4,542 research outputs found
Recurrence in generic staircases
The straight-line flow on almost every staircase and on almost every square
tiled staircase is recurrent. For almost every square tiled staircase the set
of periodic orbits is dense in the phase space
Topological defects in flat nanomagnets: the magnetostatic limit
We discuss elementary topological defects in soft magnetic nanoparticles in
the thin-film geometry. In the limit dominated by magnetostatic forces the
low-energy defects are vortices (winding number n = +1), cross ties (n = -1),
and edge defects with n = -1/2. We obtain topological constraints on the
possible composition of domain walls. The simplest domain wall in this regime
is composed of two -1/2 edge defects and a vortex, in accordance with
observations and numerics.Comment: 3 pages, eps figures. Proceedings of MMM 0
Spin Transfer Torque for Continuously Variable Magnetization
We report quantum and semi-classical calculations of spin current and
spin-transfer torque in a free-electron Stoner model for systems where the
magnetization varies continuously in one dimension.Analytic results are
obtained for an infinite spin spiral and numerical results are obtained for
realistic domain wall profiles. The adiabatic limit describes conduction
electron spins that follow the sum of the exchange field and an effective,
velocity-dependent field produced by the gradient of the magnetization in the
wall. Non-adiabatic effects arise for short domain walls but their magnitude
decreases exponentially as the wall width increases. Our results cast doubt on
the existence of a recently proposed non-adiabatic contribution to the
spin-transfer torque due to spin flip scattering.Comment: 11 pages, 9 figure
Dimensional analysis using toric ideals: Primitive invariants
© 2014 Atherton et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Classical dimensional analysis in its original form starts by expressing the units for derived quantities, such as force, in terms of power products of basic units M, L, T etc. This suggests the use of toric ideal theory from algebraic geometry. Within this the Graver basis provides a unique primitive basis in a well-defined sense, which typically has more terms than the standard Buckingham approach. Some textbook examples are revisited and the full set of primitive invariants found. First, a worked example based on convection is introduced to recall the Buckingham method, but using computer algebra to obtain an integer K matrix from the initial integer A matrix holding the exponents for the derived quantities. The K matrix defines the dimensionless variables. But, rather than this integer linear algebra approach it is shown how, by staying with the power product representation, the full set of invariants (dimensionless groups) is obtained directly from the toric ideal defined by A. One candidate for the set of invariants is a simple basis of the toric ideal. This, although larger than the rank of K, is typically not unique. However, the alternative Graver basis is unique and defines a maximal set of invariants, which are primitive in a simple sense. In addition to the running example four examples are taken from: a windmill, convection, electrodynamics and the hydrogen atom. The method reveals some named invariants. A selection of computer algebra packages is used to show the considerable ease with which both a simple basis and a Graver basis can be found.The third author received funding from Leverhulme Trust Emeritus Fellowship (1-SST-U445) and United Kingdom EPSRC grant: MUCM EP/D049993/1
Demagnetization Borne Microscale Skyrmions
Magnetic systems are an exciting realm of study that is being explored on
smaller and smaller scales. One extremely interesting magnetic state that has
gained momentum in recent years is the skyrmionic state. It is characterized by
a vortex where the edge magnetic moments point opposite to the core. Although
skyrmions have many possible realizations, in practice, creating them in a lab
is a difficult task to accomplish. In this work, new methods for skyrmion
generation and customization are suggested. Skyrmionic behavior was numerically
observed in minimally customized simulations of spheres, hemisphere,
ellipsoids, and hemi-ellipsoids, for typ- ical Cobalt parameters, in a range
from approximately 40 nm to 120 nm in diameter simply by applying a field
Probing fractal magnetic domains on multiple length scales in Nd2Fe14B
Using small-angle neutron scattering, we demonstrate that the complex
magnetic domain patterns at the surface of Nd2Fe14B, revealed by quantitative
Kerr and Faraday microscopy, propagate into the bulk and exhibit structural
features with dimensions down to 6 nm, the domain wall thickness. The observed
fractal nature of the domain structures provides an explanation for the
anomalous increase in the bulk magnetization of Nd2Fe14B below the
spin-reorientation transition. These measurements open up a rich playground for
studies of fractal structures in highly anisotropic magnetic systems.Comment: Accepted for publication in Phys. Rev. Lett. (4 pages, 4 figures
Asymmetric magnetization splitting in diamond domain structure: Dependence on exchange interaction and anisotropy
The distributions of magnetization orientation for both Landau and diamond
domain structures in nano-rectangles have been investigated by micromagnetic
simulation with various exchange coefficient and anisotropy constant. Both
symmetric and asymmetric magnetization splitting are found in diamond domain
structure, as well as only symmetric magnetization splitting in Landau
structure. In the Landau structure, the splitting angle increases with the
exchange coefficient but decreases slightly with the anisotropy constant,
suggesting that the exchange interaction mainly contributes to the
magnetization splitting in Landau structure. However in the diamond structure,
the splitting angle increases with the anisotropy constant but derceases with
the exchange coefficient, indicating that the magnetization splitting in
diamond structure is resulted from magnetic anisotropy.Comment: 5 pages, 5 figure
Continuous Neel to Bloch Transition as Thickness Increases: Statics and Dynamics
We analyze the properties of Neel and Bloch domain walls as a function of
film thickness h, for systems where, in addition to exchange, the dipole-dipole
interaction must be included. The Neel to Bloch phase transition is found to be
a second order transition at hc, mediated by a single unstable mode that
corresponds to oscillatory motion of the domain wall center. A uniform
out-of-plane rf-field couples strongly to this critical mode only in the Neel
phase. An analytical Landau theory shows that the critical mode frequency
varies as the square root of (hc - h) just below the transition, as found
numerically.Comment: 4 pages, 4 figure
Angular-dependence of magnetization switching for a multi-domain dot: experiment and simulation
We have measured the in-plane angular variation of nucleation and
annihilation fields of a multi-domain magnetic single dot with a microsquid.
The dots are Fe/Mo(110) self-assembled in UHV, with sub-micron size and a
hexagonal shape. The angular variations were quantitatively reproduced by
micromagnetic simulations. Discontinuities in the variations are observed, and
shown to result from bifurcations related to the interplay of the non-uniform
magnetization state with the shape of the dot.Comment: 4 pages, 4 figures, for submission as a regular articl
Smoking Status and Public Responses to Ambiguous Scientific Risk Evidence
Situations in which individuals receive information seldom involve scientific consensus over the level of the risk. When scientific experts disagree, people may process the information in an unpredictable manner. The original data presented here for environmental risk judgments indicate a tendency to place disproportionate weight on the high risk assessment, irrespective of its source, particularly when the experts disagree. Cigarette smokers differ in their risk information processing from nonsmokers in that they place less weight on the high risk judgment when there is a divergence in expert opinion. Consequently, they are more likely to simply average competing risk assessments
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