Browsing by Author "Tallon, JL"

Now showing 1 - 3 of 3
  • No Thumbnail Available
    Item
    HTS 5 tesla synchrotron and neutron beamline magnets
    (Institute of Electrical and Electronics Engineers (IEEE), 2009-06) Pooke, DM; Chamritski, V; Fee, M; Gibson, S; King, BT; Tallon, JL; Meissner, M; Feyerherm, R; Olsen, SR; Kennedy, SJ; Robinson, RA
    Two world-first High-Tc Superconducting (HTS) beamline magnets have been designed, manufactured and commissioned for synchrotron and neutron research facilities. One, for the Hahn-Meitner Institut, is for use with their high-resolution diffraction and resonant magnetic scattering instrument MAGS at the Berlin electron synchrotron facility BESSY. The key features of this 5 tesla split-pair magnet include field performance within a given confined geometry, and low weight. Mounting in a 6-circle goniometer and employing conduction-cooled HTS coils, it operates through 90 degrees of rotation with respect to the beamline axis and scattering plane. The neutron beamline magnet is also a 5 tesla split pair, designed for both neutron reflectometry and small-angle neutron scattering research at the new OPAL neutron facility of the Australian Nuclear Science and Technology Organisation. This much larger magnet offers wide neutron beam accessibility angles in both axial and transverse directions, large (50 mm) sample capability, and mounts on tilt stages operating in two axes. Both magnets use pulse-tube refrigeration for the HTS coils, which have been constructed from high performance BSCCO wire, and both are compatible with separate commercial cryofurnaces providing sample temperatures from 1.5 K (MAGS) or 4 K (OPAL) to over 600 K. © 2009, Institute of Electrical and Electronics Engineers (IEEE)
  • Thumbnail Image
    Item
    Oxygen isotope effect in high temperature superconductors - phonons or magnons?
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2012-11-07) Barber, FJ; Tallon, JL; Mallett, BPP; Avdeev, M
    The oxygen isotope effect displayed in the critical temperature, Tϲ, by high temperature superconducting (HTS) cuprates was studied using polycrystalline samples of YBa2Cu3O6. The dominant perspective within the community is that this isotope effect is resultant from lattice vibrations or 'phonons'. In contrast, our treatment assesses the viability of explaining this effect by an indirectly acting magnetic mechanism. This proposed mechanism involves an increase in the anti- ferromagnetic super-exchange interaction energy, J, between adjacent Cu(2) ions as a result of decreasing unit cell basal area from a decrease in lattice vibration magnitudes. Polycrystalline YBa2Cu3O6 samples of similar mass were synthesised under identical conditions, then co-annealed under 16O and 18O gas respectively. Neutron diffraction experiments were performed on these samples at 3K, 50K and 100K to accurately determine structural parameters. Rietveld refinement produced inferred critical temperature isotope exponents, α = of α-(3K)=(3.6±0.4)x103, α(50K)=(4.3±0.6)x103, and α (100K)=(7.2±0.9)x103. Comparison with the experimentally measured isotope effect of α=0.06 in the overdoped region indicates that our mechanism is unlikely to be sufficient to account for the isotope effect in Tϲ. We note, however, that there remains a possibility of a secondary contribution from this mechanism. This project plans for further back-exchange and repeated diffraction measurements, which will aid in confirming and comparing these results.
  • Thumbnail Image
    Item
    Thermal and pressure-induced spin crossover in a cobalt (II) imide complex
    (Australian Institute of Physics, 2012-01-31) Narayanaswamy, S; Cowan, MG; Olguin, J; Tallon, JL; Brooker, S
    We report the thermal- and pressure-induced spin-crossover (SCO) in the Co(II) imide complex [CoII(dpzca)2] using magnetic susceptibility measurements and high-pressure Raman spectroscopy. This complex displays thermally-induced reversible, complete, abrupt and hysteretic SCO, with T1/2↓ = 168 K upon cooling and T1/2↑ = 179 K upon heating with a thermal hysteresis of ~11K and a pressure-induced reversible high spin (HS) ⇄ low spin (LS) transition at ~0.3 GPa at room temperature.