Magnetic and crystal structures of the one-dimensional ferromagnetic chain pyroxene NaCrGe2O6

International audienceWe investigated the magnetic and structural properties of the ferromagnetic pyroxene NaCrGe2O6 by superconducting quantum interference device magnetometry and powder neutron diffraction. This material is the only ferromagnetic member of the recently reported multiferroic pyroxene family AMX2O6 A=Li,Na; M =Fe, Cr; X=Si,Ge . Below TC=6 K, the magnetic structure is characterized by one-dimensional magnetic chains with spins aligned along the c axis of the monoclinic cell. The magnetic moment of Cr3+ is significantly reduced by about 25%. We show that this is likely the result of the low dimensionality of the system. The associated magnetic space group is C2 /c . This symmetry does not allow a linear magnetoelectric effect. No structural phase transition was observed down to 1.8 K

Origin of the metamagnetic transitions in Y1-xErxFe2(H,D)4.2 compounds

The structural and magnetic properties of Y1-xErxFe2 intermetallic compounds and their hydrides and deuterides Y1-xErxFe2H(D)4.2 have been investigated using X-ray diffraction and magnetic measurements under static and pulsed magnetic field up to 60 T. The intermetallics crystallize in the C15 cubic structure , whereas corresponding hydrides and deuterides crystallize in a monoclinic structure. All compounds display a linear decrease of the unit cell volume versus Er concentration; the hydrides have a 0.8% larger cell volume compared to the deuterides with same Er content. They are ferrimagnetic at low field and temperature with a compensation point at x = 0.33 for the intermetallics and x = 0.57 for the hydrides and deuterides. A sharp first order ferromagnetic-antiferromagnetic (FM-AFM) transition is observed upon heating at TFM-AFM for both hydrides and deuterides. These compounds show two different types of field induced transitions, which have different physical origin. At low temperature (T < 50 K), a forced ferri-ferromagnetic metamagnetic transition with Btrans1 = 8 T, related to the change of the Er moments orientation from antiparallel to parallel Fe moment, is observed. Btrans1 is not sensitive to Er concentration, temperature and isotope effect. A second metamagnetic transition resulting from antiferromagnetic to ferrimagnetic state is also observed. The transition field Btrans2 increases linearly versus temperature and relates to the itinerant electron metamagnetic behavior of the Fe sublattice. An onset temperature TM0 is obtained by extrapolating TFM-AFM (B) at zero field. TM0 decreases linearly versus the Er content and is 45(5) K higher for the hydrides compared to the corresponding deuteride. The evolution of TM0 versus cell volume shows that it cannot be attributed exclusively to a pure volume effect and that electronic effects should also be considered.Comment: 22 pages, 10 figure

Magnetic and crystal structures of the magnetoelectric pyroxene LiCrSi2O6

International audienceWe investigated the magnetic and crystal structures of the recent reported magnetoelectric system LiCrSi2O6 by powder neutron diffraction. Below TN=11.5 K, an antiferromagnetic order appears. It is characterized by an antiferromagnetic coupling within the CrO6 octahedra chains and a ferromagnetic coupling between the chains. The magnetic order is commensurate with the lattice with k=0. The associated magnetic space group is P21 /c. This symmetry is in agreement with the reported magnetoelectric effect. We show that the magnetic frustration in this system is small. Finally, we discuss our results using a Landau phenomenological model and in the light of the literature

Magnetic behavior of the NixFe1-xNb2O6 quasi-one-dimensional system: Isolation of Ising chains by frustration

International audiencePhysical properties of the NixFe1-xNb2O6 compounds are investigated combining x-ray and neutron powder diffraction with magnetic and calorimetry measurements as well as 57Fe Mössbauer spectroscopy. This system is known to present quasi-one-dimensional magnetism with the magnetic moments arranged along weakly interacting Ising chains. Partial substitution of the magnetic ion tends to suppress the magnetic ordering observed in the end members of the series. When this happens, the low-temperature magnetic specific heat agrees well with what is expected for isolated Ising chains. The lowest temperature powder neutron-diffraction patterns exhibit evidence for the occurrence of short-range order, and analysis of these diffuse neutron-scattering patterns allow us to obtain information on the magnetic correlations. The suppression of magnetism is consistently interpreted as resulting from the magnetic-cation disorder induced by substitution, which enhances the system's tendency for frustration of geometrical origin

Suppression of magnetic ordering in quasi-one-dimensional FexCo1-xNb2O6 compounds

International audienceWe present a systematic investigation of the series of compounds FexCo1-xNb2O6 by means of x-ray and neutron powder diffraction combined with magnetic measurements, carried out in the paramagnetic as well as in the ordered state, to probe the stability of the magnetic ordering against the composition changes in this model Ising system. Fe for Co substitution induces a continuous lattice volume increase, preserving the orthorhombic crystal structure. The unit-cell expansion is anisotropic and occurs mainly in the ab plane. The observed magnetic structures for x=0,0.8, and 1 are described by the propagation vectors (0,1/2,0) and (1/2,1/2,0), and are consistent with the picture of ferromagnetic Ising-type chains of Fe/Co spins antiferromagnetically coupled by weak interchain interactions. We find out that for

Plant height and hydraulic vulnerability to drought and cold

Understanding how plants survive drought and cold is increasingly important as plants worldwide experience dieback with drought in moist places and grow taller with warming in cold ones. Crucial in plant climate adaptation are the diameters of water-transporting conduits. Sampling 537 species across climate zones dominated by angiosperms, we find that plant size is unambiguously the main driver of conduit diameter variation. And because taller plants have wider conduits, and wider conduits within species are more vulnerable to conduction-blocking embolisms, taller conspecifics should be more vulnerable than shorter ones, a prediction we confirm with a plantation experiment. As a result, maximum plant size should be short under drought and cold, which cause embolism, or increase if these pressures relax. That conduit diameter and embolism vulnerability are inseparably related to plant size helps explain why factors that interact with conduit diameter, such as drought or warming, are altering plant heights worldwide

Magnetic phases of the quasi-two-dimensional compounds FexCo1 - xTa2O6

International audienceWe report new results on the magnetic properties of the FexCo1 - xTa2O6 series of compounds. Essentially using neutron-diffraction and magnetic measurements we study, in more detail, the low-x limit of the temperature versus x phase diagram, where a new bicritical point is observed. The complete phase diagram shows three different magnetic phases at low temperature, for a high, intermediate and very low iron content. These phases consist of distinct antiferromagnetic orderings, characterized by different pairs of propagation vectors. We obtain information about the intraplane exchange interactions by fitting a high-temperature series of the magnetic susceptibility. Here we improve on a previously employed model, showing that two non-equivalent next-nearest-neighbor interactions must be taken into account in order to allow for in-plane magnetic orderings that are consistent with the neutron-diffraction results

Clinical applications of B-type natriuretic peptide (BNP) testing

Many claims have been made in recent years regarding the utility of plasma B-type natriuretic peptide (BNP) concentration measurements in the diagnosis, risk stratification and monitoring of patients with heart failure. This paper summarizes the current evidence and provides guidance for practising clinicians. Overall, plasma BNP testing appears to be of most value in the diagnostic arena, where it is likely to improve the performance of non-specialist physicians in diagnosing heart failure. In clinical practice, BNP testing is best used as a ‘rule out' test for suspected cases of new heart failure in breathless patients presenting to either the outpatient or emergency care settings; it is not a replacement for echocardiography and full cardiological assessment, which will be required for patients with an elevated BNP concentration. Although work is ongoing in establishing the ‘normal' values of BNP, heart failure appears to be highly unlikely below a plasma concentration of 100pg/ml. However, as BNP levels rise with age and are affected by gender, comorbidity and drug therapy, the plasma BNP measurement should not be used in isolation from the clinical contex

Utilization of a deoxynucleoside diphosphate substrate by HIV reverse transcriptase

Background: Deoxynucleoside triphosphates (dNTPs) are the normal substrates for DNA sysnthesis is catalyzed by polymerases such as HIV-1 reverse transcriptase (RT). However, substantial amounts of deoxynucleoside diphosphates (dNDPs) are also present in the cell. Use of dNDPs in HIV-1 DNA sysnthesis could have significant implications for the efficacy of nucleoside RT inhibitors such as AZT which are first line therapeutics fro treatment of HIV infection. Our earlier work on HIV-1 reverse transcriptase (RT) suggested that the interaction between the γ phosphate of the incoming dNTP and RT residue K65 in the active site is not essential for dNTP insertion, implying that this polymerase may be able to insert dNPs in addition to dNTPs. Methodology/Principal Findings: We examined the ability of recombinant wild type (wt) and mutant RTs with substitutions at residue K65 to utilize a dNDP substrate in primer extension reactions. We found that wild type HIV-1 RT indeed catalyzes incorporation of dNDP substrates whereas RT with mutations of residue K645 were unable to catalyze this reaction. Wild type HIV-1 RT also catalyzed the reverse reaction, inorganic phosphate-dependent phosphorolysis. Nucleotide-mediated phosphorolytic removal of chain-terminating 3′-terminal nucleoside inhibitors such as AZT forms the basis of HIV-1 resistance to such drugs, and this removal is enhanced by thymidine analog mutations (TAMs). We found that both wt and TAM-containing RTs were able to catalyze Pi-mediated phosphorolysis of 3′-terminal AZT at physiological levels of Pi with an efficacy similar to that for ATP-dependent AZT-excision. Conclusion: We have identified two new catalytic function of HIV-1 RT, the use of dNDPs as substrates for DNA synthesis, and the use of Pi as substrate for phosphorolytic removal of primer 3′-terminal nucleotides. The ability to insert dNDPs has been documented for only one other DNA polymerase The RB69 DNA polymerase and the reverse reaction employing inorganic phosphate has not been documented for any DNA polymerase. Importantly, our results show that Pi-mediated phosphorolysis can contribute to AZT resistance and indicates that factors that influence HIV resistance to AZT are more complex than previously appreciated. © 2008 Garforth et al