7,495 research outputs found
Role of interface coupling inhomogeneity in domain evolution in exchange bias
Models of exchange-bias in thin films have been able to describe various
aspects of this technologically relevant effect. Through appropriate choices of
free parameters the modelled hysteresis loops adequately match experiment, and
typical domain structures can be simulated. However, the use of these
parameters, notably the coupling strength between the systems' ferromagnetic
(F) and antiferromagnetic (AF) layers, obscures conclusions about their
influence on the magnetization reversal processes. Here we develop a 2D
phase-field model of the magnetization process in exchange-biased CoO/(Co/Pt)xn
that incorporates the 10 nm-resolved measured local biasing characteristics of
the antiferromagnet. Just three interrelated parameters set to measured
physical quantities of the ferromagnet and the measured density of
uncompensated spins thus suffice to match the experiment in microscopic and
macroscopic detail. We use the model to study changes in bias and coercivity
caused by different distributions of pinned uncompensated spins of the
antiferromagnet, in application-relevant situations where domain wall motion
dominates the ferromagnetic reversal. We show the excess coercivity can arise
solely from inhomogeneity in the density of biasing- and anti-biasing pinned
uncompensated spins in the antiferromagnet. Counter to conventional wisdom,
irreversible processes in the latter are not essential
Model-based testing for space-time interaction using point processes: An application to psychiatric hospital admissions in an urban area
Spatio-temporal interaction is inherent to cases of infectious diseases and
occurrences of earthquakes, whereas the spread of other events, such as cancer
or crime, is less evident. Statistical significance tests of space-time
clustering usually assess the correlation between the spatial and temporal
(transformed) distances of the events. Although appealing through simplicity,
these classical tests do not adjust for the underlying population nor can they
account for a distance decay of interaction. We propose to use the framework of
an endemic-epidemic point process model to jointly estimate a background event
rate explained by seasonal and areal characteristics, as well as a superposed
epidemic component representing the hypothesis of interest. We illustrate this
new model-based test for space-time interaction by analysing psychiatric
inpatient admissions in Zurich, Switzerland (2007-2012). Several socio-economic
factors were found to be associated with the admission rate, but there was no
evidence of general clustering of the cases.Comment: 21 pages including 4 figures and 5 tables; methods are implemented in
the R package surveillance (https://CRAN.R-project.org/package=surveillance
Accelerated expansion from structure formation
We discuss the physics of backreaction-driven accelerated expansion. Using
the exact equations for the behaviour of averages in dust universes, we explain
how large-scale smoothness does not imply that the effect of inhomogeneity and
anisotropy on the expansion rate is small. We demonstrate with an analytical
toy model how gravitational collapse can lead to acceleration. We find that the
conjecture of the accelerated expansion being due to structure formation is in
agreement with the general observational picture of structures in the universe,
and more quantitative work is needed to make a detailed comparison.Comment: 44 pages, 1 figure. Expanded treatment of topics from the Gravity
Research Foundation contest essay astro-ph/0605632. v2: Added references,
clarified wordings. v3: Published version. Minor changes and corrections,
added a referenc
Advectional enhancement of eddy diffusivity under parametric disorder
Frozen parametric disorder can lead to appearance of sets of localized
convective currents in an otherwise stable (quiescent) fluid layer heated from
below. These currents significantly influence the transport of an admixture (or
any other passive scalar) along the layer. When the molecular diffusivity of
the admixture is small in comparison to the thermal one, which is quite typical
in nature, disorder can enhance the effective (eddy) diffusivity by several
orders of magnitude in comparison to the molecular diffusivity. In this paper
we study the effect of an imposed longitudinal advection on delocalization of
convective currents, both numerically and analytically; and report subsequent
drastic boost of the effective diffusivity for weak advection.Comment: 14 pages, 6 figures, for Topical Issue of Physica Scripta "2nd Intl.
Conf. on Turbulent Mixing and Beyond
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