Publications (91)166.88 Total impact

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    ABSTRACT: We report measurements performed on a polycrystalline sample of the pyrochlore compound Nd2Sn2O7. It undergoes a second order magnetic phase transition at Tc ~ 0.91 K to a noncoplanar all-in-all-out magnetic structure of the Nd3+ magnetic moments. The thermal behavior of the low temperature specific heat fingerprints excitations with linear dispersion in a three-dimensional lattice. The temperature independent spin-lattice relaxation rate measured below Tc and the anomalously slow paramagnetic spin dynamics detected up to ~ 30 Tc are suggested to be due to magnetic short-range correlations in unidimensional spin clusters, i.e., spin loops. The observation of a spontaneous field in muon spin relaxation measurements is associated with the absence of a divergence-free field for the ground state of an all-in-all-out pyrochlore magnet as predicted recently.
    Full-text · Article · Oct 2015 · Physical Review B
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    ABSTRACT: We report a positive muon spin relaxation and rotation (\muSR) study of the quantum spin ice materials Yb2Ti2O7 and Yb2Sn2O7 focusing on the low field response. In agreement with earlier reports, data recorded in small longitudinal fields evidence anomalously slow spin dynamics in the microsecond range below the temperature T_c at which the specific heat displays an intense peak, namely T_c = 0.24 K and 0.15 K, respectively, for the two systems. We found that slow dynamics extends above T_c up to at least 0.7 K for both compounds. The conventional dynamical Gaussian Kubo-Toyabe model describes the \muSR spectra recorded above T_c. At lower temperatures a published analytical extension of the Gaussian Kubo-Toyabe model provides a good description, consistent with the existence of short-range magnetic correlations. While the physical response of the two systems is qualitatively the same, Yb2Ti2O7 exhibits a much larger local magnetic susceptibility than Yb2Sn2O7 below T_c. Considering previously reported ac susceptibility, neutron scattering and \muSR results, we suggest the existence of anomalously slow spin dynamics to be a common physical property of pyrochlore magnetic materials. The possibility of molecular spin substructures to be associated to the slow dynamics and therefore the short-range correlations is mentioned. The slow spin dynamics observed under field does not exclude the presence of much faster dynamics detected in extremely low or zero field.
    Full-text · Article · Sep 2015 · Physical Review B
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    ABSTRACT: We report Muon Spin Resonance spectra for Si0.06Ge0.94 that can be attributed to undissociated shallow muonium acceptors. This effective mass acceptor state is a paramagnetic center with the core consisting of either the negative charge state of muonium trapped in a tetrahedral interstitial void or a negatively charged Si-Mu complex. Two shallow acceptors are observed with differing hyperfine interactions and symmetries suggesting two preferential sites with different local chemical environments, implying that both states are present.
    No preview · Article · Sep 2014 · Applied Physics Letters
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    ABSTRACT: This work describes the development of a state-of-the-art muon spectrometer for the ISIS pulsed muon source. Conceived as a major upgrade of the highly successful EMU instrument, emphasis has been placed on making effective use of the enhanced flux now available at the ISIS source. This has been achieved both through the development of a highly segmented detector array and enhanced data acquisition electronics. The pulsed nature of the ISIS beam is particularly suited to the development of novel experiments involving external stimuli, and therefore the ability to sequence external equipment has been added to the acquisition system. Finally, the opportunity has also been taken to improve both the magnetic field and temperature range provided by the spectrometer, to better equip the instrument for running the future ISIS user programme.
    Preview · Article · Apr 2014 · Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
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    ABSTRACT: The current paper discusses possible designs for a stand alone muon target for MuSR studies of condensed matter science. Considering the ISIS 7 mm graphite target as a reference, Geant4 simulations have been performed in order to optimize the target parameters with respect to muon and pion yield. Previous studies suggested that the muon production can be optimized by using a thin graphite slab target with an incident proton energy significantly lower than initially considered. Surface muon production obtained by firing an 800 MeV proton beam energy onto the target is simulated and potential improvements to the target material, geometry and angle orientation with respect to the incoming proton beam as well as an estimated performance of the muon target are presented in this paper. Implications for the ISIS muon facility are also discussed. A comparison of the pion production cross section between experimental data and three theoretical models for the latest four Geant4 versions is also included in this paper.
    No preview · Article · Feb 2014 · Physical Review Special Topics - Accelerators and Beams
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    ABSTRACT: This paper marks the first 25 years of muon production at ISIS and the creation in that time of a facility dedicated to the use of these elementary particles as unique microscopic probes in condensed matter and molecular science. It introduces the basic techniques of muon spin rotation, relaxation and resonance, collectively known as μSR, that were already in use by specialist groups at other accelerator labs by the mid-1980s. It describes how these techniques have been implemented and made available at ISIS, beginning in 1987, and how they have evolved and improved since then. Ever widening applications embrace magnetism, superconductivity, interstitial diffusion and charge transport, semiconductors and dielectrics, chemical physics and radical chemistry. Over these first 25 years, a fully supported user facility has been established, open to all academic and industrial users. It presently comprises four scheduled instruments, optimized for different types of measurement, together with auxiliary equipment for radiofrequency or microwave spin manipulation and future plans for pump-probe laser excitation.
    No preview · Article · Dec 2013 · Physica Scripta
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    ABSTRACT: From magnetic, specific heat, ^{170}Yb Mössbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline Yb_{2}Sn_{2}O_{7}, we show that below the first order transition at 0.15 K all of the Yb^{3+} ions are long-range magnetically ordered and each has a moment of 1.1μ_{B} which lies at ≃10° to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [Phys. Rev. Lett. 108, 037202 (2012)] for a pyrochlore lattice of Kramers 1/2 effective spins.
    Full-text · Article · Mar 2013 · Physical Review Letters
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    ABSTRACT: In this paper we present the results of GEANT4 simulations of the production of surface muons as a function of energy of the incident protons on a graphite target. A validation of the GEANT4 hadronic physics models has been performed by comparing the results with experimental data from the Lawrence Radiation Laboratory, United States. Considering the ISIS muon target as a reference, simulations have been performed to optimize the pion and muon production. Of particular significance, we predict that optimal surface muon production occurs at a relatively modest proton energy of 500 MeV. This will be of importance for the development of future μSR facilities.
    Full-text · Article · Jan 2013 · Physical Review Special Topics - Accelerators and Beams
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    ABSTRACT: From magnetic, specific heat, 170Yb Mössbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline Yb2Sn2O7, we show that below the first order transition at 0.15 K all of the Yb3+ ions are long-range magnetically ordered and each has a moment of 1.1μB which lies at ≃10° to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [ Phys. Rev. Lett. 108 037202 (2012)] for a pyrochlore lattice of Kramers 1/2 effective spins.
    Full-text · Article · Jan 2013 · Physical Review Letters
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    ABSTRACT: ISIS is a world leading centre for research in the physical and life sciences at the Rutherford Appleton Laboratory. ISIS is used primarily for neutron production but it is also an intense source of pulsed muons. Muon target optimisation studies were performed with the Geant4 code as restrictions are imposed on target thickness, as this will affect the proton transmission to the second neutron target. In this paper we investigate the impact of incident proton energy on muon production and possible target geometries and target materials
    Full-text · Article · Dec 2012 · Physics Procedia
  • Adrian Hillier · Peter Baker · Philip King

    No preview · Article · Nov 2012 · Neutron News
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    ABSTRACT: Standard muon spin rotation (μSR) spectroscopy implants 4 MeV spin-polarized positive muons to investigate the bulk properties of matter. Success in producing epithermal muons opens interesting possibilities for studying ultrathin films, interfaces, and even surfaces. At the ISIS Facility, Rutherford Appleton Laboratory (Chilton, UK), we have produced a pulsed ultraslow muon beam (E< 20 eV) and have performed the first μSR experiments. Due to the pulsed feature, the implantation time is automatically determined and, by adjusting the final muon energy between about 8 and 20 eV, depth slicing experiments are possible down to monolayers distances. We describe slicing experiments across a 20 nm copper film on quartz substrate with evidence for a 2 nm copper oxide surface layer. A preliminary experiment on a hexagonal cobalt film suggests the existence of muon precession in the local magnetic field. The results are discussed in relation to the morphological features of the film.
    Full-text · Article · Apr 2012 · Applied Magnetic Resonance
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    ABSTRACT: Originally investigated as a candidate common data format, NeXus was an easy choice when ISIS developed the present PC-based acquisition system. After over ten years of use there was a clear need to revisit the original instrument definition to improve utility, and this work is discussed in the paper. Using NeXus has opened up the possibility of accessing analysis codes developed by the wider scientific community. The application of Mantid for analysingmuon data is particularly interesting, as this offers the muon community access to an analysis framework that is attracting broad international support. Recently, we have worked with the Mantid development team to program an interface for manipulatingmuon data that has confirmed the platform as an ideal tool for mu SR analysis.
    No preview · Article · Jan 2012 · Physics Procedia
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    ABSTRACT: The high magnetic field (HiFi) muon instrument at the ISIS pulsed neutron and muon source is a state-of-the-art spectrometer designed to provide applied magnetic fields up to 5 T for muon studies of condensed matter and molecular systems. The spectrometer is optimised for time-differential muon spin relaxation studies at a pulsed muon source. We describe the challenges involved in its design and construction, detailing, in particular, the magnet and detector performance. Commissioning experiments have been conducted and the results are presented to demonstrate the scientific capabilities of the new instrument.
    No preview · Article · Jul 2011 · Review of Scientific Instruments
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    ABSTRACT: In the pyrochlore Yb(2)GaSbO(7), the Yb(3+) sublattice forms a network of corner sharing tetrahedra and the second sublattice is made up of disordered, non-magnetic Ga(3+) and Sb(5+) ions. We have examined this compound using magnetic susceptibility, (170)Yb Mössbauer spectroscopy (down to 0.03 K) and muon spin relaxation (μSR) (down to 0.02 K) measurements. We establish the size of the Yb(3+) magnetic moments and that of the Yb(3+)-Yb(3+) coupling. At low temperatures, the correlated moments fluctuate between directions that are well tilted relative to the local [111] axis. The lattice disorder does not quench the frustration induced low temperature spin fluctuations but it does remove the first order dynamic transition that is present in the crystallographically ordered counterpart Yb(2)Ti(2)O(7). Below 1.0 K, the fluctuation rate of the correlated moments decreases progressively as the temperature is reduced and the moments remain dynamic down to 0.02 K where the rate is 7 × 10(7) s(-1). Magnetic frustration is operative in Yb(2)GaSbO(7) where the Yb(3+)-Yb(3+) interaction is antiferromagnetic as it is in Yb(2)Ti(2)O(7) where the interaction is ferromagnetic.
    No preview · Article · Apr 2011 · Journal of Physics Condensed Matter
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    Eran Amit · Amit Keren · James Lord · Philip King
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    ABSTRACT: A limiting factor in the ability to interpret isotope effect measurements in cuprates is the absence of sufficiently accurate data for the whole phase diagram; there is precise data for Tc, but not for the antiferromagnetic transition temperature T_N. Extreme sensitivity of T_N to small changes in the amount of oxygen in the sample is the major problem. This problem is solved here by using the novel compound (Ca 0.1 La 0.9)(Ba 1.65 La 0.35)Cu 3 O y for which there is a region where T_N is independent of oxygen doping. Meticulous measurements of T_N for samples with oxygen 16 and oxygen 18 find the absence of an oxygen isotope effect on T_N with unprecedented accuracy. A possible interpretation of our finding and existing data is that isotope substitution affects the normal state charge carrier density.
    Preview · Article · Feb 2011 · Advances in Condensed Matter Physics
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    ABSTRACT: We report Muonium (Mu) donor and acceptor levels in Czochralski-grown Silicon Germanium alloys (Cz-Si1−xGex). Measurement of these defect energies provides an analogous examination of Hydrogen defects that are otherwise inaccessible. Temperature-dependent Muonium fractions in several alloy samples (x=0.20,0.45,0.77,0.81,0.84,0.90,0.91,0.94,0.98) show charge-state transitions assigned to Mu donor and acceptor ionizations. Our results indicate a deep Mu donor level across the alloy system. The Mu acceptor level is deep in pure Si and valence-band resonant in pure Ge; we specifically examine the compositional dependence of the MuT0 acceptor ionization energy in Ge-rich alloys, where this level crosses into the valence band.
    No preview · Article · Nov 2010 · Physical review. B, Condensed matter
  • Philip King · Francis Pratt · James Lord · Iain McKenzie

    No preview · Article · Apr 2010 · Neutron News
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    A Bungau · R Cywinski · J Lord · P King · C Bungau

    Full-text · Article · Jan 2010
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    A Bungau · R Cywinski · J Lord · P King · C Bungau
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    ABSTRACT: GEANT4 provides an extensive set of alternative hadronic models. Simulations of the ISIS muon production using three such models applicable in the energy range of interest are presented in this paper and compared with the experimental data.
    Full-text · Article · Jan 2010