RELAXATION MEASUREMENTS IN ZnS : Fe2+ FROM ABSORBER AND SOURCE EXPERIMENTS

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Evidence for a K/T impact event in the Pacific Ocean

International audienc

Mössbauer study of 57^{57}Fe-doped La2−x_{2-x}Srx_xCuO4_4 : local magnetic properties in the semiconducting, superconducting and normal metallic regions of the phase diagram

From Mössbauer and resistivity measurements on $^57$Fe-doped La$_{2-x}$Sr$_x$CuO$_4$, we have explored the phase diagram of the series for $0\leq x \leq 0.30$ and $1.5~{\rm K} \leq T \leq 295$ K and we have studied the local magnetic properties in the semiconducting, superconducting, and normal metallic phases. The most outstanding result concerns the coexistence over part ($0.06 \lesssim x \lesssim 0.16$) of the total ($0.06 \lesssim x \lesssim 0.25$) superconducting range of two different fractions showing different local behaviours. The first fraction corresponds to intrinsic spin-freezing, below $\sim 8$ K, in the Cu sublattice. The relative sample volume of this fraction which is attributed to semiconducting domains decreases as $x$ increases. In the second fraction which is presumably superconducting, there is no evidence of intrinsic Cu moments, however iron induced local magnetic order is present. In the semiconducting fraction the Cu magnetic moments remain essentially parallel to the basal plane, while in the superconducting fraction the iron-induced local magnetization moves towards the c axis as $x$ increases. We have performed accurate $^{57}$Fe hyperfine measurements in an argon annealed quasi-stoichiometric La$_2$CuO$_4$ sample and we have observed an abrupt variation of the magnetization near the Néel temperature. In an air annealed La$_2$CuO$_{4+y}$ sample, we have evidenced the presence of oxygen interstitials and we have shown that the effective interlayer magnetic coupling is strongly reduced with respect to the $y=0$ sample.A partir de mesures de spectrométrie Mössbauer et de résistivité, nous avons exploré le diagramme de phase de la série de composés La$_{2-x}$Sr$_x$CuO$_4$ dopés en $^{57}$Fe et nous avons étudié les propriétés magnétiques locales dans les phases semiconductice, supraconductrice et métallique normale. Nous avons notamment observé la coexistence de deux fractions présentant un comportement local différent, dans une partie importante ($0,06\lesssim x \lesssim 0,16$) de la zone supraconductrice totale ($0,06\lesssim x \lesssim 0,25$) : dans la première fraction, dont le volume relatif détroit quand $x$ augmente, un gel de spins intrinsèque apparaît au-dessous de 8 K dans le sous réseau du cuivre; nous attribuons cette fraction à des domaines semiconducteurs. Par contre, il n'apparaît pas de moment magnétique intrinsèque du cuivre dans la seconde fraction, présumée supraconductice, mais on y observe un ordre magnétique local induit par le fer. Dans les régions semiconductices, les moments magnétiques de Cu restent essentiellement parallèles au plan de base, alors que dans la fraction supraconductrice l'aimantation locale induite par le fer se rapproche de l'axe c lorsque $x$ augmente. Nous avons réalisé des mesures hyperfines précises sur $^{57}$Fe dans un échantillon quasi-stoechiométrique de La$_2$CuO$_4$ recuit sous argon et observé une décroissance abrupte de l'aimantation à la température de Néel. Dans un échantillon de La$_2$CuO$_{4+y}$ recuit dans l'air, nous avons mis en évidence la présence d'intersticiels d'oxygène et montré que le couplage magnétique effectif entre les couches était fortement réduit par rapport à l'échantillon où $y\simeq 0$

A displacive periodic antiphase model for one-dimensional platinum chain compounds

From room temperature investigations we propose a displacive periodic antiphase model for the 1-dimensional compound COdef[Pt(C2O 4)2].xH2O. This model, which interrelates in a simple way the various discrete satellites, accounts for the 3-dimensional distortions present in the structure independently of the uncorrelated modulation of the Pt chains evidenced by diffuse planes at room temperature.A partir d'une étude faite à la température ambiante nous proposons un modèle d'antiphase périodique displacive pour le composé unidimensionnel COdef[Pt(C2O4)2].xH 2O. Ce modèle, qui relie de manière simple les divers satellites discrets, rend compte des distorsions 3-dimensionnelles présentes dans la structure indépendamment de la modulation non corrélée des chaines de platine, laquelle se traduit par des plans de diffusion continue à la température ambiante

Formation of spinels in cosmic objects during atmospheric entry: a clue to the Cretaceous-Tertiary boundary event

International audienc

The stratigraphic distribution of Ni-rich spinels in Cretaceous-Tertiary boundary rocks at El Kef (Tunisia), Caravaca (Spain) and Hole 761C (Leg 122)

International audienc

Crystalline micrometeorites from Greenland blue lakes: Their chemical composition, mineralogy and possible origin

International audienc

The Cretaceous-Tertiary boundary at Beloc, Haiti: No evidence for an impact in the Caribbean area

International audienc

The Cretaceous-Tertiary boundary at Gubbio revisited: vertical extent of the Ir anomaly

The classical Gubbio (Italy) Cretaceous-Tertiary boundary section has been resampled for both magnetostratigraphy and iridium. Paleomagnetic samples were taken over 7 m in the Maastrichtian and 6 m in the Paleocene. Previous results obtained a decade ago are confirmed. The reversal sequence is well defined and the individual reversals are somewhat more precisely located. A noticeable difference is the location of the position of the 29N/29R limit which may be lowered by 20-30 cm in the stratigraphic column. This would imply that the KTB occurs near the middle of chron 29R. Iridium measurements were made on samples from both shales and surrounding limestone beds from 2 m below to 3 m above the boundary: these measurements indicate that Ir is associated with clay minerals. Concentrations in the two types of samples are indeed compatible when reduced to a carbonate-free basis. Iridium concentrations stand above background over almost 3 m of section, corresponding to half a million years based on magnetostratigraphy. This is likely to indicate a protracted duration of the (external or internal) source of iridium, on top of which the main (short-lived) KTB anomaly proper stands. © 1990.link_to_subscribed_fulltex