these five states, and on
related decay processes can all be well described by such an approach.
We also make predictions, compare them with the results of earlier studies, and
highlight observables that if measured can be used to distinguish different
pictures of these resonances.Comment: 9 pages; Invited talk at workshop CHIRAL'10, Valencia (Spain), June
21-24, 201
Spin-Torque-Induced Rotational Dynamics of a Magnetic Vortex Dipole
We study, both experimentally and by numerical modeling, the magnetic
dynamics that can be excited in a magnetic thin-film nanopillar device using
the spin torque from a spatially localized current injected via a
10s-of-nm-diameter aperture. The current-driven magnetic dynamics can produce
large amplitude microwave emission at zero magnetic field, with a frequency
well below that of the uniform ferromagnetic resonance mode. Micromagnetic
simulations indicate that the physical origin of this efficient microwave
nano-oscillator is the nucleation and subsequent steady-state rotational
dynamics of a magnetic vortex dipole driven by the localized spin torque. These
results show this novel implementation of a spintronic nano-oscillator is a
promising candidate for microwave technology applications.Comment: 19 pages, 4 figures
Micromagnetic understanding of stochastic resonance driven by spin-transfertorque
In this paper, we employ micromagnetic simulations to study non-adiabatic
stochastic resonance (NASR) excited by spin-transfer torque in a
super-paramagnetic free layer nanomagnet of a nanoscale spin valve. We find
that NASR dynamics involves thermally activated transitions among two static
states and a single dynamic state of the nanomagnet and can be well understood
in the framework of Markov chain rate theory. Our simulations show that a
direct voltage generated by the spin valve at the NASR frequency is at least
one order of magnitude greater than the dc voltage generated off the NASR
frequency. Our computations also reproduce the main experimentally observed
features of NASR such as the resonance frequency, the temperature dependence
and the current bias dependence of the resonance amplitude. We propose a simple
design of a microwave signal detector based on NASR driven by spin transfer
torque.Comment: 25 pages 8 figures, accepted for pubblication on Phys. Rev.
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