2005

Nov 28, 2005

Spin polarized transport in artificial structures. Iron and aluminium oxides epitaxial growth

DSM/DRECAM/SPCSI - Oxides group

Spin electronics is a new field of research which associates two domains of physics: magnetism and electronics. In the last decade, effects related to spin dependent transport like giant magnetoresistance (GMR) and tunnelling magnetoresistance (TMR) in magnetic heterostructures have stimulated a considerable interest. These physical properties can be exploited in a variety of advanced devices such as highly sensitive magnetic sensors (e.g. read heads for magnetic recording) and non volatile magnetic memories (MRAM). Some ferromagnetic oxides have a 100 % spin polarization (there are half metallic) and thus are promising candidates for applications in spin electronics. The use of half metallic ferromagnets electrodes in magnetic tunnel junctions (MTJs) can lead to TMR effects much higher than those obtained with usual ferromagnetic electrodes. One of the predicted half metallic oxide is magnetite Fe3O4 which exhibits a high Curie temperature (Tc= 858 K) so that one can expect the half metallic character to remain significant at room temperature.

Jul 20, 2005

Antiferromagnetism on the surface and volume: NiO(111)

In an antiferromagnetic material the magnetizations carried by the atoms are gradually tail; the antiferromagnetic order is established on twice lattices (at least in a direction) compared to the structural lattices (figure 1b). It results a zero global magnetization from it and consequently a great difficulty for the measurement of the magnetic properties, in particular the classical methods of magnetometry are unsuited. Moreover, if one carries the interest on the surface (some nanometers) the situation becomes even more complicate because of the reduction of the probed matter; within this limit even the experiments based on the interaction with the neutrons become difficult. However, the antiferromagnetic substrates play a significant part in the magnetic sensors (read head of hard disks, permanent magnetic memories ...) because they ensure the magnetic hardening of one of the ferromagnetic layers of the sensor; the comprehension of its properties is a significant goal as well from a fundamental point of view as in an approach directed towards the applications of spin-electronic. In order to overcome these difficulties and to apprehend the antiferromagnetism on a material like NiO(111) [ A.Barbier and coll, Physical Review Letters 84 (2000) 2897 and Physical Review B 62 (2000) 16056 ], one proposed a new experimental approach based on the synchrotron radiation. Indeed, this one is strongly and naturally polarized in the plan of the orbit of the particles circulating in the synchrotron ring and only magnetization can, while interacting with the photons, to cause a modification (phase rotation) of polarization. The experimental configuration (figure 1a) is inspired at the same time by the surface diffraction for which the incidence angle is maintained fixes and of magnetic diffraction where a analyzer crystal makes it possible to identify the polarization of the diffracted photons. The experiments were carried out except resonance with a PG(006) analyzer crystal (photons energy of 7981 eV). The incidence angle makes it possible to choose the probed depth and polarization analyzes it makes possible the separation of the structural signal from the magnetic signal (figure 1c) which much weaker (minimum of diffusion due to surface is approximately 100 times more intense than the maximum of magnetic diffraction).

Jul 19, 2005

Resonant tunneling diodes on silicon

Molecular electronics aims at using individual molecules or small groups of organized molecules as the active part of electronic devices. It takes advantage of the size, the diversity, the quantum properties and the self-organization properties of organic molecules.

In this framework, our goal is to develop circuits based on molecular resonant tunneling diodes (RTDs) on silicon. One of the electrodes is deoxygenated silicon, in order to improve the charge injection within the molecular system. The other electrode is a carbon nanotube, in direct contact with the organic monolayer that constitutes the “active” part of the diode. The whole device will be built thanks to lithographic techniques, both optical and electronic (Figure 1). The organic monolayer is sigma-pi-sigma type, so that the charge transfer from silicon to the carbon nanotube behaves non linearly with respect to the applied bias. That resonant tunnelling effect has already been observed on similar organic systems sandwiched between two metallic electrodes, or with silicon under high vacuum.

Jul 19, 2005

Reduction of the damping induced by nonlinear effects

G. de Loubens, V.V. Naletov and O. Klein

By measuring simultaneously M_z (the component parallel to the effective field direction) and absorption power, when ferromagnets are excited by microwave fields at high power levels, we found a diminution of the damping with increasing power. These changes are interpreted as reflecting the properties of longitudinal spinwaves excited above Suhl's instability.

Reduction of the damping induced by nonlinear effects

Jul 18, 2005

Epitaxial growth of nanometric Fe3O4 thin films

Magnetite Fe₃O₄ is a very attractive oxide for applications in spin electronic devices because this material has a high Curie temperature (T_c= 860 K)and is expected to be an half-metal (i.e. a material which is a conductor for one spin direction and an insulator for the other one). In this framework, a team of the SPCSI has developped a molecular beam epitaxy (MBE) setup assited by a radio frequency plasma source specially dedicated to the growth of iron oxides layers on alumina substrates. The thin films are characterized during and after the growth by different techniques: low energy (LEED) and high energy (RHEED) electron diffraction, X-ray photoemission (XPS) and Auger electron (AES) spectroscopy.

Jul 13, 2005

Effect of the antiphase boundaries on the magnetic behavior in epitaxial Fe3O4(111) thin films

Magnetite Fe₃O₄ is one of only half-metallic materials (conductor for one spin direction and insulating for the other one) to have a Curie temperature (Tc = 860 K) above the room temperature. For this reason, this ferrimagnetic oxide constitutes a material of choice for applications in spin electronics devices (magnetoresistive read heads, nonvolatile MRAM memories). These properties make it possible to predict significant tunnel magneto-resistance (TMR) effects at room temperature if one uses this oxide as electrode in a magnetic tunnel junction (JTM). Obtaining strong TMR effects passes nevertheless by the understanding of the electronic and magnetic properties of the Fe₃O₄ thin films which are very different from those of bulk material. In particular magnetic measurements show that the magnetization of thin films at high field is considerably reduced compared to that of a monocrystal and that it is impossible to obtain saturation even with strong magnetic field. The presence of antiphase boundaries (APBs), structural defect present only in thin films, could be at the origin of these behaviors. This is why we undertook to study the role of the antiphase boundaries on the magnetic properties of Fe₃O₄thin films. For this, thin films of Fe₃O₄ were worked out by molecular beam epitaxy (MBE) assisted by monoatomic oxygen plasma. The structural studies show the perfect epitaxy of the layers. Nevertheless the transmission electronic microscopy analyses reveal also the existence of antiphase booundaries, of which we could vary the density with the film thickness.

Jul 22, 2005

Organic semiconductors

The emergence of the organic semiconductors in the electronics area and the technologies of information is now on a reality. Small molecules or polymers, these materials can be to substitute for silicon in the realization of transistors, photovoltaic cells or electroluminescent diodes. Easy to deposit, mechanically flexible, they are inexpensive. Their structure can be adjusted so as to obtain the desired properties. Lastly, the recent possibility to obtain single crystals of great purity opens vast prospects towards "Plastic Electronics".

Jul 18, 2005

Active surfaces for waste treatment

Absorption on resins is often used as secondary step in the treatment of water-based effluents, in order to reach very low concentrations. The separation of the trapped effluents from the resins and the regeneration of the resins for further use create wide volumes of secondary effluents coming from the washings of the resins with chemical reagents. We propose an alternative solution based on a « surface strategy » through adsorption phenomena and electrical control of the expulsion stage. The final goal is to limit or ideally to avoid the use of chemical reagents at the expulsion (or regeneration) stage of the depolluting process.

Heavy metal ions were captured on active filters composed by a conducting surface covered by poly-4-vinylpyridine (P4VP) or polyacrylic acid (PAA). Due to pyridine or carboxylic acid groups, those polymer films have chelating properties for copper ions. Our strategy for electrical triggering of the copper expulsion in aqueous medium is based on pH sensitive chelating groups. Applying moderate electro-oxidizing conditions generates acidic conditions in the vicinity of the electrode i.e. “inside” the polymer film. This allows a “switch-off” of the complexing properties of the film from the basic forms (pyridine or acetate) to the acid ones (pyridinium or acid). Interestingly, no buffer washing is necessary to restore (or “switch-on”) the complexing properties of the polymer film because the pH of the external medium is left unchanged by the electrochemical effect that affects only the vicinity of the electrode. Switch-on/switch-off cycles are followed and attested by IR spectroscopy and EQCM method. XPS was also used to confirm the complexation and expulsion steps.

Aug 03, 2005

Dynamics of magma mixing and degassing recorded in plagioclase at Stromboli (Aeolian Archipelago, Italy)

P. Landi ¹, N. Métrich ², A. Bertagnini ¹, M. Rosi ³

Crystal-rich materials (scoriae and lava flows) emitted during the 1985-2000 activity of Stromboli were taken into consideration for systematic study of bulk rock/matrix glass chemistry and in particular for the study of chemical and textural zoning of plagioclase, the most abundant mineral phase. Over the considered time period, bulk rock composition remained fairly constant in both major (SiO₂ 49.2-50.9 wt% and K₂O 1.96-2.18 wt%) and trace elements. The quite constant chemistry of matrix glasses also indicates that the degree of crystallization of magma was maintained at around 50 vol%. Plagioclase ranges in composition between An62 and An88 and is characterized by alternating, <10-100 µm thick, bytownitic and labradoritic concentric layers, although the dominant and representative plagioclase of scoriae is An68. The labradoritic layers (An62-70) show small-scale (1-5 µm), oscillatory zoning, are free of inclusions, and appear to record episodes of slow crystal growth in equilibrium with a degassed liquid having the composition of the matrix glass. In contrast the bytownitic layers (An70-An88) are patchy zoned, show sieve structure with abundant micrometric glass inclusions and voids, and are attributed to rapid crystal growth. A key to understand the origin of bytownitic layers can be retrieved from the texture and composition of the coronas of plagioclase xenocrysts, inherited from crystal-rich magma, in nearly aphyric pumice which are erupted during more energetic event and represent a deep, volatile-rich, HK-basaltic magma. They show a continuum from fine-sieve to evident skeletal texture from the inner to the outer part of the corona associated with normal compositional zoning from An90 to An75. In the light of these observations, we propose that input of H2O-rich melt blobs, and their mixing with the residing magma, causes partial dissolution of the labradoritic layers followed by the growth of bytownitic composition whose sieve texture attests of rapid crystallization occurring under undercooling, conditions mainly induced by degassing. As a whole, the zoning of plagioclase in the scoriae records successive and discrete intrusions of volatile-rich magma blobs, its degassing and mixing with the resident degassed magma at shallow level. 1 Istituto Nazionale di Geofisica e Vulcanologia, Pisa, Italy 2 Laboratoire Pierre Süe, CEA-CNRS, CE-Saclay, 91191 Gif /Yvette, France 3 Dpto di Scienze della Terra - Universita’ degli Studi di Pisa, Pisa, Italy Publication Contributions to Mineralogy and Petrology 2004, 147, 213-237.

Aug 03, 2005

2001 flank eruption of the alkali- and volatile-rich primitive basalt responsible for Mount Etna's evolution in last three decades

N. Métrich^a, P. Allard^a,b, N. Spillaert ^a, D. Andronico^b and M. Burton^b

Since the early seventies enhanced eruptive activity of Mount Etna has been accompanied by selective geochemical changes in erupted lavas, among which a gradual increase of alkalis whose origin is still debated. Here we provide further insight into the origin of this recent evolution, based on a detailed study of the chemistry and dissolved volatile content of melt inclusions trapped in olivine crystals of unusual plagioclase-poor primitive basalt that was extruded during a highly explosive flank eruption in July-August 2001. Two types of lava were erupted simultaneously along a N-S fracture system. Trachybasalts from the upper vents (2950-2700 m) were simply drained out by fracturing of the central volcanic conduit. They are identical to summit crater lavas and contain Mg-poor olivines (Fo70-72) with evolved and volatile-poor melt inclusions that represent late-stage crystallisation during shallow open conduit degassing. In contrast, plagioclase-poor basalt (80% of total) extruded through the lower vents (2550-2100 m) derived from lateral dyke intrusion of a more primitive and volatile-rich magma across the sedimentary basement. This primitive melt is best preserved in rare Fo82.4-80.5 skeletal olivines present in lapilli deposits from the most powerful activities at the 2550 m vent. Its high dissolved contents of H2O (3.4 wt %), CO2 (0.11 to 0.41 wt %), S (0.32 wt%), Cl (0.16 wt%) and F (0.094 wt%) point to its closed system ascent from ~400 to 250 MPa (~12 to 6.5 km depth b.s.l.). However, the predominance of euhedral olivine phenocrysts with common reverse zoning (cores Fo76-78 and rims Fo78-80) and decrepited inclusions shows that most of the erupted basalt derived from a slightly more evolved, crystallizing body of the same magma that was invaded by the uprising primitive melt prior to erupting. The few preserved inclusions in these olivines indicate pre-eruptive storage of that magma body at about 5 km depth b.s.l., in coherence with seismic data. We propose that the 2001 flank eruption resulted from gradual overpressuring of Etna's shallow plumbing system due to the influx of volatile-rich primitive basalt that may have begun several months in advance. We find that this basalt is much richer in alkalis (2.0 wt% K2O) and has higher S/Cl (2.0) but lower Cl/K and Cl/F ratios than all pre-seventies Etnean lavas (1.4 wt% K2O, S/Cl =1.5), as further examplified by melt inclusions in entrained olivine xenocrysts. Combining these new observations with previously published data, we argue that the 2001 basalt represents a new alkali-rich basic end-member feeding Mt. Etna, only few amount of which had previously been extruded during a 1974 peripheral eruption and, more recently, during brief paroxysmal summit events. Over the last three decades this new magma has progressively mixed with and replaced the former K-poorer trachybasalts filling the plumbing system, leading to extrusion of gradually more primitive and alkali-richer lavas. Its geochemical singularities cannot result from shallow crustal contaminations. Instead, they suggest the involvement of an alkali-richer but Cl-poorer arc-type component during recent magma genesis beneath Etna. a.) Laboratoire Pierre Süe, CNRS-CEA, Saclay, France b.) Istituto Nazionale di Geofisica e Vulcanologia (INGV), Catania, Italy. Publication: Earth and Planetary Science Letters 2004, 228, 1-17

2001 flank eruption of the alkali- and volatile-rich primitive basalt responsible for Mount Etna's evolution in last three decades

Aug 03, 2005

A XANES study of sulfur speciation in synthetic glasses and melt inclusions

N. Metrich¹, A. Berry², H. O’Neill² and J. Susini ³

Sulfur dissolves in silicate melts in at least two and possibly three oxidation states: sulfide (S^2-), sulfate (S⁶⁺), and possibly sulfite (S⁴⁺). The solubility of sulfur in magmas is directly related to its oxidation state, and is important for understanding geological processes such as the origin of magmatic sulfide ores, sulfur degassing from volcanic eruptions and hence global climate change, and the geochemical behavior of the chalcophile trace elements (Re-Os, PGE). We have already done different series of XANES experiments on natural glass/melt inclusions and sulfide compounds [1,2]. The XANES spectra of glass inclusions, representative of basaltic magmas saturated with respect to the immiscible FeS sulfide liquid, showed the coexistence of different sulfur species (sulfate, sulfide, even sulfite, ..), difficult to interpret in terms of redox conditions (fO2, fS2), only. In order to determine the oxidation state and speciation of sulfur in silicate glasses, µXANES spectra at the sulfur K-edge, were recorded for a wide range of experimental samples. XANES experiments were performed at the European Synchrotron Radiation Facility (ESRF, France), using the X-ray microscopy beamline, ID21. Glasses ranging from typical MORB-type basalts to Ca-(Na, Mg) rich compositions were synthesized under reducing conditions (log fO2 –8.79 to – 10.92; log fS2 –1.91) at 1200-1400°C and 1 bar. The XANES spectra of all glasses show a main peak at 2476.3 eV (S^2-), while those of the Fe-bearing glasses also contain a shoulder on the absorption edge (2471.7 eV) and differ in their structure after the edge. All these XANES spectra are clearly different in both the energy position of the first peak and the structure after the main edge from those recorded on sulfide-bearing minerals. High-pressure experiments saturated with FeS display a low energy peak at 2470.5 eV, indicating a contribution from sulfide microglobules, that can be easily assessed by combining the XANES spectrum of the glass anf that of the sulfide globule. For glasses synthesized under oxidizing conditions (log fO2 –0.49 to – 0.30; log fS2 –0.18 to –0.31; log fSO2 –1.47 to –2.06) at 1200-1400°C and 1 bar, all spectra are essentially identical, with a main peak at 2482 eV (sulfate), despite large variations in composition. We show that the sulfite (S⁴⁺) species may co-exist with sulfate (S⁶⁺), in Fe-free glasses under highly oxidizing conditions. Conversely, S^2- and S⁶⁺ species were not found to coexist in the experimental glasses. The spectra of these samples differ significantly from those of melt inclusions in addition to the sulfate peak at 2482.2 eV, and an unassigned peak at 2469.4 eV. The presence of both S^2- and S⁶⁺ may reflect intermediate redox conditions, although this is unlikely given the very narrow fO2 range over which the two oxidation states are expected to coexist. An alternative explanation is post-entrapment oxidation, which is supported by experiments on melt inclusions heated as a funtion of time. [1] M. Bonnin-Mosbah N. Métrich N., J. Susini., M. Salomé, D. Massare, B. Menez (2001). Spectr. Chim. Acta B57, 711-725. [2] Metrich N., Bonnin-Mosbah M., Susini J., Menez B. and Galoisy L. (2002), Geophys. Res. Lett. 29, 11. 1. Laboratoire Pierre Sue, CEA-CNRS, Saclay, France 2. Research School of Earth Sciences, ANU, Canberra, Australia 3. European Synchrotron Radiation Facility, Grenoble, France Communication: Goldschmidt Conference 20-25 May 2005, Session S05 – Advances in in-situ microanalysis of trace elements

Aug 03, 2005

The deep feeding system of Vesuvius involved in recent violent strombolian eruptions

P. Marianelli¹, N. Métrich², A. Sbrana¹, A. Cecchetti ¹

Activity at Vesuvius stopped abruptly in 1944 after about 300 years of semi-persistent activity. The 1637-1944 period of activity brings the testimony of primitive and water-rich tephritic magmas (e.g., the 1822 eruption), trapped as melt inclusions in spinel-bearing high pressure olivines. Through time, the extents of melt differentiation increase as well as the relative proportion between carbon dioxide and water dissolved in magmas, as recorded as olivine melt inclusions. Lowering the H₂O/CO₂ ratio in magmas, particularly in the 1944 samples, would enhance crystallization of K-tephritic magma, possibly modifying their rheology and ascent rates. It could be one of the processes leading to the transition to the present resting state. A decrease in the feeding rate of magmas from the mantle source is another possible cause, but there is no evidence of change in magma chemistry and thus of mantle source process, as discussed above. A further possibility is that Vesuvius since 1944, according to its past working mode, has moved to a pre-plinian stage in which a shallow magma chamber of medium-high volume is developing. As a matter of fact, the melt inclusions data for Vesuvius point to the existence of a volatile-rich magma storage zones, located in the upper crust, where primary magmas crystallize and differentiate, modifying their volatile content. This zone is arranged as a mush column located at a depth of >8 km that appears to be the source zone for volatile-rich magma batches. The studied eruptions (VEI 2–3) have a distinct behavior with respect to the mainly effusive or weakly explosive events occurred during the open conduit activity at Vesuvius in the last 300 years. In these eruptions and in particular in deposits related to the more intense phases of lava fountains, abundant olivine containing volatile-rich melt inclusions is distinctive, clearly testifying to the involvement of deep-seated volatile-rich magmas. We propose that the uprising of deep CO₂-H₂O-rich magma blobs and the sudden release of their volatiles triggers the high VEI eruptions and controls the dynamics of these eruptions with the transition between the lava effusions of degassed magmas, residing in open conduit shallow reservoirs, and intense lava fountains and sustained columns. 1. Dipartimento di Scienze della Terra, Università di Pisa, Italy 2. Laboratoire Pierre Süe, CNRS-CEA, Saclay, France Publication: Geophysical Research Letters 2005, 32, L02306, doi:10.1029/2004GL021667

Aug 03, 2005

Triggering mechanism at the origin of paroxysmal explosions at Stromboli (Aeolian Archipelago, Italy): the April 5, 2003 eruption

N. Métrich¹, A. Bertagnini², P. Landi², M. Rosi^2,3 and O. Belhadj¹

Between 28 December 2002, and 20 July 2003, Stromboli produced one of the most important lava emission of the past three centuries. Various lava flows were extruded from the summit crater and from vents opened between 600 and 550 m a.s.l., on the northern flank of the volcano (Sciara del Fuoco). Venting of lava at low elevation resulted in the drop of magma level in the volcano conduit, the collapse of the summit craters and the temporary cessation of the ordinary explosive activity from late December to late March. During this period, however, the persistence of seismic very long period (VLP) signals (explosion-quakes) indicated that magma-gas dynamics which characterize the persistent “strombolian” activity of the volcano remained essentially unchanged. At 9:12 local time of April 5, 2003, a paroxysmal explosion occurred while outpouring of lava was in progress from a lateral vent at 600 m elevation and craters were clogged with debris. The paroxysm lasted a few minutes and consisted of two violent, cannon-like explosions followed by a vertical jet of gas and pyroclasts and by a waning phase of ash and gas emission. Pumices emitted during April 5, 2003 event indicate that the explosion was driven by the fragmentation of a gas-rich strongly mingled magma blob, in which no stable mineral paragenesis exists. A few olivines show evidence of crystallization during continuous mixing between HK-basaltic magmas slightly different in terms of degree of differentiation, crystallization, volatile contents and mainly with respect to their respective volumes. The end-members of the mixing trend are (i) the glassy matrix of the pumice and (ii) the primitive melt inclusions in olivine Fo85-87. The most primitive term is thus sizeable at micrometer scale only. The latter represents the uprising HK-basaltic melt, with a dissolved volatile content of 3.1 wt% (H₂O, CO₂, S, Cl), from which the total fluid pressure is evaluated between 230 and 330 MPa. This magma blob is most likely nearly aphyric, since the olivine crystals that have preserved this melt represent less than 1% of crystals. April 2003 pumice do not bring the testimony of the most primitive parental Ca-rich magmas, recorded as melt inclusions, in high-Mg olivine (Fo~90), that possibly entered the deep feeding system a few days before historical paroxysms [Bertagnini et al., J. Geophys. Res. 2003]. However, in each case the involvement of a melt more primitive than the emitted pumice is only testified by olivine-hosted melt inclusions, and its imprint on the composition of the erupted magma is weak. It thus evidences recurrent features in Stromboli’s paroxysms, possibly indicating that the uprising “deep” magma blobs have small volumes and that the driving force for triggering magma ascent would be chiefly supplied by buoyancy of a gas pocket originated in the range of 230-330 MPa. Such a mechanism would imply a high volume ratio between gas and melt and possibly the existence of a bubble-rich layer of basaltic melt in this range of pressure, although this hypothesis must be confirmed. During its route to the surface, the gas-rich magma blob gradually interacts and mixes with overlying melts filling the dyke-like feeding system beneath the volcano. Contemporaneously, volatile exsolution driven by decompression participates to bubble growth, and expansion. Following this line of reasoning, the magma that erupted as pumice has the composition of the glassy matrix (~1.9 wt% K₂O, 6 wt% MgO), and represents the upper part of the magma column still able to generate pumice. It means prior to significant plagioclase crystallization, implying a dissolved H2O concentration in HK-basaltic melt <1.2wt% [Landi et al., Contrib. Mineral. Petrol. 2004]. Accordingly, the limit conditions for producing pumice-generating magma would be achieved when the melt still contains a minimum of ~1.2 wt% dissolved H₂O, whereas the gas-melt mixture becomes unstable. In the final stage, immediately before magma fragmentation and explosion, this “balloon” of vesicular magma rapidly raises through and mingles with the denser, degassed magma residing in the uppermost part of the cone. In conclusion, the “perturbation” that induced the April 5, 2003 explosion was initiated at moderate pressure (230-330 MPa), without direct relationship with the lava emission. 1 Laboratoire Pierre Süe, CEA-CNRS, CE-Saclay, 91191 Gif /Yvette, France 2 Istituto Nazionale di Geofisica e Vulcanologia, Pisa, Italy 3 Dipartimento di Scienze della Terra - Universita’ degli Studi di Pisa, Italy Publication: Geophysical Research Letters, 2005, 32, L10305, doi:10.1029/2004GL022257

Aug 03, 2005

High-temperature experiments on silicate melt inclusions in olivine at one atmosphere. Inference on temperatures of homogenization and H2O concentrations

D. Massare, N. Métrich and R. Clocchiatti

A series of heating stage experiments, at ambient atmosphere and high temperature, were conducted in order to detail the change in the homogenization temperature and the behavior of H2O of melt inclusions in olivine phenocrysts (Fo82-89). The samples were collected at different volcanic sites (Piton de la Fournaise, Réunion island; Stromboli, Aeolian islands; FAMOUS zone) expected to have a range of magmatic water contents. The melt inclusions vary in composition from basaltic to shoshonitic, with H2O contents from 0.14 to 2.9 wt.%. Temperatures of homogenization (Th) of melt inclusions systematically increase with time during heating experiments, regardless their major element composition and their H2O. It is proposed that Th changes with time in response to deformation of the host crystal and change in the volume of the cavity. The FTIR spectra successively acquired on the same inclusion repeatedly heated at constant temperature clearly demonstrated that the relative absorbance (Abs.n/Abs.0) measured at 3535 cm^-1 corresponding to hydroxyl group and molecular H2O decreases with time. The H2O-rich melt inclusions may have lost from 20 to 80% H2O after the first homogenization and almost total dehydration of melt inclusions may occur within few hours or less, at one atmosphere. Water loss driven out off olivine-hosted melt inclusions possibly exists to some extents in natural large dunitic bodies or in mantle xenoliths. Laboratoire Pierre Süe, CEA-CNRS, CE-Saclay, 91191 Gif sur Yvette, France Publication: Chemical Geology 2002, 183, 87-98

Aug 03, 2005

Conditions of ascent, degassing and extrusion of volatile-rich alkali basalt during the powerful 2002 flank eruption of Mount Etna (Italy)

N. Spilliaert¹, P. Allard^1,2, N. Métrich¹and A.V. Sobolev ³

In October 2002-January 2003 Mount Etna produced a highly explosive flank eruption, with several unusual features, soon after a shorter analogous event in 2001. We decipher the origin and mechanisms of this second eruption from the chemistry and volatile (H₂O, CO₂, S, Cl) content of its bulk products and their olivine-hosted melt inclusions, focussing on the south flank main sequence. Our results demonstrate that powerful lava fountains and dense ash columns during the initial phase of the eruption were sustained by closed system ascent of a batch of primitive, volatile-rich (≥4 wt%) basaltic magma that rose from depths ≥10 km b.s.l.. This magma - the most primitive since 140 years – probably represents the alkali-rich parental end-member responsible for Etna lavas’ evolution since the early 1970s. Its deep input likely pressurised the shallow plumbing system several weeks before the eruption, even though its sudden extrusion through already opened fractures may have been facilitated by eastward flank spreading. In fact, most of the eruption was subsequently fed by the extrusion and degassing of larger but slightly more evolved amounts of the same magma that were ponding at 6-4 km b.s.l., in agreement with seismic data and with the lack of pre-eruptive SO₂ accumulation above the initial depth of sulphur exsolution (~3 km b.s.l.). While ponding, this magma was flushed and dehydrated by a concentrated CO₂-rich gas phase of deeper derivation, a process that may commonly affect the plumbing system of Etna and other alkali basaltic volcanoes elsewhere. 1 Laboratoire Pierre Süe, CNRS-CEA, Saclay, France 2 Istituto Nazionale di Geofisica e Vulcanologia (INGV), Catania, Italy. 3 Max-Planck-Institut f. Chemie, Mainz, Germany Communication: European Geoscience Union General Assembly 2004, session VGP6 Physics and chemistry of volcano degassing Submitted for publication to Journal of Geophysical Research (2005)

Jul 19, 2005

Turbulence in Rotating Flows

François Daviaud, Olivier Dauchot and Bérengère Dubrulle

Major Results

Quasi-linear model of turbulence We have developed a new quasi-linear model of turbulence in collaboration with J.-P. Laval (Lille University) and S. Nazarenko (Warwick, GB). We have shown that this model allows simple understanding of the small scale intermittency. With J. Mc Williams (UCLA), we have also shown how this model could lead to a stochastic approach of the closure problem of Navier-Stokes equations. This method has been applied to the computation of 2D and 3D energy spectra in fluid turbulence with O. Zagorosvski (Warwick, GB), and to the computation of torques in von Karman experiment. The quasi-linear model of turbulence has also been applied to convection. The scaling laws of the turbulent transport have been derived. Logarithmic corrections to scaling have been obtained. This approach has been generalized to the case of large Prandtl number convection, leading to an explanation for an experimental controversy between several experiments.

Jul 19, 2005

Granular matter

Olivier Dauchot

Introduction

Dry granular materials are very simple: they are large conglomerations of discrete macroscopic particles. If they are non-cohesive, then the forces between them are essentially only repulsive so that the shape of the material is determined by external boundaries and gravity.

Yet despite this seeming simplicity, granular materials behave differently from any of the other standard and familiar forms of matter : solids, liquids or gases, and should therefore be considered as an additional state of matter in its own right. At the root of this unique status are three important aspects: the existence of static friction, the fact that temperature is effectively zero and, for moving grains, the inelastic nature of their collisions. In some cases, such as a sandpile at rest with a slope less than the angle of repose, static friction produces solid-like behavior: the material remains at rest even though gravitational forces create macroscopic stresses on its surface. If the pile is tilted several degrees above the angle of repose grains start to flow, like in a fluid.
However, this flow is clearly not that of an ordinary fluid because it only exists in a boundary layer at the pile's surface. Also, unlike in an ordinary fluid, kT plays no role in a granular material and entropy considerations can easily be outweighted by dynamical effects that now become of paramount importance. Unless perturbed by external disturbances, each metastable configuration of the material will last indefinitely, and no thermal averaging over nearby configurations will take place. Because each configuration has its unique properties, the reproducibility of granular behavior, even on large scales and certainly near the static limit, can only be defined in terms of ensemble averages.

In order to better understand the dynamical and rheological properties of granular materials, we have investigated surface flows in a rotating drum, then studied the response of a granular pile to a localised surface disturbance. More recently, we have considered the relaxation of a granular pile, following the occurence of an avalanche. Also, we have looked at some application to dunes dynamics and industrial powders flows.

Jul 19, 2005

Magneto-Hydrodynamics and Dynamo effect

François Daviaud, Arnaud Chiffaudel et Bérengère Dubrulle & Collab. CNRS et ENS

See the IRAMIS highlight and the CEA-CNRS-ENS press release

Web site of the VKS collaboration

Introduction

Dynamo action, the spontaneous generation of a magnetic field in a flow of conducting fluid, is supposed to be at the origin of the planets and stars magnetic fields. A lot of theoretical and numerical work has been devoted to this problem and it has been demonstrated experimentally in constrained model flows in recent experiments in Riga (Ponomarenko flow) and Karlsruhe (Roberts flow). A demonstration in the case of an unconstrained, turbulent flow, closer to the natural case, is still lacking.
This is mainly due to the fact that to build a dynamo, one must use liquid sodium, with all its incumbent dangers. Moreover, the dynamo will work only if the advection of the magnetic field and generation of current are fast compared to dissipation - namely if the magnetic Reynolds number exceeds a critical value, which strongly depends on the geometry of the flow.

Besides Riga and Karlsruhe, there are several groups around the World working on different dynamo projects: Maryland, Wisconsin and Socorro in USA, Perm in Russia and Grenoble and VKS in France. In France, the dynamo community is gathered through the GDR dynamo led by B. Dubrulle.

In collaboration with S. Fauve (ENS Paris), J.F. Pinton and P. Odier (ENS Lyon), we have designed, constructed and run a liquid sodium experiment called von Karman sodium - VKS - in CEA/Cadarache (DEN/DER/STR). The first configurations have been designed after the results obtained in the water prototype associated with numerical simulations of the induction equation. No dynamo action was observed in this first set of experiments, but many new results concerning the magnetic induction in a turbulent conducting flow have been obtained.

Jul 19, 2005

Nonlinear Waves

François Daviaud and Arnaud Chiffaudel

Introduction

Nonlinear Dynamics of traveling waves in 1-D extended systems has been a central research activity in our group since about ten years. Experimental studies have been carried with hydrothermal waves produced by thermocapillary flows in thin layers of fluids. This activity is now terminated, and concluding work has been achieved within the last two years (1-D), or is under progress (2-D).

Major Results

Modulated Amplitude Waves A synthesis of the dynamical processes involved in the propagation of spatio-temporal modulations of nonlinear waves has been reported in a pair of large papers. Special attention has been given on the comparison to recent theoretical and numerical work by groups in Leiden and Firenze. A class of modulated wave solutions of the Complex Coupled Ginzburg-Landau Equation system (CCGLE), called "Modulated Amplitude Waves" (MAWs) have been successfully identified to experimental patterns: stable and turbulent modulations as well as patterns evolving in time towards topological defects (spatio-temporal dislocations) have been recognized. The experimental stability and meta-stability of Eckhaus traveling modulated patterns are understood by the collapse of a stable and an unstable branch into a Saddle-node transition point. Spatio-temporal chaotic regimes The annular 1-D wave experiment has finally been pushed into spatio-temporal chaotic regimes at high level of constraint parameter, well above the onset of primary and secondary instabilities. We observed chaotic behaviors due to the complex dynamics of topological defects, i.e., traveling holes and/or sources and sinks, which denotes a strong competition between right and left traveling-waves. At very high constraint (four times the linear instability threshold of traveling waves), the spatio-temporally chaotic system tends to become composed of both right- and left-traveling waves patches, recovering globally and statistically its basic right/left symmetry broken at the wave threshold. The individual dynamics of the topological defects may be either quite regular (sources, holes) or erratic (sinks, holes) and we tried to connect the statistics of these structures with the nonlinear stability of the local underlying traveling-wave patterns. 2D-systems Yet unpublished data related to the wave stability in 2-D systems are currently under consideration within a collaboration (Picasso Integrated Action) with colleagues at Pamplona University (J. Burguete, C. Perez-Garcia), ENS Paris (B. Etchebarria) and ENS Lyon (N. Garnier). Our goal is a quantitative comparison between 2-D experimental data of onset waves with theoretical linear analysis involving complete convective/absolute treatment of the transition.

Collaborations

H. Chaté, R. Conte : Complex Systems Modelisation Group (SPEC, CEA-Saclay).

C. Normand : Theoretical Physics Department, CEA- Saclay.

N. Garnier : Laboratoire de Physique, ENS Lyon.

J. Burguete, C. Perez Garcia and H. Mancini : Physics Department, University of Pamplona, Spain.

Jul 19, 2005

Subcritical Transition to Turbulence

François Daviaud and Olivier Dauchot

Introduction

Studying transition to turbulence, one has to consider two kinds of flows, those subjected to volume force such as gravity, and those dominated by shear. The Rayleigh-Bénard flow (a fluid layer heated from below, in the presence of gravity) as well as the co-rotating Taylor-Couette flow (a fluid layer driven by two co-rotating concentric cylinders) are two very well known instances of the first kind of flow. The volume forces acting inside the flow often generate various linear instabilities of the thermodynamical solution branch. In this case the flow restabilizes in a new state, close enough to the basic state, so that it can be described in a rather simple way, mainly based on a modal description. Usually, the transition to turbulence then occurs through successive linear instabilities, the flow becoming more and more disordered in space and time. These transition are often called super-critical transtion to turbulence. In the case of shear flows, such as the plane Couette flow, or the contra-rotating Taylor-Couette flow, turbulence must be sustained by the shear forces and the flow inertia only. In tghis later case, the transition occurs for Reynolds number (inertial forces to viscous forces ratio) much lower than the linear instability threshold, and in some case when there is no linear instability at all. The transition in this case must be driven by finite amplitude disturnbances. The bifurcated flow is very different from the basic state and already presents a high level of disorder. Up to know, it remains out of reach of most numerical studies as well as theoretical approaches. These transitons, often called subcritical transition to turbulence are our major focus here.

Experiments

We have set up two experiments to study subcritical transition to turbulence. The first one in a plane Couette flow appartatus. The plane Couette flow is known to be linearly stable for any Reynolds number, so that its transition to turbulence can be considered as the prototype of subcritical tarnsitions. The second one is a Taylor-Couette apparatus, in which both supercritical and subcritical transitions can be studied. Both visualizations, image processing and laser doppler velocimetry are performed on these flows.

Aug 22, 2005

Surface of quasicrystals

The discovery of quasicrystals in 1982 was a revolution in physics solid state. A crystal could have a 5-fold orientational symmetry prohibited by te usual crystallography. In return, its structure is aperiodic. One distinguishes quasicrystals with icosahedral symmetry (aperiodic in N=3 dimensions) and dodecagonal (N=2). The studies by x-rays and neutrons diffraction allows to describe their structure. To describe it, coordinates of atoms are numbers of the form (n+m τ, where τ is an irrational equal to the golden mean for icosaedral quasicristals). The aperiodic network can then be described in a (NxN) dimension space. Concerning surfaces of quasicrystals, neutral helium diffraction studies and scanning tunneling microscopy (STM) images of the 5-fold surface of i-AlPdMn give evidence of wide terraces separated by step heights higher than the minimal distances between atomic planes. The surface consists of aluminium rich planes. High resolution STM images of terrasses and step sequences can be interpreted by means of the NxN=6 dimensions Katz-Gratias polyhedral model. As for usual ordered alloy crystals (Cu₃Au, Cu₈₃Pd₁₇...) terraces correspond to plans rich in segregating element (aluminium). However, the in plane density being not constant, it is necessary to add a selecting criterion on the density of the topmost and second planes.

Jul 22, 2005

Silicon carbide

The silicon carbide (SiC) is an interesting semiconductor for electronics because of its capacity to be functioned at high temperature. Another interesting property of this materials is the capacity that on its surface under certain conditions to organize itself in formed atomic dimer lines of which the length can exceed the micron. Spacing can be adjusted by a heat treatment. Thus it is possible to realize a super-lattice of atomic lines massively parallel. These highly regular structures could be used as support to build nanostructures (Spcsi- Univ Paris XI). http://www.sima.u-psud.fr/sima

Jul 22, 2005

Metal surfaces and growth

The formation of structural patterns on a nanometric scale is approached using the growth of thin layers. This approach is based on comprehension, until the atomic scale, of the relevant kinetic processes. A judicious exploitation of the energy hierarchy of these mechanisms led, by spontaneous organization of the matter deposited, to the emergence of a periodic arrangement of nanostructures with a specific geometry. These nanostructurated surfaces can then be used as matrices in order to control the growth of other materials. Thus, in only one stage, a great density of identical nano-objects is elaborate, starting point with the exploration of their physical properties. In addition to the growth techniques, the interaction of ions or even of light with surfaces can also lead to a formation of nanometric patterns.

Jul 19, 2005

Vibrational properties of Cu vicinal surfaces

We have derived an N-body semi-empirical potential for FCC Cu which has some similarity with the so-called tight-binding second moment potential. However it improves the numerical values of the surface energies and gives the best results when the interactions are cut off between the second and third nearest neighbours. In particular this cut-off produces atomic relaxations at low index surfaces in good agreement with experiments. This has important consequences since any calculation of the phonon spectrum starts from the determination of the equilibrium atomic structure and the variation of the interatomic distances modifies the corresponding force constants. Its transferability is checked on a detailed study of the low index surfaces (111), (100) and (110).

Jul 19, 2005

Surface diffusion of Cu on Cu(111)

We have undertaken a systematic study of the diffusion barriers encountered by Cu adatoms or very small clusters in various atomic geometries, with the purpose of finding a simple formula able to provide a good estimation of the diffusion barriers as a function of the atomic environment of the diffusing atom. The final aim is to use them as input in a Kinetic Monte Carlo (KMC) code. Indeed, in this type of simulation it is quite risky to consider only a few diffusion mechanisms and, since it is obviously impossible to calculate a priori all of them, a simple expression is highly desired. KMC then allows one to simulate long time scale processes much faster especially when a simple expression is available to estimate barriers.

Références
M.C. Marinica, C. Barreteau, M.C. Desjonquères and D. Spanjaard. Influence of short-range adatom-adatom interactions on the surface diffusion of Cu on Cu(111). Phys. Rev. B 70. 075415 (2004)

Jul 18, 2005

Crystalline γ-Al2O3 barrier for magnetite-based magnetic tunnel junctions

Magnetite is a very attractive oxide for applications in spin electronics devices because this material has been predicted to be an half-metal. In addition the high Curie temperature of this ferrimagnetic compound (Tc = 860 K) makes it possible to hope that the half-metal property persists at room temperature. However, the values of tunnel magneto-resistance (TMR) reported to date are very low at room temperature, except for the junctions using an amorphous alumina barrier for which TMR effects of about 10 % were obtained. The deposition of the alumina barrier is a critical stage, since aluminium has a very strong reactivity with surrounding oxygen. The presence of an alumina layer generally involves the formation of a phase reduced to the Fe₃O₄/Al₂O₃ interface, and thus the disappearance of the half-metal behavior of Fe₃O₄ in the full-course last. In this framework, we carried out a study allowing on the one hand to carry out the epitaxial growth of an alumina layer of some nanometers thickness on magnetite and on the other hand to control the stoichiometry of the Fe₃O₄/Al₂O₃ interface. The Fe₃O₄ (15 nm)/Al₂O₃ (1.5 nm) bilayers were carried out by MBE. The good epitaxy of the alumina barrier on Fe₃O₄ is confirmed by the high resolution transmision electron microscopy (HRTEM) picture, showing the crystalline character of alumina deposited.