36 research outputs found
Current-induced magnetic superstructures in exchange-spring devices
We investigate the potential to use a magneto-thermo-electric instability
that may be induced in a mesoscopic magnetic multi-layer (F/f/F) to create and
control magnetic superstructures. In the studied multilayer two strongly
ferromagnetic layers (F) are coupled through a weakly ferromagnetic spacer (f)
by an "exchange spring" with a temperature dependent "spring constant" that can
be varied by Joule heating caused by an electrical dc current. We show that in
the current-in-plane (CIP) configuration a distribution of the magnetization,
which is homogeneous in the direction of the current flow, is unstable in the
presence of an external magnetic field if the length L of the sample in this
direction exceeds some critical value Lc ~ 10 \mu m. This spatial instability
results in the spontaneous formation of a moving domain of magnetization
directions, the length of which can be controlled by the bias voltage in the
limit L >> Lc. Furthermore, we show that in such a situation the
current-voltage characteristics has a plateau with hysteresis loops at its ends
and demonstrate that if biased in the plateau region the studied device
functions as an exponentially precise current stabilizer.Comment: 8 pages, 6 figure
Breathing Current Domains in Globally Coupled Electrochemical Systems: A Comparison with a Semiconductor Model
Spatio-temporal bifurcations and complex dynamics in globally coupled
intrinsically bistable electrochemical systems with an S-shaped current-voltage
characteristic under galvanostatic control are studied theoretically on a
one-dimensional domain. The results are compared with the dynamics and the
bifurcation scenarios occurring in a closely related model which describes
pattern formation in semiconductors. Under galvanostatic control both systems
are unstable with respect to the formation of stationary large amplitude
current domains. The current domains as well as the homogeneous steady state
exhibit oscillatory instabilities for slow dynamics of the potential drop
across the double layer, or across the semiconductor device, respectively. The
interplay of the different instabilities leads to complex spatio-temporal
behavior. We find breathing current domains and chaotic spatio-temporal
dynamics in the electrochemical system. Comparing these findings with the
results obtained earlier for the semiconductor system, we outline bifurcation
scenarios leading to complex dynamics in globally coupled bistable systems with
subcritical spatial bifurcations.Comment: 13 pages, 11 figures, 70 references, RevTex4 accepted by PRE
http://pre.aps.or
Detonation type waves in phase (chemical) transformation processes in condensed matter
Fast self sustained waves (autowaves) associated with chemical or phase
transformations are observed in many situations in condensed matter.
They
are governed neither by diffusion of matter or heat
(as in combustion processes)
nor by a travelling shock wave (as in gaseous detonation).
Instead, they result from a coupling between
phase transformation and the stress field, and may be classified
as gasless detonation autowaves in solids.
We propose a simple model to describe these regimes.
The model rests on the classical equations of elastic deformations in
a 1-dimensional solid bar, with the extra assumption that the phase
(chemical) transformation induces a change of the sound velocity.
The transformations are assumed to occur through a chain
branched mechanism, which
starts when the mechanical stress exceeds a given threshold.
Our investigation shows that supersonic autowaves exist in this model.
In the absence of diffusion (dissipation factor, losses), a continuum
of travelling wave solutions is found.
In the presence of diffusion, a steady state supersonic wave solution
is found, along with a slower wave controlled by diffusion