1,607 research outputs found
Quantitative MRFM characterization of the autonomous and forced dynamics in a spin transfer nano-oscillator
Using a magnetic resonance force microscope (MRFM), the power emitted by a
spin transfer nano-oscillator consisting of a normally magnetized PyCuPy
circular nanopillar is measured both in the autonomous and forced regimes. From
the power behavior in the subcritical region of the autonomous dynamics, one
obtains a quantitative measurement of the threshold current and of the noise
level. Their field dependence directly yields both the spin torque efficiency
acting on the thin layer and the nature of the mode which first
auto-oscillates: the lowest energy, spatially most uniform spin-wave mode. From
the MRFM behavior in the forced dynamics, it is then demonstrated that in order
to phase-lock this auto-oscillating mode, the external source must have the
same spatial symmetry as the mode profile, i.e., a uniform microwave field must
be used rather than a microwave current flowing through the nanopillar
Hydrodynamic equations for self-propelled particles: microscopic derivation and stability analysis
Considering a gas of self-propelled particles with binary interactions, we
derive the hydrodynamic equations governing the density and velocity fields
from the microscopic dynamics, in the framework of the associated Boltzmann
equation. Explicit expressions for the transport coefficients are given, as a
function of the microscopic parameters of the model. We show that the
homogeneous state with zero hydrodynamic velocity is unstable above a critical
density (which depends on the microscopic parameters), signaling the onset of a
collective motion. Comparison with numerical simulations on a standard model of
self-propelled particles shows that the phase diagram we obtain is robust, in
the sense that it depends only slightly on the precise definition of the model.
While the homogeneous flow is found to be stable far from the transition line,
it becomes unstable with respect to finite-wavelength perturbations close to
the transition, implying a non trivial spatio-temporal structure for the
resulting flow. We find solitary wave solutions of the hydrodynamic equations,
quite similar to the stripes reported in direct numerical simulations of
self-propelled particles.Comment: 33 pages, 11 figures, submitted to J. Phys.
Complete mapping of the spin-wave spectrum in vortex state nano-disk
We report a study on the complete spin-wave spectrum inside a vortex state
nano-disk. Transformation of this spectrum is continuously monitored as the
nano-disk becomes gradually magnetized by a perpendicular magnetic field and
encouters a second order phase transition to the uniformly magnetized state.
This reveals the bijective relationship that exists between the eigen-modes in
the vortex state with the ones in the saturated state. It is found that the
gyrotropic mode can be continuously viewed as a uniform phase precession, which
uniquely softens (its frequency vanishes) at the saturation field to transform
above into the Kittel mode. By contrast the other spin-wave modes remain finite
as a function of the applied field while their character is altered by level
anti-crossing
Comparative Measurements of Inverse Spin Hall and Magnetoresistance in YIG|Pt and YIG|Ta
We report on a comparative study of spin Hall related effects and
magnetoresistance in YIG|Pt and YIG|Ta bilayers. These combined measurements
allow to estimate the characteristic transport parameters of both Pt and Ta
layers juxtaposed to YIG: the spin mixing conductance
at the YIGnormal metal interface, the spin Hall angle , and the
spin diffusion length in the normal metal. The inverse spin Hall
voltages generated in Pt and Ta by the pure spin current pumped from YIG
excited at resonance confirm the opposite signs of spin Hall angles in these
two materials. Moreover, from the dependence of the inverse spin Hall voltage
on the Ta thickness, we extract the spin diffusion length in Ta, found to be
nm. Both the YIG|Pt and YIG|Ta systems
display a similar variation of resistance upon magnetic field orientation,
which can be explained in the recently developed framework of spin Hall
magnetoresistance.Comment: 8 pages, 5 figures, 1 tabl
Mechanical magnetometry of Cobalt nanospheres deposited by focused electron beam at the tip of ultra-soft cantilevers
Using focused-electron-beam-induced deposition, Cobalt magnetic nanospheres
with diameter ranging between 100 nm and 300 nm are grown at the tip of
ultra-soft cantilevers. By monitoring the mechanical resonance frequency of the
cantilever as a function of the applied magnetic field, the hysteresis curve of
these individual nanospheres are measured. This enables to evaluate their
saturation magnetization, found to be around 430 emu/cm^3 independently of the
size of the particle, and to infer that the magnetic vortex state is the
equilibrium configuration of these nanospheres at remanence
Detection of the microwave spin pumping using the inverse spin Hall effect
We report electrical detection of the dynamical part of the spin pumping
current emitted during ferromagnetic resonance (FMR) using the inverse Spin
Hall Effect (ISHE). The experiment is performed on a YIGPt bilayer. The
choice of YIG, a magnetic insulator, ensures that no charge current flows
between the two layers and only pure spin current produced by the magnetization
dynamics are transferred into the adjacent strong spin-orbit Pt layer via spin
pumping. To avoid measuring the parasitic eddy currents induced at the
frequency of the microwave source, a resonance at half the frequency is induced
using parametric excitation in the parallel geometry. Triggering this nonlinear
effect allows to directly detect on a spectrum analyzer the microwave component
of the ISHE voltage. Signals as large as 30 V are measured for precession
angles of a couple of degrees. This direct detection provides a novel efficient
means to study magnetization dynamics on a very wide frequency range with great
sensitivity
Interaction between static holes in a quantum dimer model on the kagome lattice
A quantum dimer model (QDM) on the kagome lattice with an extensive
ground-state entropy was recently introduced [Phys. Rev. B 67, 214413 (2003)].
The ground-state energy of this QDM in presence of one and two static holes is
investigated by means of exact diagonalizations on lattices containing up to
144 kagome sites. The interaction energy between the holes (at distances up to
7 lattice spacings) is evaluated and the results show no indication of
confinement at large hole separations.Comment: 6 pages, 3 figures. IOP style files included. To appear in J. Phys.:
Condens. Matter, Proceedings of the HFM2003 conference, Grenobl
Improved spectral stability in spin transfer nano-oscillators: single vortex versus coupled vortices dynamics
We perform a comparative study of spin transfer induced excitation of the
gyrotropic motion of a vortex core with either uniform or vortex spin
polarizers. The microwave output voltage associated with the vortex dynamics,
detected in both cases, displays a strong reduction of phase fluctuations in
the case of the vortex polarizer, with a decrease of the peak linewidth by one
order of magnitude down to 200kHz at zero field. A thorough study of rf
emission features for the different accessible vortex configurations shows that
this improvement is related to the excitation of coupled vortex dynamics by
spin transfer torques
Epidural Stimulation Induced Modulation of Spinal Locomotor Networks in Adult Spinal Rats
The importance of the in vivo dynamic nature of the circuitries within the spinal cord that generate locomotion is becoming increasingly evident. We examined the characteristics of hindlimb EMG activity evoked in response to epidural stimulation at the S1 spinal cord segment in complete midthoracic spinal cord-transected rats at different stages of postlesion recovery. A progressive and phase-dependent modulation of monosynaptic (middle) and long-latency (late) stimulation-evoked EMG responses was observed throughout the step cycle. During the first 3 weeks after injury, the amplitude of the middle response was potentiated during the EMG bursts, whereas after 4 weeks, both the middle and late responses were phase-dependently modulated. The middle- and late-response magnitudes were closely linked to the amplitude and duration of the EMG bursts during locomotion facilitated by epidural stimulation. The optimum stimulation frequency that maintained consistent activity of the long-latency responses ranged from 40 to 60 Hz, whereas the short-latency responses were consistent from 5 to 130 Hz. These data demonstrate that both middle and late evoked potentials within a motor pool are strictly gated during in vivo bipedal stepping as a function of the general excitability of the motor pool and, thus, as a function of the phase of the step cycle. These data demonstrate that spinal cord epidural stimulation can facilitate locomotion in a time-dependent manner after lesion. The long-latency responses to epidural stimulation are correlated with the recovery of weight-bearing bipedal locomotion and may reflect activation of interneuronal central pattern-generating circuits
Superfluids as Higher-form Anomalies
We recast superfluid hydrodynamics as the hydrodynamic theory of a system
with an emergent anomalous higher-form symmetry. The higher-form charge counts
the winding planes of the superfluid -- its constitutive relation replaces the
Josephson relation of conventional superfluid hydrodynamics. This formulation
puts all hydrodynamic equations on equal footing. The anomalous Ward identity
can be used as an alternative starting point to prove the existence of a
Goldstone boson, without reference to spontaneous symmetry breaking. This
provides an alternative characterization of Landau phase transitions in terms
of higher-form symmetries and their anomalies instead of how the symmetries are
realized. This treatment is more general and, in particular, includes the case
of BKT transitions. As an application of this formalism we construct the
hydrodynamic theories of conventional (0-form) and 1-form superfluids.Comment: 29 pages; v3 corrected Eq. (1.25), published versio
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