103 research outputs found
First principles study of the multiferroics BiFeO, BiFeCrO, and BiCrO: Structure, polarization, and magnetic ordering temperature
We present results of an {\it ab initio} density functional theory study of
three bismuth-based multiferroics, BiFeO, BiFeCrO, and
BiCrO. We disuss differences in the crystal and electronic structure of
the three systems, and we show that the application of the LDA+ method is
essential to obtain realistic structural parameters for BiFeCrO. We
calculate the magnetic nearest neighbor coupling constants for all three
systems and show how Anderson's theory of superexchange can be applied to
explain the signs and relative magnitudes of these coupling constants. From the
coupling constants we then obtain a mean-field approximation for the magnetic
ordering temperatures. Guided by our comparison of these three systems, we
discuss the possibilities for designing a multiferroic material with large
magnetization above room temperature.Comment: 8 Pages, 4 Figure
Ferroelectricity induced by interatomic magnetic exchange interaction
Multiferroics, where two or more ferroic order parameters coexist, is one of
the hottest fields in condensed matter physics and materials science[1-9].
However, the coexistence of magnetism and conventional ferroelectricity is
physically unfavoured[10]. Recently several remedies have been proposed, e.g.,
improper ferroelectricity induced by specific magnetic[6] or charge orders[2].
Guiding by these theories, currently most research is focused on frustrated
magnets, which usually have complicated magnetic structure and low magnetic
ordering temperature, consequently far from the practical application. Simple
collinear magnets, which can have high magnetic transition temperature, have
never been considered seriously as the candidates for multiferroics. Here, we
argue that actually simple interatomic magnetic exchange interaction already
contains a driving force for ferroelectricity, thus providing a new microscopic
mechanism for the coexistence and strong coupling between ferroelectricity and
magnetism. We demonstrate this mechanism by showing that even the simplest
antiferromagnetic (AFM) insulator MnO, can display a magnetically induced
ferroelectricity under a biaxial strain
Magnetic ground state and multiferroicity in BiMnO
We argue that the centrosymmetric symmetry in BiMnO is
spontaneously broken by antiferromagnetic (AFM) interactions existing in the
system. The true symmetry is expected to be , which is compatible with the
noncollinear magnetic ground state, where the ferromagnetic order along one
crystallographic axis coexists with the the hidden AFM order and related to it
ferroelectric polarization along two other axes. The symmetry can be
restored by the magnetic field Tesla, which switches off the
ferroelectric polarization. Our analysis is based on the solution of the
low-energy model constructed for the 3d-bands of BiMnO, where all the
parameters have been derived from the first-principles calculations. Test
calculations for isostructural BiCrO reveal an excellent agreement with
experimental data.Comment: 5 pages, 5 figure
Epitaxial Bi2FeCrO6 Multiferroic Thin Films
We present here experimental results obtained on Bi2FeCrO6 (BFCO) epitaxial
films deposited by laser ablation directly on SrTiO3 substrates. It has been
theoretically predicted, by Baettig and Spaldin, using first-principles density
functional theory that BFCO is ferrimagnetic (with a magnetic moment of 2 Bohr
magneton per formula unit) and ferroelectric (with a polarization of ~80
microC/cm2 at 0K). The crystal structure has been investigated using X-ray
diffraction which shows that the films are epitaxial with a high crystallinity
and have a degree of orientation depending of the deposition conditions and
that is determined by the substrate crystal structure. Chemical analysis
carried out by X-ray Microanalysis and X-ray Photoelectron Spectroscopy (XPS)
indicates the correct cationic stoichiometry in the BFCO layer, namely
(Bi:Fe:Cr = 2:1:1). XPS depth profiling revealed that the oxidation state of Fe
and Cr ions in the film remains 3+ throughout the film thickness and that both
Fe and Cr ions are homogeneously distributed throughout the depth.
Cross-section high-resolution transmission electron microscopy images together
with selected area electron diffraction confirm the crystalline quality of the
epitaxial BFCO films with no identifiable foreign phase or inclusion. The
multiferroic character of BFCO is proven by ferroelectric and magnetic
measurements showing that the films exhibit ferroelectric and magnetic
hysteresis at room temperature. In addition, local piezoelectric measurements
carried out using piezoresponse force microscopy (PFM) show the presence of
ferroelectric domains and their switching at the sub-micron scale.Comment: Accepted for publication in Philosophical Magazine Letter
Magnetoelectric ordering of BiFeO3 from the perspective of crystal chemistry
In this paper we examine the role of crystal chemistry factors in creating
conditions for formation of magnetoelectric ordering in BiFeO3. It is generally
accepted that the main reason of the ferroelectric distortion in BiFeO3 is
concerned with a stereochemical activity of the Bi lone pair. However, the lone
pair is stereochemically active in the paraelectric orthorhombic beta-phase as
well. We demonstrate that a crucial role in emerging of phase transitions of
the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order
types belongs to the change of the degree of the lone pair stereochemical
activity - its consecutive increase with the temperature decrease. Using the
structural data, we calculated the sign and strength of magnetic couplings in
BiFeO3 in the range from 945 C down to 25 C and found the couplings, which
undergo the antiferromagnetic-ferromagnetic transition with the temperature
decrease and give rise to the antiferromagnetic ordering and its delay in
regard to temperature, as compared to the ferroelectric ordering. We discuss
the reasons of emerging of the spatially modulated spin structure and its
suppression by doping with La3+.Comment: 18 pages, 5 figures, 3 table
Reduction in greenhouse gas emissions from national climate legislation
The international response to climate change has been inadequate, but not zero. There are 1,800 climate change laws worldwide. We use panel data on legislative activity in 133 countries over the period 1999â2016 to identify statistically the short-term and long-term impact of climate legislation. Each new law reduces annual carbon dioxide (CO2) emissions per unit of gross domestic product by 0.78% nationally in the short term (during the first three years) and by 1.79% in the long term (beyond three years). The results are driven by parliamentary acts and by countries with a strong rule of law. In 2016, current climate laws were associated with an annual reduction in global CO2 emissions of 5.9 GtCO2, more than the US CO2 output that year. Cumulative CO2 emissions savings from 1999 to 2016 amount to 38 GtCO2, or one yearâs worth of global CO2 output. The impact on other greenhouse gases is much lower
The 4Cs of adaptation tracking: consistency, comparability, comprehensiveness, coherency
Adaptation tracking seeks to characterize, monitor, and compare general trends in climate change adaptation over time and across nations. Recognized as essential for evaluating adaptation progress, there have been few attempts to develop systematic approaches for tracking adaptation. This is reflected in polarized opinions, contradictory findings, and lack of understanding on the state of adaptation globally. In this paper, we outline key methodological considerations necessary for adaptation tracking research to produce systematic, rigorous, comparable, and usable insights that can capture the current state of adaptation globally, provide the basis for characterizing and evaluating adaptations taking place, facilitate examination of what conditions explain differences in adaptation action across jurisdictions, and can underpin the monitoring of change in adaptation over time. Specifically, we argue that approaches to adaptation tracking need to (i) utilize a consistent and operational conceptualization of adaptation, (ii) focus on comparable units of analysis, (iii) use and develop comprehensive datasets on adaptation action, and (iv) be coherent with our understanding of what constitutes real adaptation. Collectively, these form the 4Cs of adaptation tracking (consistency, comparability, comprehensiveness, and coherency)
Probable metabolic interaction between methadone and fluvoxamine in addict patients.
We report five cases where fluvoxamine (FLVX) was added to maintenance treatment with methadone (MTD) in addict patients with affective disorders. In view of the implication of FLVX in several metabolic drug interactions, MTD plasma levels were measured before and after treatment with FLVX. A slight increase (approximately 20% of the MTD plasma level/dose ratio) occurred in two cases. In the remaining three patients, the interaction was more pronounced (40-100% increase of the MTD plasma level/dose ratio), with clinical manifestations of opiate withdrawal after stopping FLVX therapy in one case. Caution is needed when starting or stopping treatment with FLVX in patients receiving maintenance treatment with methadone
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