851 research outputs found

    Magnons in ultrathin Fe/Co multi-layers on W(110) and Ir(111)

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    Probing the fundamental magnetic interactions in layered magnetic structures is essential to the understanding of the properties of these systems. Such physical quantities can be experimentally measured by probing the magnon dispersion relation over the whole Brillouin zone. On this regard, high wavevector magnons in ultrathin Fe and Co multilayers grown on W(110) and Ir(111) are studied using spin polarized high resolution electron energy loss spectroscopy. In layers of Fe and Co films deposited on W(110) the magnons are probed along the [001] direction in the wavevector range of 0.25 inverse angstrom to 1.3 inverse angstrom. Experimental results reveal two magnon modes which exhibit a clear dispersion as expected. The results are described within the Heisenberg model. In the case of 1ML Co/1ML Fe/W(110) structure, the interface Fe intralayer exchange coupling constants are found to be enhanced when compared to 1ML Fe/W(110). The interlayer exchange coupling was found to be weak (4.5 meV) relative to the 2ML Fe deposited on the same substrate (7.6 meV). The antisymmteric Dzyaloshinskii-Moriya interaction (DMI) was observed to be enhanced when compared to the 2ML Fe/W(110) system. Fe films with the thickness of 2ML and 3ML showed very soft magnons when they were grown on Ir(111). This indicates rather small exchange coupling constants in these systems. For the case of 2ML Co/1ML Fe/Ir(111) two magnon modes were observed. Comparing the experimental results with those of ab initio calculations it is observed that the exchange interaction in the Fe layer at the interface is very weak, with a large antiferromagnetic contribution. The results provide a deeper understanding of the magnetic interactions in layered structures and may help to advance the understanding of the role of symmetric and antisymmetric exchange interaction in low dimensional magnetism

    Biological wastewater treatment in aeration tanks

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    Development of mathematical model for prediction of output parameters of aeration tank with account of dissolved oxygen, oxygen , sludge, substrate transfer and biological treatment. The mathematical model may be used in predicting the effectiveness of aeration tank under different regimes of work

    Magnetic susceptibility of the frustrated spinels ZnCr2O4, MgCr2O4 and CdCr2O4

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    We analyzed the magnetic susceptibilities of several Cr spinels using two recent models for the geometrically frustrated pyrochlore lattice, the Quantum Tetrahedral Mean Field model and a Generalized Constant Coupling model. Both models can describe the experimental data for ACr2 O4 (with A = Zn, Mg, and Cd) satisfactorily, with the former yielding a somewhat better agreement with experiment for A = Zn, Mg. The obtained exchange constants for nearest and next-nearest neighbors are discussed.Comment: 4 pages, 1 figure, 1 table, conferenc

    MULTICOMPONENT HYDROGELS AS SMART MATERIALS FOR DRUG DELIVERY, DISEASE MODELING AND EX VIVO TISSUE PRODUCTION

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    Evolution of optical properties of chromium spinels CdCr2_2O4_4, HgCr2_2S4_4, and ZnCr2_2Se4_4 under high pressure

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    We report pressure-dependent reflection and transmission measurements on ZnCr2_2Se4_4, HgCr2_2S4_4, and CdCr2_2O4_4 single crystals at room temperature over a broad spectral range 200-24000 cm1^{-1}. The pressure dependence of the phonon modes and the high-frequency electronic excitations indicates that all three compounds undergo a pressure-induced structural phase transition with the critical pressure 15 GPa, 12 GPa, and 10 GPa for CdCr2_2O4_4, HgCr2_2S4_4, and ZnCr2_2Se4_4, respectively. The eigenfrequencies of the electronic transitions are very close to the expected values for chromium crystal-field transitions. In the case of the chalcogenides pressure induces a red shift of the electronic excitation which indicates a strong hybridization of the Cr d-bands with the chalcogenide bands.Comment: Accepted for publication in Phys. Rev.

    Multiferroicity and colossal magneto-capacitance in Cr-thiospinels

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    The sulfur based Cr-spinels RCr2S4 with R = Cd and Hg exhibit the coexistence of ferromagnetic and ferroelectric properties together with a pronounced magnetocapacitive coupling. While in CdCr2S4 purely ferromagnetic order is established, in HgCr2S4 a bond-frustrated magnetic ground state is realized, which, however, easily can be driven towards a ferromagnetic configuration in weak magnetic fields. This paper shall review our recent investigation for both compounds. Besides the characterization of the magnetic properties, the complex dielectric permittivity was studied by means of broadband dielectric spectroscopy as well as measurements of polarization hysteresis and pyro-currents. The observed colossal magneto-capacitive effect at the magnetic transition seems to be driven by an enormous variation of the relaxation dynamics.Comment: 10 pages, 11 figure

    Low-temperature structural transition in FeCr_2S_4

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    Transmission electron microscopy studies of [110] and [111] oriented FeCr_2S_4 single crystals at different temperatures reveal a structural transition at low temperatures indicating a cubic-to-triclinic symmetry reduction within crystallographic domains. The overall crystal symmetry was found to be reduced from Fd3m to F-43m. The triclinic distortions were suggested to result from the combined actions of tetragonal distortions due to the Jahn-Teller active Fe^2+ ions and trigonal distortions due to a displacement of the Cr^3+ ions in the direction.Comment: 4 pages, 8 figure

    Tuning orbital-selective correlation effects in superconducting Rb0.75_{0.75}Fe1.6_{1.6}Se2z_{2-z}Sz_z

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    We report on terahertz time-domain spectroscopy on superconducting and metallic iron chalcogenides Rb0.75_{0.75}Fe1.6_{1.6}Se2z_{2-z}Sz_z. The superconducting transition is reduced from Tc=T_c= 32 K (z=0z=0) to 22 K (z=1.0z=1.0), and finally suppressed (z=1.4z=1.4) by isoelectronic substitution of Se with S. Dielectric constant and optical conductivity exhibit a metal-to-insulator transition associated with an orbital-selective Mott phase. This orbital-selective Mott transition appears at higher temperature TmetT_{met} with increasing sulfur content, identifying sulfur substitution as an efficient parameter to tune orbital-dependent correlation effects in iron-chalcogenide superconductors. The reduced correlations of the dxyd_{xy} charge carriers can account for the suppression of the superconductivity and the pseudogap-like feature between TcT_c and TmetT_{met} that was observed for z=0z=0.Comment: 6 pages, 4 figure

    Berezinskii-Kosterlitz-Thouless Type Scenario in Molecular Spin Liquid AACr2_2O4_4

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    The spin relaxation in chromium spinel oxides AACr2_{2}O4_{4} (A=A= Mg, Zn, Cd) is investigated in the paramagnetic regime by electron spin resonance (ESR). The temperature dependence of the ESR linewidth indicates an unconventional spin-relaxation behavior, similar to spin-spin relaxation in the two-dimensional (2D) chromium-oxide triangular lattice antiferromagnets. The data can be described in terms of a generalized Berezinskii-Kosterlitz-Thouless (BKT) type scenario for 2D systems with additional internal symmetries. Based on the characteristic exponents obtained from the evaluation of the ESR linewidth, short-range order with a hidden internal symmetry is suggested.Comment: 7 pages, 4 figure

    Multiferroic behavior in CdCr2X4 (X = S, Se)

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    The recently discovered multiferroic material CdCr2S4 shows a coexistence of ferromagnetism and relaxor ferroelectricity together with a colossal magnetocapacitive effect. The complex dielectric permittivity of this compound and of the structurally related CdCr2Se4 was studied by means of broadband dielectric spectroscopy using different electrode materials. The observed magnetocapacitive coupling at the magnetic transition is driven by enormous changes of the relaxation dynamics induced by the development of magnetic order
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