259 research outputs found
Numerical study of imperfect liquid-filled conical shells - Verifying the present design rule
Frequency shift keying in vortex-based spin torque oscillators
Vortex-based spin-torque oscillators can be made from extended spin valves
connected to an electrical nanocontact. We study the implementation of
frequency shift keying modulation in these oscillators. Upon a square
modulation of the current in the 10 MHz range, the vortex frequency follows the
current command, with easy identification of the two swapping frequencies in
the spectral measurements. The frequency distribution of the output power can
be accounted for by convolution transformations of the dc current vortex
waveform, and the current modulation. Modeling indicates that the frequency
transitions are phase coherent and last less than 25 ns. Complementing the
multi-octave tunability and first-class agility, the capability of frequency
shift keying modulation is an additional milestone for the implementation of
vortex-based oscillators in RF circuit.Comment: 6 pages, 5 figure
Agility of vortex-based nanocontact spin torque oscillators
We study the agility of current-tunable oscillators based on a magnetic
vortex orbiting around a point contact in spin-valves. Theory predicts
frequency-tuning by currents occurs at constant orbital radius, so an
exceptional agility is anticipated. To test this, we have inserted an
oscillator in a microwave interferometer to apply abrupt current variations
while time resolving its emission. Using frequency shift keying, we show that
the oscillator can switch between two stabilized frequencies differing by 25%
in less than ten periods. With a wide frequency tunability and a good agility,
such oscillators possess desirable figures of merit for modulation-based rf
applications.Comment: 3 pages, 3 figure
Quantized spin wave modes in magnetic tunnel junction nanopillars
We present an experimental and theoretical study of the magnetic field
dependence of the mode frequency of thermally excited spin waves in rectangular
shaped nanopillars of lateral sizes 60x100, 75x150, and 105x190 nm2, patterned
from MgO-based magnetic tunnel junctions. The spin wave frequencies were
measured using spectrally resolved electrical noise measurements. In all
spectra, several independent quantized spin wave modes have been observed and
could be identified as eigenexcitations of the free layer and of the synthetic
antiferromagnet of the junction. Using a theoretical approach based on the
diagonalization of the dynamical matrix of a system of three coupled, spatially
confined magnetic layers, we have modeled the spectra for the smallest pillar
and have extracted its material parameters. The magnetization and exchange
stiffness constant of the CoFeB free layer are thereby found to be
substantially reduced compared to the corresponding thin film values. Moreover,
we could infer that the pinning of the magnetization at the lateral boundaries
must be weak. Finally, the interlayer dipolar coupling between the free layer
and the synthetic antiferromagnet causes mode anticrossings with gap openings
up to 2 GHz. At low fields and in the larger pillars, there is clear evidence
for strong non-uniformities of the layer magnetizations. In particular, at zero
field the lowest mode is not the fundamental mode, but a mode most likely
localized near the layer edges.Comment: 16 pages, 4 figures, (re)submitted to PR
Spatially resolved ultrafast precessional magnetization reversal
Spatially resolved measurements of quasi-ballistic precessional magnetic
switching in a microstructure are presented. Crossing current wires allow
detailed study of the precessional switching induced by coincident longitudinal
and transverse magnetic field pulses. Though the response is initially
spatially uniform, dephasing occurs leading to nonuniformity and transient
demagnetization. This nonuniformity comes in spite of a novel method for
suppression of end domains in remanence. The results have implications for the
reliability of ballistic precessional switching in magnetic devices.Comment: 17 pages (including 4 figures), submitted to Phys. Rev. Let
Maternal anxiety during pregnancy and reactivity, self-regulation and internalizing problems in childhood and adolescence
Finite-volume two-pion energies and scattering in the quenched approximation
We investigate how L\"uscher's relation between the finite-volume energy of
two pions at rest and pion scattering lengths has to be modified in quenched
QCD. We find that this relation changes drastically, and in particular, that
``enhanced finite-volume corrections" of order and occur at
one loop ( is the linear size of the box), due to the special properties of
the in the quenched approximation. We define quenched pion scattering
lengths, and show that they are linearly divergent in the chiral limit. We
estimate the size of these various effects in some numerical examples, and find
that they can be substantial.Comment: 22 pages, uuencoded, compressed postscript fil
Understanding nanoscale temperature gradients in magnetic nanocontacts
We determine the temperature profile in magnetic nanocontacts submitted to
the very large current densities that are commonly used for spin-torque
oscillator behavior. Experimentally, the quadratic current-induced increase of
the resistance through Joule heating is independent of the applied temperature
from 6 K to 300 K. The modeling of the experimental rate of the current-induced
nucleation of a vortex under the nanocontact, assuming a thermally-activated
process, is consistent with a local temperature increase between 150 K and 220
K. Simulations of heat generation and diffusion for the actual tridimensional
geometry were conducted. They indicate a temperature-independent efficiency of
the heat sinking from the electrodes, combined with a localized heating source
arising from a nanocontact resistance that is also essentially
temperature-independent. For practical currents, we conclude that the local
increase of temperature is typically 160 K and it extends 450 nm about the
nanocontact. Our findings imply that taking into account the current-induced
heating at the nanoscale is essential for the understanding of magnetization
dynamics in nanocontact systems.Comment: 5 pages, 5 figure
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