T Buck

The Initial State of Optically Polarized 8Li+ from the β-NMR in Bismuth

Unlike the positive muon, β-NMR probe nuclei must be actively polarized. At the TRIUMF ISAC facility this is accomplished by in-flight collinear optical pumping with resonant circularly polarized laser light. This reliably produces a high degree of polarization, but the detailed state populations in the beam emerging from the optical polarizer are not well known. These populations are significant as they represent the initial state of the ensemble of probe spins implanted in a β-NMR experiment. Here we use the well-resolved quadrupolar split spectrum of 8Li+ in a high purity single crystal of bismuth to extract the sublevel populations under typical polarizer operating conditions, accounting for the spin relaxation in this semimetal.

β -NMR Investigation of the Depth-Dependent Magnetic Properties of an Antiferromagnetic Surface

By measuring the prototypical antiferromagnet α-Fe_{2}O_{3}, we show that it is possible to determine the static spin orientation and dynamic spin correlations within nanometers from an antiferromagnetic surface using the nuclear spin polarization of implanted ^{8}Li^{+} ions detected with β-NMR. Remarkably, the first-order Morin spin reorientation in single crystal α-Fe_{2}O_{3} occurs at the same temperature at all depths between 1 and 100 nm from the (110) surface; however, the implanted nuclear spin experiences an increased 1/T_{1} relaxation rate at shallow depths revealing soft-surface magnons. The surface-localized dynamics decay towards the bulk with a characteristic length of ε=11±1  nm, closely matching the finite-size thresholds of hematite nanostructures.

Communication: Chemisorption of muonium on gold nanoparticles: A sensitive new probe of surface magnetism and reactivity

Chemisorption of muonium onto the surface of gold nanoparticles has been observed. Muonium (μ+e-), a light hydrogen-like atom, reacts chemically with uncapped 7 nm gold nanoparticles embedded in mesoporous silica (SBA-15) with a strong temperature-dependent rate. The addition rate is fast enough to allow coherent spin transfer into a diamagnetic muon state on the nanoparticle surface. The muon is well established as a sensitive probe of static or slowly fluctuating magnetic fields in bulk matter. These results represent the first muon spin rotation signal on a nanoparticle surface or any metallic surface. Only weak magnetic effects are seen on the surface of these Au nanoparticles consistent with Pauli paramagnetism.

β-NMR of 8Li+ in rutile TiO2

We report preliminary low-energy β-NMR measurements of 8Li+ implanted in single crystal rutile TiO2 at an applied field of 6.55 T and 300 K. We observe a broad 12 kHz wide quadrupole split resonance with unresolved features and a sharp component at the Larmor frequency. The line broadening may be caused by overlapping multi-quantum transitions or motion of 8Li+ on the scale of its lifetime (1.21 s). We also find spin-lattice relaxation that is relatively fast compared to other wide band gap insulators. The origin of this fast relaxation is also likely quadrupolar and may be due to anisotropic 8Li+ diffusion.

A Brief Survey of β-Detected NMR of Implanted 8Li+ in Organic Polymers

Unlike the positive muon, we expect the chemistry of the implanted 8Li+β-NMR probe in organic polymers to be simply that of the monovalent ion, but almost nothing is known about the NMR of isolated Li+ in this context. Here, we present a brief survey of 8Li+β-NMR in a variety of insulating polymers at high magnetic field, including polyimide, PET, polycarbonate, polystyrene and polyethylene oxide. In all cases, we find a large-amplitude, broad Lorentzian resonance near the Larmor frequency, consistent with the expected diamagnetic charge state. We also find remarkably fast spin-lattice relaxation rates 1/T1. There is very little dependence of either linewidth or 1/T1 on the proton density, the main source of nuclear dipolar magnetic fields, leading us to conclude the main contribution to both broadening and spin relaxation at room temperature is quadrupolar in origin. This behaviour is very different from crystalline insulators such as MgO and Al2O3, and suggests that 8Li+β-NMR will be an important probe of polymer dynamics. Additionally, we note dramatically different behaviour of one sample above its glass transition, motivating the construction of a high temperature spectrometer to enable further exploration at elevated temperature.

β-NMR Study of a buried Mn δ-doped layer in a silicon host

Low temperature growth methods were used to encapsulate a buried Mn δ-doping layer into a silicon host. A β-NMR investigation was performed of the magnetic properties in the temperature range 10 – 300 K using spin-polarized 8Li+. A depth-dependent broadening and shift of the NMR resonance was detected that is consistent with internal fields distributed at depths of 10 – 30 nm beneath the surface. At low temperatures, a negative relative shift occurred and the resonance was significantly broadened. At 300 K the line-shape could be described by a single Gaussian line, however, at 10 K the line is best approximated by a two component Lorentzian shape consisting of a broad and narrow component as anticipated for a diluted magnetic alloy. The overall magnitude of the resonance shift at both temperatures is small suggesting a weak interaction between the 8Li+ and the magnetic Mn environment.

Nature of magnetism in thiol-capped gold nanoparticles investigated with Muon spin rotation

Muon spin rotation/relaxation measurements show clear evidence for magnetism in 2.2 nm gold nanoparticles capped with butanethiol. At low temperatures (1.8 K), there is significant spin relaxation which decreases as a function of both the applied longitudinal magnetic field and increasing temperature. The results indicate that there are spatially inhomogeneous electronic moments that fluctuate with a wide distribution of correlation times. Possible explanations are discussed.Muon spin rotation/relaxation measurements show clear evidence for magnetism in 2.2 nm gold nanoparticles capped with butanethiol. At low temperatures (1.8 K), there is significant spin relaxation which decreases as a function of both the applied longitudinal magnetic field and increasing temperature. The results indicate that there are spatially inhomogeneous electronic moments that fluctuate with a wide distribution of correlation times. Possible explanations are discussed.

Microscopic Dynamics of Li+ in Rutile TiO2 Revealed by 8Li β-Detected Nuclear Magnetic Resonance

Ryan M. L. McFadden,1, 2, ∗ Terry J. Buck,3 Aris Chatzichristos,2, 3 Chia-Chin Chen,4 David L. Cortie,1, 2, 3, † Kim H. Chow,5 Martin H. Dehn,2, 3 Victoria L. Karner,1, 2 Dimitrios Koumoulis,6, ‡ C. D. Philip Levy,7 Chilin Li,8 Iain McKenzie,7, 9 Rotraut Merkle,4 Gerald D. Morris,7 Matthew R. Pearson,7 Zaher Salman,10 Dominik Samuelis,4, § Monika Stachura,7 Jiyu Xiao,1 Joachim Maier,4 Robert F. Kiefl,2, 3, 7 and W. Andrew MacFarlane1, 2, 7, ¶ 1Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC V6T 1Z1, Canada 2Stewart Blusson Quantum Matter Institute, University of British Columbia, 2355 East Mall, Vancouver, BC V6T 1Z4, Canada 3Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada 4Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany 5Department of Physics, University of Alberta, 4-181 CCIS, Edmonton, AB T6G 2E1, Canada 6Department of Chemistry and Biochemistry, University of California, Los Angeles, 607 Charles E. Young Drive East, Los Angeles, CA 90095, USA 7TRIUMF, 4004 Wesbrook Mall, Vancouver, BC V6T 2A3, Canada 8Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai, P.R. China 200050 9Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada 10Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland (Dated: August 11, 2018)

Muon Spin Rotation on Treated Nb Samples in Parallel Field Geometry

Muon Spin Rotation (μSR) is a powerful tool to probe local magnetism in matter and hence can be used to diagnose the entry of magnetic flux in superconductors. First measurements on SRF samples were done with an external DC field applied perpendicular to the sample [1] (transverse geometry) with the muons applied to the sample face. Here, the results are strongly impacted by demagnetization, pinning strength and edge effects. A new spectrometer has been developed to allow sample testing with a field varying from 0 to 300 mT applied along the sample face (parallel geometry) analogous to RF fields in SRF resonators. The geometry is characterized by a small demagnetization factor reducing the impact of pinning and edge effects on field of first flux entry. The beamline installation and first results comparing transverse and parallel results will be presented.

Spin Depolarization of Muonium in Mesoporous Silica

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.