85 research outputs found
Heavily loaded ferrite-polymer composites to produce high refractive index materials at centimetre wavelengths
A cold-pressing technique has been developed for fabricating composites composed of a polytetrafluoroethylene-polymer matrix and a wide range of volume-fractions of MnZn-ferrite filler (0%â80%). The electromagnetic properties at centimetre wavelengths of all prepared composites exhibited good reproducibility, with the most heavily loaded composites possessing simultaneously high permittivity (180 ± 10) and permeability (23±2). The natural logarithm of both the relative complex permittivity and permeability shows an approximately linear dependence with the volume fraction of ferrite. Thus, this simple method allows for the manufacture of bespoke materials required in the design and construction of devices based on the principles of transformation optics
SN 2006bt: A Perplexing, Troublesome, and Possibly Misleading Type Ia Supernova
SN 2006bt displays characteristics unlike those of any other known Type Ia
supernova (SN Ia). We present optical light curves and spectra of SN 2006bt
which demonstrate the peculiar nature of this object. SN 2006bt has broad,
slowly declining light curves indicative of a hot, high-luminosity SN, but
lacks a prominent second maximum in the i band as do low-luminosity SNe Ia. Its
spectra are similar to those of low-luminosity SNe Ia, containing features that
are only present in cool SN photospheres. Light-curve fitting methods suggest
that SN 2006bt is reddened by a significant amount of dust; however, it
occurred in the outskirts of its early-type host galaxy and has no strong Na D
absorption in any of its spectra, suggesting a negligible amount of host-galaxy
dust absorption. C II is possibly detected in our pre-maximum spectra, but at a
much lower velocity than other elements. The progenitor was likely very old,
being a member of the halo population of a galaxy that shows no signs of recent
star formation. SNe Ia have been very successfully modeled as a one-parameter
family, and this is fundamental to their use as cosmological distance
indicators. SN 2006bt is a challenge to that picture, yet its relatively normal
light curves allowed SN 2006bt to be included in cosmological analyses. We
generate mock SN Ia datasets which indicate that contamination by similar
objects will both increase the scatter of a SN Ia Hubble diagram and
systematically bias measurements of cosmological parameters. However, spectra
and rest-frame i-band light curves should provide a definitive way to identify
and eliminate such objects.Comment: ApJ, accepted. 13 pages, 13 figure
Early and Late-Time Observations of SN 2008ha: Additional Constraints for the Progenitor and Explosion
We present a new maximum-light optical spectrum of the the extremely low
luminosity and exceptionally low energy Type Ia supernova (SN Ia) 2008ha,
obtained one week before the earliest published spectrum. Previous observations
of SN 2008ha were unable to distinguish between a massive star and white dwarf
origin for the SN. The new maximum-light spectrum, obtained one week before the
earliest previously published spectrum, unambiguously shows features
corresponding to intermediate mass elements, including silicon, sulfur, and
carbon. Although strong silicon features are seen in some core-collapse SNe,
sulfur features, which are a signature of carbon/oxygen burning, have always
been observed to be weak in such events. It is therefore likely that SN 2008ha
was the result of a thermonuclear explosion of a carbon-oxygen white dwarf.
Carbon features at maximum light show that unburned material is present to
significant depths in the SN ejecta, strengthening the case that SN 2008ha was
a failed deflagration. We also present late-time imaging and spectroscopy that
are consistent with this scenario.Comment: ApJL, accepted. 5 pages, 3 figure
All-optical control of spin in a 2D van der Waals magnet
Two-dimensional (2D) van der Waals magnets provide new opportunities for control of magnetism at the nanometre scale via mechanisms such as strain, voltage and the photovoltaic effect. Ultrafast laser pulses promise the fastest and most energy efficient means of manipulating electron spin and can be utilized for information storage. However, little is known about how laser pulses influence the spins in 2D magnets. Here we demonstrate laser-induced magnetic domain formation and all-optical switching in the recently discovered 2D van der Waals ferromagnet CrI(3). While the magnetism of bare CrI(3) layers can be manipulated with single laser pulses through thermal demagnetization processes, all-optical switching is achieved in nanostructures that combine ultrathin CrI(3) with a monolayer of WSe(2). The out-of-plane magnetization is switched with multiple femtosecond pulses of either circular or linear polarization, while single pulses result in less reproducible and partial switching. Our results imply that spin-dependent interfacial charge transfer between the WSe(2) and CrI(3) is the underpinning mechanism for the switching, paving the way towards ultrafast optical control of 2D van der Waals magnets for future photomagnetic recording and device technology
Glancing-angle deposition of magnetic in-plane exchange springs
Magnetic exchange springs (ESs) are composed of exchange-coupled hard and soft magnetic layers,
i.e., layers with high and low anisotropy, respectively. The moments in the soft layer can be wound up by
applying an external field, which has to be smaller than the anisotropy field of the hard layer. Alternatively,
an ES can be realized by biasing the soft magnetic layer by two adjacent hard magnetic layers with different
magnetic anisotropy directions. We have fabricated an ES layer stack by magnetron sputter deposition.
As the hard magnetic bottom layer, we used epitaxial FePt L10, and as the top layer Co with both layers
having different in-plane easy axes. These hard layers pin the moments of a soft permalloy (Ni81Fe19) layer
sandwiched between them, winding up an ES at remanence. The anisotropy of the polycrystalline top Co
layer was engineered by glancing-angle deposition to have in-plane easy axis anisotropy perpendicular to
the easy direction of the bottom layer. Using soft x-ray spectroscopy and magneto-optical measurements,
we found the in-plane ES to extend from the soft layer into the top layer of our FePt/permalloy/Co trilayer
structure
Time resolved scanning Kerr microscopy of flux beam formation in hard disk write heads
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.The underlying research dataset supporting this publication is available under a Creative
Commons Attribution-ShareAlike 4.0 International License (https://creativecommons.
org/licenses/by-sa/4.0/) and can be publicly accessed in Open Research Exeter via the
following persistent identifier: http://hdl.handle.net/10871/21108.To meet growing data storage needs, the density of data stored on hard disk drives
must increase. In pursuit of this aim the magnetodynamics of the hard disk write head
must be characterized and understood, particularly the process of âflux beamingâ. In
this study, seven different configurations of perpendicular magnetic recording (PMR)
write heads were imaged using time-resolved scanning Kerr microscopy, revealing
their detailed dynamic magnetic state during the write process. It was found that
the precise position and number of driving coils can significantly alter the formation
of flux beams during the write process. These results are applicable to the design and
understanding of current PMR and next-generation heat-assisted magnetic recording
(HAMR) devices, as well as being relevant to other magnetic devices.The authors gratefully acknowledge financial support from the Seagate Plan
Laser-induced topological spin switching in a 2D van der Waals magnet
Two-dimensional (2D) van der Waals (vdW) magnets represent one of the most
promising horizons for energy-efficient spintronic applications because their
broad range of electronic, magnetic and topological properties. Of particular
interest is the control of the magnetic properties of 2D materials by
femtosecond laser pulses which can provide a real path for low-power
consumption device platforms in data storage industries. However, little is
known about the interplay between light and spin properties in vdW layers.
Here, combining large-scale spin dynamics simulations including biquadratic
exchange interactions and wide-field Kerr microscopy (WFKM), we show that
ultrafast laser excitation can not only generate different type of spin
textures in CrGeTe vdW magnets but also induce a reversible transformation
between them in a toggle-switch mechanism. Our calculations show that
skyrmions, anti-skyrmions, skyrmioniums and stripe domains can be generated via
high-intense laser pulses within the picosecond regime. The effect is tunable
with the laser energy where different spin behaviours can be selected, such as
fast demagnetisation process (250 fs) important for information
technologies. The phase transformation between the different topological spin
textures is obtained as additional laser pulses are applied to the system where
the polarisation and final state of the spins can be controlled by external
magnetic fields. We experimentally confirmed the creation, manipulation and
toggle switching phenomena in CrGeTe due to the unique aspect of
laser-induced heating of electrons. Our results indicate laser-driven spin
textures on 2D magnets as a pathway towards ultrafast reconfigurable
architecture at the atomistic level
Effect of sink layer thickness on damping in CoMnGe (5 nm) / Ag (6 nm) / NiFe (x nm) spin valves
Poster presented at Magnetism 4 â 5 April 2016, Sheffield.In spin valve structures the damping of a ferromagnetic layer driven at resonance can be modified by the transfer of spin angular momentum into a âsinkâ ferromagnetic layer. This effect, known as spin pumping, is interface dominated and expected to increase with increasing sink layer thickness up to a saturation absorption depth, previously reported to be 1.2 nm regardless of the sink layerâs composition [1]. Using vector network analyser ferromagnetic resonance (VNA-FMR), we have studied the variation in damping as a function of sink layer thickness in a series of CoMnGe (5 nm) / Ag (6 nm) / NiFe (x nm) spin valves. These measurements show only small variations in the CoMnGe Gilbert damping parameter for x †1.8 nm, although damping is observed to increase at x = 0.3 and 0.6 nm. Element-resolved x-ray detected ferromagnetic resonance (XFMR) [2] measurements confirm spin transfer torque due to spin pumping as the origin of the damping for x = 1.5 and 1.8 nm, with both thicknesses having the same effective spin mixing conductance, supporting the findings of Ghosh et al [1]. For thicker sink layers the source and sink FMR fields are seen to coincide, hampering the identification of spin pumping.
[1] A Ghosh, et al. Physical Review Letters 109, 127202 (2012)
[2] M Marcham, et al. Physical Review B 87, 180403 (2013)We thank the Advanced Light Source for access to beamlines 4.0.2 and 6.3.1 (ALS-06433, ALS-07116). The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.We thank Diamond Light Source for access to beamlines I06 and I10 (SI8782, SI11585, SI13063) that contributed to the results presented here.This work was supported by the Engineering and Physical Sciences Research Council [grant number EP/J018767/1]
The Low-Velocity, Rapidly Fading Type Ia Supernova 2002es
SN 2002es is a peculiar subluminous Type Ia supernova (SN Ia) with a
combination of observed characteristics never before seen in a SN Ia. At
maximum light, SN 2002es shares spectroscopic properties with the underluminous
SN 1991bg subclass of SNe Ia, but with substantially lower expansion velocities
(~6000 km/s) more typical of the SN 2002cx subclass. Photometrically, SN 2002es
differs from both SN 1991bg-like and SN 2002cx-like supernovae. Although at
maximum light it is subluminous (M_B=-17.78 mag), SN 2002es has a relatively
broad light curve (Dm15(B)=1.28 +/- 0.04 mag), making it a significant outlier
in the light-curve width vs. luminosity relationship. We estimate a 56Ni mass
of 0.17 +/- 0.05 M_sun synthesized in the explosion, relatively low for a SN
Ia. One month after maximum light, we find an unexpected plummet in the
bolometric luminosity. The late-time decay of the light curves is inconsistent
with our estimated 56Ni mass, indicating that either the light curve was not
completely powered by 56Ni decay or the ejecta became optically thin to
gamma-rays within a month after maximum light. The host galaxy is classified as
an S0 galaxy with little to no star formation, indicating the progenitor of SN
2002es is likely from an old stellar population. We also present a less
extensive dataset for SN 1999bh, an object which shares similar observed
properties. Both objects were found as part of the Lick Observatory Supernova
Search, allowing us to estimate that these objects should account for ~2.5% of
SNe Ia within a fixed volume. We find that current theoretical models are
unable to explain the observed of characteristics of SN 2002es.Comment: 19 pages, 15 figures, Submitted to Ap
- âŠ