29 research outputs found
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
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
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
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
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
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
Phenomenological description of the nonlocal magnetization relaxation in magnonics, spintronics, and domain-wall dynamics
A phenomenological equation called Landau-Lifshitz-Baryakhtar (LLBar)
equation, which could be viewed as the combination of Landau-Lifshitz (LL)
equation and an extra "exchange damping" term, was derived by Baryakhtar using
Onsager's relations. We interpret the origin of this "exchange damping" as
nonlocal damping by linking it to the spin current pumping. The LLBar equation
is investigated numerically and analytically for the spin wave decay and domain
wall motion. Our results show that the lifetime and propagation length of
short-wavelength magnons in the presence of nonlocal damping could be much
smaller than those given by LL equation. Furthermore, we find that both the
domain wall mobility and the Walker breakdown field are strongly influenced by
the nonlocal damping.Comment: 10 pages, 6 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