219 research outputs found
The influence of mycotoxins on ruminant health and performance
Hrana onečišćena mikotoksinima predstavlja bitan problem za ljude i životinje. Mikotoksini su toksični metaboliti plijesni koji narušavaju zdravlje ljudi i životinja te dovode do znatnih ekonomskih gubitaka. S tog aspekta najznačajniji su: aflatoksini, ohratoksin, trihoteceni, zearalenon, fumonisini, patulin, tremorgeni toksini i ergot alkaloidi. Neke plijesni mogu proizvoditi više različitih toksina, a neke mikotoksine može proizvesti od više različitih vrsta plijesni. Krmiva se mogu zaraziti već na polju, tijekom žetve ili tijekom skladištenja. Mikotoksini imaju različite akutne ili kronične učinke i to ovisno o vrsti i rezistenciji pojedine životinje. Preživači su znatno otporniji na negativne učinke od monogastričnih životinja. Glavni razlog tome je razgradnja mikotoksina mikrobiotima buraga. Pri tome su protozoe bitnije u biodegradaciji nego bakterije. Ipak, pri dugotrajnoj konzumaciji hrane zaražene mikotoksinima i kod preživača su mogući poremećaji u proizvodnji, reprodukciji i rastu. Posebno značajan problem predstavlja mogućnost prijenosa mikotoksina i njihovih metabolita na ljude, putem jestivih životinjskih proizvoda. Najčešće istraživane vrste preživača su: tovna i mliječna goveda, ovce, koze i jeleni.Contamination of foods and feeds with mycotoxins is a significant problem. Mycotoxins are toxic metabolites of molds that have adverse effects on humans, animals, and crops. Aflatoxins, ochratoxins, trichothecenes, zearelenone, fumonisins, tremorgenic toxins, and ergot alkaloids are the most important mycotoxins in animal production. Some molds are capable of producing more than one mycotoxin and some mycotoxins are produced by more than one fungal species. Mycotoxins could be synthesised before harvest, during harvest or during storage. Mycotoxins have acute and chronic effects on animals depending on species and susceptibility of an animal within a species. Ruminants are more resistant to the adverse effects of mycotoxins than the monogastric animals. The main reason for that is microbial degradation of mycotoxines. Protozoa are even more important in biodegradation than bacteria. However, production, reproduction, and growth can be altered when ruminants consume mycotoxin-contaminated feed for extended periods of time. Special problem is possible presence of mycotoxines and their metabolites in animal products. Beef cattle, dairy cattle, sheep, goats, and deer are among ruminants that have been investigated
Prospects of high temperature ferromagnetism in (Ga,Mn)As semiconductors
We report on a comprehensive combined experimental and theoretical study of
Curie temperature trends in (Ga,Mn)As ferromagnetic semiconductors. Broad
agreement between theoretical expectations and measured data allows us to
conclude that T_c in high-quality metallic samples increases linearly with the
number of uncompensated local moments on Mn_Ga acceptors, with no sign of
saturation. Room temperature ferromagnetism is expected for a 10% concentration
of these local moments. Our magnetotransport and magnetization data are
consistnent with the picture in which Mn impurities incorporated during growth
at interstitial Mn_I positions act as double-donors and compensate neighboring
Mn_Ga local moments because of strong near-neighbor Mn_Ga-Mn_I
antiferromagnetic coupling. These defects can be efficiently removed by
post-growth annealing. Our analysis suggests that there is no fundamental
obstacle to substitutional Mn_Ga doping in high-quality materials beyond our
current maximum level of 6.2%, although this achievement will require further
advances in growth condition control. Modest charge compensation does not limit
the maximum Curie temperature possible in ferromagnetic semiconductors based on
(Ga,Mn)As.Comment: 13 pages, 12 figures, submitted to Phys. Rev.
Interstitial Mn in (Ga,Mn)As: Binding energy and exchange coupling
We present ab initio calculations of total energies of Mn atoms in various
interstitial positions. The calculations are performed by the full-potential
linearized plane-wave method. The minimum energy is found for tetrahedral
T(As4) position, but the energy of the T(Ga4) site differs by only a few meV.
The T(Ga4) position becomes preferable in the p-type materials. In samples with
one substitutional and one interstitial Mn the Mn atoms tend to form close pair
with antiparallel magnetic moments. We also use the spin-splitting of the
valence band to estimate the exchange coupling Jpd for various positions of Mn.
It is the same for the substitutional and T(As4) position and it is only
slightly reduced for the T(Ga4) position. The hybridization of Mn d-states with
six next-nearest neighbors of the interstitial Mn explains the insensitivity of
Jpd to the position of Mn.Comment: 6 pages, 3 figures, 3 tables, submitted to the Physical Review
Mn incorporation in as-grown and annealed (Ga,Mn)As layers studied by x-ray diffraction and standing-wave uorescence
A combination of high-resolution x-ray diffraction and a new technique of
x-ray standing wave uorescence at grazing incidence is employed to study the
structure of (Ga,Mn)As diluted magnetic semiconductor and its changes during
post-growth annealing steps. We find that the film is formed by a uniform,
single crystallographic phase epilayer covered by a thin surface layer with
enhanced Mn concentration due to Mn atoms at random non-crystallographic
positions. In the epilayer, Mn incorporated at interstitial position has a
dominant effect on lattice expansion as compared to substitutional Mn. The
expansion coeffcient of interstitial Mn estimated from our data is consistent
with theory predictions. The concentration of interstitial Mn and the
corresponding lattice expansion of the epilayer are reduced by annealing,
accompanied by an increase of the density of randomly distributed Mn atoms in
the disordered surface layer. Substitutional Mn atoms remain stable during the
low-temperature annealing.Comment: 9 pages, 9 figure
Prospect for room temperature tunneling anisotropic magnetoresistance effect: density of states anisotropies in CoPt systems
Tunneling anisotropic magnetoresistance (TAMR) effect, discovered recently in
(Ga,Mn)As ferromagnetic semiconductors, arises from spin-orbit coupling and
reflects the dependence of the tunneling density of states in a ferromagnetic
layer on orientation of the magnetic moment. Based on ab initio relativistic
calculations of the anisotropy in the density of states we predict sizable TAMR
effects in room-temperature metallic ferromagnets. This opens prospect for new
spintronic devices with a simpler geometry as these do not require
antiferromagnetically coupled contacts on either side of the tunnel junction.
We focus on several model systems ranging from simple hcp-Co to more complex
ferromagnetic structures with enhanced spin-orbit coupling, namely bulk and
thin film L1-CoPt ordered alloys and a monatomic-Co chain at a Pt surface
step edge. Reliability of the predicted density of states anisotropies is
confirmed by comparing quantitatively our ab initio results for the
magnetocrystalline anisotropies in these systems with experimental data.Comment: 4 pages, 2 figure
Systematic study of Mn-doping trends in optical properties of (Ga,Mn)As
We report on a systematic study of optical properties of (Ga,Mn)As epilayers
spanning the wide range of accessible substitutional Mn_Ga dopings. The growth
and post-growth annealing procedures were optimized for each nominal Mn doping
in order to obtain films which are as close as possible to uniform
uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the
mid-infrared absorption spectra whose position exhibits a prevailing blue-shift
for increasing Mn-doping. In the visible range, a peak in the magnetic circular
dichroism blue shifts with increasing Mn-doping. These observed trends confirm
that disorder-broadened valence band states provide a better one-particle
representation for the electronic structure of high-doped (Ga,Mn)As with
metallic conduction than an energy spectrum assuming the Fermi level pinned in
a narrow impurity band.Comment: 22 pages, 14 figure
Theory of ferromagnetic (III,Mn)V semiconductors
The body of research on (III,Mn)V diluted magnetic semiconductors initiated
during the 1990's has concentrated on three major fronts: i) the microscopic
origins and fundamental physics of the ferromagnetism that occurs in these
systems, ii) the materials science of growth and defects and iii) the
development of spintronic devices with new functionalities. This article
reviews the current status of the field, concentrating on the first two, more
mature research directions. From the fundamental point of view, (Ga,Mn)As and
several other (III,Mn)V DMSs are now regarded as textbook examples of a rare
class of robust ferromagnets with dilute magnetic moments coupled by
delocalized charge carriers. Both local moments and itinerant holes are
provided by Mn, which makes the systems particularly favorable for realizing
this unusual ordered state. Advances in growth and post-growth treatment
techniques have played a central role in the field, often pushing the limits of
dilute Mn moment densities and the uniformity and purity of materials far
beyond those allowed by equilibrium thermodynamics. In (III,Mn)V compounds,
material quality and magnetic properties are intimately connected. In the
review we focus on the theoretical understanding of the origins of
ferromagnetism and basic structural, magnetic, magneto-transport, and
magneto-optical characteristics of simple (III,Mn)V epilayers, with the main
emphasis on (Ga,Mn)As. The conclusions we arrive at are based on an extensive
literature covering results of complementary ab initio and effective
Hamiltonian computational techniques, and on comparisons between theory and
experiment.Comment: 58 pages, 49 figures Version accepted for publication in Rev. Mod.
Phys. Related webpage: http://unix12.fzu.cz/ms
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