58 research outputs found
Wavelet transforms in a critical interface model for Barkhausen noise
We discuss the application of wavelet transforms to a critical interface
model, which is known to provide a good description of Barkhausen noise in soft
ferromagnets. The two-dimensional version of the model (one-dimensional
interface) is considered, mainly in the adiabatic limit of very slow driving.
On length scales shorter than a crossover length (which grows with the strength
of surface tension), the effective interface roughness exponent is
, close to the expected value for the universality class of the
quenched Edwards-Wilkinson model. We find that the waiting times between
avalanches are fully uncorrelated, as the wavelet transform of their
autocorrelations scales as white noise. Similarly, detrended size-size
correlations give a white-noise wavelet transform. Consideration of finite
driving rates, still deep within the intermittent regime, shows the wavelet
transform of correlations scaling as for intermediate frequencies.
This behavior is ascribed to intra-avalanche correlations.Comment: RevTeX, 10 pages, 9 .eps figures; Physical Review E, to be publishe
Self similar Barkhausen noise in magnetic domain wall motion
A model for domain wall motion in ferromagnets is analyzed. Long-range
magnetic dipolar interactions are shown to give rise to self-similar dynamics
when the external magnetic field is increased adiabatically. The power spectrum
of the resultant Barkhausen noise is of the form , where
can be estimated from the critical exponents for interface
depinning in random media.Comment: 7 pages, RevTex. To appear in Phys. Rev. Let
Finite driving rates in interface models of Barkhausen noise
We consider a single-interface model for the description of Barkhausen noise
in soft ferromagnetic materials. Previously, the model had been used only in
the adiabatic regime of infinitely slow field ramping. We introduce finite
driving rates and analyze the scaling of event sizes and durations for
different regimes of the driving rate. Coexistence of intermittency, with
non-trivial scaling laws, and finite-velocity interface motion is observed for
high enough driving rates. Power spectra show a decay , with
for finite driving rates, revealing the influence of the internal
structure of avalanches.Comment: 7 pages, 6 figures, RevTeX, final version to be published in Phys.
Rev.
Dynamics of a ferromagnetic domain wall: avalanches, depinning transition and the Barkhausen effect
We study the dynamics of a ferromagnetic domain wall driven by an external
magnetic field through a disordered medium. The avalanche-like motion of the
domain walls between pinned configurations produces a noise known as the
Barkhausen effect. We discuss experimental results on soft ferromagnetic
materials, with reference to the domain structure and the sample geometry, and
report Barkhausen noise measurements on FeCoB amorphous
alloy. We construct an equation of motion for a flexible domain wall, which
displays a depinning transition as the field is increased. The long-range
dipolar interactions are shown to set the upper critical dimension to ,
which implies that mean-field exponents (with possible logarithmic correction)
are expected to describe the Barkhausen effect. We introduce a mean-field
infinite-range model and show that it is equivalent to a previously introduced
single-degree-of-freedom model, known to reproduce several experimental
results. We numerically simulate the equation in , confirming the
theoretical predictions. We compute the avalanche distributions as a function
of the field driving rate and the intensity of the demagnetizing field. The
scaling exponents change linearly with the driving rate, while the cutoff of
the distribution is determined by the demagnetizing field, in remarkable
agreement with experiments.Comment: 17 RevTeX pages, 19 embedded ps figures + 1 extra figure, submitted
to Phys. Rev.
Dynamics of a ferromagnetic domain wall and the Barkhausen effect
We derive an equation of motion for the the dynamics of a ferromagnetic
domain wall driven by an external magnetic field through a disordered medium
and we study the associated depinning transition. The long-range dipolar
interactions set the upper critical dimension to be , so we suggest that
mean-field exponents describe the Barkhausen effect for three-dimensional soft
ferromagnetic materials. We analyze the scaling of the Barkhausen jumps as a
function of the field driving rate and the intensity of the demagnetizing
field, and find results in quantitative agreement with experiments on
crystalline and amorphous soft ferromagnetic alloys.Comment: 4 RevTex pages, 3 ps figures embedde
Non-invasive assessment of risk for severe tachyarrhythmias by means of non-linear analysis techniques
Sudden death remains a phenomenon of disturbing proportions, displaying a mean incidence of 300,000-350,000 persons/year in the USA (0.1-0.2% of the general population). In Europe, the figures are very similar. In 90% of cases, sudden death has an arrhythmic cause. Prevention of Sudden Cardiac Death (SCD) constitutes one of the most important challenges of modern cardiology. In order to make a real progress in this field it is crucial to precisely identify increased risk for serious ventricular tachyarrhythmias. In this study the effectiveness of different methods of the non-linear analysis (NLA) of ECG in the risk stratification of patients with ventricular arrhythmias is evaluated, and these non-invasive parameters are correlated with the results of invasive electrophysiological study (EPS). We evaluated 25 patients with history of cardiac arrest, syncope, sustained or nonsustained ventricular tachycardia (VT). The study group was compared with a control group of 25 healthy subjects. All patients underwent both electrophysiologic study (EPS) and non-linear analysis (NLA) of ECG. Patients were classified through the application of a clustering procedure to the whole set of functions, and a comparison between the results of non-linear analysis of ECG and EPS was performed. Results are presented and discussed
Non-invasive assessment of risk for severe tachyarrhythmias by means of non-linear analysis techniques
Sudden death remains a phenomenon of disturbing proportions, displaying a mean incidence of 300,000-350,000 persons/year in the USA (0.1-0.2% of the general population). In Europe, the figures are very similar. In 90% of cases, sudden death has an arrhythmic cause. Prevention of Sudden Cardiac Death (SCD) constitutes one of the most important challenges of modern cardiology. In order to make a real progress in this field it is crucial to precisely identify increased risk for serious ventricular tachyarrhythmias. In this study the effectiveness of different methods of the non-linear analysis (NLA) of ECG in the risk stratification of patients with ventricular arrhythmias is evaluated, and these non-invasive parameters are correlated with the results of invasive electrophysiological study (EPS). We evaluated 25 patients with history of cardiac arrest, syncope, sustained or nonsustained ventricular tachycardia (VT). The study group was compared with a control group of 25 healthy subjects. All patients underwent both electrophysiologic study (EPS) and non-linear analysis (NLA) of ECG. Patients were classified through the application of a clustering procedure to the whole set of functions, and a comparison between the results of non-linear analysis of ECG and EPS was performed. Results are presented and discussed
Hysteresis and Avalanches in the Random Anisotropy Ising Model
The behaviour of the Random Anisotropy Ising model at T=0 under local
relaxation dynamics is studied. The model includes a dominant ferromagnetic
interaction and assumes an infinite anisotropy at each site along local
anisotropy axes which are randomly aligned. Two different random distributions
of anisotropy axes have been studied. Both are characterized by a parameter
that allows control of the degree of disorder in the system. By using numerical
simulations we analyze the hysteresis loop properties and characterize the
statistical distribution of avalanches occuring during the metastable evolution
of the system driven by an external field. A disorder-induced critical point is
found in which the hysteresis loop changes from displaying a typical
ferromagnetic magnetization jump to a rather smooth loop exhibiting only tiny
avalanches. The critical point is characterized by a set of critical exponents,
which are consistent with the universal values proposed from the study of other
simpler models.Comment: 40 pages, 21 figures, Accepted for publication in Phys. Rev.
Universal Pulse Shape Scaling Function and Exponents: A Critical Test for Avalanche Models applied to Barkhausen Noise
In order to test if the universal aspects of Barkhausen noise in magnetic
materials can be predicted from recent variants of the non-equilibrium zero
temperature Random Field Ising Model (RFIM), we perform a quantitative study of
the universal scaling function derived from the
Barkhausen pulse shape in simulations and experiment. Through data collapses
and scaling relations we determine the critical exponents and
in both simulation and experiment. Although we find agreement
in the critical exponents, we find differences between theoretical and
experimental pulse shape scaling functions as well as between different
experiments.Comment: 19 pages (in preprint format), 5 figures, 1 tabl
Properties of branes in curved spacetimes
A generic property of curved manifolds is the existence of focal points. We
show that branes located at focal points of the geometry satisfy special
properties. Examples of backgrounds to which our discussion applies are AdS_m x
S^n and plane wave backgrounds. As an example, we show that a pair of AdS_2
branes located at the north and south pole of the S^5 in AdS_5 x S^5 are half
supersymmetric and that they are dual to a two-monopole solution of N=4 SU(N)
SYM theory. Our second example involves spacelike branes in the (Lorentzian)
plane wave. We develop a modified lightcone gauge for the open string channel,
analyze in detail the cylinder diagram and establish open-closed duality. When
the branes are located at focal points of the geometry the amplitude acquires
most of the characteristics of flat space amplitudes. In the open string
channel the special properties are due to stringy modes that become massless.Comment: 41 pages; v2:typos corrected, ref adde
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