2,566 research outputs found
Harmonic oscillator model for current- and field-driven magnetic vortices
In experiments the distinction between spin-torque and Oersted-field driven
magnetization dynamics is still an open problem. Here, the gyroscopic motion of
current- and field-driven magnetic vortices in small thin-film elements is
investigated by analytical calculations and by numerical simulations. It is
found that for small harmonic excitations the vortex core performs an
elliptical rotation around its equilibrium position. The global phase of the
rotation and the ratio between the semi-axes are determined by the frequency
and the amplitude of the Oersted field and the spin torque
Topology Detection in Microgrids with Micro-Synchrophasors
Network topology in distribution networks is often unknown, because most
switches are not equipped with measurement devices and communication links.
However, knowledge about the actual topology is critical for safe and reliable
grid operation. This paper proposes a voting-based topology detection method
based on micro-synchrophasor measurements. The minimal difference between
measured and calculated voltage angle or voltage magnitude, respectively,
indicates the actual topology. Micro-synchrophasors or micro-Phasor Measurement
Units ({\mu}PMU) are high-precision devices that can measure voltage angle
differences on the order of ten millidegrees. This accuracy is important for
distribution networks due to the smaller angle differences as compared to
transmission networks. For this paper, a microgrid test bed is implemented in
MATLAB with simulated measurements from {\mu}PMUs as well as SCADA measurement
devices. The results show that topologies can be detected with high accuracy.
Additionally, topology detection by voltage angle shows better results than
detection by voltage magnitude.Comment: 5 Pages, PESGM2015, Denver, C
Arylâgroup substituted polysiloxanes with highâoptical transmission, thermal stability, and refractive index
Polysiloxanes are an important class of polymers for optoelectronic applications.
Novel polysiloxanes with high-refractive index (RI) based on phenanthrenylsubstituted monomers are prepared by a polycondensation reaction starting
from various substituted dialkoxysilanes as monomers. The substitution patterns
on the Si atom are systematically changed to vary the properties of the
linear polymers as well as the final cured material. The two monomers with
polycyclic aromatic side groups 9-phenanthrenylmethyldimethoxysilane and
9-phenanthrenylphenyldimethoxysilane are synthesized and fully characterized,
including their single crystal X-ray structures. Linear polysiloxanes with variations in hydride, methyl, vinyl, phenyl, and phenanthrenyl side group content
are prepared by acid- and base-catalyzed polycondensation reactions. Both Si H
and Si vinyl substituted polymers with molecular weights up to 30 kDa and
adjustable RI's from 1.52 to 1.63 are obtained and the thermally cured by
Pt-catalyzed hydrosilylation reactions. Polysiloxane resins are obtained with
high-RI's, optical transmittance above 95% and thermal stabilities up to 420C.
Long-term thermal stability tests show transmittance values above 85% even
after 60 days of thermal treatment at 180C
Current-Driven Domain-Wall Dynamics in Curved Ferromagnetic Nanowires
The current-induced motion of a domain wall in a semicircle nanowire with
applied Zeeman field is investigated. Starting from a micromagnetic model we
derive an analytical solution which characterizes the domain-wall motion as a
harmonic oscillation. This solution relates the micromagnetic material
parameters with the dynamical characteristics of a harmonic oscillator, i.e.,
domain-wall mass, resonance frequency, damping constant, and force acting on
the wall. For wires with strong curvature the dipole moment of the wall as well
as its geometry influence the eigenmodes of the oscillator. Based on these
results we suggest experiments for the determination of material parameters
which otherwise are difficult to access. Numerical calculations confirm our
analytical solution and show its limitations
Spin-polarized currents in exchange spring systems
We present a computational study of the magnetization dynamics of a trilayer exchange spring system in the form of a cylindrical nanopillar in the presence of an electric current. A three-dimensional micromagnetic model is used, where the interaction between the current and the local magnetization is taken into account following a recent model by Zhang and Li [Phys. Rev. Lett. 93, 127204 (2004)] We obtain a stationary rotation of the magnetization of the system around its axis, accompanied by a compression of the artificial domain wall in the direction of the electron flow
Nuclear resonant surface diffraction
Nuclear resonant x-ray diffraction in grazing incidence geometry is used to
determine the lateral magnetic configuration in a one-dimensional lattice of
ferromagnetic nanostripes. During magnetic reversal, strong nuclear
superstructure diffraction peaks appear in addition to the electronic ones due
to an antiferromagnetic order in the nanostripe lattice. We show that the
analysis of the angular distribution of the resonantly diffracted x-rays
together with the time-dependence of the coherently diffracted nuclear signal
reveals surface spin structures with very high sensitivity. This novel
scattering technique provides a unique access to laterally correlated spin
configurations in magnetically ordered nanostructures and, in perspective, also
to their dynamics
Spin precession mapping at ferromagnetic resonance via nuclear resonant scattering
We probe the spin dynamics in a thin magnetic film at ferromagnetic resonance
by nuclear resonant scattering of synchrotron radiation at the 14.4 keV
resonance of Fe. The precession of the magnetization leads to an
apparent reduction of the magnetic hyperfine field acting at the Fe
nuclei. The spin dynamics is described in a stochastic relaxation model adapted
to the ferromagnetic resonance theory by Smit and Beljers to model the decay of
the excited nuclear state. From the fits of the measured data the shape of the
precession cone of the spins is determined. Our results open a new perspective
to determine magnetization dynamics in layered structures with very high depth
resolution by employing ultrathin isotopic probe layers
Coupled vortex oscillations in spatially separated permalloy squares
We experimentally study the magnetization dynamics of pairs of micron-sized permalloy squares coupled via their stray fields. The trajectories of the vortex cores in the Landau-domain patterns of the squares are mapped in real space using time-resolved scanning transmission x-ray microscopy. After excitation of one of the vortex cores with a short magnetic-field pulse, the system behaves like coupled harmonic oscillators. The coupling strength depends on the separation between the squares and the configuration of the vortex-core polarizations. Considering the excitation via a rotating in-plane magnetic field, it can be understood that only a weak response of the second vortex core is observed for equal core polarizations
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