281 research outputs found
Auto-oscillation threshold, narrow spectral lines, and line jitter in spin-torque oscillators based on MgO magnetic tunnel junctions
We demonstrate spin torque induced auto-oscillation in MgO-based magnetic
tunnel junctions. At the generation threshold, we observe a strong line
narrowing down to 6 MHz at 300K and a dramatic increase in oscillator power,
yielding spectrally pure oscillations free of flicker noise. Setting the
synthetic antiferromagnet into autooscillation requires the same current
polarity as the one needed to switch the free layer magnetization. The induced
auto-oscillations are observed even at zero applied field, which is believed to
be the acoustic mode of the synthetic antiferromagnet. While the phase
coherence of the auto-oscillation is of the order of microseconds, the power
autocorrelation time is of the order of milliseconds and can be strongly
influenced by the free layer dynamics
Current-driven microwave oscillations in current perpendicular-to-plane spin-valve nanopillars
We study the current and temperature dependences of the microwave voltage
emission of spin-valve nanopillars subjected to an in-plane magnetic field and
a perpendicular-to-plane current. Despite the complex multilayer geometry,
clear microwave emission is shown to be possible and spectral lines as narrow
as 3.8 MHz (at 150 K) are observed.Comment: To appear in Applied Physics Letter
Phase Coherent Precessional Magnetization Reversal in Micro-scopic Spin Valve Elements
We study the precessional switching of the magnetization in microscopic spin
valve cells induced by ultra short in-plane hard axis magnetic field pulses.
Stable and highly efficient switching is monitored following pulses as short as
140 ps with energies down to 15 pJ. Multiple application of identical pulses
reversibly toggles the cell's magnetization be-tween the two easy directions.
Variations of pulse duration and amplitude reveal alter-nating regimes of
switching and non-switching corresponding to transitions from in-phase to
out-of-phase excitations of the magnetic precession by the field pulse. In the
low field limit damping becomes predominant and a relaxational reversal is
found allowing switching by hard axis fields below the in-plane anisotropy
field threshold.Comment: 17 pages, 4 figure
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
Current-driven vortex oscillations in metallic nanocontacts
We present experimental evidence of sub-GHz spin-transfer oscillations in
metallic nano-contacts that are due to the translational motion of a magnetic
vortex. The vortex is shown to execute large-amplitude orbital motion outside
the contact region. Good agreement with analytical theory and micromagnetics
simulations is found.Comment: 4 pages, 3 figure
Auto-oscillation threshold and line narrowing in MgO-based spin-torque oscillators
We present an experimental study of the power spectrum of current-driven
magnetization oscillations in MgO tunnel junctions under low bias. We find the
existence of narrow spectral lines, down to 8 MHz in width at a frequency of
10.7 GHz, for small applied fields with clear evidence of an auto-oscillation
threshold. Micromagnetics simulations indicate that the excited mode
corresponds to an edge mode of the synthetic antiferromagnet
Measuring a population of spin waves from the electrical noise of an inductively coupled antenna
We study how a population of spin waves can be characterized from the
analysis of the electrical microwave noise delivered by an inductive antenna
placed in its vicinity. The measurements are conducted on a synthetic
antiferromagnetic thin stripe covered by a micron-sized antenna that feeds a
spectrum analyser after amplification. The antenna noise contains two
contributions. The population of incoherent spin waves generates a fluctuating
field that is sensed by the antenna: this is the "magnon noise". The antenna
noise also contains the contribution of the electronic fluctuations: the
Johnson-Nyquist noise. The latter depends on all impedances within the
measurement circuit; this includes the antenna self-inductance. As a result,
the electronic noise contains information about the magnetic susceptibility,
though it does not inform on the absolute amplitude of the magnetic
fluctuations. For micrometer-sized systems at thermal equilibrium, the
electronic noise dominates and the pure magnon noise cannot be determined. If
in contrast the spinwave bath is not at thermal equilibrium with the
measurement circuit, and if the spinwave population can be changed then one
could measure a mode-resolved effective magnon temperature provided specific
precautions are implemented
Structure and regulation of mammalian S-adenosylmethionine decarboxylase
In order to understand the structure and regulation of S-adenosylmethionine decarboxylase, cDNA clones encoding this enzyme have been isolated from rat prostate and human fibroblast cDNA libraries. The authenticity of the cDNAs was verified by: (a) transfecting the Chinese hamster ovary cells with the human cDNA in the pcD vector which resulted in a transient 10-20-fold increase in S-adenosylmethionine decarboxylase activity in recipient cells; and (b) translating the mRNA formed by transcription of the cDNA insert in a reticulocyte lysate and recording an increase in S-adenosylmethionine decarboxylase activity. The amino acid sequences deduced from the cDNAs indicate that the human proenzyme for this protein contains 334 amino acids and has a molecular weight of 38,331 whereas the rat proenzyme contains 333 amino acid residues. The human and rat enzymes are very similar having only 11 amino acid differences and the cDNAs are also closely related showing over 90% homology in the 1617-nucleotide overlap which was sequenced. A further indication of the highly conserved nature of mammalian S-adenosylmethionine decarboxylases is that the amino acid sequences deduced from the human and the rat cDNAs contained peptide sequences identical to those previously reported for the purified bovine enzyme. In vitro transcription/translation experiments showed that the proenzyme is converted to two polypeptides of molecular weights about 32,000 and 6,000 in a processing reaction which generates the prosthetic pyruvate group and that the final enzyme contains both polypeptides. Two forms of S-adenosylmethionine decarboxylase mRNA (2.1 and about 3.4-3.6 kilobases) are present in human and rodent tissues and may originate from the utilization of two different polyadenylation signals. Southern blots of rat genomic DNA indicated that the S-adenosylmethionine decarboxylase gene belongs to a multigene family. Depletion of cellular polyamines by inhibitors or ornithine decarboxylase or the aminopropyltransferases led to an increase in the content of S-adenosylmethionine decarboxylase protein and mRNA, but the elevation in the mRNA was not sufficient to account for all of the change in the enzyme level, particularly in cells in which spermine was depleted
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