252 research outputs found
Spin excitations in the antiferromagnet NaNiO2
In NaNiO2, Ni3+ ions form a quasi two dimensional triangular lattice of S =
1=2 spins. The magnetic order observed below 20K has been described as an A
type antiferromagnet with ferro- magnetic layers weakly coupled
antiferromagnetically. We studied the magnetic excitations with the electron
spin resonance for frequencies 1-20 cm-1, in magnetic fields up to 14 T. The
bulk of the results are interpreted in terms of a phenomenological model
involving bi-axial anisotropy for the spins: a strong easy-plane term, and a
weaker anisotropy within the plane. The direction of the easy plane is
constrained by the collective Jahn-Teller distortion occurring in this material
at 480 K
Influence of Nd on the magnetic properties of Nd1-xCaxMnO3
The role played by the Nd ions in the magnetic properties of Nd0.5Ca0.5MnO3
and Nd0.7Ca0.3MnO3 is studied using static magnetization, neutron diffraction
and high frequency (9.4-475GHz) Electron Spin Resonance. We show that the Nd
ions are weakly coupled to the Mn ions via ferromagnetic exchange and are
responsible for the peculiar ferromagnetic resonance observed in the FM phase
of both compounds (ground state below 120K for x=0.3, high field state for
x=0.5). We then use ESR to look for magnetic phase separation in the low field,
CO phase of Nd0.5Ca0.5MnO3. We show that there is no trace of the FM phase
imbedded in the CO phase, contrary to what is observed in La0.5Ca0.5MnO3 or
Pr0.5Sr0.5MnO3.Comment: to be published in phys.Rev.B as a Rapid Com
Valence and magnetic ordering in intermediate valence compounds : TmSe versus SmB6
The intermediate valent systems TmSe and SmB6 have been investigated up to 16
and 18 GPa by ac microcalorimetry with a pressure (p) tuning realized in situ
at low temperature. For TmSe, the transition from an antiferromagnetic
insulator for p<3 GPa to an antiferromagnetic metal at higher pressure has been
confirmed. A drastic change in the p variation of the Neel temperature (Tn) is
observed at 3 GPa. In the metallic phase (p>3 GPa), Tn is found to increase
linearly with p. A similar linear p increase of Tn is observed for the
quasitrivalent compound TmS which is at ambiant pressure equivalent to TmSe at
p=7 GPa. In the case of SmB6 long range magnetism has been detected above p=8
GPa, i.e. at a pressure slightly higher than the pressure of the insulator to
metal transition. However a homogeneous magnetic phase occurs only above 10
GPa. The magnetic and electronic properties are related to the renormalization
of the 4f wavefunction either to the divalent or the trivalent configurations.
As observed in SmS, long range magnetism in SmB6 occurs already far below the
pressure where a trivalent Sm3+ state will be reached. It seems possible, to
describe roughly the physical properties of the intermediate valence
equilibrium by assuming formulas for the Kondo lattice temperature depending on
the valence configuration. Comparison is also made with the appearance of long
range magnetism in cerium and ytterbium heavy fermion compounds.Comment: 22 pages including figure
Coercivity and squareness enhancement in ball-milled hard magnetic-antiferromagnetic composites
This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.The room-temperature coercivity,HC, and squareness, MR/MS (remanence/saturation magnetizations), of permanent magnet,SmCo5powders have been enhanced by ball milling with antiferromagnetic NiO (with Néel temperature, TN=590 K). This enhancement is observed in the as-milled state. However, when the milling of SmCo5 is carried out with an antiferromagnet with TN below room temperature (e.g., for CoO, TN=290 K), the coercivity enhancement is only observed at low temperatures after field cooling through TN. The ferromagnetic-antiferromagnetic exchange coupling induced either by local heating during milling(SmCo5+NiO) or field cooling (SmCo5+CoO) is shown to be the origin of the HC increase
High magnetic field transport measurement of charge-ordered PrCaMnO strained thin films
We have investigated the magnetic-field-induced phase transition of
charge-ordered (CO) PrCaMnO thin films, deposited onto
(100)-oriented LaAlO and (100)-oriented SrTiO substrates using the
pulsed laser deposition technique, by measuring the transport properties with
magnetic fields up to 22T. The transition to a metallic state is observed on
both substrates by application of a critical magnetic field ( at 60K).
The value of the field required to destroy the charge-ordered insulating state,
lower than the bulk compound, depends on both the substrate and the thickness
of the film. The difference of the critical magnetic field between the films
and the bulk material is explained by the difference of in-plane parameters at
low temperature (below the CO transition). Finally, these results confirm that
the robustness of the CO state, depends mainly on the stress induced by the
difference in the thermal dilatations between the film and the substrate.Comment: 10 pages, 6 figures. To be published in Phys. Rev.
Interlayer Magnetic Frustration in Quasi-stoichiometric Li1-xNi1+xO2
Susceptibility, high-field magnetization and submillimeter wave electron spin
resonance measurements of layered quasi-stoichiometric Li1-xNi1+xO2 are
reported and compared to isomorphic NaNiO2. A new mechanism of magnetic
frustration induced by the excess Ni ions always present in the Li layers is
proposed. We finally comment on the possible realization of an orbital liquid
state in this controversial compound.Comment: 4 pages, 5 figures, submitted to Phys.Rev.B, Rapid Com
De l'utilisation de méthodes géophysiques pour caractériser les hétérogénéités de dépôts quaternaires alluvionnaires. Intérêt pour l'infiltration des eaux pluviales
Major improvements to the Heliconius melpomene genome assembly used to confirm 10 chromosome fusion events in 6 million years of butterfly evolution
The Heliconius butterflies are a widely studied adaptive radiation of 46 species spread across Central and South America, several of which are known to hybridize in the wild. Here, we present a substantially improved assembly of the Heliconius melpomene genome, developed using novel methods that should be applicable to improving other genome assemblies produced using short read sequencing. First, we whole-genome-sequenced a pedigree to produce a linkage map incorporating 99% of the genome. Second, we incorporated haplotype scaffolds extensively to produce a more complete haploid version of the draft genome. Third, we incorporated ~20x coverage of Pacific Biosciences sequencing, and scaffolded the haploid genome using an assembly of this long-read sequence. These improvements result in a genome of 795 scaffolds, 275 Mb in length, with an N50 length of 2.1 Mb, an N50 number of 34, and with 99% of the genome placed, and 84% anchored on chromosomes. We use the new genome assembly to confirm that the Heliconius genome underwent 10 chromosome fusions since the split with its sister genus Eueides, over a period of about 6 million yr
AK2 deficiency compromises the mitochondrial energy metabolism required for differentiation of human neutrophil and lymphoid lineages
Reticular dysgenesis is a human severe combined immunodeficiency that is primarily characterized by profound neutropenia and lymphopenia. The condition is caused by mutations in the adenylate kinase 2 (AK2) gene, resulting in the loss of mitochondrial AK2 protein expression. AK2 regulates the homeostasis of mitochondrial adenine nucleotides (ADP, ATP and AMP) by catalyzing the transfer of high-energy phosphate. Our present results demonstrate that AK2-knocked-down progenitor cells have poor proliferative and survival capacities and are blocked in their differentiation toward lymphoid and granulocyte lineages. We also observed that AK2 deficiency impaired mitochondrial function in general and oxidative phosphorylation in particular - showing that AK2 is critical in the control of energy metabolism. Loss of AK2 disrupts this regulation and leads to a profound block in lymphoid and myeloid cell differentiation
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