Browsing by Author "Whangbo, MH"

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    Cu5SbO6 – synchrotron, neutron diffraction studies and magnetic properties
    (Australian Institute of Physics, 2011-02-04) Söhnel, T; Rey, E; Ling, CD; Avdeev, M; Johannessen, B; Wallwork, KS; Kremer, RK; Whangbo, MH
    One very interesting compound in the system Cu/Sb/O is the mixed-valent Cu5SbO6 = (Cu1+(Cu2+ 2/3Sb5+ 1/3)O2) which is crystallising in the high temperature modification as a modified Delafossite structure type. Compounds like Delafossite, CuFeO2, is one of the few groups of compounds showing the rare property of multiferroic behaviour. In Cu5SbO6 the magnetically active brucite-like CuO2 layer is diluted in an ordered fashion with nonmagnetic Sb5+. Cu5SbO6 also shows a phase transition, which exhibits a rather complicated behaviour. It depends on the temperature and the reaction conditions (reactants for preparation, pressure, open or closed system). High resolution Synchrotron and neutron powder diffraction measurements could clearly distinguish between the high temperature and the low temperature modification and reveal an ordering (HT-modification) / disordering (LT-modification) effect of the Sb5+ and Cu2+ ions in the brucite-like layers. The LT-modification can also be assigned to what had wrongly been described in the literature as Cu4.5SbO5. XANES Cu-K edge measurements and NPD measurements should clarify a potential oxidation of the Cu1+ to Cu2+ and a connected additional inclusion of oxygen in the structure. According to magnetic measurements and DFT calculations the magnetic structure in Cu5SbO6 can be described with a short range ferromagnetic-antiferromagnetic interaction model of the (Cu2+) pairs in the (Cu2+ 2/3Sb5+ 1/3)O2 layers with a super-exchange via the nonmagnetic Sb5+ atoms. The systematic replacement of the non-magnetic Sb5+ with magnetically active M5+ ions should change the magnetic properties dramatically and could lead to an long range ordering in the system. First results of Mn and Mo doping will also be presented.
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    On the nature of the spin frustration in the CuO(2) ribbon chains of LiCuVO(4): crystal structure determination at 1.6 K, magnetic susceptibility analysis, and density functional evaluation of the spin exchange constants
    (American Chemical Society, 2011-04-18) Koo, HJ; Lee, CH; Whangbo, MH; McIntyre, GJ; Kremer, RK
    The spin-1/2 Cu(2+) ions of LiCuVO(4) form one-dimensional chains along the b direction, and the spin frustration in LiCuVO(4) is described in terms of the nearest-neighbor ferromagnetic exchange h and the next-nearest-neighbor antiferromagnetic exchange J(2) in these chains. Recently, it has become controversial whether or not J(1) is stronger in magnitude than J(2). To resolve this controversy, we determined the crystal structure of LiCuVO(4) at 1.6 K by neutron diffraction, analyzed the magnetic susceptibility of LiCuVO(4) to deduce the Curie-Weiss temperature theta and the J(2)/J(1) ratio, and finally extracted the spin exchange constants of LiCuVO(4) on the basis of density functional calculations. Our work shows unambiguously that the Curie-Weiss temperature theta of LiCuVO(4) is negative in the range of -20 K, so that J(2) is substantially stronger in magnitude than J(1). © 2011, American Chemical Society
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    Orthogonal antiferromagnetism to canted ferromagnetism in CaCo3Ti4O12 quadruple perovskite driven by underlying kagome lattices
    (Springer Nature, 2022-08-01) Amano Patino, M; Denis Romero, F; Koo, HJ; Avdeev, M; Injac, SDA; Goto, M; Whangbo, MH; Shimakawa, Y
    AA′3B4O12 quadruple perovskites, with magnetic A′ and non-magnetic B cations, are characterized by a wide range of complex magnetic structures. These are due to a variety of competing spin-exchange interactions up to the fourth nearest neighbours. Here, we synthesize and characterize the magnetic behaviour of the CaCo3Ti4O12 quadruple perovskite. We find that in the absence of an external magnetic field, the system undergoes antiferromagnetic ordering at 9.3 K. This magnetic structure consists of three interpenetrating mutually orthogonal magnetic sublattices. Under an applied magnetic field, this antiferromagnetic structure evolves into a canted ferromagnetic structure. In explaining these magnetic structures, as well as the seemingly unrelated magnetic structures found in other quadruple perovskites, we suggest a crucial role played by the underlying kagome lattices in these systems. All observed magnetic structures of these materials represent indeed one of the three possible ways to reduce spin frustration in the A′ site kagome layers. More specifically, our survey of the magnetic structures observed for quadruple perovskites AA′3B4O12 reveals the following three ways to reduce spin frustration, namely to make each layer ferromagnetic, to adopt a compromise 120° spin arrangement in each layer, or to have a magnetic structure with a vanishing sum of all second nearest-neighbour spin exchanges. © The Authors - Open Access CC BY 4.0
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    Synthesis and characterization of the crystal structure and magnetic properties of the new fluorophosphate LiNaCo[PO4]F
    (American Chemical Society, 2012-08-02) Ben Yahia, H; Shikano, M; Koike, S; Tatsumi, K; Kobayashi, H; Kawaji, H; Avdeev, M; Miiller, W; Ling, CD; Liu, J; Whangbo, MH
    The new compound LiNaCo[PO4]F was synthesized by a solid state reaction route, and its crystal structure was determined by single-crystal X-ray diffraction measurements. The magnetic properties of LiNaCo[PO4]F were characterized by magnetic susceptibility, specific heat, and neutron powder diffraction measurements and also by density functional calculations. LiNaCo[PO4]F crystallizes with orthorhombic symmetry, space group Pnma, with a = 10.9334(6), b = 6.2934(11), c = 11.3556(10) angstrom, and Z = 8. The structure consists of edge-sharing CoO4F2 octahedra forming CoFO3 chains running along the b axis. These chains are interlinked by PO4 tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The magnetic susceptibility follows the Curie Weiss behavior above 60 K with theta = -21 K. The specific heat and magnetization measurements show that LiNaCo[PO4]F undergoes a three-dimensional magnetic ordering at T-mag = 10.2(5) K. The neutron powder diffraction measurements at 3 K show that the spins in each CoFO3 chain along the b-direction are ferromagnetically coupled, while these FM chains are antiferromagnetically coupled along the a-direction but have a noncollinear arrangement along the c-direction. The noncollinear spin arrangement implies the presence of spin conflict along the c-direction. The observed magnetic structures are well explained by the spin exchange constants determined from density functional calculations. © 2012, American Chemical Society.