190 research outputs found
Rotating vortex dipoles in ferromagnets
Vortex-antivortex pairs are localized excitations and have been found to be
spontaneously created in magnetic elements. In the case that the vortex and the
antivortex have opposite polarities the pair has a nonzero topological charge,
and it behaves as a rotating vortex dipole. We find theoretically, and confirm
numerically, the form of the energy as a function of the angular momentum of
the system and the associated rotation frequencies. We discuss the process of
annihilation of the pair which changes the topological charge of the system by
unity while its energy is monotonically decreasing. Such a change in the
topological charge affects profoundly the dynamics in the magnetic system. We
finally discuss the connection of our results with Bloch Points (BP) and the
implications for BP dynamics.Comment: 6 pages, 2 figure
The Formation and Coarsening of the Concertina Pattern
The concertina is a magnetization pattern in elongated thin-film elements of
a soft material. It is a ubiquitous domain pattern that occurs in the process
of magnetization reversal in direction of the long axis of the small element.
Van den Berg argued that this pattern grows out of the flux closure domains as
the external field is reduced. Based on experimental observations and theory,
we argue that in sufficiently elongated thin-film elements, the concertina
pattern rather bifurcates from an oscillatory buckling mode. Using a reduced
model derived by asymptotic analysis and investigated by numerical simulation,
we quantitatively predict the average period of the concertina pattern and
qualitatively predict its hysteresis. In particular, we argue that the
experimentally observed coarsening of the concertina pattern is due to
secondary bifurcations related to an Eckhaus instability. We also link the
concertina pattern to the magnetization ripple and discuss the effect of a weak
(crystalline or induced) anisotropy
Out-of-surface vortices in spherical shells
The interplay of topological defects with curvature is studied for
out-of-surface magnetic vortices in thin spherical nanoshells. In the case of
easy-surface Heisenberg magnet it is shown that the curvature of the underlying
surface leads to a coupling between the localized out-of-surface component of
the vortex with its delocalized in-surface structure, i.e. polarity-chirality
coupling.Comment: 6 pages, 4 figure
Vortex Polarity Switching in Magnets with Surface Anisotropy
Vortex core reversal in magnetic particle is essentially influenced by a
surface anisotropy. Under the action of a perpendicular static magnetic field
the vortex core undergoes a shape deformationof pillow- or barrel-shaped type,
depending on the type of the surface anisotropy. This deformation plays a key
point in the switching mechanism: We predict that the vortex polarity switching
is accompanied (i) by a linear singularity in case of Heisenberg magnet with
bulk anisotropy only and (ii) by a point singularities in case of surface
anisotropy or exchange anisotropy. We study in details the switching process
using spin-lattice simulations and propose a simple analytical description
using a wired core model, which provides an adequate description of the Bloch
point statics, its dynamics and the Bloch point mediated switching process. Our
analytical predictions are confirmed by spin-lattice simulations for Heisenberg
magnet and micromagnetic simulations for nanomagnet with account of a dipolar
interaction.Comment: 17 pages, 15 figure
Magnetization structure of a Bloch point singularity
Switching of magnetic vortex cores involves a topological transition
characterized by the presence of a magnetization singularity, a point where the
magnetization vanishes (Bloch point). We analytically derive the shape of the
Bloch point that is an extremum of the free energy with exchange, dipole and
the Landau terms for the determination of the local value of the magnetization
modulus.Comment: 4 pages, 2 figure
Quenched Slonczewski-Windmill in Spin-Torque Vortex-Oscillators
We present a combined analytical and numerical study on double-vortex
spin-torque nano-oscillators and describe a mechanism that suppresses the
windmill modes. The magnetization dynamics is dominated by the gyrotropic
precession of the vortex in one of the ferromagnetic layers. In the other layer
the vortex gyration is strongly damped. The dominating layer for the
magnetization dynamics is determined by the current polarity. Measurements on
Fe/Ag/Fe nano-pillars support these findings. The results open up a new
perspective for building high quality-factor spin-torque oscillators operating
at selectable, well-separated frequency bands
Static and Dynamical Properties of Antiferromagnetic Skyrmions in the Presence of Applied Current and Temperature
Skyrmions are topologically protected entities in magnetic materials which have the potential to be used in spintronics for information storage and processing. However, Skyrmions in ferromagnets have some intrinsic difficulties which must be overcome to use them for spintronic applications, such as the inability to move straight along current. We show that Skyrmions can also be stabilized and manipulated in antiferromagnetic materials. An antiferromagnetic Skyrmion is a compound topological object with a similar but of opposite sign spin texture on each sublattice, which, e.g., results in a complete cancellation of the Magnus force. We find that the composite nature of antiferromagnetic Skyrmions gives rise to different dynamical behavior due to both an applied current and temperature effects
A symmetry breaking mechanism for selecting the speed of relativistic solitons
We propose a mechanism for fixing the velocity of relativistic soliton based
on the breaking of the Lorentz symmetry of the sine-Gordon (SG) model. The
proposal is first elaborated for a molecular chain model, as the simple
pendulum limit of a double pendulums chain. It is then generalized to a full
class of two-dimensional field theories of the sine-Gordon type. From a
phenomenological point of view, the mechanism allows one to select the speed of
a SG soliton just by tuning elastic couplings constants and kinematical
parameters. From a fundamental, field-theoretical point of view we show that
the characterizing features of relativistic SG solitons (existence of conserved
topological charges and stability) may be still preserved even if the Lorentz
symmetry is broken and a soliton of a given speed is selected.Comment: 23 pages, no figure
The information needs of people living with ankylosing spondylitis: a questionnaire survey
<p>BACKGROUND:Today, health care is patient-centred with patients more involved in medical decision making and taking an active role in managing their disease. It is important that patients are appropriately informed about their condition and that their health care needs are met. We examine the information utilisation, sources and needs of people with ankylosing spondylitis (AS).</p>
<p>METHODS: Participants in an existing AS cohort study were asked to complete a postal or online questionnaire containing closed and open-ended questions, regarding their information access and needs. Participants were stratified by age and descriptive statistics were performed using STATA 11, while thematic analysis was performed on open-ended question narratives. Qualitative data was handled in Microsoft Access and explored for emerging themes and patterns of experiences.</p>
<p>RESULTS: Despite 73% of respondents having internet access, only 49% used the internet to access information regarding AS. Even then, this was only infrequently. Only 50% of respondents reported accessing written information about AS, which was obtained mainly in specialist clinics. Women were more likely than men to access information (63% (women) 46% (men)) regardless of the source, while younger patients were more likely to use online sources. The main source of non-written information was the rheumatologist. Overall, the respondents felt there was sufficient information available, but there was a perception that the tone was often too negative. The majority (95%) of people would like to receive a regular newsletter about AS, containing positive practical and local information. Suggestions were also made for more information about AS to be made available to non-specialist medical professionals and the general public.</p>
<p>CONCLUSIONS: There appears to be sufficient information available for people with AS in the UK and this is mostly accessed by younger AS patients. Many patients, particularly men, choose not to access AS information and concerns were raised about its negative tone. Patients still rely on written and verbal information from their specialists. Future initiatives should focus on the delivery of more positive information, targeting younger participants in particular and increasing the awareness in the general population and wider non-specialist medical community.</p>
First signs and symptoms of spondyloarthritis—data from an inception cohort with a disease course of two years or less (REGISPONSER-Early)
Objective. To determine the first signs and symptoms, and the clinical, biological and radiological characteristics of patients with early SpA
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