Headlines 2008
Delphine Lebeugle, Dorothée Colson, Anne Forget, Michel Viret (IRAMIS/SPEC CEA-Saclay)
Arsen Goukassov, Alexandre Bataille (IRAMIS/LLB CEA-Saclay)
Magnetic materials are heavily used in the dynamic storage of information (hard disk drive, head for reading). For these applications, they are most often designed in the form of thin films. This was achieved after the birth of the spin electronics or "spintronics" and the discovery of the giant magnetoresistance. More recently, a new research topic has opened about multiferroic materials in which magnetic and ferroelectric order coexist, both aspects being coupled. With these materials, the processing of information in RAM memories could be achieved through the magnetic and electric polarization (modifying or measuring the local magnetization by applying an electric field, or the local electric polarization with a magnetic field). However, fundamental research remains to be done for understanding the nature of the interactions and mechanisms responsible for the coupling between the two types of order. It is in this context, that recent results were obtained in IRAMIS/SPEC showing that an electric field may influence the magnetism in the BiFeO3 compound.
Potential applications require high-purity, high resistivity multiferroic compounds, with coupled magnetic and electrical order and the highest possible temperatures of order-disorder transitions (magnetic and electric). In that way, the BiFeO3 compound appears highly interesting because it is the only multiferroic oxide with transition temperatures well above room temperature. It was therefore very studied experimentally during the past three years.
G. Lambert1,2,3, T. Hara2,4, D. Garzella1, T. Tanikawa2, M. Labat1,3, B. Carre1, H. Kitamura2,4, T. Shintake2,4, M. Bougeard1, S. Inoue4, Y. Tanaka2,4, P. Salieres1, H. Merdji1, O. Chubar3, O. Gobert1, K. Tahara2, M.-E. Couprie3
1Service des Photons, Atomes et Molécules, DSM/DRECAM, CEA-Saclay, 91191 Gif-sur-Yvette, France
2RIKEN SPring-8 Centre, Harima Institute, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
3Groupe Magnétisme et Insertion, Synchrotron Soleil, L'Orme des Merisiers, Saint Aubin, 91192 Gif-sur-Yvette, France
4XFEL Project Head Office/RIKEN, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
The Editors of the journal Physical Review Letters recently awarded the label "Editor's suggestion" to an article from the Quantronics group of SPEC (Laboratory of Condensed Matter Physics) : "Phase Controlled Superconducting Proximity Effect Probed by Tunneling Spectroscopy, Phys. Rev. Lett. 100 (2008) 197002". By this label publishers aim to put forward a small number of items they regard as particularly clear and likely to attract readers outside their specialty.
This paper provides for the first time a neat, clear and complete overview of "the superconducting proximity effect." Such an effect occurs at the interface between metal superconductor (S) and metals with "normal" resistivity (N), in which superconductivity can locally expand within the normal metal and make it non-resistive.
LLB researchers have published several papers in 2008 in the prestigious journal Science [1-2] and Nature [3]. These results show the full potential of the neutron diffraction techniques at the forefront of research on new materials.
P. Bourges3, G.J. McIntyre4, M.D. Hollingsworth5, T. Breczewski6
1 Institut de Physique de Rennes, CNRS UMR 6251, Univ. Rennes 1, 35042 Rennes, France
2 CNRS, UMR 6251, IPR, 263 Avenue du Général Leclerc, 35042 Rennes Cedex, France.
3 Laboratoire Léon Brillouin, CEA-CNRS, CEA Saclay, 91191 Gif-sur-Yvette, France
4 Institut Laue-Langevin, 38042 Grenoble Cedex 9, France.
5 Department of Chemistry, Kansas State University, Manhattan, KS 66506, USA.
6 Facultad de Ciencias, Universidad del Pais Vasco, Apdo 644, Bilbao, Spain.
Symmetry and order are essential properties in the description of the various states of matter and their physical properties. These have long been synonymous with periodicity for solids, also used to define the notion of crystal, paradigm for many topics of condensed matter during the XXth century.
One way to probe the structure of matter at the atomic scale is to use radiation diffraction (such as X-rays, or the wave associated with particles: neutrons, electrons ...). The presence of a long-distance order is then characterized by the presence of diffraction peaks, forming an image reflecting the symmetry of the object. This led to the discovery in the recent decades of non-periodic materials, but still with long range order as evidenced by discrete peaks in their diffraction spectrum. Physicists represent this type of crystals as periodic crystals but in a super-space (with dimension 3 + d, which represent the 3 usual dimensions of space and the dimension d of the internal space).
The systems understudy are aperiodic supramolecular model systems consisting of an urea single crystal (host structure) and alkane molecules (invited molecules), whose length is determined by the number nC of carbon atoms in the molecule (nC>7). Crystalline plane of urea have a structure of hexagonal symmetry. Along the C perpendicular axis, the structure exhibits a double helix forming channels in which alkane molecules may be inserted. At room temperature, these molecules are ordered along the C axis of the urea lattice, but according to an aperiodic order.
Neutron diffraction spectrum along the C* axis. Crossing the first critical temperature Tc1 superstructure rods (h = 1/2) along C* appear that indicates a change in the symmetry of the system. Along the rods, incommensurate discrete diffraction peaks are characteristic of the new aperiodic ordered structure. This mesh doubling is completely original because the observed phase transition between the two incommensurate phases can be only properly described by means of the 4-dimensional space. A second structural phase appears below Tc2.
At a time when we question the fossil fuel reserves of our planet and the consequences of the greenhouse effect on global warming, hydrogen is seen as the future energy vector for transportation. Research within CEA cover all stages of this chain: production, storage, transport, distribution and use. In that field, hydrogen produced from primary energy, solar, nuclear, wind, chemical ... is stored in the tank of a vehicle and a fuel cell, allowing the clean conversion (without CO2 emission) of chemical energy into electrical energy, combined with an electric engine can replace the gasoline engine of our cars.
Among the various types of fuel cells suitable for transport applications, the most interesting are of PEMFC type (Proton Exchange Membrane Fuel Cell). These fuel cells contain, in particular, a polymer playing the role of the solid electrolyte. The Dupont De Nemours company sells a sulfonated perfluorocarbon membrane named Nafion®. However, this membrane presents some drawbacks as a mediocre autonomy (< 5000h operating), a mechanical weakness or the inability to function in anhydrous media… The "Irradiated Polymer" research group within LSI is trying to address these problems by proposing a new type of membrane.
