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Browsing by Author "Gardner, JS"

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Chemical pressure effects on crystal and magnetic structures of bilayer manganites PrA2Mn2O7 (A = Sr or Ca)
(AIP Publishing, 2016-06-03) Deng, G; Sheptyakov, D; Pomjakushin, V; Medarde, M; Pomjakushina, E; Conder, K; Kenzelmann, M; Studer, AJ; Gardner, JS; McIntyre, GJ
The crystal and magnetic structures of the bilayer manganites PrSr2Mn2O7 (PSMO) and PrCa2Mn2O7 (PCMO) have been studied by neutron powder diffraction. It was found that PSMO crystallizes in space group I4/mmm, while PCMO adopts space group Cmc21 at room temperature. The difference in the structure arises from chemical pressure induced by the Ca substitution for Sr on the A sites, which causes different Jahn-Teller distortions. In PSMO, the MnO6 octahedra suffer a small elongated distortion, while those in PCMO adopt strong compressed distortion along the axial direction. In addition, the octahedra in PCMO show a+b0c0 rotation and a0b+c+ tilting in the Glazer notation in comparison to PSMO. As a result, these two compounds adopt very different magnetic structures: The magnetic structure of PSMO is an A-type magnetic structure (Im'm'm) with propagation vector k = (0, 0, 1) and magnetic moments in the ab plane. In contrast, a C-type antiferromagnetic magnetic structure (Cm'c2′1) with the multiple propagation vectors (k = (0, 12, 12) and (0, 12, 0)) and magnetic moments mainly along the b axis is found in PCMO. The critical exponent of the magnetic phase transition is around 0.345 for PSMO and 0.235 for PCMO, indicating 3D and 2D XY transitions, respectively. The strong Jahn-Teller distortion induced by the chemical pressure is believed to suppress the double exchange and favour super-exchange in PCMO, leading to the dramatic difference in the magnetic structure. © 2016 Author(s). Published by AIP Publishing.
The cold-neutron triple-axis spectrometer SIKA at OPAL
(Australian Institute of Physics, 2018-01-31) Deng, G; Yano, SI; Wu, CM; Peng, JC; Gardner, JS; Imamovic, E; Vorderwisch, P; Li, WH; McIntyre, GJ
SIKA is a high-flux cold-neutron triple-axis spectrometer funded by Ministry of Science and Technology of Taiwan and currently being operated by National Synchrotron Radiation Research Center. It is located on the OPAL reactor face at the Australian Nuclear Science and Technology Organization (ANSTO). Its incident energy ranges from 2.6meV to 30meV with the highest flux at ~8meV. SIKA is equipped with a multiplexing analyzer consisting of an array of 13 PG crystal blades, a multi-wire detector, a single detector and a diffraction detector. The most frequently-used single-detector mode and the multi-Q constant-Ef mode are demonstrated by using the standard samples, namely, MnF2 and Pb single crystals, respectively. The spin-wave excitation of MnF2, the phonon dispersion of thermoelectric material SeSn, the spin dynamics of the spin-glass system (Ni0.40Mn0.60)TiO3, and other experimental data from SIKA are demonstrated as examples of SIKA’s capabilities and performance. The spin-wave excitation was observed in the quasi-one-dimensional spinladder compound SrCa13Cu24O41, indicating the low background of SIKA. These results indicate that SIKA is a highly-flexible cold triple-axis spectrometer with reasonably low background.
The cold-neutron triple-axis spectrometer SIKA at OPAL
(International Conference on Neutron Scattering, 2017-07-12) Deng, G; Yano, S; Wu, CM; Peng, JC; Imamovic, E; Vorderwisch, P; Li, WH; Gardner, JS
SIKA is a high-flux cold-neutron triple-axis spectrometer built on the cold source CG4 of the 20MW Open Pool Australian Light-water (OPAL) reactor at Australian Nuclear Science and Technology Organization (ANSTO) by Taiwan.[1] As a state-of-the-art triple-axis spectrometer, SIKA is equipped with a large double-focusing pyrolytic graphite (PG) monochromator, a multiblade PG analyser and a multi-detector system. The design, functions, and capabilities of SIKA are presented. The spin wave excitation of MnF2, the phonon dispersion of thermoelectric material SeSn,[2] the spin dynamics of spin glass system (Ni0.40Mn0.60)TiO3[3] and other experimental data from SIKA are demonstrated as examples of SIKA’s capabilities and performance.
Dynamics of linarite: observations of magnetic excitations
(American Physical Society, 2017-01-26) Rule, KC; Willenberg, B; Schäpers, M; Wolter, AUB; Büchner, B; Drechsler, SL; Ehlers, G; Tennant, DA; Mole, RA; Gardner, JS; Süllow, S; Nishimoto, S
Here we present inelastic neutron scattering measurements from the frustrated, quantum spin-1/2 chain material linarite, PbCuSO4(OH)2. Time of flight data, taken at 0.5 K and zero applied magnetic field reveals low-energy dispersive spin wave excitations below 1.5 meV both parallel and perpendicular to the Cu-chain direction. From this we confirm that the interchain couplings within linarite are around 10% of the nearest neighbor intrachain interactions. We analyze the data within both linear spin-wave theory and density matrix renormalization group theories and establish the main magnetic exchange interactions and the simplest realistic Hamiltonian for this material. ©2017 American Physical Society.
Low energy spin dynamics in the spin ice Ho2Sn2O7
(Intitite of Physics Publishing Ltd, 2012-02-22) Ehlers, G; Huq, A; Diallo, SO; Adriano, C; Rule, KC; Cornelius, AL; Fouquet, P; Pagliuso, PG; Gardner, JS
The magnetic properties of Ho 2 Sn 2 O 7 have been investigated and compared to other spin ice compounds. Although the lattice has expanded by 3% relative to the better studied Ho 2 Ti 2 O 7 spin ice, no significant changes were observed in the high temperature properties, T greater than or similar to 20 K. As the temperature is lowered and correlations develop, Ho 2 Sn 2 O 7 enters its quantum phase at a slightly higher temperature than Ho 2 Ti 2 O 7 and is more antiferromagnetic in character. Below 80 K a weak inelastic mode associated with the holmium nuclear spin system has been measured. The hyperfine field at the holmium nucleus was found to be ≈700 T. (c) 2012 IOP Publishing LTD
Magnetic correlations in the intermetallic antiferromagnet Nd3Co4Sn13
(IOP Publishing, 2017-09-21) Wang, CW; Lin, JW; Lue, CS; Liu, HF; Kuo, CN; Mole, RA; Gardner, JS
Specific heat, magnetic susceptibility, and neutron scattering have been used to investigate the nature of the spin system in the antiferromagnet Nd3Co4Sn13. At room temperature Nd3Co4Sn13 has a cubic, Pm-3n structure similar to Yb3Rh4Sn13. Antiferromagnetic interactions between, Nd3+ ions dominate the magnetic character of this sample and at 2.4 K the Nd spins enter a long range order state with a magnetic propagation vector q = (0 0 0) with an ordered moment of 1.78(2) μB at 1.5 K. The magnetic Bragg intensity grows very slowly below 1 K, reaching ∼2.4 μB at 350 mK. The average magnetic Nd3+ configuration corresponds to the 3D irreducible representation F7. This magnetic structure can be viewed as three sublattices of antiferromagnetic spin chains coupled with each other in the 120°-configuration. A well-defined magnetic excitation was measured around the 1 1 1 zone centre and the resulting dispersion curve is appropriate for an antiferromagnet with a gap of 0.20(1) meV. © 2017 IOP Publishing Ltd.
Magnetic structure and spin dynamics of multiferroic system Co4Nb2O9
(Australian Institute of Physics, 2017-02-01) Deng, GC; Cao, YM; Ren, W; Cao, SX; Gauthier, N; Kenzelmann, M; Studer, AJ; Rule, KC; Gardner, JS; Davison, G; Imperia, P; McIntyre, GJ
Co4Nb2O9, was recently reported to have large magneto-dielectric coupling effect under a certain magnetic field. This compound has a corundum-type crystal structure of space group P-3c1 of ref. and undergoes antiferromagnetic phase transition around 27 K. It was previously believed that the magnetic moments of Co2+ order into a collinear antiferromagnetic structure in which Co2+ spins order parallel to the c-direction and form ferromagnetic chains with antiparallel inter-chain coupling. However, the recent study has shown that this magnetic structure model is incorrect. In this study, we found that the Co2+ magnetic moments align in the ab plane with a non-collinear configuration. Using inelastic neutron scattering, we measured the spin wave excitation from its magnetic phase along (h00) and (00l). A magnetic model was proposed to explain the observed spin dynamical behavior. There are two inequivalent Co sites, which form spin chains in an alternative way along c axis. Each Co2+ moment couples with its two inequivalent neighbors on the same chain with ferromagnetic interactions. Co2+ moments from each site form a zig-zag hexagonal ring perpendicular to the c axis, where antiferromagnetic interactions dominate. On the basis of this model, the observed spin wave spectra can be well simulated by SpinW.
Origin of magnetoelectric coupling effect and spin dynamics of multiferroic system Co4Nb2O9
(International Conference on Neutron Scattering, 2017-07-12) Deng, GC; Cao, YM; Ren, W; Cao, SX; Studer, AJ; Gauthier, N; Kenzelmann, M; Davison, G; Rule, KC; Gardner, JS; Imperia, P; Ulrich, C; McIntyre, GJ
Co4Nb2O9,was recently reported to have large magnetoelectric coupling effect under a certain magnetic field. This compound has a crystal structure (space group P-3c1) derived from corundum structure and undergoes antiferromagnetic phase transition around 27K. It was previously believed that the magnetic moments of Co2+ order into a collinear antiferromagnetic structure in which magnetic moments are parallel to the c axis and form ferromagnetic chains with antiparallel inter-chain coupling. However, the recent study has shown that this magnetic structure model is incorrect. In this study, we found that the Co2+magnetic moments on both Co1 and Co2 sites align in the ab plane with a non-collinear configuration. Using inelastic neutron scattering, we measured the spin wave excitation from its magnetic phase along (h00) and (00l). A spin dynamic model proposed in this study is able to explain the observed spin dynamical behavior quite well. The nearest and next nearest neighbor interactions (NN and NNN) along the c axis are ferromagnetic. The interaction on the zig-zag ring of Co1 perpendicular to the c axis is highly frustrated while that of the zig-zag ring of Co2 is antiferromagnetic. The single ion anisotropy and Dzyaloshinskii-Moriya (DM) interaction contribute to the spin dynamics of Co4Nb2O9 as well. The simulated spin wave excitation by using SpinW[5] matches the experimental data very well. The DM interaction, which is most probably due to the triangle Co2-O-Co2 bond, was found to be the origin of the magnetoelectric coupling in this compound.
SIKA - the cold-neutron triple-axis spectrometer with multiplexing analyzer at Bragg Institute
(Asia - Oceania Neutron Scattering Association, 2015-07-19) Deng, GC; McIntyre, GJ; Wu, CM; Gardner, JS; Vorderwisch, P; Li, WH
SIKA is a high-flux cold-neutron triple-axis spectrometer funded by Ministry of Science and Technology of Taiwan and currently being operated by National Synchrotron Radiation Research Center. It locates at the OPAL reactor face at the Australian Nuclear Science and Technology Organisation (ANSTO). Its incident energy ranges from 2.5meV to 30meV with the highest flux at ~8meV. With an advanced design, SIKA is equipped with an analyzer array of 13 PG(002) blades (Fig. 1), a multi-wire detector, and a separate diffraction detector. Such a design allows SIKA to run in a traditional step-by-step mode or in various mapping (or dispersive) modes by changing the configuration of analyzers and detectors. Several typical mapping modes are analyzed and simulated using Monte Carlo ray-tracing package SIMRES of RESTRAX. [1] The performance of different mapping modes are demonstrated and evaluated, providing the dispersion relations of these operation modes as references for experimental studies. In hotcommissioning, a multiplexing mode with constant Ef was used to measure the phonon dispersion in a Pb single crystal. The simulation and experiment results demonstrate the flexibility and fast data-collecting potential of SIKA as a next generation cold neutron triple-axis spectrometer.
Spin dynamics and magnetoelectric coupling mechanism of Co4Nb2O9
(American Physical Society, 2018-02-28) Deng, GC; Cao, YM; Ren, W; Cao, SX; Studer, AJ; Gauthier, N; Kenzelmann, M; Davidson, G; Rule, KC; Gardner, JS; Imperia, P; Ulrich, C; McIntyre, GJ
Neutron powder diffraction experiments reveal that Co4Nb2O9 forms a noncollinear in-plane magnetic structure with Co2+ moments lying in the ab plane. The spin-wave excitations of this magnet were measured by using inelastic neutron scattering and soundly simulated by a dynamic model involving nearest- and next-nearest-neighbor exchange interactions, in-plane anisotropy, and the Dzyaloshinskii-Moriya interaction. The in-plane magnetic structure of Co4Nb2O9 is attributed to the large in-plane anisotropy, while the noncollinearity of the spin configuration is attributed to the Dzyaloshinskii-Moriya interaction. The high magnetoelectric coupling effect of Co4Nb2O9 in fields can be explained by its special in-plane magnetic structure. ©2018 American Physical Society
Spin dynamics of edge-sharing spin chains in SrCa13Cu24O41
(American Physical Society, 2018-11-12) Deng, GC; Yu, DH; Mole, RA; Pomjakushina, E; Conder, K; Kenzelmann, M; Yano, SI; Wang, CW; Rule, KC; Gardner, JS; Luo, HQ; Li, S; Ulrich, C; Imperia, P; Ren, W; Cao, SX; McIntyre, GJ
The low-energy magnetic excitation from the highly Ca-doped quasi-one-dimensional magnet SrCa13Cu24O41 was studied in the magnetic ordered state by using inelastic neutron scattering. We observed the gapless spin-wave excitation, dispersive along the a and c axes but nondispersive along the b axis. Such excitations are attributed to the spin wave from the spin-chain sublattice. Model fitting to the experimental data gives the nearest-neighbor interaction Jc as 5.4 meV and the interchain interaction Ja=4.4meV. Jc is antiferromagnetic and its value is close to the nearest-neighbor interactions of the similar edge-sharing spin-chain systems such as CuGeO3. Comparing with the hole-doped spin chains in Sr14Cu24O41, which shows a spin gap due to spin dimers formed around Zhang-Rice singlets, the chains in SrCa13Cu24 O41 show a gapless excitation in this paper. We ascribe such a change from gapped to gapless excitations to holes transferring away from the chain sublattice into the ladder sublattice upon Ca doping. ©2018 American Physical Society
Spin dynamics of quasi-one-dimensional spin-ladder system SrCa13Cu24O41 in the long-range magnetic ordering state
(Australian Institute of Physics, 2018-01-31) Deng, GC; Yu, DH; Mole, RA; Yano, SI; Wang, CW; Rule, KC; Gardner, JS; Luo, H; Li, S; Ulrich, C; Imperia, P; Ren, W; Cao, SX; Pomjakushina, E; Conder, K; Kenzelmann, M; McIntyre, GJ
Sr14-xCaxCu24O41 is a quasi-one-dimensional magnet, which consists of two sublattices: spin ladder and spin chain, forming an incommensurate crystal structure along the c axis, namely, the ladder leg or chain direction. The highly Ca-doped compounds undergo a superconducting phase transition under hydrostatic pressure ~ 3GPa, which is really intriguing since the hole-doped even-leg spin-ladder system was theoretically predicted as superconductors by charge-pairing mechanism through antiferromagnetic interaction on the rungs of spin ladders. In the previous study, we discovered that all compounds with different Ca content have a singlet ground state with a spin-gap ~ 32meV. In the highly Ca-doped sample SrCa13Cu24O41, a long-range magnetic ordering takes place at ~ 4.2K. Interestingly, the singlet spin-liquid state and the long-range magnetic ordering coexist in this compound. In this study, we further investigated its spin dynamics in the ordered phase by using inelastic neutron scattering on PELICAN and SIKA at OPAL. We observed the gapless spin-wave excitation, dispersive along the a and c axes but nondispersive along the b axis, indicating the nature of a 2D magnet. A dynamic model has been proposed to fit the experimental data, indicating three major exchange interactions along rungs (JR), legs (JL) and between neighbor ladders (JInter). This study helps us to understand the origin of the spin liquid ground state in this low-dimensional magnet, in which hole-doping should be attributed to induce the long-range magnetic ordering due to the disorder-induced order effect.
Status report on SIKA - Taiwan's cold neutron triple-axis spectrometer at OPAL
(Australian Institute of Physics, 2014-02-06) Wu, CM; Gardner, JS; Deng, G
We will report on the current status of SIKA, the triple-axis spectrometer with the view of the cold source from the reactor beam hall in OPAL, at ANSTO. SIKA is funded by the National Science Council of Taiwan and currently being commissioned by the National Synchrotron Radiation Research Center. To provide the flexibility for scientific applications, SIKA’s analyser can operate in a flat or in a multiplexing mode when coupled to the one-dimensional PSD consisting of 48 vertical position-sensitive wires, allowing the simultaneous data collection over a specified range in (Q, E). This analyser can also operate in a horizontal focusing mode that directs the scattered neutrons into a single-detector. The entire analyser-detector system as is packed into a single, well shielded secondary spectrometer housing which significantly reduces the background. As a state-of-the-art triple-axis spectrometer, SIKA is also equipped with a full automated sample stage and a series of collimations (both soller and radial). Neutron polarisation will be available for the incident and scattered beams through 3He polarisers.
Striped magnetic ground state of the ideal kagomé lattice compound Fe4Si2Sn7O16
(Society of Crystallographers in Australia and New Zealand, 2017-12-03) Ling, CD; Allison, MC; Schmid, S; Avdeev, M; Gardner, JS; Ryan, DH; Soehnel, T
We have used representational symmetry analysis of neutron powder diffraction data to determine the magnetic ground state of Fe4Si2Sn7O16. We recently reported a long-range antiferromagnetic (AFM) Néel ordering transition in this compound at TN = 3.0 K, based on magnetisation measurements [1]. The only magnetic ions present are layers of high-spin Fe2+ (d6, S = 2) arranged on a perfect kagomé lattice (trigonal space group P-3m1). Below TN = 3.0 K, the spins on 2/3 of these magnetic ions order into canted antiferromagnetic chains, separated by the remaining 1/3 which are geometrically frustrated and show no long-range order down to at least T = 0.1 K [2]. Moessbauer spectroscopy shows that there is no static order on the latter 1/3 of the magnetic ions — i.e., they are in a liquid-like rather than a frozen state – down to at least 1.65 K. A heavily Mn-doped sample Fe1.45Mn2.55Si2Sn7O16 has the same ground state. Although the magnetic propagation vector k = (0, ½, ½) breaks hexagonal symmetry, we see no evidence for magnetostriction in the form of a lattice distortion within the resolution of our data. To the best of our knowledge, this type of magnetic order on a kagomé lattice has no precedent experimentally and has not been explicitly predicted theoretically. We will discuss the relationship between our experimental result and a number of theoretical models that predict symmetry-breaking ground states for perfect kagomé lattices.
Striped magnetic ground state of the kagome lattice in Fe4Si2Sn7O16
(American Physical Society, 2017-11-15) Ling, CD; Allison, MC; Schmid, S; Avdeev, M; Gardner, JS; Wang, CW; Ryan, DH; Zbiri, M; Söhnel, T
We have experimentally identified a different magnetic ground state for the kagome lattice, in the perfectly hexagonal Fe2+ (3d6,S=2) compound Fe4Si2Sn7O16. A representational symmetry analysis of neutron diffraction data shows that below TN=3.5 K, the spins on 23 of the magnetic ions order into canted antiferromagnetic chains, separated by the remaining 13 which are geometrically frustrated and show no long-range order down to at least T=0.1 K. Mössbauer spectroscopy confirms that there is no static order on the latter 13 of the magnetic ions—i.e., they are in a liquidlike rather than a frozen state—down to at least 1.65 K. A heavily Mn-doped sample Fe1.45Mn2.55Si2Sn7O16 has the same magnetic structure. Although the propagation vector q=(0,12,12) breaks hexagonal symmetry, we see no evidence for magnetostriction in the form of a lattice distortion within the resolution of our data. We discuss the relationship to partially frustrated magnetic order on the pyrochlore lattice of Gd2Ti2O7, and to theoretical models that predict symmetry breaking ground states for perfect kagome lattices. ©2017 American Physical Society
Weak trimerization in the frustrated two-dimensional triangular Heisenberg antiferromagnet LuyY1−yMnO3
(American Physical Society, 2023-06-02) Yano, S; Wang, CW; Gardner, JS; Chen, WT; Iida, K; Mole, RA; Louca, D
To understand the 2D triangular Heisenberg antiferromagnetic system, we investigated the magnetic structures and the dynamics of LuyY1-yMnO3 in detail. The substitutions are adjusted to the Mn atomic position close to xMn=13. The neutron powder diffraction data claims that the magnetic structure of LuyY1-yMnO3 is described as a mixture of Γ3 (P63′cm′) and Γ4 (P63′c′m) at the xMn position for y=0.15, 0.30, and 0.45. The ratio of Γ3 and Γ4 depends on temperature and composition and the fraction of Γ3 increases upon cooling, while no clear trimerization was observed at the xMn position. We estimated exchange parameters from the analysis of the low-energy part of the spin waves. The results showed a weak trimerization effect on cooling because the nearest-neighbor exchange interaction is slightly enhanced. The temperature dependence of the spin-wave dispersion around the Γ point shows that the spin gap closes with increasing temperature because the exchange interactions in the nearest Mn-Mn neighbor become smaller. Gapless diffusive magnetic excitation from a Mn triangular lattice has been observed in a wide range in Q and E space of LuyY1-yMnO3. We found that Lu0.3Y0.7MnO3 could be an ideal case to investigate the trimerization, frustrated magnetism, and magnetoelastic coupling often observed in two-dimensional triangular lattice Heisenberg antiferromagnet systems. ©2023 American Physical Society

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