Browsing by Author "Park, JG"

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    3d-electron Heisenberg pyrochlore Mn2Sb2O7
    (American Physical Society, 2016-11-21) Peets, DC; Sim, H; Avdeev, M; Park, JG
    In frustrated magnetic systems, geometric constraints or the competition among interactions introduce extre-mely high degeneracy and prevent the system from readily selecting a low-temperature ground state. The most frustrated known spin arrangement is on the pyrochlore lattice, but nearly all magnetic pyrochlores have unquenched orbital angular momenta, constraining the spin directions through spin-orbit coupling. Pyrochlore Mn2Sb2O7 is an extremely rare Heisenberg pyrochlore system with directionally unconstrained spins and low chemical disorder. We show that it undergoes a spin-glass transition at 5.5 K, which is suppressed by disorder arising from Mn vacancies, indicating this ground state to be a direct consequence of the spins' interactions. The striking similarities to 3d transition-metal pyrochlores with unquenched angular momenta suggests that the low spin-orbit coupling in the 3d block makes Heisenberg pyrochlores far more accessible than previously imagined. ©2016 American Physical Society
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    Crystal structure of a non-centrosymmetric system, CuY2Ge2O8
    (International Conference on Neutron Scattering, 2017-07-12) Cho, H; Sim, H; Lee, S; Avdeev, M; Noda, Y; Park, JG
    A non-centrosymmeric material doesn’t have an inversion symmetry, which could lead to the intrinsic electric polarization. When this system consists of magnetic ions, the electric polarization of the system can be modified by a so-called magnetoelectric coupling under magnetic field. This multiferroic property has been widely studied not only for physical interest but also for industrial application. We synthesized both single crystal and polycrystalline samples of new multiferroic systems, Cu(Y,La) Ge and determined the crystal structure at 300 and 450 K using high-resolution powder and single crystal diffraction studies. According to our studies, both samples having magnetic Cu2+ ions has a monoclinic unit cell (space group I 1 1) without inversion symmetry. We also determined the high temperature structure that has a more symmetric orthorhombic unit cell (I m m 2). Using the structural information, we could calculate the theoretical electric polarization at room temperature phase and found that it reaches the electric polarization of 9.88 μC/cm2 along the a-axis.
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    Magnetic transitions in the chiral armchair-kagome system Mn2Sb2O7
    (American Physical Society, 2017-01-20) Peets, DC; Sim, H; Choi, S; Avdeev, M; Lee, S; Kim, SJ; Kang, H; Ahn, D; Park, JG
    The competition between interactions in frustrated magnets allows a wide variety of new ground states, often exhibiting emergent physics and unique excitations. Expanding the suite of lattices available for study enhances our chances of finding exotic physics. Mn2Sb2O7 forms in a chiral, kagome-based structure in which a fourth member is added to the kagome-plane triangles to form an armchair unit and link adjacent kagome planes. This structural motif may be viewed as intermediate between the triangles of the kagome network and the tetrahedra in the pyrochlore lattice. Mn2Sb2O7 exhibits two distinct magnetic phase transitions, at 11.1 and 14.2 K, at least one of which has a weak ferromagnetic component. The magnetic propagation vector does not change through the lower transition, suggesting a metamagnetic transition or a transition involving a multicomponent order parameter. Although previously reported in the P3121 space group, Mn2Sb2O7 actually crystallizes in P2, which allows ferroelectricity, and we show clear evidence of magnetoelectric coupling indicative of multiferroic order. The quasi-two-dimensional “armchair-kagome” lattice presents a promising platform for probing chiral magnetism and the effect of dimensionality in highly frustrated systems. ©2017 American Physical Society
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    Properties of spin-1/2 triangular-lattice antiferromagnets CuY2Ge2O8 and CuLa2Ge2O8
    (American Physical Society, 2017-04-05) Cho, H; Kratochvílová, M; Sim, H; Choi, KY; Kim, CH; Paulsen, C; Avdeev, M; Peets, DC; Jo, Y; Lee, S; Noda, Y; Lawler, MJ; Park, JG
    We found new two-dimensional (2D) quantum (S=1/2) antiferromagnetic systems: CuRE2Ge2O8 (RE=Y and La). According to our analysis of high-resolution x-ray and neutron diffraction experiments, the Cu network of CuRE2Ge2O8 (RE=Y and La) exhibits a 2D triangular lattice linked via weak bonds along the perpendicular b axis. Our bulk characterizations from 0.08 to 400 K show that they undergo a long-range order at 0.51(1) and 1.09(4) K for the Y and La systems, respectively. Interestingly, they also exhibit field induced phase transitions. For theoretical understanding, we carried out the density functional theory (DFT) band calculations to find that they are typical charge-transfer-type insulators with a gap of Eg≅2eV. Taken together, our observations make CuRE2Ge2O8 (RE=Y and La) additional examples of low-dimensional quantum spin triangular antiferromagnets with the low-temperature magnetic ordering. ©2017 American Physical Society
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    Publisher's Note: 3d-electron Heisenberg pyrochlore Mn2Sb2O7 [Phys. Rev. B 94, 174431 (2016)]
    (American Physical Society, 2017-03-21) Peets, DC; Sim, H; Avdeev, M; Park, JG
    This paper was published online on 21 November 2016 with a typographical error on page 2. On page 2, the fourth line of the left-hand column should read as “prepared from SbCl5 (Alfa Aesar, 99.997%) and deionized . . . ” The paper has been corrected as of 10 March 2017. The text is incorrect in the printed version of the journal.