546 research outputs found
Super-harmonic injection locking of nano-contact spin-torque vortex oscillators
Super-harmonic injection locking of single nano-contact (NC) spin-torque
vortex oscillators (STVOs) subject to a small microwave current has been
explored. Frequency locking was observed up to the fourth harmonic of the STVO
fundamental frequency in microwave magneto-electronic measurements. The
large frequency tunability of the STVO with respect to allowed the
device to be locked to multiple sub-harmonics of the microwave frequency
, or to the same sub-harmonic over a wide range of by tuning
the DC current. In general, analysis of the locking range, linewidth, and
amplitude showed that the locking efficiency decreased as the harmonic number
increased, as expected for harmonic synchronization of a non-linear oscillator.
Time-resolved scanning Kerr microscopy (TRSKM) revealed significant differences
in the spatial character of the magnetization dynamics of states locked to the
fundamental and harmonic frequencies, suggesting significant differences in the
core trajectories within the same device. Super-harmonic injection locking of a
NC-STVO may open up possibilities for devices such as nanoscale frequency
dividers, while differences in the core trajectory may allow mutual
synchronisation to be achieved in multi-oscillator networks by tuning the
spatial character of the dynamics within shared magnetic layers.Comment: 21 pages, 8 figure
Expanding the Lorentz concept in magnetism
This is the final version. Available on open access from IOP Publishing via the DOI in this recordIn 1878, the Dutch physicist Hendrik Antoon Lorentz first addressed the calculation of the local electric field at an atomic site in a ferroelectric material, generated by all the other electric dipoles within the sample. This calculation, which applies equally well to ferromagnets, is taught in Universities around the World. Here we demonstrate that the Lorentz concept can be used to speed up calculations of the local dipolar field in square, circular, and elliptical shaped monolayers and thin films, not only at the center of the film, but across the sample. Calculations show that long elliptical and rectangular films should exhibit the narrowest ferromagnetic resonance (FMR) linewidth. In addition, discrete dipole calculations show that the Lorentz cavity field does not hold in tetragonal films. Depending on the ratio (b/a), the local field can be either less/greater than an observation that has implications for FMR. 3D simple cubic (SC) systems are also examined. For example, while most texts discuss the Lorentz cavity field in terms of a Lorentz sphere, the Lorentz cavity field still holds when a Lorentz sphere is replaced by a the Lorentz cube, but only in cubic SC, FCC and BCC systems. Finally, while the primary emphasis is on the discrete dipole–dipole interaction, contact is made with the continuum model. For example, in the continuous SC dipole model, just one monolayer is required to generate the Lorentz cavity field. This is in marked contrast to the discrete dipole model, where a minimum of five adjacent monolayers is required.Engineering and Physical Sciences Research Council (EPSRC
Direct observation of magnetization dynamics generated by nano-contact spin-torque vortex oscillators
Time-resolved scanning Kerr microscopy has been used to directly image the
magnetization dynamics of nano-contact (NC) spin-torque vortex oscillators
(STVOs) when phase-locked to an injected microwave (RF) current. The Kerr
images reveal free layer magnetization dynamics that extend outside the NC
footprint, where they cannot be detected electrically, but which are crucial to
phase-lock STVOs that share common magnetic layers. For a single NC, dynamics
were observed not only when the STVO frequency was fully locked to that of the
RF current, but also for a partially locked state characterized by periodic
changes in the core trajectory at the RF frequency. For a pair of NCs, images
reveal the spatial character of dynamics that electrical measurements show to
have enhanced amplitude and reduced linewidth. Insight gained from these images
may improve understanding of the conditions required for mutual phase-locking
of multiple STVOs, and hence enhanced microwave power emission.Comment: 10 pages, 3 figure
Investigation of the rise time and damping of spin excitations in Ni81Fe19 thin films
Copyright © 2001 American Institute of PhysicsThe rise and damping of spin excitations in three Ni81Fe19 films of thickness 50, 500, and 5000 Å have been studied with an optical pump–probe technique in which the sample is pumped with an optically triggered magnetic field pulse. The motion of the magnetization was described by the uniform mode solution of the Landau–Lifshitz–Gilbert equation. The rise time of the pulsed field within the film was smallest in the 50 Å sample and was generally greater when the pulsed field was perpendicular to the film plane. The damping constant was smallest in the 500 Å sample. The variations in the rise time and damping are attributed to the presence of eddy currents and structural disorder in the films. Under certain excitation conditions a second mode was observed in the 5000 Å sample which we believe to be a magnetostatic surface mode
Multi-color Optical and NIR Light Curves of 64 Stripped-Envelope Core-Collapse Supernovae
We present a densely-sampled, homogeneous set of light curves of 64 low
redshift (z < 0.05) stripped-envelope supernovae (SN of type IIb, Ib, Ic and
Ic-bl). These data were obtained between 2001 and 2009 at the Fred L. Whipple
Observatory (FLWO) on Mt. Hopkins in Arizona, with the optical FLWO 1.2-m and
the near-infrared PAIRITEL 1.3-m telescopes. Our dataset consists of 4543
optical photometric measurements on 61 SN, including a combination of UBVRI,
UBVr'i', and u'BVr'i', and 2142 JHKs near-infrared measurements on 25 SN. This
sample constitutes the most extensive multi-color data set of stripped-envelope
SN to date. Our photometry is based on template-subtracted images to eliminate
any potential host galaxy light contamination. This work presents these
photometric data, compares them with data in the literature, and estimates
basic statistical quantities: date of maximum, color, and photometric
properties. We identify promising color trends that may permit the
identification of stripped-envelope SN subtypes from their photometry alone.
Many of these SN were observed spectroscopically by the CfA SN group, and the
spectra are presented in a companion paper (Modjaz et al. 2014). A thorough
exploration that combines the CfA photometry and spectroscopy of
stripped-envelope core-collapse SN will be presented in a follow-up paper.Comment: 26 pages, 17 figures, 8 tables. Revised version resubmitted to ApJ
Supplements after referee report. Additional online material is available
through http://cosmo.nyu.edu/SNYU
Improved cosmological constraints on the curvature and equation of state of dark energy
We apply the Constitution compilation of 397 supernova Ia, the baryon
acoustic oscillation measurements including the parameter, the distance
ratio and the radial data, the five-year Wilkinson microwave anisotropy probe
and the Hubble parameter data to study the geometry of the universe and the
property of dark energy by using the popular Chevallier-Polarski-Linder and
Jassal-Bagla-Padmanabhan parameterizations. We compare the simple
method of joined contour estimation and the Monte Carlo Markov chain method,
and find that it is necessary to make the marginalized analysis on the error
estimation. The probabilities of and in the
Chevallier-Polarski-Linder model are skew distributions, and the marginalized
errors are ,
, , and
. For the Jassal-Bagla-Padmanabhan model, the
marginalized errors are ,
, , and
. The equation of state parameter of dark energy
is negative in the redshift range at more than level.
The flat CDM model is consistent with the current observational data
at the level.Comment: 10 figures, 12 pages, Classical and Quantum Gravity in press; v2 to
match the pulished versio
Imaging magnetisation dynamics in nano-contact spin-torque vortex oscillators exhibiting gyrotropic mode splitting
This is the author accepted manuscript. The final version is available from IOP Publishing via the DOI in this record.Nano-contact spin-torque vortex oscillators (STVOs) are anticipated to find application as nanoscale sources of microwave emission in future technological applications. Presently the output power and phase stability of individual STVOs are not competitive with existing oscillator technologies. Synchronisation of multiple nano-contact STVOs via magnetisation dynamics has been proposed to enhance the microwave emission. The control of device-to-device variations, such as mode splitting of the microwave emission, is essential if multiple STVOs are to be successfully synchronised. In this work a combination of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM) was used to demonstrate how mode splitting in the microwave emission of STVOs was related to the magnetisation dynamics that are generated. The free-running STVO response to a DC current only was used to identify devices and bias magnetic field configurations for which single and multiple modes of microwave emission were observed. Stroboscopic Kerr images were acquired by injecting a small amplitude RF current to phase lock the free-running STVO response. The images showed that the magnetisation dynamics of a multimode device with moderate splitting could be controlled by injecting an RF current so that they exhibit similar spatial character to that of a single mode. Significant splitting was found to result from a complicated equilibrium magnetic state that was observed in Kerr images as irregular spatial characteristics of the magnetisation dynamics. Such dynamics were observed far from the nano-contact and so their presence cannot be detected in electrical measurements. This work demonstrates that TRSKM is a powerful tool for the direct observation of the magnetisation dynamics generated by STVOs that exhibit complicated microwave emission. Characterisation of such dynamics outside the nano-contact perimeter permits a deeper insight into the requirements for optimal phase-locking of multiple STVOs that share common magnetic layers.The authors gratefully acknowledge the financial support of the Engineering and Physical Sciences Research Council under grants EP/I038470/1 and EP/K008501/1, the Royal Society under grant UF080837, the Swedish Research Council (VR), the Swedish Foundation for Strategic Research (SSF), and the Knut and Alice Wallenberg Foundation (KAW). The authors and co-authors declare that there are no conflicts of interes
Optical Spectra of 73 Stripped-Envelope Core-Collapse Supernovae
We present 645 optical spectra of 73 supernovae (SNe) of Types IIb, Ib, Ic,
and broad-lined Ic. All of these types are attributed to the core collapse of
massive stars, with varying degrees of intact H and He envelopes before
explosion. The SNe in our sample have a mean redshift = 4200 km/s. Most of
these spectra were gathered at the Harvard-Smithsonian Center for Astrophysics
(CfA) between 2004 and 2009. For 53 SNe, these are the first published spectra.
The data coverage range from mere identification (1-3 spectra) for a few SNe to
extensive series of observations (10-30 spectra) that trace the spectral
evolution for others, with an average of 9 spectra per SN. For 44 SNe of the 73
SNe presented here, we have well-determined dates of maximum light to determine
the phase of each spectrum. Our sample constitutes the most extensive spectral
library of stripped-envelope SNe to date. We provide very early coverage (as
early as 30 days before V-band max) for photospheric spectra, as well as
late-time nebular coverage when the innermost regions of the SNe are visible
(as late as 2 years after explosion, while for SN1993J, we have data as late as
11.6 years). This data set has homogeneous observations and reductions that
allow us to study the spectroscopic diversity of these classes of stripped SNe
and to compare these to SNe associated with gamma-ray bursts. We undertake
these matters in follow-up papers.Comment: Published by the Astronomical Journal in May 2015. All spectra are
publicly available at the CfA SN archive:
http://www.cfa.harvard.edu/supernova/SNarchive.html . A companion paper on
constructing SNID templates based on these spectra is by Liu & Modjaz (2014)
and the resulting SNID templates are available from the NYU website:
http://cosmo.nyu.edu/SNYU/spectra
Supernovae as seen by off-center observers in a local void
Inhomogeneous universe models have been proposed as an alternative
explanation for the apparent acceleration of the cosmic expansion that does not
require dark energy. In the simplest class of inhomogeneous models, we live
within a large, spherically symmetric void. Several studies have shown that
such a model can be made consistent with many observations, in particular the
redshift--luminosity distance relation for type Ia supernovae, provided that
the void is of Gpc size and that we live close to the center. Such a scenario
challenges the Copernican principle that we do not occupy a special place in
the universe. We use the first-year Sloan Digital Sky Survey-II supernova
search data set as well as the Constitution supernova data set to put
constraints on the observer position in void models, using the fact that
off-center observers will observe an anisotropic universe. We first show that a
spherically symmetric void can give good fits to the supernova data for an
on-center observer, but that the two data sets prefer very different voids. We
then continue to show that the observer can be displaced at least fifteen
percent of the void scale radius from the center and still give an acceptable
fit to the supernova data. When combined with the observed dipole anisotropy of
the cosmic microwave background however, we find that the data compells the
observer to be located within about one percent of the void scale radius. Based
on these results, we conclude that considerable fine-tuning of our position
within the void is needed to fit the supernova data, strongly disfavouring the
model from a Copernican principle point of view.Comment: 20 pages, 6 figures, matches the published versio
- …