Browsing by Author "Koo, HJ"

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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