Browsing by Author "McKenzie, I"

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    Direct measurements of the temperature, depth and processing dependence of phenyl ring dynamics in polystyrene thin films by β-detected NMR
    (Royal Society of Chemistry, 2018-05-24) McKenzie, I; Chai, Y; Cortie, DL; Forrest, JA; Fujimoto, D; Karner, VL; Kiefl, RF; Levy, CDP; MacFarlane, WA; McFadden, RML; Morris, GD; Pearson, MR; Zhu, S
    There is indirect evidence that the dynamics of a polymer near a free surface are enhanced compared with the bulk but there are few studies of how dynamics varies with depth. β-Detected nuclear spin relaxation of implanted 8Li+ has been used to directly probe the temperature and depth dependence of the γ-relaxation mode, which is due to phenyl rings undergoing restricted rotation, in thin films of atactic deuterated polystyrene (PS-d8) and determine how the depth dependence of dynamics is affected by sample processing, such as annealing, floating on water and the inclusion of a surfactant, and by the presence of a buried interface. The activation energy for the γ-relaxation process is lower near the free surface. Annealing the PS-d8 films and then immersing in water to mimic the floating procedure used to transfer films had negligible effects on the thickness of the region near the free surface with enhanced mobility. Measurements on a bilayer film indicate enhanced phenyl ring dynamics near the buried interface compared with a single film at the same depth. PS-d8 films annealed with the surfactant sodium dodecyl sulfate (SDS) deposited on the surface show enhanced dynamics in the bulk compared with a pure PS-d8 film and a PS-d8 film where the SDS was washed away. There is less contrast between the surface and bulk in the SDS-treated sample, which could account for the elimination of the Tg confinement effect observed in films containing SDS [Chen and Torkelson, Polymer, 2016, 87, 226]. © The Royal Society of Chemistry 2018.
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    Interaction of the Mu-cyclohexadienyl radical with metallic (Au, Pt) nanoparticles in mesoporous silica
    (IOP Publishing, 2014-12-16) Xiao, J; Arseneau, DJ; Bridges, MD; Cortie, DL; Cottrell, SP; Dehn, M; Fleming, DG; Kelly, J; Kiefl, RF; MacFarlane, WA; MacLachlan, M; McKenzie, I
    μSR and ALCR techniques have been used to investigate the structure and dynamics of the Mu-cyclohexadienyl radical interacting with Au and Pt metal nanoparticles (MNPs) supported in mesoporous silica (SBA-15). Surprisingly, coherent precession signals are observed and the isotropic hyperfine coupling constants are almost the same in loaded and unloaded samples, implying that the electronic structure of MuC6H6 is only weakly perturbed by the presence of the MNPs. We propose the observed radicals are shielded from the metallic surfaces by a benzene coating on the MNPs. The Δ1 resonance is observable in MNP-loaded samples at higher temperatures than in the unloaded SBA-15. This is attributed to stronger binding of MuC6H6 to the benzene coated MNPs.. © The Authors - Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
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    Local electronic and magnetic properties of the doped topological insulators Bi2Se3:Ca and Bi2Te3 : Mn investigated using ion-implanted 8Li β-NMR
    (American Physical Society (APS), 2020-12-28) McFadden, RML; Chatzichristos, A; Cortie, DL; Fujimoto, D; San Hor, Y; Ji, H; Karner, VL; Kiefl, RF; Levy, CDP; Li, RH; McKenzie, I; Morris, GD; Pearson, MR; Stachura, M; Cava, RJ; MacFarlane, WA
    We report β-NMR measurements in Bi2Se3:Ca and Bi2Te3:Mn single crystals using Li+8 implanted to depths on the order of 100 nm. Above ∼200K, spin-lattice relaxation reveals diffusion of Li+8, with activation energies of ∼0.4eV (∼0.2eV) in Bi2Se3:Ca (Bi2Te3:Mn). At lower temperatures, the NMR properties are those of a heavily doped semiconductor in the metallic limit, with Korringa relaxation and a small, negative, temperature-dependent Knight shift in Bi2Se3:Ca. From this, we make a detailed comparison with the isostructural tetradymite Bi2Te2Se [McFadden, Phys. Rev. B 99, 125201 (2019)2469-995010.1103/PhysRevB.99.125201]. In the magnetic Bi2Te3:Mn, the effects of the dilute Mn moments predominate, but remarkably the Li+8 signal is not wiped out through the magnetic transition at 13 K, with a prominent critical peak in the spin-lattice relaxation that is suppressed in a high applied field. This detailed characterization of the Li+8 NMR response is an important step toward using depth-resolved β-NMR to study the low-energy properties of the chiral topological surface state in the Bi2Ch3 tetradymite topological insulator. ©2024 American Physical Society.
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    The Site and High Field βNMR Properties of 8Li+ Implanted in α-Al2O3
    (IOP Publishing, 2023-03-01) MacFarlane, WA; Shenton, JK; Salman, Z; Chatzichristos, A; Cortie, DL; Dehn, M; Fujimoto, D; Karner, VL; Kiefl, RF; Koumoulis, D; Levy, CDP; McFadden, RML; McKenzie, I; Morris, GD; Pearson, MR; Stachura, M; Ticknor, JO
    We present high (> 2 T) magnetic field βNMR measurements of 8Li+ implanted in single crystals of sapphire, a commonly used backing material for other samples. From the well-resolved quadrupolar splitting, we extract the electric field gradient (EFG) at the implanted 8Li+ site. Comparison with supercell density functional theory calculations of the EFG allows us to identify the octahedral interstitial site as the most likely candidate. In contrast to the zero field βNQR spectra, where multiple singals are detected, only a single site is evident at high field. We discuss possible explanations for this discrepancy. The spin lattice relaxation is extremely slow (1/T1 < 0.02 s−1 ) over a broad temperature range from 4 to 300 K, demonstrating that cross relaxation with the 27Al nuclear spins is quenched in such high magnetic fields.
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    Spin depolarization of Muonium in Mesoporous Silica
    (IOP Publishing, 2014-12-16) Dehn, MH; Arseneau, DJ; Bridges, MD; Buck, T; Cortie, DL; Cottrell, SP; Fleming, DG; Kelly, JA; MacFarlane, WA; MacLachlan, MJ; Morris, GD; McKenzie, I; Xiao, J; Kiefl, RF
    We report muon spin rotation/relaxation measurements of muonium in mesoporous silica (SBA-15) with a high specific surface area of 600 m2/g. Up to 70 percent of the incoming muons form muonium and escape efficiently into the open pores at all temperatures between 3 and 300K. We present evidence that the interaction with the silica surfaces involves both spin exchange and a transition to a diamagnetic state, possibly due to dangling bonds on the surface. At very low temperatures, below 20K, the interaction between muonium and the silica surfaces is suppressed due to a He film coating the surfaces. These results indicate that it should be possible to use muonium to probe the surfaces of uncapped nanoparticles supported in silica. © 2014 The Authors - Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.