Browsing by Author "Kennedy, SJ"

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    Anomalous time-decay in neutron depolarization by high-Tc superconductors
    (Australian and New Zealand Institutes of Physics, 1994-11-09) Miles, PA; Kennedy, SJ; Taylor, KNR; Russell, GJ; Wang, J; Gu, GD; Takamara, K; Koshizuka, N
    In studies of the- field and time dependence of the polarisation state of a polarised neutron beam transmitted through single crystals of YBa2Cu- (YBCO) and BI2Sr2CaCu202-6 (BSCCO), we have observed some unusual behaviour for temperatures > 60*K for YBCO and 30>T>40K. The observed results suggest that at these temperatures, after removing an external magnetic field, the trapped flux density seen by the neutrons is essentially zero for all time less than = 200msecs. With increasing tune, the sampled flux density increases in magnitude, reaching the values observed in long time (20secs) measurements only after a delay of several seconds. At this time, the trapped flux density reaches a maximum and subsequently decays logarithmically at a rate determined by the effective pinning energy U(H,T). Possible mechanisms responsible for the inverse flux decay are considered. Large anisotropy in this behaviour is observed with the effect not being observed at all with Hlab for BSCCO and no marked temperature onset with YBCO with Hlab. For both YBCO and BSCCO the Hjfc observed flux decay show this behaviour. We conclude that the observed results follow, not from a change in the magnitude of the trapped flux, but rather from a change in the flux distribution within the sample. We interpret this in terms of a domain structure in the flux lattice system which is associated with the fringing closure flux which remains once the external applied field is removed.
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    A comparative study of magnetic behaviors in TbNi2, TbMn2 and TbNi2Mn
    (AIP Publishing LLC., 2014-01-01) Wang, JL; Din, MFM; Kennedy, SJ; Hong, F; Campbell, SJ; Studer, AJ; Wu, GH; Cheng, ZX; Dou, SX
    All TbNi2, TbMn2, and TbNi2Mn compounds exhibit the cubic Laves phase with AB2-type structure in spite of the fact that the ratio of the Tb to transition-metal components in TbNi2Mn is 1:3. Rietveld refinement indicates that in TbNi2Mn the Mn atoms are distributed on both the A (8a) and B (16d) sites. The values of the lattice constants were measured to be a¼14.348A ° (space group F-43 m), 7.618A ° , and 7.158A ° (space group Fd-3 m) for TbNi2, TbMn2, and TbNi2Mn, respectively. The magnetic transition temperatures TC were found to be TC¼38K and TC¼148K for TbNi2 and TbNi2Mn, respectively, while two magnetic phase transitions are detected for TbMn2 at T1¼20K and T2¼49 K. Clear magnetic history effects in a low magnetic field are observed in TbMn2 and TbNi2Mn. The magnetic entropy changes have been obtained. © 2014 AIP Publishing LLC.
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    Construction of the neutron beam facility at Australia's OPAL research reactor
    (Elsevier B. V., 2006-11-15) Kennedy, SJ
    Australia's new research reactor, OPAL, has been designed principally for neutron beam science and radioisotope production. It has a capacity for 18 neutron beam instruments, located at the reactor face and in a neutron guide hall. The neutron beam facility features a 20 l liquid deuterium cold neutron source and cold and thermal supermirror neutron guides. Nine neutron beam instruments are under development, of which seven are scheduled for completion in early 2007. The project is approaching the hot-commissioning stage, when criticality will be demonstrated. Installation of the neutron beam transport system and neutron beam instruments in the neutron guide hall and at the reactor face is underway, and the path to completion of this project is relatively clear. This paper will outline the key features of the OPAL reactor, and will describe the neutron beam facility in particular. The status of the construction and a forecast of the program to completion, including commissioning and commencement of routine operation in 2007 will also be discussed. © 2006 Elsevier B.V.
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    Construction of the neutron beam facility at Australia's OPAL Research Reactor
    (The Bragg Institute, Australian Nuclear Science and Technology Organisation, 2005-11-27) Kennedy, SJ
    Australia's new research reactor, OPAL, has been designed for high quality neutron beam science and radioisotope production. It has a capacity for eighteen neutron beam instruments to be located at the reactor face and in a neutron guide hall. The new neutron beam facility features a 20 litre liquid deuterium cold neutron source and supermirror neutron reflecting guides for intense cold and thermal neutron beams. Nine neutron beam instruments are under development, of which seven are scheduled for completion in early 2007. The project is approaching the hot-commissioning stage, where criticality will be demonstrated. Installation of the neutron beam transport system and neutron beam instruments in the neutron guide hall and at the reactor face is underway, and the path to completion of this project is relatively clear. The lecture will outline Australia's aspirations for neutron science at the OPAL reactor, and describe the neutron beam facility under construction. The status of this project and a forecast of the program to completion, including commissioning and commencement of routine operation in 2007 will also be discussed. This project is the culmination of almost a decade of effort. We now eagerly anticipate catapulting Australia's neutron beam science capability to meet the best in the world today. © The Author
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    Correction of optical aberrations in elliptic neutron guides
    (Elsevier Science BV, 2012-11-21) Bentley, PM; Kennedy, SJ; Andersen, KH; Rodriguez, DM; Mildner, DFR
    Modern, nonlinear ballistic neutron guides are an attractive concept in neutron beam delivery and instrumentation because they offer increased performance over straight or linearly tapered guides. However, like other ballistic geometries they have the potential to create significantly non-trivial instrumental resolution functions. We address the source of the most prominent optical aberration, namely coma, and we show that for extended sources the off-axis rays have a different focal length from on-axis rays, leading to multiple reflections in the guide system. We illustrate how the interplay between coma, sources of finite size, and mirrors with non-perfect reflectivity can therefore conspire to produce uneven distributions in the neutron beam divergence, a source of complicated resolution functions. To solve these problems, we propose a hybrid elliptic-parabolic guide geometry. Using this new kind of neutron guide shape, it is possible to condition the neutron beam and remove almost all of the aberrations, whilst providing the same performance in beam current as a standard elliptic neutron guide. We highlight the positive implications for neutron scattering instruments that this new shape can bring. © 2012, Elsevier Ltd.
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    Critical behaviour of Ho2Fe17-xMnx - magnetisation and Mössbauer spectroscopy
    (Springer, 2012-11-20) Wang, JL; Campbell, SJ; Kennedy, SJ; Dou, SX
    The magnetic properties of Ho2Fe17 − xMnx compounds (x = 0–2) of ferromagnetic ordering temperatures up to TC ~344 K have been investigated by DC magnetization and Mössbauer effect measurements. The nature of the magnetic phase transitions and the critical behaviour around TC has been investigated by analysis of the magnetisation data and the critical exponents β, γ and δ determined. The critical exponents are found to be similar to the theoretical values of the mean-field model for which β = 0.5 and γ = 1.0, indicating the existence of a long-range ferromagnetic interactions. The isothermal entropy changes ΔS around TC have been determined as a function of temperature in different magnetic fields. © 2012, Springer
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    Critical magnetic transition in TbNi2Mn-magnetization and Mössbauer spectroscopy
    (IOP Publishing LTD, 2011-06-01) Wang, JL; Campbell, SJ; Kennedy, SJ; Zeng, R; Dou, SX; Wu, GH
    The structural and magnetic properties of the TbNi2Mnx series (0.9 ≤ x ≤ 1.10) have been investigated using x-ray diffraction, field- and temperature-dependent AC magnetic susceptibility, DC magnetization (5–340 K; 0–5 T) and 57Fe Mössbauer spectroscopy (5–300 K). TbNi2Mnx crystallizes in the MgCu2-type structure (space group Fd\bar {3}m ). The additional contributions to the magnetic energy terms from transition-metal–transition-metal interactions (T–T) and rare-earth–transition-metal interactions (R–T) in RNi2Mn compounds contribute to their increased magnetic ordering temperatures compared with RNi2 and RMn2. Both the lattice constant a and the Curie temperature TC exhibit maximal values at the x = 1 composition indicating strong magnetostructural coupling. Analyses of the AC magnetic susceptibility and DC magnetization data of TbNi2Mn around the Curie temperature TC = 147 K confirm that the magnetic transition is second order with critical exponents β = 0.77 ± 0.12, γ = 1.09 ± 0.07 and δ = 2.51 ± 0.06. These exponents establish that the magnetic interactions in TbNi2Mn are long range despite mixed occupancies of Tb and Mn atoms at the 8a site and vacancies. The magnetic entropy − ΔSM around TC is proportional to (μ0H/TC)2/3 in agreement with the critical magnetic analyses. The Mössbauer spectra above TC are fitted by two sub-spectra in agreement with refinement of the x-ray data while below TC three sub-spectra are required to represent the three inequivalent local magnetic environments.(c) 2011 IOP Publishing LTD
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    Determination of the deuterium order-disorder structure transition in Pd-DO.65 at '50K'
    (Australian and New Zealand Institutes of Physics, 1994-02-09) Kennedy, SJ; Wu, E; Krisi, EH; Gray, EM; Kennedy, BJ
    Anomalies in the heat capacity and the electrical resistance of Palladium Hydride at ~50K were first reported in the 1950's(l). Since then a number of experimental techniques have been applied in attempts to determine the origin of these anomalies. These techniques have included XRD and neutron diffraction. All crystallographic studies have been hampered by the sluggishness of this transition (i.e. it takes several days to complete). Early powder diffraction results suggested a phase transition involving migration of H (or D) atoms from sites of octahedral symmetry to sites of tetrahedral symmetry(2). Later evidence from single crystal neutron diffraction studies indicated an order-disorder transition within the octahedral sites(3). Neither description has found global support because conclusive evidence has not been produced. We have just completed a time resolved powder neutron diffraction study of the structural transition in b-Pd-DO 6 5 at S4K using the new Medium Resolution Neutron Powder Diffractometer (MRPD) at the HIFAR research reactor. Our results show that the latter description of an order-disorder transition of the octahedral sites is correct. The 'ordered' crystal structure can be accurately described with space group Pm3n and by doubling the lattice constant from the high temperature structure (Fm3m). The transition to the ordered phase involves diffusion from specific 'D' sites from nearest 'D' neighbour positions to second nearest 'D' neighbour positions. This eventually produces 97% and 33% occupancy of the favoured and unfavoured sites respectively.
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    Diffuse neutron scattering studies of single crystal specimens of Fe3-x MnxSi
    (Australian and New Zealand Institutes of Physics, 1994-02-09) Ersez, T; Kennedy, SJ; Hicks, TJ; Kepa, H
    Polarized neutron diffuse scattering measurements have been made of two single crystal specimens of the ordered intermetallic compounds Fe2.45Mno.55Si (at room temperature and Fe2MnSi (above and below the re-ordering temperature). The difference between spin-flip and non-spin-flip scattering cross-sections for the (111) position gave a negative diffuse peak around a positive Bragg peak, being more pronounced for the Fe2.45Mno.55Si crystal. This is consistent with a loss of moment on the B(Fe/Mn) site with addition of Mn. Preliminary model fitting has suggested that Mn atoms cluster on their preferred B-site.
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    Driving magnetostructural transitions in layered intermetallic compounds
    (American Physical Society, 2013-05-23) Wang, JL; Caron, L; Campbell, SJ; Kennedy, SJ; Hofmann, M; Cheng, ZX; Din, MFM; Studer, AJ; Brück, E; Dou, SX
    We report the dramatic effect of applied pressure and magnetic field on the layered intermetallic compound Pr0.5Y0.5Mn2Ge2. In the absence of pressure or magnetic field this compound displays interplanar ferromagnetism at room temperature and undergoes an isostructural first order magnetic transition (FOMT) to an antiferromagnetic state below 158 K, followed by another FOMT at 50 K due to the reemergence of ferromagnetism as praseodymium orders (T-C(Pr)). The application of a magnetic field drives these two transitions towards each other, whereas the application of pressure drives them apart. Pressure also produces a giant magnetocaloric effect such that a threefold increase of the entropy change associated with the lower FOMT (at T-C(Pr)) is seen under a pressure of 7.5 kbar. First principles calculations, using density functional theory, show that this remarkable magnetic behavior derives from the strong magnetoelastic coupling of the manganese layers in this compound. © 2013, American Physical Society.
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    Echidna—the new high-resolution powder diffractometer being built at OPAL
    (Elsevier B. V., 2006-11-15) Liss, KD; Hunter, BA; Hagen, ME; Noakes, TJ; Kennedy, SJ
    A new powder diffractometer aiming for high angular, and thus high reciprocal space, resolution is being constructed within the Neutron Beam Instrumentation Project at the upcoming Australian Neutron Source OPAL, near Sydney. The neutron flux at the sample can be expected to be up to 107 n/cm2/s. With an array of 128 position sensitive detectors, each equipped with a 30 cm high Söller collimator of 5 arc min acceptance this instrument will have one of the highest performances of its kind. In addition to classical applications in powder diffraction, the quasi two-dimensional detector will be used for rapid texture measurements, where high separation of peaks is necessary. Even single crystal reciprocal space mapping is envisaged. The article compiles an overview of the design, status of the project and potential research activities. Crown Copyright © 2006 Published by Elsevier B.V
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    Echidna—the new high-resolution powder diffractometer being built at OPAL
    (The Bragg Institute, Australian Nuclear Science and Technology Organisation, 2005-11-27) Liss, KD; Hunter, BA; Hagen, ME; Noakes, TJ; Kennedy, SJ
    A new powder diffractometer, named Echidna, aiming for high angular and thus high reciprocal space resolution is being constructed within the Neutron Beam Instrumentation Project at the upcoming Australian Neutron Source OPAL near Sydney. The neutron flux at the sample can be expected to be more than 107 n/s. With an array of 128 position sensitive detectors, each equipped with a 30 cm high Soeller collimator of 5 arc min acceptance this instrument will have one of the highest performances of its kind. In addition to classical applications in powder diffraction, the quasi two-dimensional detector will be used for rapid texture measurements where high separation of peaks is necessary. Even single crystal reciprocal space mapping will be envisaged. The poster compiles an overview of the design, status of the project and potential research to be achieved. © The Authors
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    Effects of Cr substitution on structural and magnetic properties in La0.7Pr0.3Fe11.4Si1.6 compound
    (AIP Scitation, 2014-04-03) Din, MFM; Wang, JL; Studer, AJ; Gu, QF; Zeng, R; Debnath, JC; Shamba, P; Kennedy, SJ; Dou, SX
    In an effort to explore the effect of substitution Fe by Cr in NaZn13-type La0.7Pr0.3Fe11.4−xCrxSi1.6 (x = 0, 0.06, 0.12, 0.26, and 0.34) compounds, the structure and magnetic properties have been investigated by high intensity of x-ray and neutron diffraction, scanning electron microscopy, specific heat, and magnetization measurement. It has been found that a substitution of Cr for Fe in this compounds leads to decrease in the lattice parameter a at room temperature but variation on Curie temperature (TC). While the first order nature of magnetic phase transition around TC does not change with increasing Cr content up to x = 0.34. High intensity x-ray and neutron diffraction study at variable temperatures for highest Cr concentration x = 0.34 confirmed the presence of strong magneto-volume effect around TC and indicated the direct evident of coexistence across magnetic transition as characteristic of first order nature. The values of −ΔSM around TC decrease from 17 J kg−1 K−1 for x = 0 to 12 J kg−1 K−1 for x = 0.06 and then increases with further increasing Cr content up to 17.5 J kg−1 K−1 for x = 0.34 under a change of 0–5 T magnetic field. Similar behavior on relative cooling power which is decrease from 390 J kg−1 for x = 0 to 365 J kg−1 for x = 0.06 at the beginning and then increases up to 400 J kg−1 for x = 0.34. From the point of this view with the highest Cr concentration (x = 0.34) exhibits favourable material candidate for magnetic refrigerator application therefore should inspire further study concerning on higher Cr concentration in this compound. © 2014, AIP Publishing LLC.
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    The elusive magnetic structure of FePS3
    (Australian Institute of Physics, 2003-02-04) Rule, KC; McIntyre, GJ; Kennedy, SJ; Hicks, TJ
    Laue diffraction patterns from a single crystal of FePS3 were recorded at temperatures above and below the Néel temperature TN = 120 K on the new thermal Laue diffractometer VIVALDI at the Institut Laue Langevin. Magnetic peaks were weaker and more extended than the nuclear peaks. The directions in reciprocal space of the magnetic reflections were found by reference to the nuclear peaks, and the magnetic reflections could then be readily located on the monochromatic diffractometer D19. The strongest magnetic peaks were found at 0.5, -0.5, 0.34 and 1.5, -0.5, 0.34 and symmetry related positions.
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    Evolution of the neutron-scattering capability on the OPAL reactor at ANSTO
    (Taylor & Francis Online, 2016-04-29) Klose, F; Constantine, P; Kennedy, SJ; Schulz, JC; Robinson, RA; Holden, PJ; McIntyre, GJ
    Australia is currently in the very privileged position of having the world's newest, fully operating research reactor, OPAL. As a consequence, the suite of neutron-beam instruments is also amongst the youngest and most advanced in the world, with full advantage taken in their construction of lessons learned at reactors elsewhere to develop state-of-the-art instruments that are best suited to the local and regional user communities. There are two thermal-neutron beam ports, two cold-neutron beam ports, and two (future) hot-neutron beam ports around the OPAL reactor core (see Fig. 3 in [1] and [2]). One each of the thermal-neutron and cold-neutron ports feed into a set of three guide bundles that serve the present (first) guide hall. In the initial construction phase, completed in 2007, only the outer two (TG1 and TG3) of the thermal guides, and the outer two (CG1 and CG3) of the cold guides were installed. The other thermal-neutron (TG4) and cold-neutron (CG4) guides are relatively short, to serve single instruments within the reactor beam hall with the highest flux but without eliminating line-of-sight [2]. Table 1 lists the current suite of instruments at the OPAL reactor, with brief technical details and the principal features. Figure 1 shows the layout of the current suite, with the anticipated location of the next instrument, BioRef (vide infra), indicated. We now briefly outline the evolution of the suite. © 2021 Informa UK Limited
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    Ground state of (Pb0.94Sr0.06)(Zr0.530Ti0.470)O3 in the morphotropic phase boundary region: evidence for a monoclinic Cc space group
    (American Physical Society, 2011-10-24) Solanki, RS; Singh, AK; Mishra, SK; Kennedy, SJ; Suzuki, T; Kuroiwa, Y; Moriyoshi, C; Pandey, D
    The antiferrodistortive (AFD) phase transition for a pseudotetragonal composition of Pb(Zr(0.530)Ti(0.470))O(3) doped with 6% Sr has been investigated using sound velocity (4-320 K), high-resolution synchrotron x-ray powder diffraction (100-800 K), and high-resolution as well as high-flux neutron powder diffraction measurements (4 K) to settle the existing controversies about the true ground state of Pb(Zr(x)Ti(1-x))O(3) (PZT) in the morphotropic phase boundary (MPB) region. The multiplet character of the neutron diffraction profiles of (3/2 1/2 1/2)(pc) (pseudocubic or pc indices) and (3/2 3/2 1/2)(pc) superlattice peaks, appearing below the AFD transition temperature, rules out the rhombohedral R3c space group. The true ground state is confirmed to be monoclinic in the Cc space group, in agreement with the predictions of the first-principles calculations and earlier findings for pure PZT in the MPB region. Here, 6% Sr(2+) substitution and the use of high-wavelength (lambda = 2.44 angstrom) neutrons have played a key role in settling the existing controversies about the true ground state of PZT in the MPB region. © 2011, American Physical Society
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    High luminosity time of flight with polarisation analysis: CeCu6
    (Elsevier, 2007-07-15) Harker, SJ; Hicks, TJ; Kennedy, SJ; Yu, DH
    The magnetic excitation spectrum of the heavy fermion system CeCu6 has been obtained by pseudo-randomly chopping the polarisation of the beam on LONGPOL and measuring the time of flight with polarisation analysis before the detectors. Separation of the magnetic part of the spectrum was made by changing the polarisation direction at the specimen. This method achieves a greater luminosity at the specimen compared with conventional methods at the expense of a background proportional to the total scattering from the specimen. The form of the spectrum is as expected with two transitions and considerable inelastic intensity centred oil zero energy transfer. The crystal field spectrum of the Ce3+ ion is considerably broadened by the Kondo nature of the system. An analysis of the experimental accuracy was made based on the statistical variation between the runs which make up the experiment. LONGPOL is a very slow instrument, based as it is on a thermal source with an incident wavelength of 3.6 A. The demonstrated possibility to obtain a spectrum incorporating polarisation analysis provides a basis to evaluate the usefulness of future implementation. © 2007, Elsevier Ltd.
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    High-temperature thermal stability of Ti2AlN and Ti4AlN3: a comparative diffraction study
    (Elsevier, 2011-01) Low, IM; Pang, WK; Kennedy, SJ; Smith, RI
    The susceptibility of Ti2AlN and Ti4AlN3 to high-temperature thermal dissociation in a dynamic environment of high-vacuum has been investigated using in situ neutron diffraction. Under high vacuum, these ternary nitrides decomposed above 1400°C through the sublimation of Al, and possibly Ti, to form a surface coating of TiNx (0.5 ≤ x ≤ 0.75). The kinetics of isothermal phase decomposition were modelled using the Avrami equation and the Avrami exponents (n) of isothermal decomposition of Ti2AlN and Ti4AlN3 were determined to be 0.62 and 0.18, respectively. The characteristics of thermal stability and phase transitions in Ti2AlN and Ti4AlN3 are compared in terms of the rate of decomposition, phase relations and microstructures. © 2010, Elsevier Ltd.
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    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)
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    In situ diffraction study on decomposition of Ti2AlN at 1500–1800°C in vacuum
    (Elsevier, 2010-11-25) Pang, WK; Low, IM; Kennedy, SJ; Smith, RI
    The susceptibility of Ti2AlN to thermal dissociation at 1500–1800°C in high-vacuum has been studied using in situ neutron diffraction. Above 1500°C, Ti2AlN decomposed to TiNx primarily through the sublimation of aluminium (Al). The kinetics of isothermal phase decomposition at 1550°C was modelled using a modified Avrami equation. An Avrami exponent (n) of 0.338 was determined, indicative of the highly restricted out-diffusion of Al between the channels of Ti6N octahedra. The characteristics of thermal stability and phase transition in Ti2AlN are discussed. Characterisation of surface compositions and examination of cross-sectional microstructures of decomposed Ti2AlN, using synchrotron radiation diffraction and SEM, respectively, verify the findings of the neutron diffraction; that Ti2AlN decomposes to TiNx at the surface primarily via the sublimation of Al from grain boundaries. © 2010, Elsevier Ltd.