111 research outputs found
Functional Integration Approach to Hysteresis
A general formulation of scalar hysteresis is proposed. This formulation is
based on two steps. First, a generating function g(x) is associated with an
individual system, and a hysteresis evolution operator is defined by an
appropriate envelope construction applied to g(x), inspired by the overdamped
dynamics of systems evolving in multistable free energy landscapes. Second, the
average hysteresis response of an ensemble of such systems is expressed as a
functional integral over the space G of all admissible generating functions,
under the assumption that an appropriate measure m has been introduced in G.
The consequences of the formulation are analyzed in detail in the case where
the measure m is generated by a continuous, Markovian stochastic process. The
calculation of the hysteresis properties of the ensemble is reduced to the
solution of the level-crossing problem for the stochastic process. In
particular, it is shown that, when the process is translationally invariant
(homogeneous), the ensuing hysteresis properties can be exactly described by
the Preisach model of hysteresis, and the associated Preisach distribution is
expressed in closed analytic form in terms of the drift and diffusion
parameters of the Markovian process. Possible applications of the formulation
are suggested, concerning the interpretation of magnetic hysteresis due to
domain wall motion in quenched-in disorder, and the interpretation of critical
state models of superconducting hysteresis.Comment: 36 pages, 9 figures, to be published on Phys. Rev.
The effect of additive noise on dynamical hysteresis
We investigate the properties of hysteresis cycles produced by a
one-dimensional, periodically forced Langevin equation. We show that depending
on amplitude and frequency of the forcing and on noise intensity, there are
three qualitatively different types of hysteresis cycles. Below a critical
noise intensity, the random area enclosed by hysteresis cycles is concentrated
near the deterministic area, which is different for small and large driving
amplitude. Above this threshold, the area of typical hysteresis cycles depends,
to leading order, only on the noise intensity. In all three regimes, we derive
mathematically rigorous estimates for expectation, variance, and the
probability of deviations of the hysteresis area from its typical value.Comment: 30 pages, 5 figure
Exact Solution of Return Hysteresis Loops in One Dimensional Random Field Ising Model at Zero Temperature
Minor hysteresis loops within the main loop are obtained analytically and
exactly in the one-dimensional ferromagnetic random field Ising-model at zero
temperature. Numerical simulations of the model show excellent agreement with
the analytical results
Magnetization Reversal and Nanoscopic Magnetic Phase Separation in Doped La1-xSrxCoO3
The doped perovskite cobaltite La1-xSrxCoO3 (LSCO) has been advanced as a
model system for studying intrinsic magnetic phase separation. We have employed
a first-order reversal curve (FORC) method to probe the amount of irreversible
switching in bulk polycrystalline LSCO as a function of Sr doping, field
cooling procedure, and temperature. The value of the FORC distribution, rho, is
used as a measure of the extent of irreversible switching. For x < 0.18, the
small values of rho and its ridge-like distribution along local coercivity (Hc)
and zero bias (Hb), are characteristic of non-interacting single domain
particles. This is consistent with the formation of an array of isolated
nanoscopic ferromagnetic clusters, as observed in previous work. For x >= 0.18,
the much larger values of rho, the tilting of its distribution towards negative
bias field, and the emergence of regions with negative rho, are consistent with
increased long-range ferromagnetic ordering. The FORC distributions display
little dependence on the cooling procedure. With increasing temperature, the
fraction of irreversible switching determined from the FORC distribution
follows closely the ferromagnetic phase fraction measured by La nuclear
magnetic resonance. Our results furthermore demonstrate that the FORC method is
a valuable first-pass characterization tool for magnetic phase separation.Comment: 30 pages, 8 figures, to appear in PR
Concave Plasmonic Particles: Broad-Band Geometrical Tunability in the Near Infra-Red
Optical resonances spanning the Near and Short Infra-Red spectral regime were
exhibited experimentally by arrays of plasmonic nano-particles with concave
cross-section. The concavity of the particle was shown to be the key ingredient
for enabling the broad band tunability of the resonance frequency, even for
particles with dimensional aspect ratios of order unity. The atypical
flexibility of setting the resonance wavelength is shown to stem from a unique
interplay of local geometry with surface charge distributions
Magnetoelastic effects in Jahn-Teller distorted CrF and CuF studied by neutron powder diffraction
We have studied the temperature dependence of crystal and magnetic structures
of the Jahn-Teller distorted transition metal difluorides CrF and CuF
by neutron powder diffraction in the temperature range 2-280 K. The lattice
parameters and the unit cell volume show magnetoelastic effects below the
N\'eel temperature. The lattice strain due to the magnetostriction effect
couples with the square of the order parameter of the antiferromagnetic phase
transition. We also investigated the temperature dependence of the Jahn-Teller
distortion which does not show any significant effect at the antiferromagnetic
phase transition but increases linearly with increasing temperature for CrF
and remains almost independent of temperature in CuF. The magnitude of
magnetovolume effect seems to increase with the low temperature saturated
magnetic moment of the transition metal ions but the correlation is not at all
perfect
Quantification of magnetic force microscopy images using combined electrostatic and magnetostatic imaging
A method for calibrating the force gradients and probe magnetic moment in phase-contrast magnetic
force microscopy ~MFM! is introduced. It is based upon the combined electrostatic force
microscopy EFM and MFM images of a conducting non magnetic metal strip. The behavior of the
phase contrast in EFM is analyzed and modeled as a finite area capacitor. This model is used in
conjunction with the imaging data to derive the proportionality constant between the phase and the
force gradient. This calibration is further used to relate the measured MFM images with the field
gradient from the same conducting strip to derive the effective magnetic moment of the probe. The
knowledge of the phase-force gradient proportionality constant and the probe’s effective moment is
essential to directly quantify field derivatives in MFM images
Hysteresis and hierarchies: dynamics of disorder-driven first-order phase transformations
We use the zero-temperature random-field Ising model to study hysteretic
behavior at first-order phase transitions. Sweeping the external field through
zero, the model exhibits hysteresis, the return-point memory effect, and
avalanche fluctuations. There is a critical value of disorder at which a jump
in the magnetization (corresponding to an infinite avalanche) first occurs. We
study the universal behavior at this critical point using mean-field theory,
and also present preliminary results of numerical simulations in three
dimensions.Comment: 12 pages plus 2 appended figures, plain TeX, CU-MSC-747
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