Browsing by Author "Graham, PJ"

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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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    Electromagnons in multiferroics probed by Raman light scattering comparison to neutron scattering investigations
    (Australian Institute of Nuclear Science and Engineering, 2012-11-15) Rovillain, P; Graham, PJ; Reynolds, N; Narayanan, N; Gallis, Y; Sacuto, A; Measson, MA; Sakata, H; McIntyre, GJ; Mulders, AM; Ulrich, C; Cazayous, M
    In multiferroic materials the two antagonistic effects, magnetic and ferroelectric orders, exist simultaneously. The switching of these orders is known as magnetoelectric coupling. Thereby, magnetoelectric materials can potentially be used to control spins or electric polarization with the application of an external electric or magnetic field, respectively. This makes them promising candidates for applications in spintronics or magnonics that use magnetic excitations for information processing. BiFe03, is the rare case where both orders coexist at room temperature. Using Raman scattering, we show that in BiFe03 the spin-wave energy can be tuned electrically by over 30%, in a non-volatile way with virtually no power dissipation. In TbMnO3 (and RMn2O5) the coupling of the orders gives rise to a hybrid excitation: the electromagnon. Electromagnons are spin wave excitations which possess an electric dipole. We have identified the magnetic excitation underneath the electromagnon by comparison with neutron measurement and further the phonon mode at the origin of the dipole activity. We have extended our investigations to Raman scattering and inelastic neutron scattering on DyMn03. The combination of both techniques offers the opportunity to obtain more information on the electromagnetic interaction in this type of multiferroic material.
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    Inelastic neutron scattering in multiferroic materials
    (Australian Institute of Physics, 2012-02-02) Reynolds, NM; Graham, PJ; Mulders, AM; McIntyre, GJ; Dainlkin, SA; Fujioka, J; Tokura, Y; Keimer, B; Reehuis, M; Ulrich, C
    In order to obtain a deeper understanding of the spin interactions between the magnetic moments of the Tb-ions and the Mn-ions in multiferroic TbMnO3, inelastic neutron scattering experiments (at the ILL in Grenoble and the Bragg Institute at ANSTO) are performed on isostructural, non-multiferroic TbVO3. Acoustic and optical magnon branches are identified at energies comparable to the spin wave excitation spectrum of YVO3. In addition, a crystal field excitation arising from the Tb-ions is identified at the energy of 14.9 meV. This is substantially larger than the crystal field excitation at 4.5 meV in TbMnO3.
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    Low pressure synchrotron x-ray powder diffraction of Cu5-xMxSbO6 (M=Cr, Mn, W)
    (Australian Institute of Physics, 2016-02-04) Wilson, DJ; Söhnel, T; Smith, KL; Brand, HEA; Ulrich, C; Graham, PJ; Chang, FF; Allison, MC; Vyborna, NH
    The large crystallographic and chemical diversity of copper-based metal oxides is one of their highlighting features and cause for pursuit into copper based material research. An interesting feature seen in copper based metal oxides is the coexistence of different copper oxidation states, in different crystallographic positions, within the same compound. This can lead to a mixture of magnetically active Cu2+ and magnetically inactive Cu1+ within the same compound, with different structural motifs. One interesting compound that demonstrates this coexistence of mixed copper oxidation states is Cu5SbO6, which crystallises in a modified delafossite structure type (CuFeO2). Here, the magnetically active brucite-like CuO2 layer was diluted in an ordered fashion with non-magnetic Sb5+. These layers were separated by linearly coordinated, magnetically inactive Cu1+. Rietveld refinements on a range of preparation temperatures revealed a low-temperature (LT) and high-temperature modification (HT) phase transition. This is related to an ordering (HT)/disordering (LT) effect of the Sb5+/Cu2+ brucite-like layers between the Cu1+ ions. Substituting the Cu2+ or Sb5+ in the layers with other transition metals (Cr, Mn, W) could present interesting changes to the properties of the material, and potentially influence the ordered/disordered stacking of the layers. By using solid-state Raman spectroscopy, we could show that this structure displayed a pressure-induced phase transition at room temperature for the ordered modification, which was not observed for the disordered modification. Lowering the pressure from ambient down to 20 mbar showed phonon modes at about 700 cm-1 and 550 cm-1 disappeared almost completely. Neutron powder diffraction experiments were conducted at atmospheric and low pressure on both ordered and disordered modifications. On analysis of the neutron diffraction patterns, we could show a very small shift in the reflections, and thus changes in the unit cell parameters, for the ordered modification, while these shifts were not observed for the disordered modification. These shifts should also be observed in synchrotron powder diffraction patterns. Therefore, we investigated the nature of this phase transition with variable pressure synchrotron X-ray powder diffraction.
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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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    Oxygen nonstoichiometry in (Ca2CoO3)0.62(CoO2): a combined experimental and computational study
    (American Chemical Society, 2014-07-28) Schrade, M; Casolo, S; Graham, PJ; Ulrich, C; Li, S; Løvvik, OM; Finstad, TG; Norby, T
    The oxygen nonstoichiometry in the misfit calcium cobaltite (Ca2CoO3)0.62(CoO2) has been studied experimentally and by density functional theory (DFT) calculations. The standard oxidation enthalpy ΔH0Ox of oxygen deficient (Ca2CoO3)0.62(CoO2) was measured directly using simultaneous thermogravimetry and differential scanning calorimetry. ΔH0Ox was found to be in agreement with the prediction from a previously published defect chemical model based on purely thermogravimetrical analysis. A series of samples with different oxygen vacancy concentration was prepared by annealing in air, followed by rapid quenching. Room-temperature Raman spectroscopy showed a sharp mode at 700 cm–1 decreasing in intensity with increasing vacancy concentration. We discuss this observation as evidence for oxygen vacancies being preferably formed within the central layer of the Ca2CoO3 subsystem. DFT calculations demonstrated that the calculated electronic structure is sensitive to the chosen model of the crystal structure. Still, for all investigated models, the standard formation enthalpy of oxygen vacancies within the Ca2CoO3 moiety was much lower than that for a site within the CoO2 layer, in agreement with the presented experimental data. © 2014, American Chemical Society.
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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