Enhanced iron magnetic moment in the ThFe11C2 intermetallic compound

International audienceDetailed theoretical investigations on the electronic and magnetic properties of the ThFe11C2 compound have been performed using both the linear muffin-tin orbital and Korringa-Kohn-Rostocker methods of band structure calculation. The structure of the ThFe11C2 compound has three inequivalent iron sites with different local environment. A strongly enhanced magnetic moment is observed on certain Fe positions, coexisting with much lower magnetic moments on other iron positions of the lattice. Band structure calculations indeed show that the Fe magnetic moments depend strongly on the local environment. The average Fe magnetic moment obtained from these calculations is in good agreement with the experimental average Fe moment obtained from magnetization measurements. The orbital contribution to the magnetic moment is found to be especially large on the Fe 4b position. Comparing calculated hyperfine fields with experimental results, it is found that the calculated and experimental hyperfine fields are correlated. However, similarly to the results reported before for elemental Fe, the magnitude of all calculated Fe hyperfine fields is about 25% smaller. The agreement with the Mössbauer measurements is improved by scaling the core polarization contribution and by estimating the orbital valence d-electrons contribution to the magnetic hyperfine fields using the local spin density approximation + dynamical mean field theory calculated orbital moments

Metastable and transient states of chemical ordering in Fe-V nanocrystalline alloys

Chemical ordering of the disordered alloys Fe0.78V0.22, Fe0.53V0.47, Fe0.39V0.61, and Fe0.37V0.63 was performed by annealing at temperatures from 723 to 973 K. The initial state of chemical disorder was produced by high-energy ball milling, and the evolution of order was measured by neutron diffractometry and by 57Fe Mössbauer spectrometry. The hyperfine magnetic field distributions obtained from the Mössbauer spectra provided quantitative measurements of the number of antisite Fe atoms in the partially ordered alloys. The long-range order parameters in steady state after long annealing times were used as states of metastable equilibrium for a generally successful comparison with the metastable Fe-V phase diagram calculated by Sanchez et al. [Phys. Rev. B 54, 8958 (1996)]. For the metastable equilibrium state of order in Fe0.53V0.47 at low temperatures, the order parameters were smaller than expected. This corresponded to an abundance of antisite atoms, which were not removed effectively by annealing at the lower temperatures

EFFECT OF STARTING POWDER PREMIXING ON THE INTERPHASE EXCHANGE COUPLING IN Nd₂Fe₁₄B + 10 WT % Fe NANOCOMPOSITES OBTAINED TROUGH MECHANICAL MILLING

In the frame of optimizing the potential, of had /soft nanocomposites for high performance applications, we report on the effect of starting powder premixing on the structure, microstructure and exchange coupling in hard/soft magnetic phases. Nd2Fe14B powder was first mixed with Fe powder using two different means: by hand or by using a Turbula Mixer. Moreover, in the hand mixed powders the Fe particle size was changed from 100 to 1 μm, while the samples mixed using the Turbula Mixer contained only Fe particles of around 1 μm in size. The mixed powders were subsequently milled for 6 hours in a planetary ball mill; the calculated impact energy was 77 mJ/impact and 10 kJ/g for the entire duration of process. Good exchange coupling was obtained in all three cases. The samples prepared with Fe < 1 μm particles yielding slightly better results due to better dispersion of the two phases in the final nanocomposite material. The highest energy product was achieved for the sample premixed with the Turbula Mixer ((BH)max=125 kJ/m3) after being annealed at 800 °C for 1.5 min

Monitoring long-term evolution of engineered barrier systems using magnets:Magnetic response

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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

STRUCTURAL, MAGNETIC AND ELECTRONIC PROPERTIES OF THE INTERMETALLIC COMPOUNDS FROM THE TERNARY SYSTEM Ce – Co – Mn

XRD, magnetic and XPS measurements of the three classes of intermetallic compounds with different crystallographic structures are reported. XPS spectra of Ce2Co15Mn3, CeCo7Mn5 and CeCo8Mn4 compounds pointed out the intermediate valence state of Ce atoms and that both Co and Mn atoms carry magnetic moments. The complex magnetic structure is determined by the competition between the ferromagnetic (Co–Co pairs) and antiferromagnetic (Co–Mn and Mn–Mn pairs) interactions. In CeCoMn, Ce3Co3Mn4 and Ce3Co6Mn compounds both Ce and Co atoms are non-magnetic like in the isostructural compound CeCo2. Magnetic behavior is mostly due to the Mn moments and depends essentially on the Mn - Mn distances

SOFT MAGNETIC NANOCRYSTALLINE Ni-Fe-X-Y AND MeFe₂O₄ POWDERS OBTAINED BY MECHANOSYNTHESIS

The soft magnetic nanocrystalline powders, alloys (Ni3Fe, 79Ni16Fe5Mo, 77Ni14Fe5Cu4Mo, wt. %) and zinc ferrite, were obtained by dry and wet mechanical alloying and reactive milling, followed by different heat treatments. The powders were characterised by X-ray diffraction, scanning electron microscopy, X-ray microanalysis, differential scanning calorimetry, thermomagnetic and magnetic measurements. The X-ray diffraction shown the progressive new phases formation. The crystallite size is between 18-7 nm depending on materials and milling conditions. The particle size is smaller for wet-mechanical alloying comparing with dry-milling. The thermomagnetic measurement shown the Curie temperature of the alloys. The spontaneous magnetisation of the wet-milled and annealed samples is higher than of the molten alloys

NANOCRYSTALLINE MAGNETITE - Fe₃O₄ PARTICLES SYNTHESIZED BY MECHANICAL MILLING

Nanocrystalline magnetite – Fe3O4 has been synthesized in nanocrystalline state by mechanical milling of well crystallized magnetite samples obtained by heat treatment of a stoichiometric mixture of iron and hematite. Upon increasing the milling time, the mean crystallite size of magnetite is diminishing. After only 5 minutes of mechanical milling the crystallites are 110 nm. The milling up to 120 minutes leads to a continuous reduction of crystallites size up to 8 nm. The mechanical milling process induces strains into the lattice and the lattice strains increase drastically in the first 60 minutes of milling, after that a saturation of lattice strains is noticed. During milling a contamination of the powder with elemental iron occurs for milling times larger than 30 minutes. A decrease of the saturation magnetisation is noticed upon increasing the milling time and the decrease is associated with the structural disorder and defects that are induced into the material during milling. A tendency of the magnetisation to become unsaturated is also noticed and it is related to canted spins effect

Identification of metabolic pathways influenced by the G-protein coupled receptors GprB and GprD in Aspergillus nidulans

Heterotrimeric G-protein-mediated signaling pathways play a pivotal role in transmembrane signaling in eukaryotes. Our main aim was to identify signaling pathways regulated by A. nidulans GprB and GprD G-protein coupled receptors (GPCRs). When these two null mutant strains were compared to the wild-type strain, the DeltagprB mutant showed an increased protein kinase A (PKA) activity while growing in glucose 1% and during starvation. In contrast, the DeltagprD has a much lower PKA activity upon starvation. Transcriptomics and (1)H NMR-based metabolomics were performed on two single null mutants grown on glucose. We noted modulation in the expression of 11 secondary metabolism gene clusters when the DeltagprB and DeltagprD mutant strains were grown in 1% glucose. Several members of the sterigmatocystin-aflatoxin gene cluster presented down-regulation in both mutant strains. The genes of the NR-PKS monodictyphenone biosynthesis cluster had overall increased mRNA accumulation in DeltagprB, while in the DeltagprD mutant strain the genes had decreased mRNA accumulation. Principal component analysis of the metabolomic data demonstrated that there was a significant metabolite shift in the DeltagprD strain. The (1)H NMR analysis revealed significant expression of essential amino acids with elevated levels in the DeltagprD strain, compared to the wild-type and DeltagprB strains. With the results, we demonstrated the differential expression of a variety of genes related mainly to secondary metabolism, sexual development, stress signaling, and amino acid metabolism. We propose that the absence of GPCRs triggered stress responses at the genetic level. The data suggested an intimate relationship among different G-protein coupled receptors, fine-tune regulation of secondary and amino acid metabolisms, and fungal development