Browsing by Author "Mulders, AM"
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- Item18O isotope substitution on the multiferroic compound DyMnO3(Australian Institute of Physics, 2013-02-06) Narayanan, N; Li, F; Hutchison, WD; Reynolds, NM; Rovillain, P; Ulrich, C; Hester, JR; McIntyre, GJ; Mulders, AMNot available
- ItemAb 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, AMStrontium 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.
- ItemCircularly polarized soft x-ray diffraction study of helical magnetism in hexaferrite(American Physical Society, 2010-03-01) Mulders, AM; Lawrence, SM; Princep, AJ; Staub, U; Bodenthin, Y; García-Fernández, M; Garganourakis, M; Hester, JR; Macquart, RB; Ling, CDMagnetic spiral structures can exhibit ferroelectric moments as recently demonstrated in various multiferroic materials. In such cases the helicity of the magnetic spiral is directly correlated with the direction of the ferroelectric moment and measurement of the helicity of magnetic structures is of current interest. Soft x-ray resonant diffraction is particularly advantageous because it combines element selectivity with a large magnetic cross-section. We calculate the polarization dependence of the resonant magnetic x-ray cross-section (electric dipole transition) for the basal plane magnetic spiral in hexaferrite Ba0.8Sr1.2Zn2Fe12O22 and deduce its domain population using circular polarized incident radiation. We demonstrate there is a direct correlation between the diffracted radiation and the helicity of the magnetic spiral. © 2010, 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.
- ItemDirect observation of charge order and an orbital glass state in multiferroic LuFe2O4(American Physical Society, 2009-08-14) Mulders, AM; Lawrence, SM; Staub, U; García-Fernández, M; Scagnoli, V; Mazzoli, C; Pomjakushina, E; Conder, K; Wang, YDGeometrical frustration of the Fe ions in LuFe2O4 leads to intricate charge and magnetic order and a strong magnetoelectric coupling. Using resonant x-ray diffraction at the Fe K edge, the anomalous scattering factors of both Fe sites are deduced from the (h/3 k/3 l/2) reflections. The chemical shift between the two types of Fe ions equals 4.0(1) eV corresponding to full charge separation into Fe2+ and Fe3+. The polarization and azimuthal angle dependence of the superlattice reflections demonstrate the absence of differences in anisotropic scattering revealing random orientations of the Fe2+ orbitals characteristic of an orbital glass state. © 2009, American Physical Society
- ItemDoping and temperature dependence of Mn 3d states in A-site ordered manganites(American Physical Society, 2010-12-06) García-Fernández, M; Staub, U; Bodenthin, Y; Pomjakushin, V; Mirone, A; Fernández-Rodríguez, J; Scagnoli, V; Mulders, AM; Lawrence, SM; Pomjakushina, EWe present a systematic study of the electronic structure in A-site ordered manganites as function of doping and temperature. The energy dependencies observed with soft x-ray resonant diffraction (SXRD) at the Mn L2,3 edges are compared with structural investigations using neutron powder diffraction as well as with cluster calculations. The crystal structures obtained with neutron powder diffraction reflect the various orbital and charge ordered phases, and show an increase in the Mn-O-Mn bond angle as function of doping and temperature. Cluster calculations show that the observed spectral changes in SXRD as a function of doping are more pronounced than expected from an increase in bandwitdh due to the increase in Mn-O-Mn bond angle and are best described by holes that are distributed at the neighboring oxygen ions. These holes are not directly added to the Mn 3d shell but centered at the Mn site. In contrast, the spectral changes in SXRD as function of temperature are best described by an increase of magnetic correlations. This demonstrates the strong correlations between orbitals and magnetic moments of the 3d states. © 2010, American Physical Society
- ItemElectromagnons 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, MIn 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.
- ItemEvolution 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, KThe 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.
- ItemFerroelectric 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, KNeutron 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
- ItemFerroelectric 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, KNeutron 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.
- ItemHigh/low-moment phase transition in hexagonal Mn-Fe-P-Si compounds(American Physical Society, 2012-07-30) Dung, NH; Zhang, L; Ou, ZQ; Zhao, L; van Eijck, L; Mulders, AM; Avdeev, M; Suard, E; van Dijk, NH; Brück, EUsing high-resolution neutron diffraction measurements for Mn-rich hexagonal Mn-Fe-P-Si compounds, we show that the substitution of Mn for Fe on the 3f sites results in a linear decrease of the Fe/Mn(3f) magnetic moments, while the Mn(3g) magnetic moments remain constant. With increasing temperature, the Mn(3g) magnetic moments show almost no change, while the Fe/Mn(3f) moments decrease quickly when the transition temperature is approached. The reduction of the magnetic moments at the transition temperature and in the high-temperature range is discussed based on changes in interatomic distances and lattice parameters and high-temperature magnetic-susceptibility measurement. © 2012, American Physical Society.
- ItemInduced noncollinear magnetic order of Nd3+ in NdNiO3 observed by resonant soft x-ray diffraction(American Physical Society, 2008-03) Scagnoli, V; Staub, U; Bodenthin, Y; García-Fernández, M; Mulders, AM; Meijer, GI; Hammerl, GSoft x-ray resonant magnetic diffraction at the Nd M edges was performed on a NdNiO3 epitaxial film to investigate the magnetic ordering of the Nd ions below the metal-insulator transition. A noncollinear magnetic structure induced by the Ni magnetic moments best describes the azimuthal angle dependency of the (1/2, 0, 1/2) reflection. This confirms the Ni spin structure observed with soft x-ray diffraction experiments performed at the Ni L edge, providing further evidence of charge disproportionation without orbital order below the metal-insulator transition in NdNiO3. © 2008, American Physical Society
- ItemInelastic neutron scattering in multiferroic materials(Australian Institute of Physics, 2012-02-02) Reynolds, NM; Graham, P; Mulders, AM; McIntyre, G; Danilkin, SI; Fujioka, J; Tokura, Y; Keimer, B; Reehuis, M; Ulrich, CMagnetism and ferroelectricity are both exciting physical properties and are used in everyday life in sensors and data storage. Multiferroic materials are materials where both properties coexist. They offer a great potential for future technological applications like the increase of data storage capacity or in novel senor applications. The coupling mechanism between both antagonistic effects, electrical polarization and magnetic polarization, is not fully understood yet. The aim of the project is the systematic study of multiferroic materials such as TbMnO3 and related materials by inelastic neutron scattering (INS) in order to obtain a deeper insight into the interplay between the two interacting effects. We have started our investigations with TbVO3, which is isostructural to TbMnO3, but has a collinear antiferromagnetic spin arrangement [1] instead of a cycloidal spin structure [2]. By using inelastic neutron scattering (INS) we have obtained the spin wave dispersion relation and the crystal field excitations of the Tb-ions in TbVO3. These data will be compared with previously obtained data of D. Senff on TbMnO3 [3]. Experiments were performed at the ILL in Grenoble, France and at the research reactor OPAL at ANSTO, Australia.
- ItemInelastic 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, CIn 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.
- ItemMagnetic and electronic co states in the layered cobaltate GdBaCo2O5.5-x(American Physical Society, 2008-08) García-Fernández, M; Scagnoli, V; Staub, U; Mulders, AM; Janousch, M; Bodenthin, Y; Meister, D; Patterson, BD; Mirone, A; Tanaka, Y; Nakamura, T; Grenier, S; Huang, YJ; Conder, KWe have performed nonresonant x-ray diffraction, resonant soft and hard x-ray magnetic diffraction, soft x-ray absorption, and x-ray magnetic circular dichroism measurements to clarify the electronic and magnetic high-spin (HS) state at the states of the Co3+, ions in GdBaCo2O5.5. Our data are consistent with a Co-Py(3+) pyramidal sites and a Co-Oc(3+), low-spin (LS) state at the octahedral sites. The structural distortion with a doubling of the a axis (2a(p)X2a(p)X2a(p) cell) shows alternating elongations and contractions of the pyramids, and indicates that the metal-insulator transition is associated with orbital order in the t(2g) orbitals of the Co-Py(3+) HS state. This distortion corresponds to an alternating ordering of xz and yz orbitals along the a and c axes for the Co-Py(3+). The orbital ordering and pyramidal distortion lead to deformation of the octahedra but the Co-Oc(3+) LS state does not allow an orbital order to occur for the Co-Oc(3+), ions. The soft x-ray magnetic diffraction results indicate that the magnetic moments are aligned in the ab plane but are not parallel to the crystallographic a or b axes. The orbital order and the doubling of the magnetic unit cell along the c axis support a noncollinear magnetic structure. The x-ray magnetic circular dichroism data indicate that there is a large orbital magnetic contribution to the total ordered Co moment. © 2008, American Physical Society
- ItemMagnetically 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, WDIn 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
- ItemMagnetically 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, WDIn 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.
- ItemMagnetoelectric 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, CMultiferroic 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.
- ItemMagnetoelectric 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, CNot available
- ItemNeutron diffraction study on the magnetic structure of Fe2P-based Mn0.66Fe1.29P1-xSix melt-spun ribbons(Elsevier Science BV., 2013-08-01) Ou, ZQ; Zhang, L; Dung, NH; van Eijck, L; Mulders, AM; Avdeev, M; van Dijk, NH; Brück, EWe report on the magnetic and structural properties of Mn0.66Fe1.29P1−xSix melt-spun ribbons with 0.34≤x≤0.42 that are promising candidates for high-temperature magnetocaloric applications. A magnetic moment of up to 4.57 μB/f.u. for x=0.34 indicates high magnetic density in the system, which is certainly advantageous for the magnetocaloric effects. Introducing site disorder at the 3g site by replacing 1/3 of Fe with Mn appears to enhance the magnetic interaction, while the strong magnetoelastic coupling is maintained. This site disorder also shows a stabilizing effect on the hexagonal crystal structure, which is maintained to a high Si content. The moment alignment within the crystallographic unit cell is also affected when the Si content is increased from x=0.34 to 0.42 in the Mn0.66Fe1.29P1−xSix compounds as the canting angle with respect to the c-direction increases. © 2013, Elsevier Ltd.