4,368 research outputs found
Magnetization profile for impurities in graphene nanoribbons
The magnetic properties of graphene-related materials and in particular the
spin-polarised edge states predicted for pristine graphene nanoribbons (GNRs)
with certain edge geometries have received much attention recently due to a
range of possible technological applications. However, the magnetic properties
of pristine GNRs are not predicted to be particularly robust in the presence of
edge disorder. In this work, we examine the magnetic properties of GNRs doped
with transition-metal atoms using a combination of mean-field Hubbard and
Density Functional Theory techniques. The effect of impurity location on the
magnetic moment of such dopants in GNRs is investigated for the two principal
GNR edge geometries - armchair and zigzag. Moment profiles are calculated
across the width of the ribbon for both substitutional and adsorbed impurities
and regular features are observed for zigzag-edged GNRs in particular. Unlike
the case of edge-state induced magnetisation, the moments of magnetic
impurities embedded in GNRs are found to be particularly stable in the presence
of edge disorder. Our results suggest that the magnetic properties of
transition-metal doped GNRs are far more robust than those with moments arising
intrinsically due to edge geometry.Comment: submitte
Impurity segregation in graphene nanoribbons
The electronic properties of low-dimensional materials can be engineered by
doping, but in the case of graphene nanoribbons (GNR) the proximity of two
symmetry-breaking edges introduces an additional dependence on the location of
an impurity across the width of the ribbon. This introduces energetically
favorable locations for impurities, leading to a degree of spatial segregation
in the impurity concentration. We develop a simple model to calculate the
change in energy of a GNR system with an arbitrary impurity as that impurity is
moved across the ribbon and validate its findings by comparison with ab initio
calculations. Although our results agree with previous works predicting the
dominance of edge disorder in GNR, we argue that the distribution of adsorbed
impurities across a ribbon may be controllable by external factors, namely an
applied electric field. We propose that this control over impurity segregation
may allow manipulation and fine-tuning of the magnetic and transport properties
of GNRs.Comment: 5 pages, 4 figures, submitte
Deflection of (anti)ferromagnetic skyrmions at heterochiral interfaces
Devising magnetic nanostructures with spatially heterogeneous
Dzyaloshinskii-Moriya interaction (DMI) is a promising pathway towards advanced
confinement and control of magnetic skyrmions in potential devices. Here we
discuss theoretically how a skyrmion interacts with a heterochiral interface
using micromagnetic simulations and analytic arguments. We show that a
heterochiral interface deflects the trajectory of ferromagnetic (FM) skyrmions,
and that the extent of such deflection is tuned by the applied spin-polarized
current and the difference in DMI across the interface. Further, we show that
this deflection is characteristic for the FM skyrmion, and is completely absent
in the antiferromagnetic (AFM) case. In turn, we reveal that the AFM skyrmion
achieves much higher velocities than its FM counterpart, yet experiences far
stronger confinement in nanoengineered heterochiral tracks, which reinforces
AFM skyrmions as a favorable choice for skyrmion-based devices
Manipulation of Magnetic Skyrmions by Superconducting Vortices in Ferromagnet-Superconductor Heterostructures
Dynamics of magnetic skyrmions in hybrid ferromagnetic films harbors novel
physical phenomena and holds promise for technological applications. In this
work, we discuss the behavior of magnetic skyrmions when coupled to
superconducting vortices in a ferromagnet-superconductor heterostructure. We
use numerical simulations and analytic arguments to reveal broader
possibilities for manipulating the skyrmion-vortex dynamic correlations in the
hybrid system, that are not possible in its separated constituents. We explore
the thresholds of particular dynamic phases, and quantify the phase diagram as
a function of the relevant material parameters, applied current and induced
magnetic torques. Finally, we demonstrate the broad and precise tunability of
the skyrmion Hall-angle in presence of vortices, with respect to currents
applied to either or both the superconductor and the ferromagnet within the
heterostructure
Accelerated freshening of Antarctic Bottom Water over the last decade in the Southern Indian Ocean
© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Science Advances 3 (2017): e1601426, doi:10.1126/sciadv.1601426.Southern Ocean abyssal waters, in contact with the atmosphere at their formation sites around Antarctica, not only bring signals of a changing climate with them as they move around the globe but also contribute to that change through heat uptake and sea level rise. A repeat hydrographic line in the Indian sector of the Southern Ocean, occupied three times in the last two decades (1994, 2007, and, most recently, 2016), reveals that Antarctic Bottom Water (AABW) continues to become fresher (0.004 ± 0.001 kg/g decade−1), warmer (0.06° ± 0.01°C decade−1), and less dense (0.011 ± 0.002 kg/m3 decade−1). The most recent observations in the Australian-Antarctic Basin show a particularly striking acceleration in AABW freshening between 2007 and 2016 (0.008 ± 0.001 kg/g decade−1) compared to the 0.002 ± 0.001 kg/g decade−1 seen between 1994 and 2007. Freshening is, in part, responsible for an overall shift of the mean temperature-salinity curve toward lower densities. The marked freshening may be linked to an abrupt iceberg-glacier collision and calving event that occurred in 2010 on the George V/Adélie Land Coast, the main source region of bottom waters for the Australian-Antarctic Basin. Because AABW is a key component of the global overturning circulation, the persistent decrease in bottom water density and the associated increase in steric height that result from continued warming and freshening have important consequences beyond the Southern Indian Ocean.The 2016 I08S cruise and the analysis and science performed at sea, as well as the individual principal investigators were funded through multiple National Oceanic and Atmospheric Administration (NOAA) and NSF grants including NSF grant OCE-1437015. The research for this article was mainly completed at sea. For land-based work, V.V.M. relied on her postdoctoral funding through NSF grant OCE-1435665, and A.M.M. was supported in part by NSF grant OCE-1356630 and NOAA grant NA11OAR4310063
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