Browsing by Author "Yethiraj, M"

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    Ab initio determination of the structure of the ferroelectric phase of SrTi18O3
    (Australian Institute of Physics, 2011-02-02) Bartkowiak, M; Kearley, GJ; Yethiraj, M; Mulders, AM
    Strontium titanate (SrTi18O3) is known to display a quantum paraelectric behavior. Its dielectric constant saturates at low temperatures and does not increase with cooling due to quantum fluctuations present in the system. Only in 1999 Itoh et al [1] discovered that substituting regular 16O with the 18O isotope stabilizes the system and allows a transition into a ferroelectric phase below 23 K. The mechanism of the transition and the structure of the new phase have not been conclusively determined by experiment. The new phase displays ferroelectric properties and there are new peaks present in the Raman spectrum. However, diffraction experiments indicate that the structural distortion accompanying the transition is minimal, while Raman and NMR measurements provide evidence for both the order-disorder mechanism and the displacive mechanism to be an applicable explanation of the transition. We applied density functional theory calculations and lattice dynamics analysis to show that the paraelectric tetragonal phase of the regular SrTiO3 is inherently unstable. By distorting the structure along the direction of the soft mode present at the centre of the Brillouin zone we obtained an orthorhombic, ferroelectric structure of SrTiO3 which is energetically favourable over the paraelectric one. Lattice dynamics calculations show that our new structure is stable and the frequencies of the phonon modes present in it are in good agreement with the experimental values published so far.
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    Anisotropic three-dimensional magnetism in CaFe2As2
    (American Physical Society, 2008-11-28) McQueeney, RJ; Diallo, SO; Antropov, VP; Samolyuk, GD; Broholm, C; Ni, N; Nandi, S; Yethiraj, M; Zarestky, JL; Pulikkotil, JJ; Kreyssig, A; Lumsden, MD; Harmon, BN; Canfield, PC; Goldman, AI
    Inelastic neutron scattering measurements of the magnetic excitations in CaFe2As2 indicate that the spin wave velocity in the Fe layers is exceptionally large and similar in magnitude to the cuprates. However, the spin wave velocity perpendicular to the layers is at least half as large that in the layer, so that the magnetism is more appropriately categorized as anisotropic three-dimensional, in contrast to the two-dimensional cuprates. Exchange constants derived from band structure calculations predict spin wave velocities that are consistent with the experimental data. © 2008, American Physical Society
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    Artificially 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, A
    We 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
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    Composition dependence of the in-plane Cu-O bond-stretching LO phonon mode in YBa2Cu3O6+x
    (American Physical Society, 2008-01) Stercel, F; Egami, T; Mook, HA; Yethiraj, M; Chung, JH; Arai, M; Frost, CD; Dogan, F
    An inelastic pulsed neutron scattering study was performed on the dependence of the dispersion and spectral intensity of the in-plane Cu-O bond-stretching LO phonon mode on doped charge density. The measurements were made in the time-of-flight mode with the multiangle position sensitive spectrometer of the ISIS facility on single crystals of YBa2Cu3O6+x (x=0.15, 0.35, 0.6, 0.7, and 0.95). The focus of the study is the in-plane Cu-O bond-stretching LO phonon mode, which is known for strong electron-phonon coupling and unusual dependence on composition and temperature. It is shown that the dispersions for the samples with x=0.35, 0.6, and 0.7 are similar to the superposition of those for x=0.15 and 0.95 samples, and cannot be explained in terms of the structural anisotropy. It is suggested that the results are consistent with the model of nanoscale electronic phase separation, with the fraction of the phases being dependent on the doped charge density. © 2008, American Physical Society
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    Copper selenide: soft phonon modes and superionic phase transition
    (Australian Institute of Physics, 2010-02-03) Danilkin, SA; Yethiraj, M; Kearley, GJ
    This 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.
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    Effects of 18O isotope substitution in multiferroic RMnO3 (R = Tb, Dy)
    (Australian Institute of Physics, 2016-02-05) Graham, PJ; Narayanan, N; McIntyre, GJ; Hutchison, WD; Ulrich, C; Reynolds, N; Rovillain, P; Hester, JR; Kimpton, JA; Yethiraj, M; Pomjakushina, E; Condor, K; Kenzelmann, M
    Multiferroic materials demonstrate desirable attributes for next-generation multifunctional devices as they exhibit coexisting ferroelectric and magnetic orders. In type-II multiferroics, coupling exists that allows ferroelectricity to be manipulated via magnetic order and vice versa, offering potential in high-density information storage and sensor applications. Despite extensive investigations into the subject, questions of the physics of magnetoelectric coupling in multiferroics remain, and competing theories propose different mechanisms. The aim of this investigation was to study changes in the statics and dynamics of structural, ferroelectric and magnetic orders with oxygen-18 isotope substitution to shine light into the coupling mechanism in multiferroic RMnO3 (R=Tb, Dy) systems. We have performed Raman spectroscopy on 16O and 18O-substituted TbMnO3 single crystals. Oxygen-18 isotope substitution reduces all phonon frequencies significantly. However, specific heat measurements determine no changes in Mn3+ (28 and 41 K) magnetic phase transition temperatures. Pronounced anomalies in peak position and linewidth at the magnetic and ferroelectric phase transitions. While the anomalies at the sinusoidal magnetic phase transition (41 K) are in accordance to the theory of spin-phonon coupling, further deviations develop upon entering the ferroelectric phase (28 K). Furthermore, neutron diffraction measurements on 16O and 18O-substituted DyMnO3 powders show structural deviations at the ferroelectric phase transition (17 K) in the order of 100 fm in the b direction. The Pbnm space group is centrosymmetric and therefore does not allow ferroelectricity via atomic displacements, however our Reitveld analysis for the subgroup P21 shows significant displacements and polarisation along b that is comparable to the experimental value, making it the most promising candidate for ionic displacement induced polarisation in DyMnO3. These combined results demonstrate that structure is an important consideration in the emergence of ferroelectricity in these materials.
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    Effects of 18O isotope substitution in multiferroic RMnO3 (R=Tb, Dy)
    (Australian Institute of Physics, 2015-02-02) Graham, PJ; Narayanan, N; Reynolds, NM; Li, F; Rovillain, P; Bartkowiak, M; Hester, JR; Kimpton, JA; Yethiraj, M; Pomjakushina, E; Conder, K; Kenzelmann, M; McIntyre, GJ; Hutchison, WD; Ulrich, C
    Multiferroic materials demonstrate desirable attributes for next-generation multifunctional devices as they exhibit coexisting ferroelectric and magnetic orders. In type-II multiferroics, coupling exists that allows ferroelectricity to be manipulated via magnetic order and vice versa, offering potential in high-density information storage and sensor applications. Despite extensive investigations into the subject, questions of the physics of magnetoelectric coupling in multiferroics remain, and competing theories propose different mechanisms. The aim of this investigation was to study changes in the statics and dynamics of structural, ferroelectric and magnetic orders with oxygen-18 isotope substitution to shine light into the coupling mechanism in multiferroic RMnO3 (R=Tb, Dy) systems. We have performed Raman spectroscopy on 16O and 18O-substituted TbMnO3 single crystals. Oxygen-18 isotope substitution reduces all phonon frequencies significantly. However, specific heat measurements determine no changes in Mn3+ (28 and 41 K) magnetic phase transition temperatures. Pronounced anomalies in peak position and linewidth at the magnetic and ferroelectric phase transitions are seen. While the anomalies at the sinusoidal magnetic phase transition (41 K) are in accordance to the theory of spin-phonon coupling, further deviations develop upon entering the ferroelectric phase (28 K). Furthermore, neutron diffraction measurements on 16O and 18O-substituted DyMnO3 powders show structural deviations at the ferroelectric phase transition (17 K) in the order of 100 fm. These results indicate that the structure is actively involved in the emergence of ferroelectricity in these materials.
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    Flux line lattice structure in YNi2B2C
    (Physical Society of Japan, 2008-10) Kawano-Furukawa, H; Ohira-Kawamura, S; Tsukagoshi, H; Kobayashi, C; Nagata, T; Sakiyama, N; Yoshizawa, H; Yethiraj, M; Suzuki, J; Takeya, H
    Recently Nakai et at. reported a theoretical H-T phase diagram of flux line lattice (FLL) structure in which successive transitions from a triangular, a square (square(v)), a triangular and another square (square(g)) occur with increasing a magnetic field. Here square(v) and square(g) indicate the FLL structures reflecting anisotropies in the Fermi velocity and the superconducting gap, respectively. In the case of YNi2B2C, square(v) and square(g) should rotate by 45 degrees. The low field transition from triangular to square(v) is observed in RENi2B2C (RE = Er, Tm, Lu, and Y). However, there is no experimental evidence for the appearance of square(g) phase so far. We studied the FLL structure of YNi2B2C in the higher field region by small-angle neutron scattering. Our results show that a large area of the H-T phase diagram is occupied by square(v) phase and there is no evidence for the appearance of square(g) lattice. © 2008, Physical Society of Japan
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    Ground state of a quantum critical system: neutron scattering on Ce(Ru1-xFex)2Ge2
    (American Physical Society, 2007-08) Montfrooij, W; Lamsal, J; Aronson, M; Bennett, M; De Visser, A; Kai, HY; Huy, NT; Yethiraj, M; Lumsden, MD; Qui, YM
    We present neutron scattering data on Ce(Ru1−xFex)2Ge2 that show that magnetic ordering in the vicinity of a quantum critical point is restricted to short length scales even though all moments that are present have lined up with their neighbors. We argue that order is not limited by crystallographic defects but rather that quantum fluctuations disorder the system and dilute the magnetic moments to such an extent that a percolation network forms. The latter naturally explains how E/T scaling is possible in a system whose apparent dimensionality is above the upper critical dimension. © 2007, American Physical Society
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    Intrinsically localized vibrations and the mechanical properties of α-uranium.
    (Elsevier, 2007-10-11) Manley, ME; Yethiraj, M; Sinn, H; Volz, HM; Alatas, A; Lashley, JC; Hults, WL; Lander, GH; Thoma, DJ; Smith, JL
    Recent experiments have indicated that the high-temperature properties of α-uranium may be strongly influenced by the formation of randomly distributed intrinsically localized vibrational modes, just a few atoms across in size. One observation was a loss of mechanical ductility that coincided with the formation of the intrinsically localized mode (ILM). Here, we consider this observation in more detail. In particular, we use the anisotropic thermal expansion behaviour to estimate the strains associated with each ILM and consider the implications for the forces between ILMs and the defects responsible for mechanical deformation. In the process we also suggest that an unusual transition from positive to negative thermal expansion along the [0 1 0] direction may be caused by the formation of ILMs. © 2007, Elsevier Ltd.
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    Magnetoelectric coupling in isotopically substituted TbMn16/18O3 and RMn2O5 (R=Tb, Ho and Y) explored by Raman light spectroscopy
    (Australian Institute of Physics, 2014-02-04) Graham, PJ; Rovillian, P; Mulders, AM; Yethiraj, M; Argyriou, D; Pomjakushina, E; Condor, K; Kenzelmann, M; Ulrich, C
    Multiferroic materials demonstrate excellent potential for next-generation multifunctional devices, as they exhibit coexisting ferroelectric and magnetic orders. In magnetoelectric multiferroics, the existing coupling between both properties offers a unique possibility to manipulate ferroelectricity via magnetic order and vice versa opening unexpected new potential for high-density information storage and sensor applications. At present, the underlying physics of the magnetoelectric coupling is not fully understood, and competing theories propose conflicting experimental outcomes. By studying the lattice and magnetic excitations via Raman light scattering, we have obtained insight into the various coupling mechanism in multiferroic materials like TbMnO3 and RMn2O5 (R = Tb, Ho, and Y). Raman light scattering experiments were performed on TbMn16/18O3 oxygen-isotopesubstituted single crystals. Pronounced anomalies in sign and strength of the phonon shifts at the magnetic phase transition at 43 K and the ferroelectric phase transition at 28 K indicate an interaction between the lattice and the magnetic and electric ordering, providing information about the nature of the competing magnetic interactions present in this compound. Our Raman light scattering experiments on RMn2O5 (R = Tb, Ho, and Y) revealed opposite spin-phonon interactions for R = magnetic Tb and Ho, in contrast to non-magnetic Y. This offers a unique insight in the various competing spin exchange interactions, which lead to the highly frustrated spin structure and finally the multiferroic properties of RMn2O5. Using single crystal neutron diffraction at high magnetic fields (up to 11 T) we were able to determine a theoretically proposed but hitherto unobserved crystallographic phase transition, which naturally explains the origin of the ferroelectric polarization.
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    Magnetoelectric coupling in TbMnO3 explored via Raman spectroscopy
    (Australian Institute of Physics, 2013-02-06) Graham, PJ; Bartkowiak, M; Rovillain, P; Mulders, AM; Yethiraj, M; Pomjakushina, E; Conder, K; Kenzelmann, M; Ulrich, C
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    A new approach to the creation of magnetically modulated structures
    (Australian Institute of Physics, 2010-02-03) Saerbeck, T; Klose, F; Lott, D; Mankey, GJ; Lu, Z; LeClair, PR; Stampfl, APJ; Danilkin, SA; Yethiraj, M; Schreyer, A
    The plethora of structural and magnetic properties observed in many transition metal alloys has attracted a great deal of interest in both the pure and applied sciences [1]. One key attribute of these alloys is that their electronic and magnetic properties are extremely sensitive to not only stoichiometry but order as well. In this paper we report on a new approach of creating a magnetically modulated structure, without changing composition or lattice structure, namely by artificially controlling the degree of chemical order in the material. The compound FePt3, as it is well known from bulk crystals, has the extraordinary property to evolve ferromagnetic (FM) or antiferromagnetic (AFM) phases determined by the degree of chemical ordering [2]. We succeeded in preparing epitaxial FePt3 superlattices of homogeneous composition consisting of an artificially modulated ferro/antiferromagnetic layering sequence simply by alternating the growth temperature. A direct effect of such an exotic FM/AFM interface is the observation of a high exchange bias upon field cooling through the Nèel temperature. In order to quantify the degree of antiferromagnetic ordering, high angle neutron diffraction has been performed using the triple axis spectrometer IN12 (Institute Laue Langevin, Grenoble) and TAIPAN (Australian Nuclear Science and Technology Organisation). Similar to chemically ordered bulk FePt3 the superlattice exhibits the onset of a (½ ½ 0) AFM Bragg peak below a temperature of TN=140 K (Bulk TN=160 K [2]). Using the polarized neutron reflectometry technique at the German research facility GKSS, Geesthacht, a detailed layer resolved magnetic characterization of the superlattice was carried out.
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    Phonon dispersion in superionic copper selenide - observation of soft phonon modes in superionic phase transition
    (Physical Society of Japan, 2010-03-01) Danilkin, SA; Yethiraj, M; Kearley, GJ
    This paper reports lattice dynamical measurements of Cu1.8Se superionic conductor in the superionic α-phase at ambient temperature. Measurements of phonon dispersion curves were performed with the new triple-axis spectrometer, TAIPAN, at the OPAL reactor. We found that TA [100], TA [111] and TA1 [110] phonon branches demonstrate a decrease in frequency at wavevectors q > 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 observed 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 > 0.5 observed in the experiment. © 2010 The Physical Society of Japan
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    Phonon mode softening at the ferroelectric transition in Eu0.5Ba0.5TiO3
    (Springer Nature, 2010-11-10) Rowan-Weetaluktuk, WN; Ryan, DH; Sushkov, AO; Eckel, S; Lamoreaux, SK; Sushkov, OP; Cadogan, JM; Yethiraj, M; Studer, AJ
    We have observed the effects of phonon mode softening at the ferroelectric transition in Eu0.5Ba0.5TiO3 by 151Eu Mössbauer spectroscopy. Both Eu2+ and Eu3+ spectral components are observed in the relative area ratio of 90% : 10% and both show a decrease in subspectral area at the transition, centred at 175 K, due to phonon mode softening. Surprisingly, the temperature dependence of the f-factor shows a much stronger response in the Eu3+ component than in the Eu2+ one. Preliminary analysis of neutron powder diffraction data rules out the possibility that some of the europium might be located on titanium sites. © 2010 Springer Science Business Media B.V.
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    Raman scattering on multiferroic TbMnO3
    (Australian Institute of Physics, 2012-02-02) Graham, PJ; Bartkowiak, M; Mulders, AM; Yethiraj, M; Pomjakushina, E; Ulrich, C
    Multiferroic materials are promising for their technological potential in next-generation microelectronics. They are materials that possess coexisting ferroelectric polarisation and magnetic order, and in particular cases they exhibit coupling between these parameters. This offers the possibility of manipulating ferroelectric polarisation via magnetic order and vice versa, leading to low-powered, ultra-high-capacity solid-state memory or sensor applications. At present, the physics that underpin magnetoelectric coupling in multiferroics is not entirely understood. Competing theories exist that propose different experimental outcomes. In studying the nature of excitations via Raman scattering, this research intends to provide deeper insight into such behaviour in TbMnO3 and for multiferroic materials in general. We have performed Raman spectroscopy measurements on a TbMnO3 crystal and two oxygen-isotope-substituted powder samples. Anomalies in oxygen-octohedra stretching modes have been examined in respect to the sinusoidal and multiferroic phases in this material. Anomalies at TC ~28 K may be ascribed to spin-phonon coupling or to other effects related to the coupled cycloidal-spin and ferroelectric order in the multiferroic phase. Results for anomalies between oxygen-isotope substituted samples indicate that the physical origin for these anomalies is sensitive to oxygen mass. If spin-phonon coupling is responsible for anomalies in the multiferroic phase, our results may suggest that the Dzyaloshinskii-Moriya model, as opposed to the spin-current model, more correctly describes magnetoelectric coupling in TbMnO3. Further experimental and theoretical work is in preparation to explore the implications of our results for magnetoelectric coupling in this material.
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    Subpicometer-scale atomic displacements and magnetic properties in the oxygen-isotope substituted multiferroic DyMn O3
    (American Physical Society, 2017-02-27) Narayanan, N; Graham, PJ; Reynolds, N; Li, F; Rovillain, P; Hester, JR; Kimpton, JA; Yethiraj, M; McIntyre, GJ; Hutchison, WD; Ulrich, C
    We have investigated DyMn16O3 and its isotopically substituted counterpart DyMn18O3 by neutron powder diffraction, x-ray diffraction, and heat capacity measurements to investigate the mechanism leading to its magnetically induced electric polarization. 18O isotope substitution does not influence the magnetic ordering temperature of the Mn ions TN,Mn or the multiferroic ordering temperature Tl coinciding with the onset of the spin spiral phase; however, it does reduce the ordering temperature of Dy into its incommensurate magnetic state TN,Dy from 7.0(1) K to 5.9(1) K. The temperature dependence of the magnetic propagation vector, qIC, changes with 18O substitution, while Tl remains almost constant, independent of qIC. Pronounced changes in the lattice parameters occur at the various phase transitions. Furthermore, distinct subpicometer-scale distortions of the MnO6 octahedra and displacements of the Dy ions are observed below the ferroelectric phase transition at Tl in both samples, pointing toward the mechanism for electric polarization and its coupling to the orbital degrees of freedom. ©2017 American Physical Society
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    Symmetry of ferroelectric phase of SrTi18O3 determined by ab initio calculations
    (American Physical Society, 2011-02-11) Bartkowiak, M; Kearley, GJ; Yethiraj, M; Mulders, AM
    Substitution of more than 33% of the naturally abundant 16O in strontium titanate SrTiO3 by 18O causes the system to become ferroelectric at low temperatures. The ferroelectricity has been observed via susceptibility measurements, but to date the details of the ferroelectric phase and the phase transition are unclear. Using ab initio density functional theory and lattice-dynamics calculations, we find that the stable structure of the ferroelectric phase is orthorhombic with Ima2 symmetry. The Ima2 point group is noncentrosymmetric and the proposed structure exhibits an electric dipole moment of (0.57 0 0) eÅ. The Ima2 symmetry is consistent with the limited structural details that are reported using neutron diffraction and Raman spectroscopy. © 2011, American Physical Society
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    Zener double exchange from local valence fluctuations in magnetite
    (American Physical Society, 2007-12-14) McQueeney, RJ; Yethiraj, M; Chang, S; Montfrooij, W; Perring, TG; Honig, JM; Metcalf, P
    Magnetite (Fe3O4) is a mixed valent system where electronic conductivity occurs on the B site (octahedral) iron sublattice of the spinel structure. Below Tv=123 K, a metal-insulator transition occurs which is argued to arise from the charge ordering of 2+ and 3+ iron valences on the B sites (Verwey transition). Inelastic neutron scattering measurements show that optical spin waves propagating on the B site sublattice (similar to 80 meV) are shifted upwards in energy above Tv due to the occurrence of B-B ferromagnetic double exchange in the mixed valent phase. The double exchange interaction affects only spin waves of Δ5 symmetry, not all modes, indicating that valence fluctuations are slow and the double exchange is constrained by short-range electron correlations above Tv. © 2007, American Physical Society