240 research outputs found
A strategy for the design of skyrmion racetrack memories
Magnetic storage based on racetrack memory is very promising for the design
of ultra-dense, low-cost and low-power storage technology. Information can be
coded in a magnetic region between two domain walls or, as predicted recently,
in topological magnetic objects known as skyrmions. Here, we show the
technological advantages and limitations of using Bloch and Neel skyrmions
manipulated by spin current generated within the ferromagnet or via the
spin-Hall effect arising from a non-magnetic heavy metal underlayer. We found
that the Neel skyrmion moved by the spin-Hall effect is a very promising
strategy for technological implementation of the next generation of skyrmion
racetrack memories (zero field, high thermal stability, and ultra-dense
storage). We employed micromagnetics reinforced with an analytical formulation
of skyrmion dynamics that we developed from the Thiele equation. We identified
that the excitation, at high currents, of a breathing mode of the skyrmion
limits the maximal velocity of the memory
Nonlinear dispersion relation in anharmonic periodic mass-spring and mass-in-mass systems
The study of wave propagation in chains of anharmonic periodic systems is of
fundamental importance to understand the response of dynamical absorbers of
vibrations and acoustic metamaterials working in nonlinear regime. Here, we
derive an analytical nonlinear dispersion relation for periodic chains of
anharmonic mass-spring and mass-in-mass systems resulting from considering the
hypothesis of weak anharmonic energy and a periodic distribution function as
ansatz of a general solution of the nonlinear equations of motion. Numerical
simulations show that this expression is valid for anharmonic potential energy
up to 50% of the harmonic one. This work provides a simple tool to design and
study nonlinear dynamics for a class of seismic metamaterials.Comment: 18 pages, 5 figure
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
Combined frequency-amplitude nonlinear modulation: theory and applications
In this work we formulate a generalized theoretical model to describe the
nonlinear dynamics observed in combined frequency-amplitude modulators whose
characteristic parameters exhibit a nonlinear dependence on the input
modulating signal. The derived analytical solution may give a satisfactory
explanation of recent laboratory observations on magnetic spin-transfer
oscillators and fully agrees with results of micromagnetic calculations. Since
the theory has been developed independently of the mechanism causing the
nonlinearities, it may encompass the description of modulation processes of any
physical nature, a promising feature for potential applications in the field of
communication systems.Comment: 8 pages, 4 figures, to be published on IEEE Transactions on Magnetic
Single-shot time-domain studies of spin-torque-driven switching in magnetic tunnel junctions
We report single-shot measurements of resistance versus time for thermally
assisted spin-torque-driven switching in magnetic tunnel junctions. We achieve
sufficient sensitivity to resolve the resistance signals leading up to
switching, including the variations between individual switching events.
Analyses of pre-switching thermal fluctuations allow detailed measurements of
coherence times and variations in magnetization precession amplitude. We find
that with a small in-plane hard-axis magnetic field the magnetization dynamics
are more spatially coherent than for the case of zero field.Comment: 13 pages, 4 figure
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
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