Browsing by Author "Danilkin, SA"
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- ItemAnomalous precursive behaviour for the martensitic material Ni 0.625 Al 0.375(Australian Institute of Physics, 2011-02-02) Finlayson, TR; Danilkin, SA; Studer, AJ; Whitfield, RENixAl1-x alloys for 0.615 < x < 0.64 undergo a martensitic transformation upon cooling from a CsCl-type structure to a pseudo-orthorhombic structure [1]. The transformation temperature is extremely composition dependent [2] and for Ni0.625Al0.375 is ~ 80 K [1]. In previous research using an approximate cube of single crystal having x = 0.625 [3], significant strain anisotropy was detected above 80 K. It was suggested that this anomalous strain anisotropy indicated the presence of a martensite precursor within the cubic “parent” phase. The aim of the current research project is to investigate the precursive structural behaviour in the above Ni0.625Al0.375 single crystal using both elastic and inelastic neutron scattering. Results from initial experiments at both the Wombat and Taipan instruments at the Opal Research Reactor will be presented and discussed in relation to previously published strain anisotropy data determined using variable temperature, capacitance dilatometry [3].
- ItemApparent critical phenomena in the superionic phase transition of Cu2-xSe(IOP Science, 2016-01-11) Kang, SD; Danilkin, SA; Aydemir, U; Avdeev, M; Studer, AJ; Snyder, GJThe superionic phase transition of Cu2-xSe accompanies drastic changes in transport properties. The Seebeck coefficient increases sharply while the electrical conductivity and thermal diffusivity drops. Such behavior has previously been attributed to critical phenomena under the assumption of a continuous phase transition. However, applying Landau's criteria suggests that the transition should be first order. Using the phase diagram that is consistent with a first order transition, we show that the observed transport properties and heat capacity curves can be accounted for and modeled with good agreement. The apparent critical phenomena is shown to be a result of compositional degree-of-freedom. Understanding of the phase transition allows to explain the enhancement in the thermoelectric figure-of-merit that is accompanied with the transition. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
- ItemArtificially modulated chemical order in thin films: a different approach to create ferro/antiferromagnetic interfaces(American Physical Society, 2010-10-06) Saerbeck, T; Klose, F; Lott, D; Mankey, GJ; Lu, Z; LeClair, PR; Schmidt, W; Stampfl, APJ; Danilkin, SA; Yethiraj, M; Schreyer, AWe report on a unique magnetic exchange interaction in a thin film of FePt3, comprising an artificially created ferromagnetic (FM)/antiferromagnetic (AFM) modulation, but homogeneous chemical composition and epitaxy throughout the film. The chemical order, on the other hand, is modulated resulting in the formation of alternating FM/AFM layers. To determine the existence and form of the magnetic structure within the monostoichiometric thin film, we use a unique combination of polarized neutron reflectometry, x-ray/neutron diffraction, and conventional magnetometry. This artificial stratified AFM/FM FePt3 exhibits a high magnetic exchange bias thus opening up possibilities to study such magnetic phenomena in a perfectly lattice-matched system. © 2010, American Physical Society
- ItemCoexistence of long-range magnetic ordering and singlet ground state in the spin-ladder superconductor SrCa13Cu24O41(American Physical Society, 2013-11-26) Deng, GC; Kenzelmann, M; Danilkin, SA; Studer, AJ; Pomjakushin, V; Imperia, P; Pomjakushina, E; Conder, KA long-range magnetic order was discovered in the quasi-one-dimensional spin-ladder compound SrCa13Cu24O41 by susceptibility, specific heat, and neutron diffraction experiments. The temperature dependence of the magnetic Bragg peak intensity could be well fitted to the power law with a transition temperature TN = 4.23 K and a critical exponent β = 0.28, indicating a three-dimensional phase transition for a low-dimensional magnet. A computer program was coded and found two possible magnetic structure models fitting best with all the observed magnetic peaks. These models suggest the spin-ladder sublattice is magnetically ordered with Cu moments aligning along the a axis. The spin interactions are primarily antiferromagnetic along rungs and legs, while there are ferromagnetic clusters along legs. Surprisingly, the singlet-triplet spin-gap excitation is observable above and below TN, indicating a coexistence of the spin-singlet ground state and long-range magnetic ordering state in this compound. © 2013 American Physical Society.
- ItemComparison of the magnetic and crystal field excitations in orthorhombically distorted vanadates and multiferroic manganites(Australian Institute of Nuclear Science and Engineering, 2012-11-15) Reynolds, N; Rovillain, P; Narayanan, N; Fujioka, F; Tokura, Y; Danilkin, SA; Mulders, AM; McIntyre, GJ; Ulrich, CMagnetism and ferroelectricity are both exciting physical properties and are used in everyday life in sensors and data storage. In multiferroic materials both properties coexist. They offer a great potential for future technological applications like the increase of data storage capacity or in novel senor applications. We have performed a comparative inelastic neutron scattering (INS) investigation on a series of vanadates, in particularly TbV0{sub 3} DyV0{sub 3}, PrV0{sub 3}, and CeV0{sub 3}, with their multiferroic Mn-counterparts. The Vanadates are isostructural to the multiferroic materials TbMnO{sub 3} and DyMn0{sub 3}, but posses a collinear antiferromagnetic spin arrangement below TN ≈110 K instead of a cycloidal spin structure below TFE 28 ≈K. By using inelastic neutron scattering we have obtained the spin wave dispersion relation and the crystal field excitations of the V-sublattice and the rare earth ions, respectively. The data will be compared with previously obtained INS data of D. Senff on TbMnO{sub 3} and our INS data on DyMnO{sub 3} with the intention of uncovering information about the complex interplay between the magnetic moments of the rare earth ions its role in the formation of the multiferroic phase.
- ItemComplex magnetic structure in strained nanoscale bismuth ferrite thin films(Australian Institute of Physics, 2016-02-02) Ulrich, C; Bertinshaw, J; Maran, R; Callori, SJ; Ramesh, V; Cheng, J; Danilkin, SA; Hu, S; Siedel, J; Valanoor, NMultiferroic 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 coexist far beyond room temperature, making it the ideal candidate for technological applications. In particular, multiferroic thin films are the most promising pathway for spintronics applications. Therefore we have investigated BiFeO3 thin films by neutron diffraction. At present, the underlying physics of the magnetoelectric coupling is not fully understood and competing theories exist with partly conflicting predictions. For example, the existence of spin cycloid is a mandatory requirement to establish a direct magnetoelectric coupling. Thus far internal strain in epitaxially grown films has limited the stability of the spin cycloid for BiFeO3 films with less than 300 nm thickness, causing the spin cycloid to collapses to a collinear G-type antiferromagnetic structure. Our neutron diffraction experiments have demonstrated that we were able to realize a spin cycloid in films of just 100 nm thickness through improved electrostatic and epitaxial constraints. This underlines the importance of the correct mechanical and electrical boundary conditions required to achieve emergent spin properties in mutiferroic thin film systems. The discovery of a large scale uniform cycloid in thin film BiFeO3 opens new avenues for fundamental research and technical applications that exploit the spin cycloid in spintronic or magnonic devices.
- ItemCopper selenide: soft phonon modes and superionic phase transition(Australian Institute of Physics, 2010-02-03) Danilkin, SA; Yethiraj, M; Kearley, GJThis paper reports lattice dynamical measurements of Cu1.8Se superionic conductor having structure of the superionic α-phase at ambient temperature. Cu2δSe is a mixed ionic-electronic conductor with a superionic transition at 414K in stoichiometric compound Cu2Se. At room temperature the superionic α -phase exists in the composition range from δ = 0.15 to 0.25. The important features of the Cu1.8Se compound is the ordering of Cu atoms observed at ambient temperature [1] which is described as “disordered” α- phase in the literature and presence of low-energy transverse acoustic (TA) modes [2]. Measurements of phonon dispersion curves were performed with the new triple-axis spectrometer, TAIPAN, at the OPAL reactor [3]. We found that TA [100], TA [111] and TA1 [110] phonon branches demonstrate a decrease in frequency at wavevectors q/qm > 0.5 rather than the flattening observed previously. Results are compared with calculated density functional theoretical calculations showing the presence of unstable soft mode related to ordering of Cu atoms in Cu1.8Se at room temperature followed by α- β phase transition at a lower temperature. Superstructure arising from the ordering causes effects similar to the folding of the Brillouin zone, although phonon intensities at new Brillouin zone centres are weak. The coupling of low-energy phonon modes with displacement of mobile ions can explain the strong damping of phonons at q/qm > 0.5.
- ItemCrystal structure of R10Mo6O33 (R = Nd, Pr) from 3 K to 973 K by neutron powder diffraction(Elsevier, 2016-05-01) Ishikawa, Y; Danilkin, SA; Avdeev, M; Voronkova, VI; Sakuma, TOxygen ionic conductors, R10Mo6O33 (R = Nd, Pr), were investigated by neutron powder diffraction in the wide temperature range from 3 K to 973 K. The Rietveld analyses and peak profile examination confirmed that there are no structural phase transitions in whole temperature range, keeping a cubic structure. The nuclear density distribution calculated by maximum entropy method and the Bond Valence Sum mapping analysis shows that a possible diffusion pathway connects the neighboring 48i oxygen ions at general positions. These results are consistent with the previous results of a one-particle Coulomb potential calculation and electric conductivity measurements. © 2015 Elsevier B.V.
- ItemDerivation of inter-atomic force constants of Cu2O from diffuse neutron scattering measurement(Atom Indonesia, 2013-04-15) Makhsun, T; Sakuma, T; Kartini, E; Takahashi, H; Igawa, N; Danilkin, SA; Sakai, RNeutron scattering intensity from Cu2O compound has been measured at 10 K and 295 K with High Resolution Powder Diffractometer at JRR-3 JAEA. The oscillatory diffuse scattering related to correlations among thermal displacements of atoms was observed at 295 K. The correlation parameters were determined from the observed diffuse scattering intensity at 10 and 295 K. The force constants between the neighboring atoms in Cu2O were estimated from the correlation parameters and compared to those of Ag2O. (c) 2016 Atom Indonesia.
- ItemDiffuse scattering and lattice dynamics of superionic copper chalcogenides(Elsevier, 2009-05-14) Danilkin, SAThe energy-resolved neutron diffraction and inelastic neutron scattering study of diffuse scattering in copper chalcogenides was performed in order to clarify the role of static disorder versus low-energy phonons. Neutron diffraction patterns taken from Cu1.75Se, Cu1.98Se and Ag0.25Cu1.75Se powders in superionic phase show a broad maximum related to diffuse scattering. This diffuse background is suppressed in the energy-resolved experiment which indicates a strong contribution from inelastic scattering coming from correlated thermal displacements of the ions in the superionic phase. Diffraction experiments on a single crystal of α-Cu1.8Se have revealed an ordered structure with superstructure reflections at the G +/- 1/2 < 111 > and G +/- 1/3 < 220 > positions of reciprocal space at room temperature. In addition to superstructure spots, diffuse scattering was observed along the < 111 > direction which is considered as a possible diffusion path of mobile Cu ions. In inelastic neutron scattering measurements with this single crystal sample strong inelastic scattering was observed along < 111 >. This shows that the diffuse scattering found in conventional diffraction experiment is mainly inelastic and most probably comes from low-energy phonons. © 2009, Elsevier Ltd.
- ItemDiffuse scattering and QENS study of copper chalcogenides(Australian Institute of Physics, 2009-02-05) Danilkin, SA; Avdeev, M; Studer, AJ; Ling, CD; Macquart, RB; Russina, M; Izaola, ZNot available
- ItemDynamical mechanism of phase transitions in a-site ferroelectric relaxor (Na1/2Bi1/2)TiO3(APS Physics, 2014-10-13) Deng, GC; Danilkin, SA; Zhang, HW; Imperia, P; Li, XB; Zhao, X; Luo, HQThe dynamical phase transition mechanism of (Na1/2Bi1/2)TiO3 (NBT) was studied using inelastic neutron scattering. Softening was observed of multiple phonon modes in the phase transition sequence of NBT. As usual, the softening of the zone center transverse optical modes Δ5 and Σ3 was observed in the (200) and (220) zones, showing the Ti vibration instabilities in TiO6 octahedra for both cubic-tetragonal (C-T) and tetragonal-rhombohedral (T-R) phase transitions. In these two phase transitions, however, Ti4+ has different preferential displacement directions. Surprisingly, the longitudinal optic mode also softens significantly toward the zone center in the range of the transition temperature, indicating the Na+/Bi3+ vibration instability against TiO6 octahedra during the T-R phase transition. Strong inelastic diffuse scattering shows up near M(1.5, 0.5, 0) and R(1.5, 1.5, 0.5) in the tetragonal and rhombohedral phases, respectively, indicating the condensations of the M3 and R25 optic modes for the corresponding transitions. This reveals the different rotation instabilities of TiO6 in the corresponding transition temperature range. Bottleneck or waterfall features were observed in the dispersion curves at certain temperatures but did not show close correlations to the formation of polar nanoregions. Additional instabilities could be the origin of the complexity of phase transitions and crystallographic structures in NBT. © 2014 American Physical Society.
- ItemDynamical mechanism of phase transitions in a-site ferroelectric relaxor (Na1/2Bi1/2)TiO3(Australian Institute of Physics, 2016-02-02) Deng, GC; Danilkin, SA; Imperia, P; Li, X; Zhao, XB; Luo, HQ
- ItemDynamical mechanism of phase transitions in a-site ferroelectric relaxor (Na1/2Bi1/2)TiO3(Australian Institute of Nuclear Science and Engineering, 2016-11-29) Deng, GC; Danilkin, SA; Zhang, HW; Imperia, P; Li, XB; Zhao, X; Luo, HQThe dynamical phase transition mechanism of (Na1/2Bi1/2)TiO3 (NBT) was studied using inelastic neutron scattering. Softening of multiple phonon modes were observed to correlate with the phase transition sequence of NBT. As usual, the softening of the zone centre transverse optic (TO) modes Δ5 and Σ3 was observed in (200) and (220) zone, showing the Ti vibration instabilities in TiO6 octahera for both cubic-tetragonal (C-T) and tetragonal rhombohedral (T-R) phase transitions. In this two phase transitions, however, Ti4+ has different preferential displacement directions. Surprisingly, the longitudinal optic (LO) mode also soften significantly toward zone centre in the vicinity range of the transition temperature, indicating the Na+/Bi3+ vibration instability against TiO6 octahera during the R-T phase transition. Strong inelastic diffuse scattering shows up near M(1.5, 0.5, 0) and R(1.5, 1.5, 0.5) in the tetragonal and rhombohedral phases, respectively, indicating the condensations of the M3 and R25 optic modes for the corresponding phase transitions. This reveals the rotation instabilities of TiO6 in the corresponding phase transition temperature range. Bottleneck or waterfall features were observed in the dispersion curves at certain temperatures, but did not show the close correlations to the formation of polar nanoregions (PNRs). Additional instabilities are the origin of the complexity of phase transitions and crystallographic structures in NBT.
- ItemElectron doping effects on the spin spectroscopy of BaFe2-xNixAs2 superconductors(International Conference on Neutron Scattering, 2017-07-12) Luo, HQ; Gong, DL; Xie, T; Lu, XY; Kamazawa, K; Iida, K; Kajimoto, R; Ivanov, AS; Adroja, DT; Kulda, J; Danilkin, SA; Deng, GC; Li, SL; Dai, PCHigh-temperature superconductivity in iron pnictides emerges from electron or hole doped parent compounds with antiferromagnetic order, which is argued to be associated with both the presence of high-energy spin excitations and a coupling between low-energy spin excitations and itinerant electrons. With more than 6 years\' efforts, we have used time-of-flight neutron spectroscopy to extensively map out the spin excitations in the electron-doped BaFe2-xNixAs2 especially around the overdoped zone boundary of superconductivity. We have found that the high energy spin fluctuations survive in the extremely high doping x=0.6 far beyond the superconducting dome, but the low energy spin excitations including the spin resonance mode is very sensitive to the electron dopings, by finally forming a large spin gap just after the disappearance of superconductivity in the overdoped regime. Further polarized neutron analysis indicate that the spin gap actually is anisotropic, and the longitudinal mode of spin fluctuations, as a hallmark of the itinerant magnetism from Fermi surface nesting, is totally eliminated together with the hole pockets near the electron-overdoped zone boundary of superconductivity.Our results suggest that the strong fluctuations from local moments give framework for magnetic interaction, while itinerant spin excitations originated from Fermi surface nesting are crucial to the superconductivity in iron pnictides.
- ItemElectron doping evolution of the anisotropic spin excitations in BaFe(2-x)NixAs2(Americal Physical Society, 2012-07-10) Luo, HQ; Yamani, Z; Chen, YC; Lu, XY; Wang, M; Li, SL; Maier, TA; Danilkin, SA; Adroja, DT; Dai, PCWe use inelastic neutron scattering to systematically investigate the Ni-doping evolution of the low-energy spin excitations in BaFe(2-x)NixAs2 spanning from underdoped antiferromagnet to overdoped superconductor (0.03 <= x <= 0.18). In the undoped state, BaFe2As2 changes from paramagnetic tetragonal phase to orthorhombic antiferromagnetic (AF) phase below about 138 K, where the low-energy (<=similar to 80 meV) spin waves form transversely elongated ellipses in the [H, K] plane of the reciprocal space. Upon Ni doping to suppress the static AF order and induce superconductivity, the c-axis magnetic exchange coupling is rapidly suppressed and the momentum distribution of spin excitations in the [H, K] plane is enlarged in both the transverse and longitudinal directions with respect to the in-plane AF ordering wave vector of the parent compound. As a function of increasing Ni-doping x, the spin excitation widths increase linearly but with a larger rate along the transverse direction. These results are in general agreement with calculations of dynamic susceptibility based on the random phase approximation (RPA) in an itinerant electron picture. For samples near optimal superconductivity at x approximate to 0.1, a neutron spin resonance appears in the superconducting state. Upon further increasing the electron doping to decrease the superconducting transition temperature T-c, the intensity of the low-energy magnetic scattering decreases and vanishes concurrently with vanishing superconductivity in the overdoped side of the superconducting dome. Comparing with the low-energy spin excitations centered at commensurate AF positions for underdoped and optimally doped materials (x <= 0.1), spin excitations in the overdoped side (x = 0.15) form transversely incommensurate spin excitations, consistent with the RPA calculation. Therefore, the itinerant electron approach provides a reasonable description to the low-energy AF spin excitations in BaFe(2-x)NixAs2. © 2012, American Physical Society.
- ItemElectron doping evolution of the magnetic excitations in BaFe(2-x)NixAs2(American Physical Society., 2013-10-25) Luo, HQ; Lu, XY; Zhang, R; Wang, M; Goremychkin, EA; Adroja, DT; Danilkin, SA; Deng, GC; Yamani, Z; Dai, PCWe use inelastic neutron scattering (INS) spectroscopy to study the magnetic excitations spectra throughout the Brillouin zone in electron-doped iron pnictide superconductors BaFe2-xNixAs2 with x = 0.096,0.15,0.18. While the x = 0.096 sample is near optimal superconductivity with T-c = 20 K and has coexisting static incommensurate magnetic order, the x = 0.15,0.18 samples are electron overdoped with reduced T-c of 14 and 8 K, respectively, and have no static antiferromagnetic (AF) order. In previous INS work on undoped (x = 0) and electron optimally doped (x = 0.1) samples, the effect of electron doping was found to modify spin waves in the parent compound BaFe2As2 below similar to 100 meV and induce a neutron spin resonance at the commensurate AF ordering wave vector that couples with superconductivity. While the new data collected on the x = 0.096 sample confirm the overall features of the earlier work, our careful temperature dependent study of the resonance reveals that the resonance suddenly changes its Q width below T-c similar to that of the optimally hole-doped iron pnictides Ba0.67K0.33Fe2As2. In addition, we establish the dispersion of the resonance and find it to change from commensurate to transversely incommensurate with increasing energy. Upon further electron doping to overdoped iron pnictides with x = 0.15 and 0.18, the resonance becomes weaker and transversely incommensurate at all energies, while spin excitations above similar to 100 meV are still not much affected. Our absolute spin excitation intensity measurements throughout the Brillouin zone for x = 0.096,0.15,0.18 confirm the notion that the low-energy spin excitation coupling with itinerant electron is important for superconductivity in these materials, even though the high-energy spin excitations are weakly doping dependent. © 2013, American Physical Society.
- ItemEstimation of force constants of Al from diffuse neutron scattering measurement(The Physical Society of Japan, 2014-06-19) Makhsun; Hashimoto, T; Takahashi, H; Kamishima, O; Igawa, N; Danilkin, SA; Sakuma, TNeutron diffraction measurement of an aluminum powder sample at 290 K was carried out at the high resolution powder diffractometer installed at JRR-3. Broad oscillations of the diffuse scattering intensity were observed and explained by the correlation effects among the thermal displacements of atoms. The interatomic force constants were determined from the correlation effects using a newly introduced equation. The derived force constants and the crystal structure of Al were used to estimate the phonon dispersion relations, phonon density of states, and specific heat by computer simulation. The calculated phonon dispersion relations and specific heat of Al are similar to those obtained from inelastic neutron scattering and specific heat measurements, respectively.© 2014, The Physical Society of Japan.
- ItemIn situ monochromator alignment on ANSTO’s thermal spectrometer, TAIPAN(Australian Institute of Physics, 2018-01-30) Sutton, C; Rule, KC; McIntyre, GJ; Danilkin, SA; Stampfl, APJTAIPAN is the thermal triple axis spectrometer at ANSTO and also hosts an alternate secondary spectrometer called the Be-filter. Both instruments use a common graphite monochromator (PG002) to select a single wavelength or energy of the neutrons. By rotating the monochromator with respect to the incident beam of neutrons from the reactor, the energy of the neutrons from the monochromator can vary between 5-70 meV. The PG002 monochromator is made from 63 individual crystals of highly oriented pyrolytic graphite and the neutrons are scattered from the 002 lattice planes. The benefit of separate crystals is that they can be oriented to allow for vertical focusing of the neutron beam, or horizontal focusing or double focusing, depending on the experimental requirements. Recent measurements at TAIPAN have indicated that the crystals may not be well aligned on the double focusing mechanism, leading to broad peaks that indicate an increased mosaicity. We will present our work to develop a rapid in situ laser alignment-checking procedure for the 63-crystal TAIPAN graphite monochromator. Due to the small aperture, translation, rotation and tilt of the laser, as well as rotation and tilt of the monochromators would be needed, which will involve derivation of the geometrical mathematics, construction from what we have available of a stage with the large translations needed, testing ex situ, then in situ.
- ItemInelastic neutron scattering studies on the crystal field excitations in Superconducting NdFeAsO0.85F0.15(Hindawi, 2015-03-02) Cheng, P; Bao, W; Danilkin, SA; Zhao, TS; Sheng, JM; Liu, JJ; Luo, W; Wang, JCInelastic neutron scattering experiments were performed on polycrystalline samples of NdFeAsO0.85F0.15 over a wide temperature range (3 K–250 K). Based on the analysis of the experimental data, a Nd3+ CF energy level scheme is proposed to give a consistent explanation about the observed CF transitions. The observed extra ground-state CF transitions could not be simply explained by the transitions between five Kramers doublets split from the Nd3+ ground state in the point symmetry. A reliable explanation would be a superposition of crystal fields due to different local symmetries around the Nd3+ ions induced by the fluorine doping.© 2015, Peng Cheng et al.
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