134 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
Dynamic hysteresis in Finemet thin films
We performed a series of dynamic hysteresis measurements on three series of
Finemet films with composition FeCuNbSiB, using
both the longitudinal magneto-optical Kerr effect (MOKE) and the inductive
fluxometric method. The MOKE dynamic hysteresis loops show a more marked
variability with the frequency than the inductive ones, while both measurements
show a similar dependence on the square root of frequency. We analyze these
results in the frame of a simple domain wall depinning model, which accounts
for the general behavior of the data.Comment: 3 pages, 3 figure
Beyond power laws: Universality in the average avalanche shape
We report the measurement of multivariable scaling functions for the temporal
average shape of Barkhausen noise avalanches, and show that they are consistent
with the predictions of simple mean-field theories. We bypass the confounding
factors of time-retarded interactions (eddy currents) by measuring thin permal-
loy films, and bypass thresholding effects and amplifier distortions by
applying Wiener deconvolution. We find experimental shapes that are
approximately symmetric, and track the evolution of the scaling function. We
solve a mean- field theory for the magnetization dynamics and calculate the
form of the scaling function in the presence of a demagnetizing field and a
finite field ramp-rate, yielding quantitative agreement with the experiment.Comment: 13 pages, 14 figure
String spectra near some null cosmological singularities
We construct cosmological spacetimes with null Kasner-like singularities as
purely gravitational solutions with no other background fields turned on. These
can be recast as anisotropic plane-wave spacetimes by coordinate
transformations. We analyse string quantization to find the spectrum of string
modes in these backgrounds. The classical string modes can be solved for
exactly in these time-dependent backgrounds, which enables a detailed study of
the near singularity string spectrum, (time-dependent) oscillator masses and
wavefunctions. We find that for low lying string modes(finite oscillation
number), the classical near-singularity string mode functions are non-divergent
for various families of singularities. Furthermore, for any infinitesimal
regularization of the vicinity of the singularity, we find a tower of string
modes of ultra-high oscillation number which propagate essentially freely in
the background. The resulting picture suggests that string interactions are
non-negligible near the singularity.Comment: Latex, 30pgs; v2. minor clarifications, references adde
noise and avalanche scaling in plastic deformation
We study the intermittency and noise of dislocation systems undergoing shear
deformation. Simulations of a simple two-dimensional discrete dislocation
dynamics model indicate that the deformation rate exhibits a power spectrum
scaling of the type . The noise exponent is far away from a
Lorentzian, with . This result is directly related to the
way the durations of avalanches of plastic deformation activity scale with
their size.Comment: 6 pages, 5 figures, submitted to Phys. Rev.
Is demagnetization an efficient optimization method?
Demagnetization, commonly employed to study ferromagnets, has been proposed
as the basis for an optimization tool, a method to find the ground state of a
disordered system. Here we present a detailed comparison between the ground
state and the demagnetized state in the random field Ising model, combing exact
results in and numerical solutions in . We show that there are
important differences between the two states that persist in the thermodynamic
limit and thus conclude that AC demagnetization is not an efficient
optimization method.Comment: 2 pages, 1 figur
Investigation of scaling properties of hysteresis in Finemet thin films
We study the behavior of hysteresis loops in Finemet
FeCuNbSiB thin films by using a fluxometric setup
based on a couple of well compensated pickup coils. The presence of scaling
laws of the hysteresis area is investigated as a function of the amplitude and
frequency of the applied field, considering sample thickness from about 20 nm
to 5 m. We do not observe any scaling predicted by theoretical models,
while dynamic loops show a logarithmic dependence on the frequency.Comment: 2 pages, 2 figure
Helium condensation in aerogel: avalanches and disorder-induced phase transition
We present a detailed numerical study of the elementary condensation events
(avalanches) associated to the adsorption of He in silica aerogels. We use
a coarse-grained lattice-gas description and determine the nonequilibrium
behavior of the adsorbed gas within a local mean-field analysis, neglecting
thermal fluctuations and activated processes. We investigate the statistical
properties of the avalanches, such as their number, size and shape along the
adsorption isotherms as a function of gel porosity, temperature, and chemical
potential. Our calculations predict the existence of a line of critical points
in the temperature-porosity diagram where the avalanche size distribution
displays a power-law behavior and the adsorption isotherms have a universal
scaling form. The estimated critical exponents seem compatible with those of
the field-driven Random Field Ising Model at zero temperature.Comment: 16 pages, 14 figure
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.
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