Magnetic excitations in a new anisotropic Kagom\'{e} antiferromagnet

The Nd-langasite compound contains planes of magnetic Nd3+ ions on a lattice topologically equivalent to a kagom\'{e} net. The magnetic susceptibility does not reveal any signature of long-range ordering down to 2 K but rather a correlated paramagnetism with significant antiferromagnetic interactions between the Nd and a single-ion anisotropy due to crystal field effect. Inelastic neutron scattering on Nd-langasite powder and single-crystal allowed to probe its very peculiar low temperature dynamical magnetic correlations. They present unusual dispersive features and are broadly localized in wave-vector Q revealing a structure factor associated to characteristics short range-correlations between the magnetic atoms. From comparison with theoretical calculations, these results are interpreted as a possible experimental observation of a spin liquid state in an anisotropic kagom\'{e} antiferromagnet.Comment: to appear in Physica

ELECTRONIC STRUCTURE OF THE GROUND STATE OF THE COVALENT AND IONIC SOLIDS : USE OF LOCALIZED ORBITALS. APPLICATION TO THE EQUATION OF STATE OF THE DIAMOND

Dans la détermination de la structure électronique des molécules ou des solides, on trouve deux grandes catégories de méthodes basées sur l'emploi : - soit d'orbitales moléculaires délocalisées dans les molécules ou de bandes dans les solides, - soit d'orbitales localisées. Nous utilisons cette seconde catégorie de méthodes pour établir, dans le cas des solides ioniques ou covalents, l'expression de l'énergie totale de leur état fondamental. Cette méthode est appliquée au calcul de l'équation d'état du diamant.In the determination of the electronic structure of molecules or solids, we can find two great categories of methods based on the use : - whether of molecular orbitals delocalized in molecules or bands in solids, - or of localized orbitals. We use the second category of methods to set up, in the case of ionic or covalent solids, the expression of the total energy of the ground state. This method is applied to the calculation of the state equation of the diamond

Comportement non linéaire de l'aimantation de FeCl2, FeBr2 et FeI2 en champ perpendiculaire a l'axe d'anisotropie

We present, for ferrous halogenides, FeCl2, FeBr2, FeI2, the magnetization isotherms obtained, at low temperature, in a perpendicular magnetic field (static and pulsed fields). These experimental results show a divergence from the linear magnetization law. We give a natural interpretation of our results by using a rigorous treatment of a theoretical model in which we consider an assembly of triplet ions with a single ion uniaxial anisotropy, the magnitude of which is of the same order as the intralayer ferromagnetic exchange interaction (J1) and the intralayer antiferromagnetic one (J2) (molecular field-approximation). This study allows us to calculate the divergence from the linear law for all values of D, J1 and J2 and shows the fundamental role played by the anisotropy. The theoretical model gives a more satisfactory explanation of the experimental results obtained for FeCl 2, FeBr2 and FeI2 than given by previous models.Nous présentons, pour les halogénures ferreux FeCl2, FeBr 2, FeI2, les isothermes d'aimantation obtenues, à basse température, en champ magnétique perpendiculaire (champ statique et champ pulsé). Ces résultats expérimentaux mettent en évidence un écart à la loi linéaire d'aimantation. Nous donnons une interprétation naturelle de nos résultats par le traitement rigoureux d'un modèle théorique où l'on considère une assemblée d'ions triplets caractérisés par une anisotropie uniaxiale dont l'amplitude D est du même ordre de grandeur que les interactions d'échange, ferromagnétiques dans le plan (J 1) et antiferromagnétiques entre plans (J2) (approximation du champ moléculaire). Cette étude théorique permet d'évaluer l'écart à la loi linéaire pour toute valeur de D, J1 et J 2 et montre le rôle fondamental joué par l'anisotropie. Ce modèle théorique donne une explication plus satisfaisante des résultats obtenus pour FeCl2, FeBr2 et FeI2 que ceux donnés par les modèles précédents

Magnetic Frustration on a Kagomé Lattice in R3_{3}Ga5_{5}SiO14_{14} Langasites with R = Nd, Pr

9 pages. 1 table, 5 figures.International audienceIn the R$_{3}$Ga$_{5}$SiO$_{14}$ compounds, the network R of rare earth cations form well separated planes of corner sharing triangles topologically equivalent to a kagomé lattice. Powder samples and single crystals with R = Nd and Pr were prepared and magnetostatic measurements were performed under magnetic field up to 10 T in the temperature range from 1.6 K to 400 K. Analysis of the magnetic susceptibility at the high temperatures where only the quadrupolar term of the crystal electric field prevails, suggests that the Nd and Pr magnetic moments can be modeled as coplanar elliptic rotators perpendicular to the three fold axis of the crystal structure that interact antiferromagnetically. Nonetheless, a disordered phase that can be ascribed to geometric frustration persists down to the lowest temperature which is about 25 times smaller than the energy scale for the exchange interactions

Nanodomains in multiferroic hexagonal RMnO3 films (R = Y,Dy,Ho,Er)

International audienceA homogeneous distribution of ferroelectric nanoinclusions was observed by optical second harmonic generation in epitaxial films of hexagonal RMnO3 (R = Y, Dy, Ho, Er) grown on ZrO2(Y2O3) substrates. The inclusions correspond to secondary orientations in the c-axis-oriented films. Their volume density is in the range of 10(-3) and their occurrence is independent of growth technique and film thickness in the range of 20-1000 nm. The inclusions behave as preferentially polarized ferroelectric nanodomains whereas the epitaxial film is in a ferroelectric single-domain state. In addition, the antiferromagnetic phase exhibits nanodomains of < 100 nm which is three orders of magnitude below the extension of bulk antiferromagnetic domains in RMnO3. Indications for a polarization-induced magnetic order different from that of the bulk are discussed

Epitaxial growth of germanium on silicon using a Gd2O3/Si (111) crystalline template

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