95 research outputs found
Premartensitic transition driven by magnetoelastic interaction in bcc ferromagnetic
We show that the magnetoelastic coupling between the magnetization and the
amplitude of a short wavelength phonon enables the existence of a first order
premartensitic transition from a bcc to a micromodulated phase in .
Such a magnetoelastic coupling has been experimentally evidenced by AC
susceptibility and ultrasonic measurements under applied magnetic field. A
latent heat around 9 J/mol has been measured using a highly sensitive
calorimeter. This value is in very good agreement with the value predicted by a
proposed model.Comment: 4 pages RevTex, 3 Postscript figures, to be published in Physical
Review Letter
Lattice dynamics and phonon softening in Ni-Mn-Al Heusler alloys
Inelastic and elastic neutron scattering have been used to study a single
crystal of the NiMnAl Heusler alloy over a broad
temperature range. The paper reports the first experimental determination of
the low-lying phonon dispersion curves for this alloy system. We find that the
frequencies of the TA modes are relatively low. This branch exhibits an
anomaly (dip) at a wave number , which softens with
decreasing temperature. Associated with this anomalous dip at , an
elastic central peak scattering is also present. We have also observed
satellites due to the magnetic ordering.Comment: 6 pages, 6 figures. Accepted for publication in the Physical Review
On the athermal character of structural phase transitions
The significance of thermal fluctuations on nucleation in structural
first-order phase transitions has been examined. The prototype case of
martensitic transitions has been experimentally investigated by means of
acoustic emission techniques. We propose a model based on the mean
first-passage time to account for the experimental observations. Our study
provides a unified framework to establish the conditions for isothermal and
athermal transitions to be observed.Comment: 5 pages, 4 figures, accepted in Phys. Rev. Let
Some properties of the k-dimensional Lyness' map
This paper is devoted to study some properties of the k-dimensional Lyness'
map. Our main result presentes a rational vector field that gives a Lie
symmetry for F. This vector field is used, for k less or equal to 5 to give
information about the nature of the invariant sets under F. When k is odd, we
also present a new (as far as we know) first integral for F^2 which allows to
deduce in a very simple way several properties of the dynamical system
generated by F. In particular for this case we prove that, except on a given
codimension one algebraic set, none of the positive initial conditions can be a
periodic point of odd period.Comment: 22 pages; 3 figure
Lattice dynamics in magnetic superelastic Ni-Mn-In alloys. Neutron scattering and ultrasonic experiments
Neutron scattering and ultrasonic methods have been used to study the lattice
dynamics of two single crystals of Ni-Mn-In Heusler alloys close to
NiMnIn magnetic superelastic composition. The paper
reports the experimental determination of the low-lying phonon dispersion
curves and the elastic constants for this alloy system. We found that the
frequencies of the TA branch are relatively low and it exhibits a small
dip anomaly at a wave number , which softens with
decreasing temperature. Associated with the softening of this phonon, we also
observed the softening of the shear elastic constant .
Both temperature softenings are typical for bcc based solids which undergo
martensitic transformations and reflect the dynamical instability of the cubic
lattice against shearing of planes along directions.
Additionally, we measured low-lying phonon dispersion branches and elastic
constants in applied magnetic fields aimed to characterize the magnetoelastic
coupling.Comment: 8 pages, 7 figures. Accepted for publication in the Physical Review
First-principles study of lattice instabilities in the ferromagnetic martensite NiMnGa
The phonon dispersion relations and elastic constants for ferromagnetic
NiMnGa in the cubic and tetragonally distorted Heusler structures are
computed using density-functional and density-functional perturbation theory
within the spin-polarized generalized-gradient approximation. For
, the TA tranverse acoustic branch along and
symmetry-related directions displays a dynamical instability at a wavevector
that depends on . Through examination of the Fermi-surface nesting and
electron-phonon coupling, this is identified as a Kohn anomaly. In the parent
cubic phase the computed tetragonal shear elastic constant,
C=(CC)/2, is close to zero, indicating a marginal
elastic instability towards a uniform tetragonal distortion. We conclude that
the cubic Heusler structure is unstable against a family of energy-lowering
distortions produced by the coupling between a uniform tetragonal distortion
and the corresponding modulation. The computed relation between the
ratio and the modulation wavevector is in excellent agreement with
structural data on the premartensitic ( = 1) and martensitic ( =
0.94) phases of NiMnGa.Comment: submitted to Phys. Rev.
Inverse magnetocaloric effect in ferromagnetic Ni-Mn-Sn alloys
The magnetocaloric effect (MCE) in paramagnetic materials has been widely
used for attaining very low temperatures by applying a magnetic field
isothermally and removing it adiabatically. The effect can be exploited also
for room temperature refrigeration by using recently discovered giant MCE
materials. In this letter, we report on an inverse situation in Ni-Mn-Sn
alloys, whereby applying a magnetic field adiabatically, rather than removing
it, causes the sample to cool. This has been known to occur in some
intermetallic compounds, for which a moderate entropy increase can be induced
when a field is applied, thus giving rise to an inverse magnetocaloric effect.
However, the entropy change found for some ferromagnetic Ni-Mn-Sn alloys is
just as large as that reported for giant MCE materials, but with opposite sign.
The giant inverse MCE has its origin in a martensitic phase transformation that
modifies the magnetic exchange interactions due to the change in the lattice
parameters.Comment: 12 pages, 4 figures, to appear in Nature Materials (online published,
15 May 2005
Morbidity, outcomes and cost-benefit analysis of wildlife rehabilitation in Catalonia (Spain)
Background There are few studies of careful examination of wildlife casualties in Wildlife Rehabilitation Centers. These studies are essential for detecting menaces to wild species and providing objective criteria about cost-benefit of treatments in those centers. The release rate is considered the main outcome indicator, but other parameters such as length of stay at the center and a cost-benefit index expressed as number of released animals per euro and day, could be used as reliable estimators of the rehabilitation costs. Methodology A retrospective study based on 54772 admissions recorded from 1995-2013 in the database of the Wildlife Rehabilitation Center of Torreferrussa (Catalonia, NW Spain) assessed the morbidity, outcomes and cost-benefits of the rehabilitation practices. Results Three hundred and two species were included: 232 birds (n = 48633), 37 mammals (n = 3293), 20 reptiles (n = 2705) and 13 amphibians (n = 141). The most frequent causes of admission were: 39.8% confiscation of protected species (89.4% passerines), 31.8% orphaned young animals (35.3% swifts, 21.7% diurnal raptors and owls) and 17.4% trauma casualties (46.7% raptors and owls). The highest proportion of releases was found in the captivity confiscation category [87.4% passerines (median time of stay: 12 days)], followed by the orphaned category [78% owls (66 days), 76.5% diurnal birds of prey (43 days), 75.6% hedgehogs (49 days), 52.7% swifts (19 days) and 52% bats (55 days)]. For the trauma group, 46.8% of releases were hedgehogs (44 days) and 25.6% owls (103 days). As regards the cost-benefit index, the trauma casualties and infectious diseases had the worse values with 1.3 and 1.4 released animals/euro/day respectively, and were particularly low in raptors, waders, marine birds and chiroptera. On the contrary, captivity (4.6) and misplacement (4.1) had the best index, particulary in amphibian, reptiles and passerines. Conclusions/significance Cost-benefit studies including the release rate, the time of stay at the center and the costbenefit index should be implemented for improving management efficiency of the Wildlife Rehabilitation Centers
Habitat determinants of golden‐headed lion tamarin (Leontopithecus chrysomelas) occupancy of cacao agroforests: Gloomy conservation prospects for management intensification
Organismal distributions in human‐modified landscapes largely depend on the capacity of any given species to adapt to changes in habitat structure and quality. The golden‐headed lion tamarin (GHLT; Leontopithecus chrysomelas) is an Endangered primate from the Brazilian Atlantic Forest whose remaining populations occupy heterogeneous landscapes consisting primarily of shade cacao (Theobroma cacao) agroforestry, locally known as cabrucas. This cash crop can coexist with high densities of native tree species and holds a significant proportion of the native fauna, but its widely extolled wildlife‐friendly status is increasingly threatened by management intensification. Although this potentially threatens to reduce the distribution of GHLTs, the main determinants of tamarin's occupancy of cabrucas remain unknown, thereby limiting our ability to design and implement appropriate conservation practices. We surveyed 16 cabruca patches in southern Bahia, Brazil, and used occupancy modeling to identify the best predictors of GHLT patch occupancy. Key explanatory variables included vegetation structure, critical resources, landscape context, human disturbance, and predation pressure. We found a negative relationship between GHLT occupancy and the prevalence of jackfruit trees (Artocarpus heterophylus), which is likely associated with the low representation of other key food species for GHLTs. Conversely, cabrucas retaining large‐diameter canopy trees have a higher probability of GHLT occupancy, likely because these trees provide preferred sleeping sites. Thus, key large tree resources (food and shelter) are currently the main drivers of GHLT occupancy within cabruca agroecosystems. Since both factors can be directly affected by crop management practices, intensification of cabrucas may induce significant habitat impacts on GHLT populations over much of their remaining range‐wide distribution
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