71 research outputs found
Multiscale and multimodel simulation of Bloch point dynamics
We present simulation results on the structure and dynamics of micromagnetic
point singularities with atomistic resolution. This is achieved by embedding an
atomistic computational region into a standard micromagnetic algorithm. Several
length scales are bridged by means of an adaptive mesh refinement and a
seamless coupling between the continuum theory and a Heisenberg formulation for
the atomistic region. The code operates on graphical processing units and is
able to detect and track the position of strongly inhomogeneous magnetic
regions. This enables us to reliably simulate the dynamics of Bloch points,
which means that a fundamental class of micromagnetic switching processes can
be analyzed with unprecedented accuracy. We test the code by comparing it with
established results and present its functionality with the example of a
simulated field-driven Bloch point motion in a soft-magnetic cylinder
Numerical micromagnetism of strong inhomogeneities
The size of micromagnetic structures, such as domain walls or vortices, is
comparable to the exchange length of the ferromagnet. Both, the exchange length
of the stray field and the magnetocrystalline exchange length are
material-dependent quantities that usually lie in the nanometer range. This
emphasizes the theoretical challenges associated with the mesoscopic nature of
micromagnetism: the magnetic structures are much larger than the atomic lattice
constant, but at the same time much smaller than the sample size. In computer
simulations, the smallest exchange length serves as an estimate for the largest
cell size admissible to prevent appreciable discretization errors. This general
rule is not valid in special situations where the magnetization becomes
particularly inhomogeneous. When such strongly inhomogeneous structures
develop, micromagnetic simulations inevitably contain systematic and numerical
errors. It is suggested to combine micromagnetic theory with a Heisenberg model
to resolve such problems. We analyze cases where strongly inhomogeneous
structures pose limits to standard micromagnetic simulations, arising from
fundamental aspects as well as from numerical drawbacks
Magnetization dynamics in a three-dimensional interconnected nanowire array
Three-dimensional magnetic nanostructures have recently emerged as artificial
magnetic material types with unique properties bearing potential for
applications, including magnonic devices. Interconnected magnetic nanowires are
a sub-category within this class of materials that is attracting particular
interest. We investigate the high-frequency magnetization dynamics in a cubic
array of cylindrical magnetic nanowires through micromagnetic simulations based
on a frequency-domain formulation of the linearized Landau-Lifshitz-Gilbert
equation. The small-angle high-frequency magnetization dynamics excited by an
external oscillatory field displays clear resonances at distinct frequencies.
These resonances are identified as oscillations connected to specific geometric
features and micromagnetic configurations. The geometry- and
configuration-dependence of the nanowire array's absorption spectrum
demonstrates the potential of such magnetic systems for tuneable and
reprogrammable magnonic applications.Comment: 7 pages, 5 figure
Spin-Transfer Torque Induced Vortex Dynamics in Fe/Ag/Fe Nanopillars
We report experimental and analytical work on spin-transfer torque induced
vortex dynamics in metallic nanopillars with in-plane magnetized layers. We
study nanopillars with a diameter of 150 nm, containing two Fe layers with a
thickness of 15 nm and 30 nm respectively, separated by a 6 nm Ag spacer. The
sample geometry is such that it allows for the formation of magnetic vortices
in the Fe disks. As confirmed by micromagnetic simulations, we are able to
prepare states where one magnetic layer is homogeneously magnetized while the
other contains a vortex. We experimentally show that in this configuration
spin-transfer torque can excite vortex dynamics and analyze their dependence on
a magnetic field applied in the sample plane. The center of gyration is
continuously dislocated from the disk center, and the potential changes its
shape with field strength. The latter is reflected in the field dependence of
the excitation frequency. In the second part we propose a novel mechanism for
the excitation of the gyrotropic mode in nanopillars with a perfectly
homogeneously magnetized in-plane polarizing layer. We analytically show that
in this configuration the vortex can absorb energy from the spin-polarized
electric current if the angular spin-transfer efficiency function is
asymmetric. This effect is supported by micromagnetic simulations.Comment: The article has been sent to J. Phys. D. Submitted on August 9, 2010.
(7 pages and 4 figures.
Ultrafast dynamics of a magnetic antivortex - Micromagnetic simulations
The antivortex is a fundamental magnetization structure which is the
topological counterpart of the well-known magnetic vortex. We study here the
ultrafast dynamic behavior of an isolated antivortex in a patterned Permalloy
thin-film element. Using micromagnetic simulations we predict that the
antivortex response to an ultrashort external field pulse is characterized by
the production of a new antivortex as well as of a temporary vortex, followed
by an annihilation process. These processes are complementary to the recently
reported response of a vortex and, like for the vortex, lead to the reversal of
the orientation of the antivortex core region. In addition to its fundamental
interest, this dynamic magnetization process could be used for the generation
and propagation of spin waves for novel logical circuits.Comment: 4 pages, 4 figures. To be published in Physical Review B (R
Finite element calculations on the single-domain limit of a ferromagnetic cube – a solution to µmag standard problem
Abstract Zero field states of the magnetization in a uniaxial ferromagnetic sample of cubic shape are calculated by means of micromagnetic finite element modeling. With increasing size the minimum energy arrangement changes from a singledomain configuration (flower state) to a vortex configuration. An intermediate arrangement (twisted flower state) between the flower state and the vortex state is observed. A further magnetization state resulting in the calculation is a vortex state with a singularity of the directional field of the magnetization. This work provides our solution to the micromagnetic Standard Problem No. 3 posed by the mMAG micromagnetic modeling activity group at the National Institute of Standards and Technology (NIST).
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