174 research outputs found
Dynamical influence of vortex-antivortex pairs in magnetic vortex oscillators
We study the magnetization dynamics in a nanocontact magnetic vortex
oscillators as function of temperature. Low temperature experiments reveal that
the dynamics at low and high currents differ qualitatively. At low currents, we
excite a temperature independent standard oscillation mode, consisting in the
gyrotropic motion of a free layer vortex about the nanocontact. Above a
critical current, a sudden jump of the frequency is observed, concomitant with
a substantial increase of the frequency versus current slope factor. Using
micromagnetic simulation and analytical modeling, we associate this new regime
to the creation of a vortex-antivortex pair in the pinned layer of the spin
valve. The vortex-antivortex distance depends on the Oersted field which favors
a separation, and on the exchange bias field, which favors pair merging. The
pair in the pinned layer provides an additional spin torque altering the
dynamics of the free layer vortex, which can be quantitatively accounted for by
an analytical model
Current-driven microwave oscillations in current perpendicular-to-plane spin-valve nanopillars
We study the current and temperature dependences of the microwave voltage
emission of spin-valve nanopillars subjected to an in-plane magnetic field and
a perpendicular-to-plane current. Despite the complex multilayer geometry,
clear microwave emission is shown to be possible and spectral lines as narrow
as 3.8 MHz (at 150 K) are observed.Comment: To appear in Applied Physics Letter
Probing the Dzyaloshinskii-Moriya interaction in CoFeB ultrathin films using domain wall creep and Brillouin light spectroscopy
We have characterized the strength of the interfacial Dyzaloshinskii-Moriya
interaction (DMI) in ultrathin perpendicularly magnetized CoFeB/MgO films,
grown on different underlayers of W, TaN, and Hf, using two experimental
methods. First, we determined the effective DMI field from measurements of
field-driven domain wall motion in the creep regime, where applied in-plane
magnetic fields induce an anisotropy in the wall propagation that is correlated
with the DMI strength. Second, Brillouin light spectroscopy was employed to
quantify the frequency non-reciprocity of spin waves in the CoFeB layers, which
yielded an independent measurement of the DMI. By combining these results, we
show that DMI estimates from the different techniques only yield qualitative
agreement, which suggests that open questions remain on the underlying models
used to interpret these results.Comment: 8 page
Unidirectionality of spin waves in Synthetic Antiferromagnets
We study the frequency non-reciprocity of the spin waves in symmetric
CoFeB/Ru/CoFeB synthetic antiferromagnets stacks set in the scissors state by
in-plane applied fields. Using a combination of Brillouin Light Scattering and
propagating spin wave spectroscopy experiments, we show that the acoustical
spin waves in synthetic antiferromagnets possess a unique feature if their
wavevector is parallel to the applied field: the frequency non-reciprocity can
be so large that the acoustical spin waves transfer energy in a unidirectional
manner for a wide and bipolar interval of wavevectors. Analytical modeling and
full micromagnetic calculations are conducted to account for the dispersion
relations of the optical and acoustical spin waves for arbitrary field
orientations. Our formalism provides a simple and direct method to understand
and design devices harnessing propagating spin waves in synthetic
antiferromagnets
Measuring a population of spin waves from the electrical noise of an inductively coupled antenna
We study how a population of spin waves can be characterized from the
analysis of the electrical microwave noise delivered by an inductive antenna
placed in its vicinity. The measurements are conducted on a synthetic
antiferromagnetic thin stripe covered by a micron-sized antenna that feeds a
spectrum analyser after amplification. The antenna noise contains two
contributions. The population of incoherent spin waves generates a fluctuating
field that is sensed by the antenna: this is the "magnon noise". The antenna
noise also contains the contribution of the electronic fluctuations: the
Johnson-Nyquist noise. The latter depends on all impedances within the
measurement circuit; this includes the antenna self-inductance. As a result,
the electronic noise contains information about the magnetic susceptibility,
though it does not inform on the absolute amplitude of the magnetic
fluctuations. For micrometer-sized systems at thermal equilibrium, the
electronic noise dominates and the pure magnon noise cannot be determined. If
in contrast the spinwave bath is not at thermal equilibrium with the
measurement circuit, and if the spinwave population can be changed then one
could measure a mode-resolved effective magnon temperature provided specific
precautions are implemented
Interface magnetic anisotropy in cobalt clusters embedded in a platinum or niobium matrix
A low concentration of cobalt clusters with a fcc structure and containing
almost one thousand atoms are embedded in two different metallic matrices:
platinum and niobium. Samples have been prepared using a co-deposition
technique. Cobalt clusters preformed in the gas phase and matrix atoms are
simultaneously deposited on a silicon substrate under Ultra High Vacuum
conditions. This original technique allows to prepare nanostructured systems
from miscible elements such as Co/Pt and Co/Nb in which clusters keep a pure
cobalt core surrounded with an alloyed interface. Magnetic measurements
performed using a Vibrating Sample Magnetometer (VSM) reveal large differences
in the magnetic properties of cobalt clusters in Pt and Nb pointing out the key
role of cluster/matrix interfaces.Comment: 7 pages (LaTeX), 12 PostScript figures, 1 PostScript tabl
Gradient methods for problems with inexact model of the objective
We consider optimization methods for convex minimization problems under inexact information on the objective function. We introduce inexact model of the objective, which as a particular cases includes inexact oracle [19] and relative smoothness condition [43]. We analyze gradient method which uses this inexact model and obtain convergence rates for convex and strongly convex problems. To show potential applications of our general framework we consider three particular problems. The first one is clustering by electorial model introduced in [49]. The second one is approximating optimal transport distance, for which we propose a Proximal Sinkhorn algorithm. The third one is devoted to approximating optimal transport barycenter and we propose a Proximal Iterative Bregman Projections algorithm. We also illustrate the practical performance of our algorithms by numerical experiments
Voltage control of magnetism in ferromagnetic structures (Conference Paper)
San Diego, California, United StatesInternational audienceUntil now, spintronics devices have relied on polarized currents, which still generate relatively high dissipation, particularly for nanodevices based on DW motion. A novel solution to further reduce power consumption is emerging, based on electric field (E) gating to control the magnetic state. Here, we will describe the state of the art and our recent experiments on voltage induced changes in the magnetic properties of ferromagnetic metals. A thorough description of the advances in terms of control of intrinsic properties such as magnetic anisotropy and ferromagnetic transition temperature as well as in intrinsic properties like coercive field and domain wall motion will be presented. Additionally, a section will be dedicated to the summary of the key aspects concerning the fabrication and performance of magneto-electric field-effect devices
QUantification of Erosion/Sedimentation patterns to Trace the natural versus anthropogenic sediment dynamics QUEST4D
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