148 research outputs found
Unified description of bulk and interface-enhanced spin pumping
The dynamics of non-equilibrium spin accumulation generated in metals or
semiconductors by rf magnetic field pumping is treated within a diffusive
picture. The dc spin accumulation produced in a uniform system by a rotating
applied magnetic field or by a precessing magnetization of a weak ferromagnet
is in general given by a (small) fraction of hbar omega, where omega is the
rotation or precession frequency. With the addition of a neighboring,
field-free region and allowing for the diffusion of spins, the spin
accumulation is dramatically enhanced at the interface, saturating at the
universal value hbar omega in the limit of long spin relaxation time. This
effect can be maximized when the system dimensions are of the order of sqrt(2pi
D omega), where D is the diffusion constant. We compare our results to the
interface spin pumping theory of A. Brataas et al. [Phys. Rev. B 66, 060404(R)
(2002)]
Microwave spectroscopy on magnetization reversal dynamics of nanomagnets with electronic detection
We demonstrate a detection method for microwave spectroscopy on magnetization
reversal dynamics of nanomagnets. Measurement of the nanomagnet anisotropic
magnetoresistance was used for probing how magnetization reversal is resonantly
enhanced by microwave magnetic fields. We used Co strips of 2 um x 130 nm x 40
nm, and microwave fields were applied via an on-chip coplanar wave guide. The
method was applied for demonstrating single domain-wall resonance, and studying
the role of resonant domain-wall dynamics in magnetization reversal
Coupling efficiency for phase locking of a spin transfer oscillator to a microwave current
The phase locking behavior of spin transfer nano-oscillators (STNOs) to an
external microwave signal is experimentally studied as a function of the STNO
intrinsic parameters. We extract the coupling strength from our data using the
derived phase dynamics of a forced STNO. The predicted trends on the coupling
strength for phase locking as a function of intrinsic features of the
oscillators i.e. power, linewidth, agility in current, are central to optimize
the emitted power in arrays of mutually coupled STNOs
Dynamics of two coupled vortices in a spin valve nanopillar excited by spin transfer torque
We investigate the dynamics of two coupled vortices driven by spin transfer.
We are able to independently control with current and perpendicular field, and
to detect, the respective chiralities and polarities of the two vortices. For
current densities above , a highly coherent signal
(linewidth down to 46 kHz) can be observed, with a strong dependence on the
relative polarities of the vortices. It demonstrates the interest of using
coupled dynamics in order to increase the coherence of the microwave signal.
Emissions exhibit a linear frequency evolution with perpendicular field, with
coherence conserved even at zero magnetic field
Domain wall displacement by remote spin-current injection
We demonstrate numerically the ability to displace a magnetic domain wall by
a remote spin current injection. We consider a long and narrow magnetic
nanostripe with a single domain wall (DW). The spin-polarized current is
injected perpendicularly to the plane of the film (CPP) through a small
nanocontact which is located at certain distance from the domain wall initial
position. We show theoretically that the DW motion can be initiated not only by
conventional spin-transfer torque but also by indirect spin-torque, created by
a remote spin-current injection and then transferred to the DW by the
exchange-spring mechanism. An analytical description of this effect is
proposed. This finding may lead to a solution of bottleneck problems of DW
motion-based spintronic and neuromorphic devices with perpendicular
spin-current injection.Comment: 6 pages, 4 figure
Parallel pumping of magnetic vortex gyrations in spin-torque nano-oscillators
We experimentally demonstrate that large magnetic vortex oscillations can be
parametrically excited in a magnetic tunnel junction by the injection of
radio-frequency (rf) currents at twice the natural frequency of the gyrotropic
vortex core motion. The mechanism of excitation is based on the parallel
pumping of vortex motion by the rf orthoradial field generated by the injected
current. Theoretical analysis shows that experimental results can be
interpreted as the manifestation of parametric amplification when rf current is
small, and of parametric instability when rf current is above a certain
threshold. By taking into account the energy nonlinearities, we succeed to
describe the amplitude saturation of vortex oscillations as well as the
coexistence of stable regimes.Comment: Submitted to Phys. Rev. Let
Field dependence of magnetization reversal by spin transfer
We analyse the effect of the applied field (Happl) on the current-driven
magnetization reversal in pillar-shaped Co/Cu/Co trilayers, where we observe
two different types of transition between the parallel (P) and antiparallel
(AP) magnetic configurations of the Co layers. If Happl is weaker than a rather
small threshold value, the transitions between P and AP are irreversible and
relatively sharp. For Happl exceding the threshold value, the same transitions
are progressive and reversible. We show that the criteria for the stability of
the P and AP states and the experimentally observed behavior can be precisely
accounted for by introducing the current-induced torque of the spin transfer
models in a Landau-Lifschitz-Gilbert equation. This approach also provides a
good description for the field dependence of the critical currents
Microwave Oscillations of a Nanomagnet Driven by a Spin-Polarized Current
We describe direct electrical measurements of microwave-frequency dynamics in
individual nanomagnets that are driven by spin transfer from a DC
spin-polarized current. We map out the dynamical stability diagram as a
function of current and magnetic field, and we show that spin transfer can
produce several different types of magnetic excitations, including small-angle
precession, a more complicated large-angle motion, and a high-current state
that generates little microwave signal. The large-angle mode can produce a
significant emission of microwave energy, as large as 40 times the
Johnson-noise background.Comment: 12 pages, 3 figure
Mechanisms of spin-polarized current-driven magnetization switching
The mechanisms of the magnetization switching of magnetic multilayers driven
by a current are studied by including exchange interaction between local
moments and spin accumulation of conduction electrons. It is found that this
exchange interaction leads to two additional terms in the
Landau-Lifshitz-Gilbert equation: an effective field and a spin torque. Both
terms are proportional to the transverse spin accumulation and have comparable
magnitudes
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