52 research outputs found
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
Thermal stability and topological protection of skyrmions in nanotracks
Magnetic skyrmions are hailed as a potential technology for data storage and
other data processing devices. However, their stability against thermal
fluctuations is an open question that must be answered before skyrmion-based
devices can be designed. In this work, we study paths in the energy landscape
via which the transition between the skyrmion and the uniform state can occur
in interfacial Dzyaloshinskii-Moriya finite-sized systems. We find three
mechanisms the system can take in the process of skyrmion nucleation or
destruction and identify that the transition facilitated by the boundary has a
significantly lower energy barrier than the other energy paths. This clearly
demonstrates the lack of the skyrmion topological protection in finite-sized
magnetic systems. Overall, the energy barriers of the system under
investigation are too small for storage applications at room temperature, but
research into device materials, geometry and design may be able to address
this
Hysteresis of nanocylinders with Dzyaloshinskii-Moriya interaction
The potential for application of magnetic skyrmions in high density storage
devices provides a strong drive to investigate and exploit their stability and
manipulability. Through a three-dimensional micromagnetic hysteresis study, we
investigate the question of existence of skyrmions in cylindrical
nanostructures of variable thickness. We quantify the applied field and
thickness dependence of skyrmion states, and show that these states can be
accessed through relevant practical hysteresis loop measurement protocols. As
skyrmionic states have yet to be observed experimentally in confined
helimagnetic geometries, our work opens prospects for developing viable
hysteresis process-based methodologies to access and observe skyrmionic states.Comment: 4 pages, 2 figure
Ground state search, hysteretic behaviour, and reversal mechanism of skyrmionic textures in confined helimagnetic nanostructures
Magnetic skyrmions have the potential to provide solutions for low-power,
high-density data storage and processing. One of the major challenges in
developing skyrmion-based devices is the skyrmions' magnetic stability in
confined helimagnetic nanostructures. Through a systematic study of equilibrium
states, using a full three-dimensional micromagnetic model including
demagnetisation effects, we demonstrate that skyrmionic textures are the lowest
energy states in helimagnetic thin film nanostructures at zero external
magnetic field and in absence of magnetocrystalline anisotropy. We also report
the regions of metastability for non-ground state equilibrium configurations.
We show that bistable skyrmionic textures undergo hysteretic behaviour between
two energetically equivalent skyrmionic states with different core orientation,
even in absence of both magnetocrystalline and demagnetisation-based shape
anisotropies, suggesting the existence of Dzyaloshinskii-Moriya-based shape
anisotropy. Finally, we show that the skyrmionic texture core reversal dynamics
is facilitated by the Bloch point occurrence and propagation.Comment: manuscript: 14 pages, 7 figures; supplementary information: 8 pages,
7 figure
Skyrmions in thin films with easy-plane magnetocrystalline anisotropy
We demonstrate that chiral skyrmionic magnetization configurations can be
found as the minimum energy state in B20 thin film materials with easy-plane
magnetocrystalline anisotropy with an applied magnetic field perpendicular to
the film plane. Our observations contradict results from prior analytical work,
but are compatible with recent experimental investigations. The size of the
observed skyrmions increases with the easy-plane magnetocrystalline anisotropy.
We use a full micromagnetic model including demagnetization and a
three-dimensional geometry to find local energy minimum (metastable)
magnetization configurations using numerical damped time integration. We
explore the phase space of the system and start simulations from a variety of
initial magnetization configurations to present a systematic overview of
anisotropy and magnetic field parameters for which skyrmions are metastable and
global energy minimum (stable) states.Comment: 5 pages, 3 figure
Stable and manipulable Bloch point
The prediction of magnetic skyrmions being used to change the way we store
and process data has led to materials with Dzyaloshinskii-Moriya interaction
coming into the focus of intensive research. So far, studies have looked mostly
at magnetic systems composed of materials with single chirality. In a search
for potential future spintronic devices, combination of materials with
different chirality into a single system may represent an important new avenue
for research. Using finite element micromagnetic simulations, we study an FeGe
disk with two layers of different chirality. We show that for particular
thicknesses of layers, a stable Bloch point emerges at the interface between
two layers. In addition, we demonstrate that the system undergoes hysteretic
behaviour and that two different types of Bloch point exist. These
`head-to-head' and `tail-to-tail' Bloch point configurations can, with the
application of an external magnetic field, be switched between. Finally, by
investigating the time evolution of the magnetisation field, we reveal the
creation mechanism of the Bloch point. Our results introduce a stable and
manipulable Bloch point to the collection of particle-like state candidates for
the development of future spintronic devices.Comment: 8 pages, 4 figure
Fidimag – A Finite Difference Atomistic and Micromagnetic Simulation Package
Fidimag is an open-source scientific code for the study of magnetic materials at the nano- or micro-scale using either atomistic or finite difference micromagnetic simulations, which are based on solving the Landau-Lifshitz-Gilbert equation. In addition, it implements simple procedures for calculating energy barriers in the magnetisation through variants of the nudged elastic band method. This computer software has been developed with the aim of creating a simple code structure that can be readily installed, tested, and extended. An agile development approach was adopted, with a strong emphasis on automated builds and tests, and reproducibility of results. The main code and interface to specify simulations are written in Python, which allows simple and readable simulation and analysis configuration scripts. Computationally costly calculations are written in C and exposed to the Python interface as Cython extensions. Docker containers are shipped for a convenient setup experience. The code is freely available on GitHub and includes documentation and examples in the form of Jupyter notebooks. Funding Statement: We acknowledge financial support from EPSRC’s Centre for Doctoral Training in Next Generation Computational Modelling, (EP/L015382/1), EPSRC’s Doctoral Training Centre in Complex System Simulation (EP/G03690X/1), CONICYT Chilean scholarship programme Becas Chile (72140061), Horizon 2020 European Research Infrastructure project OpenDreamKit (676541), National Natural Science Foundation of China (11604169), and the Gordon and Betty Moore Foundation through Grant GBMF #4856, by the Alfred P. Sloan Foundation and by the Helmsley Trust
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