19 research outputs found
Engineering Curvature Induced Anisotropy in Thin Ferromagnetic Films
The large curvature effects on micromagnetic energy of a thin ferromagnetic
film with nonlocal dipolar energy are considered. We predict that the dipolar
interaction and surface curvature can produce perpendicular anisotropy which
can be controlled by engineering a special type of periodic surface shape
structure. Similar effects can be achieved by a significant surface roughness
in the film. We show that in general the anisotropy can point in an arbitrary
direction depending on the surface curvature. We provide simple examples of
these periodic surface structures to demonstrate how to engineer particular
anisotropies in the film.Comment: 5 pages, 4 figure
Excitation of mixed Rossby-gravity waves by wave-mean flow interactions on the sphere
The equatorial mixed Rossby-gravity wave (MRGW) is an important contributor
to tropical variability. Its excitation mechanism capable of explaining the
observed MRGW variance peak at synoptic scales remains elusive. This study
investigates wave-mean flow interactions as a generation process for the MRGWs
using the barotropic version of the global Transient Inertia-Gravity And Rossby
wave dynamics model (TIGAR), which employs Hough harmonics as the basis of
spectral expansion, thereby representing MRGWs as prognostic variables. High
accuracy numerical simulations manifest that interactions between waves
emanating from a tropical heat source and zonal mean jets in the subtropics
generate MRGWs with the variance spectra resembling the one observed in the
tropical troposphere. Quantification of spectral tendencies associated with the
MRGW energy growth underscores the significance of wave-mean flow interactions
in comparison to excitation mechanisms driven by external forcing and wave-wave
interactions. The MRGW growth and amplitude depend on the asymmetry in the
zonal mean flow that may explain not only seasonal variability but also
differences between the troposphere and the middle atmosphere.Comment: 20 pages, 13 figures, 1 table, submitted to Quarterly Journal of the
Royal Meteorological Societ
Dynamics of ferromagnetic domain walls under extreme fields
We report the existence of a new regime for domain wall motion in uniaxial
and near-uniaxial ferromagnetic nanowires, characterised by applied magnetic
fields sufficiently strong that one of the domains becomes unstable. There
appears a new stable solution of the Landau-Lifshitz-Gilbert equation,
describing a nonplanar domain wall moving with constant velocity and precessing
with constant frequency. Even in the presence of thermal noise, the new
solution can propagate for distances on the order of 500 times the field-free
domain wall width before fluctuations in the unstable domain become
appreciable.Comment: 7 pages, 5 figures (including Supplemental Material
Magnetization in narrow ribbons:Curvature effects
A ribbon is a surface swept out by a line segment turning as it moves along a central curve. For narrow magnetic ribbons, for which the length of the line segment is much less than the length of the curve, the anisotropy induced by the magnetostatic interaction is biaxial, with hard axis normal to the ribbon and easy axis along the central curve. The micromagnetic energy of a narrow ribbon reduces to that of a one-dimensional ferromagnetic wire, but with curvature, torsion and local anisotropy modified by the rate of turning. These general results are applied to two examples, namely a helicoid ribbon, for which the central curve is a straight line, and a Möbius ribbon, for which the central curve is a circle about which the line segment executes a 180° twist. In both examples, for large positive tangential anisotropy, the ground state magnetization lies tangent to the central curve. As the tangential anisotropy is decreased, the ground state magnetization undergoes a transition, acquiring an in-surface component perpendicular to the central curve. For the helicoid ribbon, the transition occurs at vanishing anisotropy, below which the ground state is uniformly perpendicular to the central curve. The transition for the Möbius ribbon is more subtle; it occurs at a positive critical value of the anisotropy, below which the ground state is nonuniform. For the helicoid ribbon, the dispersion law for spin wave excitations about the tangential state is found to exhibit an asymmetry determined by the geometric and magnetic chiralities