Finite size effects in the Verwey transition of magnetite thin films

We report on the finite size effects in the Verwey transition of stress-free magnetite Fe3O4 thin films. A limit thickness of 20 nm is evidenced, above which the transition temperature TV is constant and close to 120 K (bulk value) and below which no genuine transition is observed. Field Cooled and Zero Field Cooled measurements evidence irreversibilities for all thicknesses. This irreversible behavior abruptly disappears around TV for the thicker films, when the magnetic anisotropy vanishes. These behaviors are interpreted in terms of assemblies of interacting magnetic Fe3O4 clusters, which are smaller than the antiphase domains present in the films

Ewald methods for polarizable surfaces with application to hydroxylation and hydrogen bonding on the (012) and (001) surfaces of alpha-Fe2O3

We present a clear and rigorous derivation of the Ewald-like method for calculation of the electrostatic energy of the systems infinitely periodic in two-dimensions and of finite size in the third dimension (slabs) which is significantly faster than existing methods. Molecular dynamics simulations using the transferable/polarizable model by Rustad et al. were applied to study the surface relaxation of the nonhydroxylated, hydroxylated, and solvated surfaces of alpha-Fe2O3 (hematite). We find that our nonhydroxylated structures and energies are in good agreement with previous LDA calculations on alpha-alumina by Manassidis et al. [Surf. Sci. Lett. 285, L517, 1993]. Using the results of molecular dynamics simulations of solvated interfaces, we define end-member hydroxylated-hydrated states for the surfaces which are used in energy minimization calculations. We find that hydration has a small effect on the surface structure, but that hydroxylation has a significant effect. Our calculations, both for gas-phase and solution-phase adsorption, predict a greater amount of hydroxylation for the (012) surface than for the (001) surface. Our simulations also indicate the presence of four-fold coordinated iron ions on the (001) surface.Comment: 23 pages, REVTeX (LaTeX), 8 figures not included, e-mail to [email protected], paper accepted in Surface Scienc

Concentration-dependent organization of DNA by the dinoflagellate histone-like protein HCc3

The liquid crystalline chromosomes of dinoflagellates are the alternative to the nucleosome-based organization of chromosomes in the eukaryotes. These nucleosome-less chromosomes have to devise novel ways to maintain active parts of the genome. The dinoflagellate histone-like protein HCc3 has significant sequence identity with the bacterial DNA-binding protein HU. HCc3 also has a secondary structure resembling HU in silico. We have examined HCc3 in its recombinant form. Experiments on DNA-cellulose revealed its DNA-binding activity is on the C-terminal domain. The N-terminal domain is responsible for intermolecular oligomerization as demonstrated by cross-linking studies. However, HCc3 could not complement Escherichia coli HU-deficient mutants, suggesting functional differences. In ligation assays, HCc3-induced DNA concatenation but not ring closure as the DNA-bending HU does. The basic HCc3 was an efficient DNA condensing agent, but it did not behave like an ordinary polycationic compound. HCc3 also induced specific structures with DNA in a concentration-dependent manner, as demonstrated by atomic force microscopy (AFM). At moderate concentration of HCc3, DNA bridging and bundling were observed; at high concentrations, the complexes were even more condensed. These results are consistent with a biophysical role for HCc3 in maintaining extended DNA loops at the periphery of liquid crystalline chromosomes

Concentration-dependent organization of DNA by the dinoflagellate histone-like protein HCc3

The liquid crystalline chromosomes of dinoflagellates are the alternative to the nucleosome-based organization of chromosomes in the eukaryotes. These nucleosome-less chromosomes have to devise novel ways to maintain active parts of the genome. The dinoflagellate histone-like protein HCc3 has significant sequence identity with the bacterial DNA-binding protein HU. HCc3 also has a secondary structure resembling HU in silico. We have examined HCc3 in its recombinant form. Experiments on DNA-cellulose revealed its DNA-binding activity is on the C-terminal domain. The N-terminal domain is responsible for intermolecular oligomerization as demonstrated by cross-linking studies. However, HCc3 could not complement Escherichia coli HU-deficient mutants, suggesting functional differences. In ligation assays, HCc3-induced DNA concatenation but not ring closure as the DNA-bending HU does. The basic HCc3 was an efficient DNA condensing agent, but it did not behave like an ordinary polycationic compound. HCc3 also induced specific structures with DNA in a concentration-dependent manner, as demonstrated by atomic force microscopy (AFM). At moderate concentration of HCc3, DNA bridging and bundling were observed; at high concentrations, the complexes were even more condensed. These results are consistent with a biophysical role for HCc3 in maintaining extended DNA loops at the periphery of liquid crystalline chromosomes

Etude des propriétés physiques des films de Fe3O4 et de la polarisation en spin à l'interface Fe3O4/gamma-Al2O3

La magnétite Fe3O4 est un matériau prometteur pour l'électronique de spin, puisque des calculs de structure de bande la prédisent demi-métallique, c'est à dire totalement polarisée en spin au niveau de Fermi. Nous avons fait croître des films minces de Fe3O4 de 3 à 50 nm d'épaisseur sur g-Al2O3 par épitaxie par jets moléculaires. Les films sont monocristallins mais présentent un grand nombre de parois d'antiphase qui sont responsables des anomalies magnétiques observées. Nous avons reproduit le comportement magnétique des films à l'aide d'un modèle unidimensionnel et confronté les résultats avec la taille caractéristique des parois mesurée par une analyse fractale. Nous avons d'autre part étudié la transition de Verwey des films minces; les effets de taille finie font disparaître la transition pour les films d'épaisseur inférieure à 20 nm et le magnétisme de tous les films présente des effets de dynamique lente. Nous avons de plus mis au point une méthode de dépôt de couches minces de g-Al2O3 épitaxiée sur les films de Fe3O4, en contrôlant la stoechiométrie à l'interface entre oxydes. Les films de g-Al2O3 d'épaisseur supérieure à 2 nm présentent toutes les propriétés requises pour une barrière tunnel (continuité électrique, découplage magnétique entre les électrodes). Nous avons enfin étudié la polarisation en spin à l'inierface Fe3O4/ g-Al2O3 par deux méthodes distinctes: les mesures directes par photoémission résolue en spin conduisent à une polarisation en spin de -40% tandis que l'on mesure une magnétorésistancc tunnel de +3% à température ambiante pour les jonctions tunnel Fe3O4/ g-Al2O3/Co.Magnetite Fe3O4 is a promising material for spintronics since band structure calculations predict it to be half-metallic, i. e. fully spin-polarised at the Fermi level. One thus expects large magnetoresistance in magnetic tunnel junctions using Fe3O4 as electrode. We have grown 3 to 50 nm-thick Fe3O4 thin films onto g-Al2O3 by molecular beam epitaxy. The films are single crystalline but comprise a large number of antiphase boundaries which are responsible for the magnetic anomalies of Fe3O4 thin films. We have qualitatively reproduced the magnetic behavior of our samples with a one-dimensionnal model, and compared the results with the characteristic size of the antiphase boundaries measured by a fractal analysis. We also studied the Verwey transitions of our thin films. The transition is not seen for thicknesses below 20 nm because of finite size effects and all samples show slow magnetization dynamics. We have also developed a growth method allowing us to deposit g-Al2O3 epitaxially onto Fe3O4, and to control the stoichiometry at the interface. g-Al2O3 thin films of thicknesses greater than 2 nm exhibit all the characteristics required for a tunnel barrier (electrical continuity, magnetic decoupling). We finally studied the spin polarization at the Fe3O4/g-Al2O3 interface by too distinct methods. Direct measurements by spin resolved photoemission yields to -40 % for the spin polarization whereas the tunnel magnetoresistance is +3\% at room temperature for Fe3O4/ g-Al2O3/Co.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Couches minces épitaxiées de ferrite de cobalt pour le filtrage de spin à température ambiante

Spin filtering is a physical phenomenon which can potentially produce spin-polarized electron currents by the spin-selective transport of electrons across a magnetic tunnel barrier. In this thesis, we present a complete study of the material cobalt ferrite (CoFe2O4), whose insulating behavior and high Curie temperature make it a very good candidate for spin filtering at room temperature. CoFe2O4 thin films were grown by oxygen plasma-assisted molecular beam epitaxy. Their structural, chemical and magnetic properties were studied by a number of in situ and ex situ characterization techniques. CoFe2O4-based spin filter tunnel junctions were then prepared for spin polarized tunneling experiments involving either the Meservey-Tedrow technique or tunneling magnetoresistance (TMR) measurements. We also payed special attention to the magnetization reversal behavior of the CoFe2O4 spin filter barrier and its magnetic counter-electrode (Co or Fe3O4). We therefore show that CoFe2O4 tunnel barriers provide a model system to investigate spin filtering in a wide range of temperatures.Le filtrage de spin est un phénomène physique qui permet de générer des courants d'électrons polarisés en spin grâce au transport sélectif à travers une barrière tunnel magnétique. Dans cette thèse, nous présentons une étude du matériau ferrite de cobalt (CoFe2O4), dont le caractère isolant et la température de Curie élevée en font un très bon candidat pour le filtrage de spin à température ambiante. L'élaboration des couches minces de CoFe2O4 a été réalisée par épitaxie par jets moléculaires assistée par plasma d'oxygène. Les propriétés structurales, chimiques et magnétiques ont été étudiées par plusieurs méthodes de caractérisation in situ et ex situ. Des jonctions tunnel à base de CoFe2O4 ont été préparées pour des mesures de transport tunnel polarisé en spin, soit par la méthode de Meservey-Tedrow, soit par des mesures de magnétorésistance tunnel (TMR). Dans ce dernier cas, nous avons porté une attention particulière au retournement magnétique du CoFe2O4 et de la contre électrode magnétique (Co ou Fe3O4). Nous démontrons ainsi que les barrières tunnel de CoFe2O4 constituent un système modèle pour étudier le filtrage de spin dans une large gamme de températures.PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

A first principles Hartree-Fock interpretation of the X-ray oxygen K-edge spectrum of haematite (a-Fe₂O₃)

This study presents a comparison of the experimental O K-edge absorption spectrum of haematite (alpha-Fe2O3) with a theoretical spectrum obtained from all electron ab initio periodic Hartree-Fock calculations of the antiferromagnetic R (3) over bar structure. There is good overall accord between the two spectra and agreement to within about 0.2eV for the major peak-to-peak separations. From a consideration of the empty p density of states (DOS), calculations predict the first 20 eV of the absorption to result almost entirely from excitations of the type \ O I]--&gt; \ O np] with negligible participation of the Fe p states. The lower part of the spectrum is attributed to O p states hybridized with Fe d states while, at energies greater than 5eV above the absorption edge, in the region of the broad absorption at about 11 eV, calculations suggest that the predominant hybridization is with the Fe s states. Important differences are found between the near-edge empty p DOS of the antiferromagnetic R (3) over bar and ferromagnetic R (3) over bar c spin orderings which suggests that spin-selective O K-edge spectra might be sensitive to the weak canting below the Morin temperature. The satisfactory agreement between experiment and theory, which is based on the calculated ground-state conduction band, confirms the view that core-hole states in charge-transfer insulators such as alpha-Fe2O3 are screened effectively by the valence electrons, which in these systems, are predominantly O in character.</p

Time resolved XAS study of a phase transition: The polymorphic transformation of tetragonal to orthorhombic PbO

International audienceWe present a time resolved XAS study of the phase transition of tetragonal to orthorhombic lead monoxide PbOα → PbOβ, occuring at 490 °C. XAS experiments were carried out on a fast energy dispersive spectrometer and full Pb L3 XAS spectra were periodically recorded as a function of the sample temperature within the range 100–700 °C. The time resolution is about 1 s. XAS experiments give evidence for a dynamical two-step solid state transformation. The phase transition appears to be initiated by O atom displacements within the PbO4 structural pyramidal unit. Pb atom displacements seem to occur in a second stage of the phase transition