Browsing by Author "Deng, G"
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- Item90° Rotation of orbital stripes in bilayer manganite PrCa2Mn2O7 studied by in situ transmission electron microscopy(Academic Press Inc Elsevier Science, 2013-04-01) He, ZB; Deng, G; Tian, H; Xu, Q; Van Tendeloo, GWe present an in situ transmission electron microscopy study on the half-doped bilayer manganite PrCa2Mn2O7 to reveal the rotation process of the orbital stripes. Between the reported initial and final ordering phases, we identified an intermediate state with two sets of satellite spots to bridge the 90° rotation of the orbital stripes. Furthermore, we determined that the rotation of the orbital stripes does not always occur. Some restricted conditions for the orbital rotation to occur were found and reasons are discussed. © 2013, Elsevier Inc.
- ItemChemical 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, GJThe 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.
- ItemThe 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, JSSIKA 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.
- ItemThe 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, GJSIKA 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.
- ItemComplementary terahertz absorption and inelastic neutron study of the dynamic anisotropy contribution to zone-center spin waves in a canted antiferromagnet NdFeO3(APS Physics, 2014-08-19) Constable, E; Cortie, DL; Horvat, J; Lewis, RA; Cheng, Z; Deng, G; Cao, S; Yuan, S; Ma, GWe employ a combination of pulsed- and continuous-wave polarized terahertz spectroscopy techniques to probe temperature-dependent spin waves in the antiferromagnet NdFeO3. Our optical data span 1.6–467 K and reveal a conspicuous spin reorientation between 110 and 170 K, during which the lower-energy mode softens completely. Complementary inelastic neutron scattering reveals that the frequencies of the optically excited spin waves are consistent with a temperature-variable spin gap in the low-energy spin-wave dispersion of NdFeO3. The result links the temperature dependence of the spin waves to a dynamic in-plane anisotropy. The magnetic anisotropy is calculated based on the results of the optical measurements. The change observed in the anisotropy energy along the a and c crystal axes suggests that the spin reorientation evident in NdFeO3 is driven by temperature-dependent in-plane anisotropy.© 2014, American Physical Society.
- ItemCrystal growth and thermoelectric properties of CaMn0.98Nb0.02O3-delta(Elsevier Science B.V., 2013-08-15) Alfaruq, DS; Aguirre, MH; Otal, EH; Populoh, S; Karvonen, L; Yoon, S; Lu, Y; Deng, G; Ebbinghaus, SG; Weidenkaff, AThermoelectric CaMn0.98Nb0.02O3-delta single crystals were grown from sintered polycrystalline material using the traveling-solvent floating zone (TSFZ) method. The floating-zone furnace was operated at over-pressure using an Ar/O-2 mixture to prevent evaporation during the growth process. Six twin-domain variants were detected with single-crystal X-ray diffraction (XRD) and confirmed by electron diffraction (ED) and high-resolution transmission electron microscopy (HRTEM). The Seebeck coefficient (S) of the single-crystalline material indicates n-type semiconducting behavior. Within 10 K < T < 125 K a negative peak in S is observed, known to be characteristic of antiferromagnetic ordering. The ferromagnetic long-range ordering, expected on the basis of double exchange between Mn4+ and doped Mn3+ species, thus appears to remain suppressed in the single-crystalline material. © 2013, Elsevier Ltd.
- ItemDetermination of hole distribution in Sr14-xCaxCu24O41 using soft x-ray absorption spectroscopy at the Cu L3 edge.(American Physical Society, 2013-07-22) Huang, MJ; Deng, G; Chin, YY; Hu, ZW; Cheng, JG; Chou, FC; Conder, K; Zhou, JS; Pi, TW; Goodenough, JB; Lin, HJ; Chen, CTThe physical properties of Sr14-xCaxCu24O41 are determined by the hole distribution between the edge-shared CuO2 chain and the corner-shared Cu2O3 two-leg ladder, but inconsistent results on the hole distribution were obtained in various experimental works in recent decades. In this work we reinvestigate the hole distribution by soft x-ray absorption spectroscopy at the Cu L-3 edge. By comparing with the pure ladder and the pure chain systems, we can unambiguously distinguish between the hole distributions in the chain and the ladder. We have found that there are 5.3 holes in the edge-shared chain and 0.7 holes in the corner-shared ladder on average for Sr14Cu24O41. Upon Ca substitution, the holes gradually transfer from the edge-shared chain to the corner-shared ladder, and there are up to 1.21 holes in the corner-shared ladder for Sr1.8Ca12.2Cu24O41. © 2013, American Physical Society.
- ItemEvolution of crystallization and magnetic phase transition in Cu1-xZnxFe2O4 studied by neutron powder diffraction(American Physical Society, 2017-03-13) Chang, FF; Avdeev, M; Deng, G; Hester, JR; Wang, X; Ulrich, CHigh resolution and high intensity neutron powder diffraction were applied to study the crystallographic and magnetic phase transition in Cu1-xZnxFe2O4 from 4 K to 750 K. Structural phase transition from cubic to tetragonal phase was observed in CuFe2O4. Ferrimagnetic order was observed in CuFe2O4 and short-range antiferromagnetic scattering was observed below 10 K in cubic ZnFe2O4 which is strongly restrained by addition of slightly amount of Cu2+ ions. Upon doping, ferromagnetic order temperature was gradually reduced from 789 K. Collinear spin setting was observed and no indication of frustration was found even up to doping rate of x = 0.6. Highly frustrated Cu0.04Zn0.96Fe2O4 and ZnFe2O4 behave short-range antiferromagnetic order, induced by the competing between ferromagnetic interaction from first-nearest neighbor and antiferromagnetic interaction from the third-nearest neighbor in tetrahedron formed by Fe ions on B sites. © 2021 American Physical Society
- ItemFloating zone crystal growth and magnetic properties of bilayer manganites Pr(Sr1−xCax)(2)Mn2O7(Elsevier Science BV, 2012-08-15) Deng, G; Thiyagarajan, R; Radheep, DM; Pomjakushina, E; Medarde, M; Krzton-Maziopa, A; Wang, S; Arumugam, S; Conder, KSingle crystals of a bilayer manganite series Pr(Sr1-xCax)(2)Mn2O7 (x=0, 0.4, 0.9, 1) have been grown using traveling solvent floating zone technique under various oxygen partial pressure. It was found that the crystal quality is very sensitive to oxygen partial pressure applied during growth and highly Ca doped crystals could only be obtained at elevated oxygen pressure. Whereas a secondary phase was observed for PrCa2Mn2O7 grown at oxygen partial pressure below 1 bar, a pure phase can be achieved at 8 bar of oxygen. This was evidenced studying composition mapping by X-ray fluorescence spectroscopy. X-ray Laue diffraction and X-ray powder diffraction have been carried out on the samples for the quality check of the grown crystals. Susceptibility measurements indicate that all samples are antiferromagnetic at low temperature. They also reveal the existence of two anomalies at similar to 320 K and 370 K, which coincide with the charge ordering/orbital ordering transitions described in the literature. © 2012, Elsevier Ltd.
- ItemGrowth of LiCoO2 single crystals by the TSFZ method(American Chemical Society, 2018-11-08) Nakamura, S; Maljuk, A; Maruyama, Y; Nagao, M; Watauchi, S; Hayashi, T; Anzai, Y; Furukawa, Y; Ling, CD; Deng, G; Avdeev, M; Büchner, B; Tanaka, IWe have grown LiCoO2 single crystals by the traveling solvent floating zone (TSFZ) growth with Li-rich solvent, having observed the incongruent melting behavior of LiCoO2 between 1100 and 1300 °C. The optimum growth conditions in terms of atmosphere and solvent composition were determined to be Ar flow and an atomic ratio Li/Co 85:15, respectively. The crystals grown using a conventional-mirror-type furnace contained periodic inclusions of a Co–O phase due to the influence of Co–O phase segregation on the stability of the molten zone during growth. By using a tilted-mirror FZ furnace, inclusion-free LiCoO2 crystals of about 5 mm in diameter and 70 mm long were obtained at a tilting angle θ = 10°. The grown crystals were confirmed to be single-domain by neutron Laue diffraction. © 2018 American Chemical Society
- ItemHigh oxygen pressure single crystal growth of highly Ca-doped spin ladder compound Sr(14-x)Ca(x)Cu(24)O(41) (x > 12)(Elsevier, 2011-07-15) Deng, G; Radheep, DM; Thiyagarajan, R; Pomjakushina, E; Wang, S; Nikseresht, N; Arumugam, S; Conder, KLarge size high-quality Ca-doped Sr14−xCaxCu24O41 (x=12.2, 12.6 and 13) spin ladder superconducting single crystals were grown using a modified mirror floating zone furnace with oxygen pressure up to 35 bar. The qualities of the as-grown single crystals were confirmed by polarized optical microscopy, neutron diffraction and X-ray diffraction methods. The sample compositions and homogeneity were measured using micro-X-ray fluorescence spectroscopy. The lattice parameters of each sample have been refined using the Rietveld method. The oxygen stoichiometry measurements which were performed by hydrogen reduction/thermogravimetry method show that all the three samples are slightly oxygen deficient even though high oxygen pressure was applied during growth. The high oxygen pressure is indispensible for growing highly Ca-doped Sr14−xCaxCu24O41 single crystals. The susceptibility was measured along c-axis for all three compositions, fitting well with the one dimensional dimer model, consistent with the reported values of undoped Sr14Cu24O41 parent compounds.(C) 2011 Elsevier B.V.
- ItemMicro-fabrication process for small transport devices of layered manganite(American Institute of Physics, 2012-04-01) Omrani, AA; Deng, G; Radenovic, A; Kis, A; Rønnow, HMDevices have been fabricated based on the bilayer manganite La(1.4)Sr(1.6)Mn(2)O(7), which in the bulk state orders magnetically below 90 K, at which point both in-plane and c-axis bulk resistivity decrease by 2-3 orders of magnitude. We provide an optimized procedure to fabricate devices to electrical transport in-and out of plane. Fabricated mesoscopic devices have dimensions comparable to a typical magnetic domain, allowing us to study structures going from a single domain to several domains. © 2012, American Institute of Physics.
- ItemMicrostructure of bilayer manganite PrCa2Mn2O7 showing charge/orbital ordering(American Institute of Physics, 2013-05-27) He, ZB; Tian, H; Deng, G; Xu, Q; Van Tendeloo, GThe microstructure of the charge/orbital ordering Ruddleden-Popper phase PrCa2Mn2O7 was studied by transmission electron microscopy along both the [001] and the [110] orientation. Three coexisting charge/orbital ordering phases CO1, CO2, and CO3 were observed along the [001] orientation at room temperature. Different from the one-dimensional modulation in the CO1 and CO2 phase, the CO3 phase is characterized by two sets of mutually perpendicular structural modulations. From [110] high angle annular dark field-scanning transmission electron microscopy, we found that the Pr atoms locate in-between the bilayer MnO6 octahedra, which is different from the previous reports. © 2013, American Institute of Physics.
- ItemSpin-gap evolution upon Ca doping in the spin-ladder series Sr(14−x)CaxCu24O41 studied by inelastic neutron scattering(Americal Physical Society, 2013-07-03) Deng, G; Tsyrulin, N; Bourges, P; Lamago, D; Rønnow, HM; Kenzelmann, M; Danilkin, SA; Pomjakushina, E; Conder, KThe spin-gap evolution upon Ca doping in Sr14-xCaxCu24O41 was systematically investigated using inelastic neutron scattering. We discover that the singlet-triplet spin-gap excitation survives in this series with x up to 13, indicating the singlet dimer ground state in these compounds. This observation corrects the previous speculation that the spin gap collapses at x similar to 13 by the NMR technique. The strong intensity modulation along Q(H) in x = 0 gradually evolves into a Q-independent feature in x > 11. This could be attributed to the localized Cu moment magnetism developing into an itinerant magnetism with increasing x. It is a surprise that the spin gap persists in the normal state of this spin-ladder system with metallic behavior, which evidences the possibility of magnetically mediated carrier pairing mechanism in a two-leg spin-ladder lattice. © 2013, American Physical Society.
- ItemStability and scaling behavior of the spin cycloid in BiFeO3 thin films(Australian Institute of Physics, 2018-01-30) Burns, SR; Sando, D; Bertinshaw, J; Russell, L; Xu, X; Maran, R; Callori, SJ; Ramash, V; Cheung, J; Danilkin, SA; Deng, G; Lee, WT; Hu, S; Bellaiche, L; Seidel, J; Valanoor, N; Ulrich, CMultiferroic materials demonstrate excellent potential for next-generation multifunctional devices, as they exhibit coexisting ferroelectric and magnetic orders. Bismuth ferrite (BiFeO3) is a rare exemption where both order parameters exist far beyond room temperature, making it the ideal candidate for technological applications. To realize magnonic devices, a robust longrange spin cycloid with well-known direction is desired, since it is a prerequisite for the magnetoelectric coupling. Despite extensive investigation, the stabilization of a large-scale uniform spin cycloid in nanoscale (<300 nm) thin BiFeO3 films has not been accomplished. Using neutron diffraction we were able to demonstrate cycloidal spin order in 100 nm BiFeO3 thin films which became stable through the careful choice of crystallographic orientation and control of the electrostatic and strain boundary conditions during growth [1]. Furthermore, Co-doping, which has demonstrated to further stabilize the spin cycloid, did allow us to obtain spin cycloid order in films of just 50 nm thickness, i.e. films thinner than the cycloidal length of about 64 nm. Interestingly, in thin films the propagation direction of the spin cycloid has changed and shows a peculiar scaling behavior for thinnest films. We were able to support these observations by Monte Carlo theory based on a first-principles effective Hamiltonian method. Our results therefore offer new avenues for fundamental research and technical applications that exploit the spin cycloid in spintronic or magnonic devices.
- ItemStatus report on SIKA - Taiwan's cold neutron triple-axis spectrometer at OPAL(Australian Institute of Physics, 2014-02-06) Wu, CM; Gardner, JS; Deng, GWe 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.
- ItemStructural evolution of one-dimensional spin-ladder compounds Sr14−xCaxCu24O41 with Ca doping and related evidence of hole redistribution(American Physical Society, 2011-10-17) Deng, G; Pomjakushin, V; Petříček, V; Pomjakushina, E; Kenzelmann, M; Conder, KIncommensurate crystal structures of spin-ladder series Sr(14-x)Ca(x)Cu(24)O(41) (x = 3, 7, 11, and 12.2) were characterized by powder neutron scattering method and refined using the superspace group Xmmm(00 gamma)ss0 [ equivalent to superspace group Fmmm(0, 0, 1 + gamma)ss0; X stands for nonstandard centering (0, 0, 0, 0), (0, 1/2, 1/2, 1/2), (1/2, 1/2, 0, 0), (1/2, 0, 1/2, 1/2)] with a modulated structure model. The Ca doping effects on the lattice parameters, atomic displacement, Cu-O distances, Cu-O bond angles, and Cu bond valence sum were characterized. The refined results show that the CuO(4) planar units in both chain and ladder sublattices become closer to square shape with an increase of Ca doping. The Cu bond valence sum calculation provided new evidence for the charge transfer from the chains to ladders (approximately 0.16 holes per Cu from x = 0 to 12.2). The charge transfer was attributed to two different mechanisms: (a) the Cu-O bond distance shrinkage on the ladder and (b) increase of the interaction between two sublattices, resulting in Cu-O bonding between the chains and ladders. The low-temperature structural refinement resulted in the similar conclusion with a slight charge backflow to the chains.© 2011, American Physical Society
- ItemSuperconductivity on a crossover phenomenon of spin–ladder system SrCa13Cu24O41 single crystals(Physical Society of Japan (JPS), 2014-01-01) Hisada, A; Matsubayashi, K; Uwatoko, Y; Fujiwara, N; Deng, G; Pomjakushina, E; Conder, C; Radheep, DM; Thiyagarajan, R; Esakkimuthu, S; Arumugam, SElectrical resistivity and alternating current susceptibility measurements were performed for the spin–ladder compound SrCa13Cu24O41 under hydrostatic pressure up to 8 GPa. The superconducting transition was observed below TC = 14.7 K at pressures above 2 GPa. By using high-quality single crystals, the bulk superconductivity with a volume fraction of 50% and temperature quadratic behavior of the normal state resistivity were observed above 3.7 GPa. They suggest that a strong interladder interaction induces the bulk superconductivity and transforms the system into a Fermi-liquid-like state. The optimal pressure was in a crossover region and the highest TC was observed without going through the bulk superconducting state. Our results suggest that the superconducting state on the crossover phenomenon presents the feature of the doped two-leg spin–ladder system because the spin gap was observed in this compound. © 2014, The Physical Society of Japan.
- ItemTaipan – a versatile thermal neutron scattering instrument for condensed matter and materials research.(Australian Institute of Physics, 2022-12-11) Rule, KC; Stampfl, APJ; Deng, GTaipan is the highest flux, thermal neutron scattering instrument at ANSTO, Australia. This poster will present some recent scientific highlights at Taipan – both as a triple axis spectrometer, and a Be-filter analyser spectrometer.