Browsing by Author "Kim, JH"

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    Competing exchange interactions on the verge of a metal-insulator transition in the two-dimensional spiral magnet Sr3Fe2O7
    (Americal Physical Society, 2014-10-03) Kim, JH; Jain, A; Reehuis, M; Khaliullin, G; Peets, DC; Ulrich, C; Park, JT; Faulhaber, E; Hoser, A; Walker, HC; Adroja, DT; Walters, AC; Inosov, DS; Maljuk, A; Keimer, B
    We report a neutron scattering study of the magnetic order and dynamics of the bilayer perovskite Sr 3 Fe 2 O 7 , which exhibits a temperature-driven metal-insulator transition at 340 K. We show that the Fe 4+ moments adopt incommensurate spiral order below T N =115  K and provide a comprehensive description of the corresponding spin-wave excitations. The observed magnetic order and excitation spectra can be well understood in terms of an effective spin Hamiltonian with interactions ranging up to third-nearest-neighbor pairs. The results indicate that the helical magnetism in Sr 3 Fe 2 O 7 results from competition between ferromagnetic double-exchange and antiferromagnetic superexchange interactions whose strengths become comparable near the metal-insulator transition. They thus confirm a decades-old theoretical prediction and provide a firm experimental basis for models of magnetic correlations in strongly correlated metals. © 2014, American Physical Society.
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    Evaluation of isotopic boron (11B) for the fabrication of low activation Mg11B2 superconductor for next generation fusion magnets
    (John Wiley & Sons, Inc., 2020-04-13) Jie, H; Luzin, V; Zaman, M; Abdulsalam, AV; Chae, KH; Choi, H; Levchenko, VA; Nijhius, A; Kim, JH; Mustapić, M; Dou, SX; Yamauchi, Y; Khan, A; Hossain, A
    In this study, we analyze the properties of boron isotope (11B)‐rich powders from three different sources, that is, American, Cambridge, and Pavezyum, to fabricate the bulk Mg11B2 superconductors and evaluate their superconducting properties. While 11B‐rich powder is an essential precursor to fabricate Mg11B2 superconductors for fusion magnet applications, the properties of the 11B powder turned out to be critical to determine the quality of the final superconducting product. Therefore, appropriate control of processing conditions is needed to comply with the requirements of the nuclear fusion application. Analysis of the B isotope ratio by accelerator mass spectroscopy and neutron transmission revealed that all three types of powder are enriched with 11B to better than 99 at % quality. In addition, Pavezyum's 11B shows the lowest crystallinity and smallest crystalline domain size as evidenced by the high‐resolution X‐ray diffractometer and scanning electron microscopy. The chemical states of the boron isotope investigated with near edge X‐ray absorption fine structure spectroscopy and X‐ray photoemission spectroscopy also reveals that Pavezyum boron has amorphous structure. Mg11B2 bulks and multi‐filamentary (12‐filament) wires have been manufactured, sintered at different temperatures and characterized via the transport critical current density. The wire with Pavezyum 11B shows three times higher current carrying capacity at a particular magnetic field compared to the wire using Cambridge 11B and hence, Pavezyum 11B boron has the potential for manufacturing fusion grade Mg11B2 based magnets. The results of this study demonstrated that Boron powders with higher purity, smaller grain size and lower crystallinity are critical for improving the superconducting and electronic properties of Mg11B2 samples fabricated from the powder. Thus, the low‐neutron‐activation Mg11B2 is possibly an affordable and technically viable candidate to replace NbTi superconductors in the low field poloidal field and correction coils for the next‐generation fusion reactors. © 1999-2020 John Wiley & Sons, Inc.
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    Graphene doping to enhance the flux pinning and supercurrent carrying ability of a magnesium diboride superconductor
    (Institute of Physics, 2010-08) Xu, X; Dou, SX; Wang, XL; Kim, JH; Stride, JA; Choucair, M; Yeoh, WK; Zheng, RK; Ringer, SP
    The effect of graphene doping on the electromagnetic properties of MgB2 has been examined, in comparison with the case for undoped MgB2. It was found that graphene doping is more efficient than other forms of carbon doping for effecting improvement in the critical current density–field performance (Jc(B)), with little change in the transition temperature of MgB2. An optimal enhancement of Jc(B) was achieved for 3.7 at.% graphene doped MgB2, by a factor of 30 at 5 K and 10 T, as compared to undoped MgB2. It is found that spatial fluctuation in Tc is responsible for the flux pinning mechanism of graphene doped MgB2. © 2010, Institute of Physics
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    LiTa2PO8: a fast lithium-ion conductor with new framework structure
    (Royal Society of Chemistry, 2018-10-29) Kim, JG; Kim, JH; Avdeev, M; Yun, H; Kim, SJ
    A new lithium-ion conducting oxide, LiTa2PO8, with a three-dimensional (3D) framework structure was synthesized. The complementary use of synchrotron XRD and neutron diffraction revealed that LiTa2PO8 crystallized into a monoclinic system (space group C2/c) with lattice parameters a ≈ 9.716 Å, b ≈ 11.536 Å, c ≈ 10.697 Å, and β = 90.04°. In this structure, the TaO6 octahedra and PO4 tetrahedra are connected via corner-sharing to form an unprecedented anionic framework, [(TaO6/2)2(PO4/2)]−, providing a 3D pathway for lithium-ion conduction. The sintered LiTa2PO8 pellets exhibited excellent lithium-ion conductivities: bulk conductivity of 1.6 × 10−3 S cm−1 and total conductivity of 2.5 × 10−4 S cm−1 at 25 °C. © Royal Society of Chemistry
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    Magnetic phase diagram of Sr3Fe2O7-delta
    (American Physical Society, 2013-06-10) Peets, DC; Kim, JH; Dosanjh, P; Reehuis, M; Maljuk, A; Aliouane, N; Ulrich, C; Keimer, B
    Magnetometry, electrical transport, and neutron scattering measurements were performed on single crystals of the Fe4+-containing perovskite-related phase Sr3Fe2O7−δ as a function of oxygen content. Although both the crystal structure and electron configuration of this compound are closely similar to those of well-studied ruthenates and manganates, it exhibits very different physical properties. The fully oxygenated compound (δ=0) exhibits a charge-disproportionation transition at TD=340 K, and an antiferromagnetic transition at TN=115 K. For temperatures T≤TD, the material is a small-gap insulator; the antiferromagnetic order is incommensurate, which implies competing exchange interactions between the Fe4+ moments. The fully deoxygenated compound (δ=1) is highly insulating, and its Fe3+ moments exhibit commensurate antiferromagnetic order below TN∼600 K. Compounds with intermediate δ exhibit different order with lower TN, likely as a consequence of frustrated exchange interactions between Fe3+ and Fe4+ sublattices. A previous proposal that the magnetic transition temperature reaches zero is not supported. © 2013, American Physical Society.
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    Magnetic proximity effect in YBa2Cu3O7/La2/3Ca1/3MnO3 and YBa2Cu3O7/LaMnO3+delta superlattices
    (Amnerican Physical Society, 2012-05-07) Satapathy, DK; Uribe-Laverde, MA; Marozau, I; Malik, VK; Das, S; Wagner, T; Marcelot, C; Stahn, J; Bruck, S; Ruhm, A; Macke, S; Tietze, T; Goering, E; Frano, A; Kim, JH; Wu, M; Benckiser, E; Keimer, B; Devishvili, A; Toperverg, BP; Merz, M; Nagel, P; Schuppler, S; Bernhard, C
    Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa(2)Cu(3)O(7) and ferromagnetic-metallic La(0.67)Ca(0.33)MnO(3) or ferromagnetic-insulating LaMnO(3+delta). We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic La(0.67)Ca(0.33)MnO(3) and almost absent for ferromagnetic-insulating LaMnO(3+delta). We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature. © 2012, American Physical Society.
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    Stress/strain induced flux pinning in highly dense MgB2 bulks
    (Institute of Electrical and Electronics Engineers (IEEE), 2009-06-01) Zeng, R; Dou, SX; Lu, L; Li, WX; Poh, CK; Kim, JH; Horvat, J; Shi, DQ; Wang, JL; Munroe, P; Wang, XF; Zheng, RK; Ringer, SP; Rindfleisch, M; Tomsic, M
    We have systematically studied the flux pinning behavior of MgB(2) bulks synthesized by direct diffusion of Mg into pressed pellets of high purity crystalline B powder, with and without mixing with C and SiC nanoparticles, at a reaction temperature of 850 degrees C for 10 hrs. All of the samples showed very high purity and high density, but their microstructure and flux pinning behavior showed significant differences. It was found that the pure MgB(2) agrees with the delta T(c) pinning model, nano-C doped MgB(2) agrees with the delta l pinning model, while the SiC + MgB(2) composite agrees with the delta epsilon pinning model (stress/strain field pinning), since the dominant micro-defects that influence the flux pinning in these three samples are different. © 2009, Institute of Electrical and Electronics Engineers (IEEE)