175 research outputs found
Current Induced Fingering Instability in Magnetic Domain Walls
The shape instability of magnetic domain walls under current is investigated
in a ferromagnetic (Ga,Mn)(As,P) film with perpendicular anisotropy. Domain
wall motion is driven by the spin transfer torque mechanism. A current density
gradient is found either to stabilize domains with walls perpendicular to
current lines or to produce finger-like patterns, depending on the domain wall
motion direction. The instability mechanism is shown to result from the
non-adiabatic contribution of the spin transfer torque mechanism.Comment: 5 pages, 3 figures + supplementary material
Magnetic wire-based sensors for the micro-rheology of complex fluids
We propose a simple micro-rheology technique to evaluate the viscoelastic
properties of complex fluids. The method is based on the use of magnetic wires
of a few microns in length submitted to a rotational magnetic field. In this
work, the method is implemented on a surfactant wormlike micellar solution that
behaves as an ideal Maxwell fluid. With increasing frequency, the wires undergo
a transition between a steady and a hindered rotation regime. The study shows
that the average rotational velocity and the amplitudes of the oscillations
obey scaling laws with well-defined exponents. From a comparison between model
predictions and experiments, the rheological parameters of the fluid are
determined.Comment: 14 pages 7 figures, accepted in Physical Review
Instability driven formation of domains in the intermediate state of type-I superconductors
The formation of normal-state domains in type-I superconducting indium films
is investigated using the high resolution magneto-optical imaging technique.
The observed patterns consist of coexisting circular and lamellar normal-phase
domains surrounded by the superconducting phase. The distribution of domain
surface areas is found to exhibit a threshold, above which only the lamellar
shape is observed. We show that this threshold coincides with the predicted
critical surface area for the elongation instability of the circular shape. The
partition of the normal phase into circular and lamellar domains is determined
by the combined effects of the elongation instability and the penetration of
magnetic flux by bursts at the early stage of pattern formation. It is not
governed by mutual interactions between domains, as usually assumed for
self-organized systems
Twisting and buckling: a new undulation mechanism for artificial swimmers
We present an artificial swimmer consisting in a long cylinder of ferrogel
which is polarized transversely and in opposite directions at each extremity.
When it is placed on a water film and submitted to a transverse oscillating
magnetic field, this artificial worm undulates and swims. Whereas symmetry
breaking is due to the field gradient, the undulations of the worm result from
a torsional buckling instability as the polarized ends tend to align with the
applied magnetic field. The critical magnetic field above which buckling and
subsequent swimming is observed may be predicted using elasticity equations
including the effect of a magnetic torque. As the length of the worm is varied,
several undulation modes are observed which are in good agreement with the
bending modes of an elastic rod with free ends
Gyromagnetic effects in dynamics of magnetic microparticles
We derive equations of motion for paramagnetic and ferromagnetic particles
fully accounting for gyromagnetic effects. Considering the Einstein-de Haas
effect for an ellipsoidal paramagnetic particle we find that starting from a
quiescent non-magnetized state, after the field is switched on a rotation along
the short axis is established. This is confirmed by the stability analysis of
the fixed points of the corresponding ordinary differential equations. In the
case of a ferromagnetic particle we integrate the equations of motion in the
dissipationless case by finding the integrals of motion. We also reformulate
the equations in a Hamiltonian framework in this case and find a period of
small nutation oscillations.Comment: 24 pages, 15 figure
Motion of magnetotactic bacteria swarms in an external field
Magnetotactic bacteria moving on circular orbits form hydrodynamically bound
states. When close to a surface and with the tilting of the field in a
direction close to the perpendicular to this surface these swarms move
perpendicularly to the tilting angle. We describe quantitatively this motion by
a continuum model with couple stress arising from the torques produced by the
rotary motors of the amphitrichous magnetotactic bacteria. The model not only
correctly describes the change of direction of swarm motion while inverting the
tangential field but also predicts reasonable value of the torque produced by
the rotary motors
Counterion-Mediated Attraction and Kinks on Loops of Semiflexible Polyelectrolyte Bundles
The formation of kinks in a loop of bundled polyelectrolyte filaments is analyzed in terms of the thermal fluctuations of charge density due to polyvalent counterions adsorbed on the polyelectrolyte filaments. It is found that the counterion-mediated attraction energy of filaments depends on their bending. By consideration of curvature elasticity energy and counterion-mediated attraction between polyelectrolyte filaments, the characteristic width of the kink and the number of kinks per loop is found to be in reasonable agreement with existing experimental data for rings of bundled actin filaments
Hydrodynamics of Monolayer Domains at the Air-Water Interface
Molecules at the air-water interface often form inhomogeneous layers in which
domains of different densities are separated by sharp interfaces. Complex
interfacial pattern formation may occur through the competition of short- and
long-range forces acting within the monolayer. The overdamped hydrodynamics of
such interfacial motion is treated here in a general manner that accounts for
dissipation both within the monolayer and in the subfluid. Previous results on
the linear stability of interfaces are recovered and extended, and a
formulation applicable to the nonlinear regime is developed. A simplified
dynamical law valid when dissipation in the monolayer itself is negligible is
also proposed. Throughout the analysis, special attention is paid to the
dependence of the dynamical behavior on a characteristic length scale set by
the ratio of the viscosities in the monolayer and in the subphase.Comment: 12 pages, RevTeX, 4 ps figures, accepted in Physics of Fluids
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