173 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
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
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
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
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
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
Modulating spin transfer torque switching dynamics with two orthogonal spin-polarizers by varying the cell aspect ratio
We study in-plane magnetic tunnel junctions with additional perpendicular
polarizer for subnanosecond-current-induced switching memories. The
spin-transfer-torque switching dynamics was studied as a function of the cell
aspect ratio both experimentally and by numerical simulations using the
macrospin model. We show that the anisotropy field plays a significant role in
the dynamics, along with the relative amplitude of the two spin-torque
contributions. This was confirmed by micromagnetic simulations. Real-time
measurements of the reversal were performed with samples of low and high aspect
ratio. For low aspect ratios, a precessional motion of the magnetization was
observed and the effect of temperature on the precession coherence was studied.
For high aspect ratios, we observed magnetization reversals in less than 1 ns
for high enough current densities, the final state being controlled by the
current direction in the magnetic tunnel junction cell.Comment: 6 pages, 7 figure
Current-driven vortex oscillations in metallic nanocontacts: Zero-field oscillations and training effects
We present an experimental and theoretical study of the low-field dynamics of
current-driven vortex oscillations in nanocontacts based on spin-valve
multilayers. These oscillations appear as low-frequency (250-500 MHz)
excitations in the electrical power spectrum which arise from to variations in
the giant-magnetoresistance. We show that the vortex oscillations, once
nucleated at large fields applied perpendicular to the film plane, persist at
zero applied magnetic fields. Some training effects on the oscillation
frequency and linewidth also observed for small in-plane magnetic fields.Comment: 6 pages, 7 figure
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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