Browsing by Author "Bartkowiak, 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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    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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    Evolution of charge order through the magnetic phase transition of LuFe2O4
    (American Physical Society, 2012-07-13) Bartkowiak, M; Mulders, AM; Scagnoli, V; Staub, U; Pomjakushina, E; Conder, K
    The charge order in multiferroic LuFe(2)O(4) has been investigated with resonant x-ray diffraction at the Fe K edge in the combined charge ordered and magnetic phase. The energy dependence of the charge order reflection (1/3 1/3 7/2) has been analyzed in detail to investigate the charge disproportionation between the iron sites as a function of temperature. It is found that the charge disproportionation is constant within 0.02e across the Neel temperature T(N). The charge order reflection exhibits a decrease in intensity with increasing temperature which is attributed to an increase in the probability of electron hopping. We confirm the increase in polarization at T(N) is not of static origin but rather dynamic. Our observations are consistent with antiferromagnetically aligned magnetic moments inhibiting the double exchange mechanism and reducing the probability of electrons hopping between Fe(2+) and Fe(3+) in the magnetic phase. © 2012, American Physical Society.
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    Ferroelectric charge order stabilized by antiferromagnetism in multiferroic LuFe(2)O(4)
    (Australian Institute of Physics, 2012-02-02) Mulders, AM; Bartkowiak, M; Hester, JR; Pomjakushina, E; Condor, K
    Neutron diffraction measurements on multiferroic LuFe2O4 show changes in the antiferromagnetic (AFM) structure characterized by wavevector q = ( 1/3 1/3 ½) as a function of electric field cooling procedures. The increase of intensity from all magnetic domains and the decrease in the 2D magnetic order observed below the Neel temperature are indicative of increased ferroelectric charge order. The AFM order changes the dynamics of the CO state, and stabilizes it. It is determined that the increase in electric polarization observed at the magnetic ordering temperature is due to a transition from paramagnetic 2D charge order to AFM 3D charge order.
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    Ferroelectric charge order stabilized by antiferromagnetism in multiferroic LuFe(2)O(4)
    (American Physical Society, 2011-10-03) Mulders, AM; Bartkowiak, M; Hester, JR; Pomjakushina, E; Conder, K
    Neutron diffraction measurements on multiferroic LuFe(2)O(4) show changes in the antiferromagnetic (AFM) structure characterized by wave vector q = (1/3 1/3 1/2) as a function of electric field cooling procedures. The increase of intensity from all magnetic domains and the decrease in the two-dimensional (2D) magnetic order observed below the Neel temperature are indicative of increased ferroelectric charge order (CO). The AFM order changes the dynamics of the CO state, and stabilizes it. It is determined that the increase in electric polarization observed at the magnetic ordering temperature is due to a transition from paramagnetic 2D charge order to AFM three-dimensional charge order.© 2011, American Physical Society
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    Magnetically driven electric polarization in frustrated magnetic oxide multiferroics
    (Australian Institute of Physics, 2014-02-04) Narayanan, N; Reynolds, NM; Li, F; Mulders, AM; Rovillian, P; Ulrich, C; Bartkowiak, M; Hester, JR; McIntyre, GJ; Hutchinson, WD
    In multiferroics more than one ferroic order can coexist and in the present case we are interested in systems which exhibit simultaneous magnetic ordering and electric polarization (EP). Of particular interest are frustrated magnetic materials that exhibit an electric polarization that is strongly coupled to the magnetism [1]. Examples of such multiferroics are RMnO3 (R= Tb, Dy), Ni3V2O8, and RbFe(MoO4)2 [2-4]. This coupling can be utilized in applications such as magnetoelectric random access memory. Although technically relevant, the coupling mechanism between these two orders is complicated [1]. Whereas the magnetic ordering results from exchange interaction of unpaired spins, origins of EP coupled to the magnetic ordering depends on the interplay between lattice, orbital, spin and charge degrees of freedom. Several mechanisms such as the inverse Dzyaloshinskii-Moriya interaction, magnetostriction and coupling of the chirality to the crystal structure or a combination of them are currently discussed depending on the compound [2-5]. Additionally EP has ionic and electronic contributions. In the present work we investigate the coupling of magnetism to EP involving all three above mechanisms, in orthorhombic DyMnO3 (DMO), Cu3Nb2O8 and Ba3NiNb2O9 with neutron powder diffraction (NPD), magnetization and heat capacity measurements focusing on the magnetic and multiferroic phase transitions. In order to investigate the role of the lattice distortion or equivalently the role of oxygen, isotope substitution of 16O with 18O was performed on DMO. All samples are prepared as single phases via the solid state route and NPD experiments are carried out at Wombat and at Echidna at OPAL.
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    Magnetically driven electric polarization in frustrated magnetic oxide multiferroics
    (Australian Institute of Physics, 2014-02-06) Narayanan, N; Reynolds, NM; Li, F; Mulders, AM; Rovillain, P; Ulrich, C; Bartkowiak, M; Hester, JR; McIntyre, GJ; Hutchison, WD
    In multiferroics more than one ferroic order can coexist and in the present case we are interested in systems which exhibit simultaneous magnetic ordering and electric polarization (EP). Of particular interest are frustrated magnetic materials that exhibit an electric polarization that is strongly coupled to the magnetism [1]. Examples of such multiferroics are RMnO3 (R= Tb, Dy), Ni3V2O8, and RbFe(MoO4)2 [2-4]. This coupling can be utilized in applications such as magnetoelectric random access memory. Although technically relevant, the coupling mechanism between these two orders is complicated [1]. Whereas the magnetic ordering results from exchange interaction of unpaired spins, origins of EP coupled to the magnetic ordering depends on the interplay between lattice, orbital, spin and charge degrees of freedom. Several mechanisms such as the inverse Dzyaloshinskii - Moriya interaction, magnetostriction and coupling of the chirality to the crystal structure or a combination of them are currently discussed depending on the compound [2-5]. Additionally EP has ionic and electronic contributions. In the present work we investigate the coupling of magnetism to EP involving all three above mechanisms, in orthorhombic DyMnO3 (DMO), Cu3Nb2O8 and Ba3NiNb2O9 with neutron powder diffraction (NPD), magnetization and heat capacity measurements focusing on the magnetic and multiferroic phase transitions. In order to investigate the role of the lattice distortion or equivalently the role of oxygen, isotope substitution of 16O with 18O was performed on DMO. All samples are prepared as single phases via the solid state route and NPD experiments are carried out at Wombat and at Echidna at OPAL
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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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    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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    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