Gilles André

Bio: Gilles André is an academic researcher from Centre national de la recherche scientifique. The author has contributed to research in topics: Neutron diffraction & Magnetic structure. The author has an hindex of 38, co-authored 328 publications receiving 5805 citations. Previous affiliations of Gilles André include ETH Zurich & University of Rennes.

Crystallographic, magnetic and electronic structures of a new layered ferromagnetic compound Cr2Ge2Te6

Abstract: Cr2Ge2Te6 is a new layered material belonging to the lamellar ternary M2X2Te6 chalcogenides family (where M is a 3+ oxidation state metal and X2 a silicon or a germanium pair). It was synthesized from a stoichiometric mixture of the elements and heated in a sealed evacuated quartz tube for 20 d at 700 degrees C. The crystal symmetry is rhombohedral, of space group R3, with the following hexagonal cell parameters: a=b=0.68275(4) nm, c=2.05619(9) nm, V=0.830 1(1) nm3 and Z=3. The crystal structure of Cr2Ge2Te6 was solved using both X-ray single-crystal diffraction and neutron powder diffraction data. Growth defects were detected and investigated using high-resolution electron microscopy. The magnetic structure and properties of Cr2Ge2Te6 have been determined by neutron powder diffraction and susceptibility and magnetization measurements. Below 61 K, Cr2Ge2Te6 is ferromagnetic with spins aligned along the c axis of the cell ( mu (Cr3+)=2.80(4)mu B at 5 K). The thermal evolution of the magnetic moments below Tc is given. At room temperature, Cr2Ge2Te6 presents a rho =0.02 Omega cm resistivity. Above Tc, the thermal evolution of the resistivity can be fitted as rho =exp(A/kT), where A=0.2 eV. Band-structure and crystal orbitals of Cr2Ge2Te6 have been calculated using the extended Huckel method and indicate no gap but highly localized Cr 3d states, giving evidence of a hopping mechanism for Cr2Ge2Te6, electrical conductivity.

Crystal and magnetic structure of Ca 2 RuO 4 : Magnetoelastic coupling and the metal-insulator transition

Abstract: The crystal and magnetic structure of ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$ (CRO) has been studied by powder neutron diffraction. CRO was synthesized in two different modifications (stoichiometric and containing excess oxygen) whose crystal structures are, in spite of strong differences in the lattice constants, closely related. Both structures are derived from the ideal ${\mathrm{K}}_{2}{\mathrm{NiF}}_{4}$ structure type by a rotation of the ${\mathrm{RuO}}_{6}$ octahedra around the long axis, combined with a tilt around an axis lying in the ${\mathrm{RuO}}_{2}$ plane. The orientation of the tilt axis seems to distinguish the two room-temperature symmetries; the excess oygen compound is characterized by a smaller tilt angle and shorter Ru-O in-plane bond distances. Stoichiometric CRO undergoes large structural changes on cooling, though no symmetry change was detected. In contrast, the excess oxygen containing compound undergoes a first-order structural phase transition accompanied by a change from metallic to insulating behavior in the electric resistivity. Both compounds exhibit antiferromagnetic order below 110--150 K; for the stoichiometric sample, the onset of magnetic order is associated with several structural anomalies.

Structural and magnetic aspects of the metal-insulator transition in Ca 2 − x Sr x RuO 4

Abstract: The phase diagram of ${\mathrm{Ca}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{RuO}}_{4}$ has been studied by neutron diffraction on powder and single-crystalline samples. The experiments reveal antiferromagnetic order and structural distortions characterized by tilts and rotations of the ${\mathrm{RuO}}_{6}$ octahedra. There is strong evidence that the structural details of the isovalent samples tune the magnetic as well as the electronic behavior. In particular we observe for low Sr concentration a metal-insulator transition associated with a structural change and magnetic ordering.

Crystal and magnetic structure of LaTiO 3 : Evidence for nondegenerate t 2 g orbitals

Abstract: The crystal and magnetic structures of ${\mathrm{LaTiO}}_{3}$ have been studied by x-ray and neutron-diffraction techniques using nearly stoichiometric samples. We find a strong structural anomaly near the antiferromagnetic ordering ${T}_{N}=146\mathrm{K}.$ In addition, the octahedra in ${\mathrm{LaTiO}}_{3}$ exhibit an intrinsic distortion, which implies a splitting of the ${t}_{2g}$ levels. Our results indicate that ${\mathrm{LaTiO}}_{3}$ should be considered as a Jahn-Teller system where the structural distortion and the resulting level splitting are enhanced by the magnetic ordering.

Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals

Abstract: We report the experimental discovery of intrinsic ferromagnetism in Cr 2 Ge 2 Te 6 atomic layers by scanning magneto-optic Kerr microscopy. In this 2D van der Waals ferromagnet, unprecedented control of transition temperature is realized via small magnetic fields.

Ferromagnetic materials

Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Comparison of silver and molybdenum microfocus X-ray sources for single-crystal structure determination

Abstract: The quality of diffraction data obtained using silver and molybdenum microsources has been compared for six model compounds with a wide range of absorption factors. The experiments were performed on two 30 W air-cooled Incoatec IµS microfocus sources with multilayer optics mounted on a Bruker D8 goniometer with a SMART APEX II CCD detector. All data were analysed, processed and refined using standard Bruker software. The results show that Ag Kα radiation can be beneficial when heavy elements are involved. A numerical absorption correction based on the positions and indices of the crystal faces is shown to be of limited use for the highly focused microsource beams, presumably because the assumption that the crystal is completely bathed in a (top-hat profile) beam of uniform intensity is no longer valid. Fortunately the empirical corrections implemented in SADABS, although originally intended as a correction for absorption, also correct rather well for the variations in the effective volume of the crystal irradiated. In three of the cases studied (two Ag and one Mo) the final SHELXL R1 against all data after application of empirical corrections implemented in SADABS was below 1%. Since such corrections are designed to optimize the agreement of the intensities of equivalent reflections with different paths through the crystal but the same Bragg 2θ angles, a further correction is required for the 2θ dependence of the absorption. For this, SADABS uses the transmission factor of a spherical crystal with a user-defined value of μr (where μ is the linear absorption coefficient and r is the effective radius of the crystal); the best results are obtained when r is biased towards the smallest crystal dimension. The results presented here suggest that the IUCr publication requirement that a numerical absorption correction must be applied for strongly absorbing crystals is in need of revision.

Magnetic metal-organic frameworks.

Abstract: The purpose of this critical review is to give a representative and comprehensive overview of the arising developments in the field of magnetic metal–organic frameworks, in particular those containing cobalt(II). We examine the diversity of magnetic exchange interactions between nearest-neighbour moment carriers, covering from dimers to oligomers and discuss their implications in infinite chains, layers and networks, having a variety of topologies. We progress to the different forms of short-range magnetic ordering, giving rise to single-molecule-magnets and single-chain-magnets, to long-range ordering of two- and three-dimensional networks (323 references).