651 research outputs found
Micromagnetic simulations of interacting dipoles on a fcc lattice: Application to nanoparticle assemblies
Micromagnetic simulations are used to examine the effects of cubic and axial
anisotropy, magnetostatic interactions and temperature on M-H loops for a
collection of magnetic dipoles on fcc and sc lattices. We employ a simple model
of interacting dipoles that represent single-domain particles in an attempt to
explain recent experimental data on ordered arrays of magnetoferritin
nanoparticles that demonstrate the crucial role of interactions between
particles in a fcc lattice. Significant agreement between the simulation and
experimental results is achieved, and the impact of intra-particle degrees of
freedom and surface effects on thermal fluctuations are investigated.Comment: 10 pages, 9 figure
A ferrofluid based neural network: design of an analogue associative memory
We analyse an associative memory based on a ferrofluid, consisting of a
system of magnetic nano-particles suspended in a carrier fluid of variable
viscosity subject to patterns of magnetic fields from an array of input and
output magnetic pads. The association relies on forming patterns in the
ferrofluid during a trainingdphase, in which the magnetic dipoles are free to
move and rotate to minimize the total energy of the system. Once equilibrated
in energy for a given input-output magnetic field pattern-pair the particles
are fully or partially immobilized by cooling the carrier liquid. Thus produced
particle distributions control the memory states, which are read out
magnetically using spin-valve sensors incorporated in the output pads. The
actual memory consists of spin distributions that is dynamic in nature,
realized only in response to the input patterns that the system has been
trained for. Two training algorithms for storing multiple patterns are
investigated. Using Monte Carlo simulations of the physical system we
demonstrate that the device is capable of storing and recalling two sets of
images, each with an accuracy approaching 100%.Comment: submitted to Neural Network
Soliton pair dynamics in patterned ferromagnetic ellipses
Confinement alters the energy landscape of nanoscale magnets, leading to the
appearance of unusual magnetic states, such as vortices, for example. Many
basic questions concerning dynamical and interaction effects remain unanswered,
and nanomagnets are convenient model systems for studying these fundamental
physical phenomena. A single vortex in restricted geometry, also known as a
non-localized soliton, possesses a characteristic translational excitation mode
that corresponds to spiral-like motion of the vortex core around its
equilibrium position. Here, we investigate, by a microwave reflection
technique, the dynamics of magnetic soliton pairs confined in lithographically
defined, ferromagnetic Permalloy ellipses. Through a comparison with
micromagnetic simulations, the observed strong resonances in the subgigahertz
frequency range can be assigned to the translational modes of vortex pairs with
parallel or antiparallel core polarizations. Vortex polarizations play a
negligible role in the static interaction between two vortices, but their
effect dominates the dynamics.Comment: supplemental movies on
http://www.nature.com/nphys/journal/v1/n3/suppinfo/nphys173_S1.htm
Magnetic phase diagram of a spin-1 condensate in two dimensions with dipole interaction
Several new features arise in the ground-state phase diagram of a spin-1
condensate trapped in an optical trap when the magnetic dipole interaction
between the atoms is taken into account along with confinement and spin
precession. The boundaries between the regions of ferromagnetic and polar
phases move as the dipole strength is varied and the ferromagnetic phases can
be modulated. The magnetization of the ferromagnetic phase perpendicular to the
field becomes modulated as a helix winding around the magnetic field direction,
with a wavelength inversely proportional to the dipole strength. This
modulation should be observable for current experimental parameters in
Rb. Hence the much-sought supersolid state, with broken continuous
translation invariance in one direction and broken global U(1) invariance,
occurs generically as a metastable state in this system as a result of dipole
interaction. The ferromagnetic state parallel to the applied magnetic field
becomes striped in a finite system at strong dipolar coupling.Comment: 11 pages, 7 figures;published versio
Quantifying energetics and dissipation in magnetohydrodynamic turbulence
We perform a suite of two- and three-dimensional magnetohydrodynamic (MHD)
simulations with the Athena code of the non-driven Kelvin-Helmholtz instability
in the subsonic, weak magnetic field limit. Focusing the analysis on the
non-linear turbulent regime, we quantify energy transfer on a scale-by-scale
basis and identify the physical mechanisms responsible for energy exchange by
developing the diagnostic known as spectral energy transfer function analysis.
At late times when the fluid is in a state of MHD turbulence, magnetic tension
mediates the dominant mode of energy injection into the magnetic reservoir,
whereby turbulent fluid motions twist and stretch the magnetic field lines.
This generated magnetic energy turbulently cascades to smaller scales, while
being exchanged backwards and forwards with the kinetic energy reservoir, until
finally being dissipated. Incorporating explicit dissipation pushes the
dissipation scale to larger scales than if the dissipation were entirely
numerical. For scales larger than the dissipation scale, we show that the
physics of energy transfer in decaying MHD turbulence is robust to numerical
effects.Comment: 23 pages, 20 figures, 4 tables, Accepted for publication in MNRA
Hysteresis and noise in ferromagnetic materials with parallel domain walls
We investigate dynamic hysteresis and Barkhausen noise in ferromagnetic
materials with a huge number of parallel and rigid Bloch domain walls.
Considering a disordered ferromagnetic system with strong in-plane uniaxial
anisotropy and in-plane magnetization driven by an external magnetic field, we
calculate the equations of motion for a set of coupled domain walls,
considering the effects of the long-range dipolar interactions and disorder. We
derive analytically an expression for the magnetic susceptivity, related to the
effective demagnetizing factor, and show that it has a logarithmic dependence
on the number of domains. Next, we simulate the equations of motion and study
the effect of the external field frequency and the disorder on the hysteresis
and noise properties. The dynamic hysteresis is very well explained by means of
the loss separation theory.Comment: 13 pages, 11 figure
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