517 research outputs found
The effect of confinement and defects on the thermal stability of skyrmions
The stability of magnetic skyrmions against thermal fluctuations and external
perturbations is investigated within the framework of harmonic transition state
theory for magnetic degrees of freedom. The influence of confined geometry and
atomic scale non-magnetic defects on the skyrmion lifetime is estimated. It is
shown that a skyrmion on a track has lower activation energy for annihilation
and higher energy for nucleation if the size of the skyrmion is comparable with
the width of the track. Two mechanisms of skyrmion annihilation are considered:
inside the track and escape through the boundary. For both mechanisms, the
dependence of activation energy on the track width is calculated. Non-magnetic
defects are found to localize skyrmions in their neighborhood and strongly
decrease the activation energy for creation and annihilation. This is in
agreement with experimental measurements that have found nucleation of
skyrmions in presence of spin-polarized current preferably occurring near
structural defects
Energy surface and lifetime of magnetic skyrmions
The stability of skyrmions in various environments is estimated by analyzing
the multidimensional surface describing the energy of the system as a function
of the directions of the magnetic moments in the system. The energy is given by
a Heisenberg-like Hamiltonian that includes Dzyaloshinskii-Moriya interaction,
anisotropy and external magnetic field. Local minima on this surface correspond
to the ferromagnetic and skyrmion states. Minimum energy paths (MEP) between
the minima are calculated using the geodesic nudged elastic band method. The
maximum energy along an MEP corresponds to a first order saddle point on the
energy surface and gives an estimate of the activation energy for the magnetic
transition, such as creation and annihilation of a skyrmion. The
pre-exponential factor in the Arrhenius law for the rate, the so-called attempt
frequency, is estimated within harmonic transition state theory where the
eigenvalues of the Hessian at the saddle point and the local minima are used to
characterize the shape of the energy surface. For some degrees of freedom,
so-called 'zero modes', the energy of the system remains invariant. They need
to be treated separately and give rise to temperature dependence of the attempt
frequency. As an example application of this general theory, the lifetime of a
skyrmion in a track of finite width for a PdFe overlayer on a Ir(111) substrate
is calculated as a function of track width and external magnetic field. Also,
the effect of non-magnetic impurities is studied. Various MEPs for annihilation
inside a track, via the boundary of a track and at an impurity are presented.
The attempt frequency as well as the activation energy has been calculated for
each mechanism to estimate the transition rate as a function of temperature
The International Surface Pressure Databank version 2
The International Surface Pressure Databank (ISPD) is the world's largest collection of global surface and sea-level pressure observations. It was developed by extracting observations from established international archives, through international cooperation with data recovery facilitated by the Atmospheric Circulation Reconstructions over the Earth (ACRE) initiative, and directly by contributing universities, organizations, and countries. The dataset period is currently 1768–2012 and consists of three data components: observations from land stations, marine observing systems, and tropical cyclone best track pressure reports. Version 2 of the ISPD (ISPDv2) was created to be observational input for the Twentieth Century Reanalysis Project (20CR) and contains the quality control and assimilation feedback metadata from the 20CR. Since then, it has been used for various general climate and weather studies, and an updated version 3 (ISPDv3) has been used in the ERA-20C reanalysis in connection with the European Reanalysis of Global Climate Observations project (ERA-CLIM). The focus of this paper is on the ISPDv2 and the inclusion of the 20CR feedback metadata. The Research Data Archive at the National Center for Atmospheric Research provides data collection and access for the ISPDv2, and will provide access to future versions
Atomic-scale modeling of the deformation of nanocrystalline metals
Nanocrystalline metals, i.e. metals with grain sizes from 5 to 50 nm, display
technologically interesting properties, such as dramatically increased
hardness, increasing with decreasing grain size. Due to the small grain size,
direct atomic-scale simulations of plastic deformation of these materials are
possible, as such a polycrystalline system can be modeled with the
computational resources available today.
We present molecular dynamics simulations of nanocrystalline copper with
grain sizes up to 13 nm. Two different deformation mechanisms are active, one
is deformation through the motion of dislocations, the other is sliding in the
grain boundaries. At the grain sizes studied here the latter dominates, leading
to a softening as the grain size is reduced. This implies that there is an
``optimal'' grain size, where the hardness is maximal.
Since the grain boundaries participate actively in the deformation, it is
interesting to study the effects of introducing impurity atoms in the grain
boundaries. We study how silver atoms in the grain boundaries influence the
mechanical properties of nanocrystalline copper.Comment: 10 pages, LaTeX2e, PS figures and sty files included. To appear in
Mater. Res. Soc. Symp. Proc. vol 538 (invited paper). For related papers, see
http://www.fysik.dtu.dk/~schiotz/publist.htm
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