416 research outputs found

    Magnetic moment suppression in Ba3CoRu2O9: hybridization effect

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    An unusual orbital state was recently proposed to explain the magnetic and transport properties of Ba3_3CoRu2_2O9_9 [Phys. Rev. B. {\bf 85}, 041201 (2012)]. We show that this state contradicts to the first Hund's rule and does not realize in the system under consideration because of a too small crystal-field splitting in the t2gt_{2g} shell. A strong suppression of the local magnetic moment in Ba3_3CoRu2_2O9_9 is attributed to a strong hybridization between the Ru 4dd and O 2pp states.Comment: 5 pages, 5 figure

    Electronic Raman scattering in metals: effects of electron-phonon coupling

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    We report the first systematic measurements of the Raman scattering by electrons in elemental metals of Al, Mo, Nb, Os, Pb, Re, Ta, Ti, V, W and metallic compound LaB6B_6. Experimental spectra are modelled on the base of the band structures, calculated within the density functional theory, taking properly into account the effects of electron-phonon scattering. The agreement between our measured and calculated spectra is excellent for the variety of metals, thus providing estimates for the electron-phonon coupling constants and temperature-dependent relaxation rates. The method can be applied for other metallic materials to evaluate an electron-phonon coupling as an alternative to the transport and optical measurements

    Ab initio investigation of the exchange interactions in Bi2_2Fe4_4O9_9: The Cairo pentagonal lattice compound

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    We present the \emph{ab initio} calculation of the electronic structure and magnetic properties of Bi2_2Fe4_4O9_9. This compound crystallizes in the orthorhombic crystal structure with the Fe3+^{3+} ions forming the Cairo pentagonal lattice implying strong geometric frustration. The neutron diffraction measurements reveal nearly orthogonal magnetic configuration, which at first sight is rather unexpected since it does not minimize the total energy of the pair of magnetic ions coupled by the Heisenberg exchange interaction. Here we calculate the electronic structure and exchange integrals of Bi2Fe4O9 within the LSDA+U method. We obtain three different in-plane (J3=36 K, J4=73 K, J5=23 K) and two interplane (J1=10 K, J2=12 K) exchange parameters. The derived set of exchange integrals shows that the realistic description of Bi2Fe4O9 needs a more complicated model than the ideal Cairo pentagonal lattice with only two exchange parameters in the plane. However, if one takes into account only two largest exchange integrals, then according to the ratio x\equiv J3/J4=0.49<\sqrt{2} (a critical parameter for the ideal Cairo pentagonal lattice, see. Ref.~1) the ground state should be the orthogonal magnetic configuration in agreement with experiment. The microscopic origin of different exchange interactions is also discussed.Comment: 6 pages, 4 figure

    Jahn-Teller distortions and charge, orbital and magnetic orders in NaMn7O12

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    With the use of the band structure calculations we demonstrate that previously reported [Nat. Materials {\bf 3}, 48 (2004)] experimental crystal and magnetic structures for NaMn7_7O12_{12} are inconsistent with each other. The optimization of the crystal lattice allows us to predict a new crystal structure for the low temperature phase, which is qualitatively different from the one presented before. The AFM-CE type of the magnetic order stabilizes the structure with the elongated, not compressed MnB3+^{3+}_BO6_6 octahedra, striking NaMn7_7O12_{12} out of the list of the anomalous Jahn-Teller systems. The orbital correlations were shown to exist even in the cubic phase, while the charge order appears only in the low temperature distorted phase.Comment: 5 page

    Covalent bonds against magnetism in transition metal compounds

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    Magnetism in transition metal compounds is usually considered starting from a description of isolated ions, as exact as possible, and treating their (exchange) interaction at a later stage. We show that this standard approach may break down in many cases, especially in 4d4d and 5d5d compounds. We argue that there is an important intersite effect -- an orbital-selective formation of covalent metal-metal bonds, which leads to an "exclusion" of corresponding electrons from the magnetic subsystem, and thus strongly affects magnetic properties of the system. This effect is especially prominent for noninteger electron number, when it results in suppression of the famous double exchange, the main mechanism of ferromagnetism in transition metal compounds. We study this novel mechanism analytically and numerically and show that it explains magnetic properties of not only several 4d5d4d-5d materials, including Nb2_2O2_2F3_3 and Ba5_5AlIr2_2O11_{11}, but can also be operative in 3d3d transition metal oxides, e.g. in CrO2_2 under pressure. We also discuss the role of spin-orbit coupling on the competition between covalency and magnetism. Our results demonstrate that strong intersite coupling may invalidate the standard single-site starting point for considering magnetism, and can lead to a qualitatively new behaviour

    Orbital structure and magnetic ordering in stoichiometric and doped crednerite CuMnO2

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    The exchange interactions and magnetic structure in layered system CuMnO2 (mineral crednerite) and in nonstoichiometric system Cu1.04Mn0.96O2, with triangular layers distorted due to orbital ordering of the Mn3+ ions, are studied by ab-initio band-structure calculations, which were performed within the GGA+U approximation. The exchange interaction parameters for the Heisenberg model within the Mn-planes and between the Mn-planes were estimated. We explain the observed in-plane magnetic structure by the dominant mechanism of the direct d-d exchange between neighboring Mn ions. The superexchange via O ions, with 90 degree Mn-O-Mn bonds, plays less important role for the in-plane exchange. The interlayer coupling is largely dominated by one exchange path between the half-filled 3z^2-r^2 orbitals of Mn3+. The change of interlayer coupling from antiferromagnetic in pure CuMnO2 to ferromagnetic in doped material is also explained by our calculations

    Suppression of magnetism in Ba5AlIr2O11: interplay of Hund's coupling, molecular orbitals and spin-orbit interaction

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    The electronic and magnetic properties of Ba5_5AlIr2_2O11_{11} containing Ir-Ir dimers are investigated using the GGA and GGA+SOC calculations. We found that strong suppression of the magnetic moment in this compound recently found in [J. Terzic {\it et al.}, Phys. Rev. B {\bf 91}, 235147 (2015)] is not due to charge-ordering, but is related to the joint effect of the spin-orbit interaction and strong covalency, resulting in the formation of metal-metal bonds. They conspire and act against the intra-atomic Hund's rule exchange interaction to reduce total magnetic moment of the dimer. We argue that the same mechanism could be relevant for other 4d4d and 5d5d dimerized transition metal compounds

    Theoretical prediction of Jahn-Teller distortions and orbital ordering in Cs2CuCl2Br2

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    With the use of the density function calculations we show that the actual crystal structure of Cs2_2CuCl2_2Br2_2 should contain elongated in the abab-plane CuCl4_4Br2_2 octahedra, in contrast to the experimentally observed compression in cc-direction. We also predict that the spins on Cu2+^{2+} ions should be ferromagnetically ordered in abab-plane, while the exchange interaction along cc-direction is small and its sign is uncertain.Comment: 4 pages, 3 figure

    Realization of anisotropic compass model on the diamond lattice of Cu2+^{2+} in CuAl2_2O4_4

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    Spin-orbit (SO) Mott insulators are regarded as a new paradigm of magnetic materials, whose properties are largely influenced by SO coupling and featured by highly anisotropic bond-dependent exchange interactions between the spin-orbital entangled Kramers doublets, as typically manifested in 5d5d iridates. Here, we propose that a very similar situation can be realized in cuprates when the Cu2+^{2+} ions reside in a tetrahedral environment, like in spinel compounds. Using first-principles electronic structure calculations, we construct a realistic model for the diamond lattice of the Cu2+^{2+} ions in CuAl2_2O4_4 and show that the magnetic properties of this compound are largely controlled by anisotropic compass-type exchange interactions that dramatically modify the magnetic ground state by lifting the spiral spin-liquid degeneracy and stabilizing a commensurate single-q\boldsymbol{q} spiral
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