2,273 research outputs found
Synchronization of spin-torque driven nanooscillators for point contacts on a quasi-1D nanowire: Micromagnetic simulations
In this paper we present detailed numerical simulation studies on the
synchronization of two spin-torque nanooscillators (STNO) in the quasi-1D
geometry: magnetization oscillations are induced in a thin NiFe nanostripe by a
spin polarized current injected via square-shaped CoFe nanomagnets on the top
of this stripe. In a sufficiently large out-of-plane field, a propagating
oscillation mode appears in such a system. Due to the absence of the
geometrically caused wave decay in 1D systems, this mode is expected to enable
a long-distance synchronization between STNOs. Indeed, our simulations predict
that synchronization of two STNOs on a nanowire is possible up to the
intercontact distance 3 mkm (for the nanowire width 50 nm). However, we have
also found several qualitatively new features of the synchronization behaviour
for this system, which make the achievement of a stable synchronization in this
geometry to a highly non-trivial task. In particular, there exist a minimal
distance between the nanocontacts, below which a synchronization of STNOs can
not be achieved. Further, when the current value in the first contact is kept
constant, the amplitude of synchronized oscillations depends non-monotonously
on the current value in the second contact. Finally, for one and the same
currents values through the contacts there might exist several synchronized
states (with different frequencies), depending on the initial conditions.Comment: 13 pages with 4 figurews, recently submitted to PR
Anatomy of Spin-Transfer Torque
Spin-transfer torques occur in magnetic heterostructures because the
transverse component of a spin current that flows from a non-magnet into a
ferromagnet is absorbed at the interface. We demonstrate this fact explicitly
using free electron models and first principles electronic structure
calculations for real material interfaces. Three distinct processes contribute
to the absorption: (1) spin-dependent reflection and transmission; (2) rotation
of reflected and transmitted spins; and (3) spatial precession of spins in the
ferromagnet. When summed over all Fermi surface electrons, these processes
reduce the transverse component of the transmitted and reflected spin currents
to nearly zero for most systems of interest. Therefore, to a good
approximation, the torque on the magnetization is proportional to the
transverse piece of the incoming spin current.Comment: 16 pages, 8 figures, submitted to Phys. Rev.
A selfconsistent theory of current-induced switching of magnetization
A selfconsistent theory of the current-induced switching of magnetization
using nonequilibrium Keldysh formalism is developed for a junction of two
ferromagnets separated by a nonmagnetic spacer. It is shown that the
spin-transfer torques responsible for current-induced switching of
magnetization can be calculated from first principles in a steady state when
the magnetization of the switching magnet is stationary. The spin-transfer
torque is expressed in terms of one-electron surface Green functions for the
junction cut into two independent parts by a cleavage plane immediately to the
left and right of the switching magnet. The surface Green functions are
calculated using a tight-binding Hamiltonian with parameters determined from a
fit to an {\it ab initio} band structure.This treatment yields the spin
transfer torques taking into account rigorously contributions from all the
parts of the junction. To calculate the hysteresis loops of resistance versus
current, and hence to determine the critical current for switching, the
microscopically calculated spin-transfer torques are used as an input into the
phenomenological Landau-Lifshitz equation with Gilbert damping. The present
calculations for Co/Cu/Co(111) show that the critical current for switching is
, which is in good agreement with experiment.Comment: 23 pages, 16 figure
Adaptive filtering of radar images for autofocus applications
Autofocus techniques are being designed at the Jet Propulsion Laboratory to automatically choose the filter parameters (i.e., the focus) for the digital synthetic aperture radar correlator; currently, processing relies upon interaction with a human operator who uses his subjective assessment of the quality of the processed SAR data. Algorithms were devised applying image cross-correlation to aid in the choice of filter parameters, but this method also has its drawbacks in that the cross-correlation result may not be readily interpretable. Enhanced performance of the cross-correlation techniques of JPL was hypothesized given that the images to be cross-correlated were first filtered to improve the signal-to-noise ratio for the pair of scenes. The results of experiments are described and images are shown
Spin-torque driven magnetization dynamics in a nanocontact setup for low external fields: numerical simulation study
We present numerical simulation studies of the steady-state magnetization
dynamics driven by a spin-polarized current in a point contact geometry for the
case of a relatively large contact diameter (D = 80 nm) and small external
field (H = 30 Oe). We show, that under these conditions the magnetization
dynamics is qualitatively different from the dynamics observed for small
contacts in large external fields. In particular, the 'bullet' mode with a
homogeneous mode core, which was the dominating localized mode for small
contacts, is not found here. Instead, all localized oscillation modes observed
in simulations correspond to different motion kinds of vortex-antivortex (V-AV)
pairs. These kinds include rotational and translational motion of pairs with
the V-AV distance d ~ D and creation/annihilation of much smaller (satellite)
V-AV pairs. We also show that for the geometry studied here the Oersted field
has a qualitative effect on the magnetization dynamics of a 'free' layer. This
effect offers a possibility to control magnetization dynamics by a suitable
electric contact setup, optimized to produce a desired Oersted field. Finally,
we demonstrate that when the magnetization dynamics of the 'fixed' layer
(induced only by the stray field interaction with the 'free' layer) is taken
into account, the threshold current for the oscillation onset is drastically
reduced and new types of localized modes appear. In conclusion, we show that
our simulations reproduce semiquantitatively several important features of the
magnetization dynamics in a point contact system for low external fields
reported experimentally.Comment: 26 pages, 12 figures, submitted to Phys. Rev.
Magnetic dynamics with spin transfer torques near the Curie temperature
We use atomistic stochastic Landau-Lifshitz-Slonczewski simulations to study
the interaction between large thermal fluctuations and spin transfer torques in
the magnetic layers of spin valves. At temperatures near the Curie temperature
, spin currents measurably change the size of the magnetization
(i.e. there is a {\it longitudinal} spin transfer effect). The change in
magnetization of the free magnetic layer in a spin valve modifies the
temperature dependence of the applied field-applied current phase diagram for
temperatures near . These atomistic simulations can be accurately
described by a Landau-Lifshitz-Bloch + Slonczewski equation, which is a
thermally averaged mean field theory. Both the simulation and the mean field
theory show that a longitudinal spin transfer effect can be a substantial
fraction of the magnetization close to .Comment: 8 pages, 6 figure
Current-induced spin-wave excitations in a single ferromagnetic layer
A new current induced spin-torque transfer effect has been observed in a
single ferromagnetic layer without resorting to multilayers. At a specific
current density of one polarity injected from a point contact, abrupt
resistance changes due to current-induced spin wave excitations have been
observed. The critical current at the onset of spin-wave excitations depends
linearly on the external field applied perpendicular to the layer. The observed
effect is due to current-driven heterogeneity in an otherwise uniform
ferromagnetic layer.Comment: 12 pages, 4 figure
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