2,436 research outputs found
Nmag micromagnetic simulation tool - software engineering lessons learned
We review design and development decisions and their impact for the open
source code Nmag from a software engineering in computational science point of
view. We summarise lessons learned and recommendations for future computational
science projects. Key lessons include that encapsulating the simulation
functionality in a library of a general purpose language, here Python, provides
great flexibility in using the software. The choice of Python for the top-level
user interface was very well received by users from the science and engineering
community. The from-source installation in which required external libraries
and dependencies are compiled from a tarball was remarkably robust. In places,
the code is a lot more ambitious than necessary, which introduces unnecessary
complexity and reduces main- tainability. Tests distributed with the package
are useful, although more unit tests and continuous integration would have been
desirable. The detailed documentation, together with a tutorial for the usage
of the system, was perceived as one of its main strengths by the community.Comment: 7 pages, 5 figures, Software Engineering for Science, ICSE201
Mixed-precision AMG as linear equation solver for definite systems
AbstractThe performance of algebraic multigrid (AMG) algorithms, implemented in 4-byte floating point arithmetic, is investigated on modern cluster architecture with multi-core CPUs. The algorithmic considerations comprise the effect of preconditioning in 4-byte floating point arithmetic on Krylov solvers using standard 8-byte floating point arithmetic. The data of basic linear algebra benchmarks are used to interpret the performance of AMG algorithms employed as linear solvers in computational fluid dynamics simulation tools
Electric quantum walks with individual atoms
We report on the experimental realization of electric quantum walks, which
mimic the effect of an electric field on a charged particle in a lattice.
Starting from a textbook implementation of discrete-time quantum walks, we
introduce an extra operation in each step to implement the effect of the field.
The recorded dynamics of such a quantum particle exhibits features closely
related to Bloch oscillations and interband tunneling. In particular, we
explore the regime of strong fields, demonstrating contrasting quantum
behaviors: quantum resonances vs. dynamical localization depending on whether
the accumulated Bloch phase is a rational or irrational fraction of 2\pi.Comment: 5 pages, 4 figure
Frequency-based nanoparticle sensing over large field ranges using the ferromagnetic resonances of a magnetic nanodisc
Using finite element micromagnetic simulations, we study how resonant
magnetisation dynamics in thin magnetic discs with perpendicular anisotropy are
influenced by magnetostatic coupling to a magnetic nanoparticle. We identify
resonant modes within the disc using direct magnetic eigenmode calculations and
study how their frequencies and profiles are changed by the nanoparticle's
stray magnetic field. We demonstrate that particles can generate shifts in the
resonant frequency of the disc's fundamental mode which exceed resonance
linewidths in recently studied spin torque oscillator devices. Importantly, it
is shown that the simulated shifts can be maintained over large field ranges
(here up to 1T). This is because the resonant dynamics (the basis of
nanoparticle detection here) respond directly to the nanoparticle stray field,
i.e. detection does not rely on nanoparticle-induced changes to the magnetic
ground state of the disk. A consequence of this is that in the case of small
disc-particle separations, sensitivities to the particle are highly mode- and
particle-position-dependent, with frequency shifts being maximised when the
intense stray field localised directly beneath the particle can act on a large
proportion of the disc's spins that are undergoing high amplitude precession.Comment: 9 pages, 9 figures. Updated version from 31.7.2016 includes minor
changes in introduction and sections III.C and III.D (additional information
linking the results to real-world bio-sensing devices
Resonant translational, breathing and twisting modes of pinned transverse magnetic domain walls
We study translational, breathing and twisting resonant modes of transverse
magnetic domain walls pinned at notches in ferromagnetic nanostrips. We
demonstrate that a mode's sensitivity to notches depends strongly on the
characteristics of that particular resonance. For example, the frequencies of
modes involving lateral motion of the wall are the ones which are most
sensitive to changes in the notch intrusion depth (especially at the narrower,
more strongly confined end of the domain wall). In contrast, the breathing
mode, whose dynamics are concentrated away from the notches is relatively
insensitive to changes in the notches' sizes. We also demonstrate a sharp drop
in the translational mode's frequency towards zero when approaching depinning
which is found, using a harmonic oscillator model, to be consistent with a
reduction in the local slope of the notch-induced confining potential at its
edge.Comment: 11 pages, 10 figures, additional data and analysi
Magnon-Driven Domain-Wall Motion with the Dzyaloshinskii-Moriya Interaction
We study domain wall (DW) motion induced by spin waves (magnons) in the
presence of Dzyaloshinskii-Moriya interaction (DMI). The DMI exerts a torque on
the DW when spin waves pass through the DW, and this torque represents a linear
momentum exchange between the spin wave and the DW. Unlike angular momentum
exchange between the DW and spin waves, linear momentum exchange leads to a
rotation of the DW plane rather than a linear motion. In the presence of an
effective easy plane anisotropy, this DMI induced linear momentum transfer
mechanism is significantly more efficient than angular momentum transfer in
moving the DW
Virtual micromagnetics: a framework for accessible and reproducible micromagnetic simulation
Computational micromagnetics requires numerical solution of partial differential equations to resolve complex interactions in magnetic nanomaterials. The Virtual Micromagnetics project described here provides virtual machine simulation environments to run open-source micromagnetic simulation packages [1]. These environments allow easy access to simulation packages that are often difficult to compile and install, and enable simulations and their data to be shared and stored in a single virtual hard disk file, which encourages reproducible research. Virtual Micromagnetics can be extended to automate the installation of micromagnetic simulation packages on non-virtual machines, and to support closed-source and new open-source simulation packages, including packages from disciplines other than micromagnetics, encouraging reuse. Virtual Micromagnetics is stored in a public GitHub repository under a three-clause Berkeley Software Distribution (BSD) license
HOAX: A Hyperparameter Optimization Algorithm Explorer for Neural Networks
Computational chemistry has become an important tool to predict and
understand molecular properties and reactions. Even though recent years have
seen a significant growth in new algorithms and computational methods that
speed up quantum chemical calculations, the bottleneck for trajectory-based
methods to study photoinduced processes is still the huge number of electronic
structure calculations. In this work, we present an innovative solution, in
which the amount of electronic structure calculations is drastically reduced,
by employing machine learning algorithms and methods borrowed from the realm of
artificial intelligence. However, applying these algorithms effectively
requires finding optimal hyperparameters, which remains a challenge itself.
Here we present an automated user-friendly framework, HOAX, to perform the
hyperparameter optimization for neural networks, which bypasses the need for a
lengthy manual process. The neural network generated potential energy surfaces
(PESs) reduces the computational costs compared to the ab initio-based PESs. We
perform a comparative investigation on the performance of different
hyperparameter optimiziation algorithms, namely grid search, simulated
annealing, genetic algorithm, and bayesian optimizer in finding the optimal
hyperparameters necessary for constructing the well-performing neural network
in order to fit the PESs of small organic molecules. Our results show that this
automated toolkit not only facilitate a straightforward way to perform the
hyperparameter optimization but also the resulting neural networks-based
generated PESs are in reasonable agreement with the ab initio-based PESs.Comment: 18 page
A Spline-Based Partial Element Equivalent Circuit Method for Electrostatics
This contribution investigates the connection between Isogeometric Analysis (IgA) and the Partial Element Equivalent Circuit (PEEC) method for electrostatic problems. We demonstrate that using the spline-based geometry concepts from IgA allows for extracting circuit elements without an explicit meshing step. Moreover, the proposed IgA-PEEC method converges for complex geometries up to three times faster than the conventional PEEC approach and, in turn, it requires a significantly lower number of degrees of freedom to solve a problem with comparable accuracy. The resulting method is closely related to the isogeometric boundary element method. However, it uses lowest-order basis functions to allow for straightforward physical and circuit interpretations. The findings are validated by an analytical example with complex geometry, i.e., significant curvature, and by a realistic model of a surge arrester
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