ANSTO :: Browsing by Author "Ulrich, C"

Browsing by Author "Ulrich, C"

Now showing 1 - 20 of 64

18O 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, AM
Not available
Anomalous spin dynamics and orbital excitations in Mott-insulating titanates
(Australian Institute of Physics, 2010-02-03) Ulrich, C; Khaliullin, G; Ament, LJP; Ghiringhelli, G; Braicovich, L; Lorenz, T; Tokura, Y; van den Brink, J; Keimer, B
Spin and orbital degrees of freedom play an important role in the various phenomena of strongly correlated electron systems like unconventional high-temperature superconductivity in cuprates or colossal magnetoresistance in manganates. Our extensive neutron scattering experiments on the cubic perovskite titanates LaTiO3 and YTiO3 lead to the discovery of a highly unusual magnetic ground state which is in contradiction to the standard Goodenough-Kanamori rules, but indicates the presence of strong orbital fluctuations [1-4]. Raman light scattering spectra of LaTiO3 and YTiO3 exhibit unexpected features in the high energy range well above the phonon spectrum [5]. Using momentum dependent resonant inelastic x-ray scattering (RIXS) experiments in combination with theoretical calculations, we were able to identify these excitations as collective orbital excitations (orbital waves termed ‘orbitons’) [6-7].
Comparison 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, C
Magnetism 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.
Competing exchange interactions on the verge of a metal-insulator transition in the two-dimensional spiral magnet Sr3Fe2O7
(Americal Physical Society, 2014-10-03) Kim, JH; Jain, A; Reehuis, M; Khaliullin, G; Peets, DC; Ulrich, C; Park, JT; Faulhaber, E; Hoser, A; Walker, HC; Adroja, DT; Walters, AC; Inosov, DS; Maljuk, A; Keimer, B
We report a neutron scattering study of the magnetic order and dynamics of the bilayer perovskite Sr 3 Fe 2 O 7 , which exhibits a temperature-driven metal-insulator transition at 340 K. We show that the Fe 4+ moments adopt incommensurate spiral order below T N =115 K and provide a comprehensive description of the corresponding spin-wave excitations. The observed magnetic order and excitation spectra can be well understood in terms of an effective spin Hamiltonian with interactions ranging up to third-nearest-neighbor pairs. The results indicate that the helical magnetism in Sr 3 Fe 2 O 7 results from competition between ferromagnetic double-exchange and antiferromagnetic superexchange interactions whose strengths become comparable near the metal-insulator transition. They thus confirm a decades-old theoretical prediction and provide a firm experimental basis for models of magnetic correlations in strongly correlated metals. © 2014, American Physical Society.
Complex 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, N
Multiferroic 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.
Crystallographic and magnetic structure study in SrCoO3-x by high resolution x-ray and neutron powder diffraction
(Australian Institute of Physics, 2016-02-04) Chang, FF; Reehuis, M; Hester, JR; Avdeev, M; Xiang, F; Wang, X; Seidel, J; Ulrich, C
Transition metal oxides (TMOs) represent a wide set of materials with a broad range of functionalities, including superconductivity, magnetism, and ferroelectricity, which can be tuned by careful choice of parameters such as strain, oxygen content, and applied electric and magnetic fields. This tunability makes TMO’s ideal candidate materials for use in developing novel information and energy technologies and SrCoO3 provides a particularly interesting system for investigation due to its propensity to form oxygen-vacancy-ordered structures as the oxygen content is decreased. The ties between structural and functional properties of this material are obvious as it undergoes simultaneously structural and magnetic phase transitions between two topotactic phases: from a ferromagnetic perovskite phase at SrCoO3.0 to the antiferromagnetic brownmillerite SrCoO2.5. In this study we have determined their crystallographic and magnetic structures of SrCoO2.50, SrCoO2.875, and cubic SrCoO3.00 using high resolution X-ray and neutron powder diffraction from 4 K to 600 K. The correct structure of oxygen-deficient end-member SrCoO2.5 was determined in space group of Imma, instead of Pnma or Ima2 proposed previously, with G-type antiferromagnetic order up to TN = 570 K. In SrCoO2.875, clear peak splitting was observed from (200) in cubic phase to (004) and (440) in tetragonal phase, indicating that the precise structure is I4/mmm with a = b = 10.829(9) Å and c = 7.684(2) Å at 95 K, and the corresponding magnetic structure is ferromagnetic with 1.86(4) μB per formula, in accordance to a spin configuration of cobalt ions with an intermediate spin state of both on Co3+ and on Co4+. The end member SrCoO3.00 possesses a simple cubic crystal structure with a = 3.817(2) Å at 95 K, and ferromagnetic order up to 280 K. The magnetic moment of 1.96(8) μB /Co4+ corresponds to an intermediate spin state of Co4+.
Direct evidence for the spin cycloid in strained nanoscale bismuth ferrite thin films
(Australian Institute of Physics, 2017-01-31) Bertinshaw, J; Maran, R; Callori, SJ; Ramesh, V; Cheung, J; Dainlkin, SA; Lee, WT; Hu, S; Seidel, J; Valanoor, N; Ulrich, C
Multiferroic 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 exist far beyond room temperature, making it the ideal candidate for technological applications. In particular, magnonic devices that utilize electric control of spin waves mediated by complex spin textures are an emerging direction in spintronics research. To realize magnonic devices, a robust long-range spin cycloid with well known direction is desired, since it is a prerequisite for the magnetoelectric coupling. Despite extensive investigation, the stabilization of a large-scale uniform spin cycloid in nanoscale (100 nm) thin BiFeO3 films has not been accomplished. Here, we demonstrate cycloidal spin order in 100 nm BiFeO3 thin films through the careful choice of crystallographic orientation, and control of the electrostatic and strain boundary conditions during growth [1]. Neutron diffraction, in conjunction with X-ray diffraction, reveals an incommensurate spin cycloid with a unique [112] propagation direction. While this direction is different from bulk BiFeO3, the cycloid length and Néel temperature remain equivalent to bulk single crystals. 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.
The effect of oxygen isotopes substitution on magnetism in multiferroic CaMn7O12
(Australian Institute of Nuclear Science and Engineering, 2012-11-15) Li, F; Narayanan, N; Hutchison, WD; Ulrich, C; McIntyre, GJ
Multiferroic materials, where ferroelectricity and ferromagnetism coexist and interact and one property can be used to drive the other, can find potential applications in spintronics and information technology and form the basis for four-state memory. However the details of coupling between these two orders are not yet understood. Competing theories of inherent electronic structure and ionic displacement are proposed to explain this coupling, but no experimental evidence currently exists to differentiate these models. To investigate the interaction between magnetic moments and electric dipoles on a fundamental level, this study will extend the isotopic pure oxygen substitution, a widely used technique for the investigation of high temperature superconductors, to multiferroics. Single crystal CaMn{sub 7}0{sub 12} showing the largest magnetically induced electric polarization measured to date is chosen as a test material and synthesized by flux method. The preliminary results show that single crystals of a size —100x100x100 μm can be obtained however a small amount of CaMn 3 0 6 impurity phase is also detected in XRD. Efforts on growing a larger single crystal are under way.
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.
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.
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.
Electronic and phononic Raman scattering in detwinned YBa2Cu3O6.95 and Y0.85Ca0.15Ba2Cu3O6.95: s-wave admixture to the d(x)(2)-y(2)-wave order parameter
(America Physical Society, 2009-08-01) Bakr, M; Schnyder, AP; Klam, L; Manske, D; Lin, CT; Keimer, B; Cardona, M; Ulrich, C
Inelastic light (Raman) scattering has been used to study electronic excitations and phonon anomalies in detwinned, slightly overdoped YBa2Cu3O6.95 and moderately overdoped Y0.85Ca0.15Ba2Cu3O6.95 single crystals. In both samples modifications of the electronic pair-breaking peaks when interchanging the a and b axis were observed. The lineshapes of several phonon modes involving plane and apical oxygen vibrations exhibit pronounced anisotropies with respect to the incident and scattered light-field configurations. Based on a theoretical model that takes both electronic and phononic contributions to the Raman spectra into account, we attribute the anisotropy of the superconductivity-induced changes in the phonon lineshapes to a small s-wave admixture to the d(x)(2)-y(2) pair wave function. Our theory allows us to disentangle the electronic Raman signal from the phononic part and to identify corresponding interference terms. We argue that the Raman spectra are consistent with an s-wave admixture with an upper limit of 20%. © 2009, American Physical Society.
Element-specific depth profile of magnetism and stoichiometry at the La0.67Sr0.33MnO3/BiFeO3 interface
(American Physical Society, 2014-07-11) Bertinshaw, J; Brück, S; Lott, D; Fritzsche, H; Khaydukov, Y; Soltwedel, O; Keller, T; Goering, E; Audehm, P; Cortie, DL; Hutchison, WD; Ramasse, QM; Arredondo, M; Maran, R; Nagarajan, V; Klose, F; Ulrich, C
Depth-sensitive magnetic, structural, and chemical characterization is important in the understanding and optimization of physical phenomena emerging at the interfaces of transition metal oxide heterostructures. In a simultaneous approach we have used polarized neutron and resonant x-ray reflectometry to determine the magnetic profile across atomically sharp interfaces of ferromagnetic La0.67Sr0.33MnO3/multiferroic BiFeO3 bilayers with subnanometer resolution. In particular, the x-ray resonant magnetic reflectivity measurements at the Fe and Mn resonance edges allowed us to determine the element-specific depth profile of the ferromagnetic moments in both the La0.67Sr0.33MnO3 and BiFeO3 layers. Our measurements indicate a magnetically diluted interface layer within the La0.67Sr0.33MnO3 layer, in contrast to previous observations on inversely deposited layers [P. Yu et al., Phys. Rev. Lett. 105, 027201 (2010)]. Additional resonant x-ray reflection measurements indicate a region of altered Mn and O content at the interface, with a thickness matching that of the magnetic diluted layer, as the origin of the reduction of the magnetic moment.© 2014, American Physical Society.
Evolution of crystallization and magnetic phase transition in Cu1-xZnxFe2O4 studied by neutron powder diffraction
(American Physical Society, 2017-03-13) Chang, FF; Avdeev, M; Deng, G; Hester, JR; Wang, X; Ulrich, C
High resolution and high intensity neutron powder diffraction were applied to study the crystallographic and magnetic phase transition in Cu1-xZnxFe2O4 from 4 K to 750 K. Structural phase transition from cubic to tetragonal phase was observed in CuFe2O4. Ferrimagnetic order was observed in CuFe2O4 and short-range antiferromagnetic scattering was observed below 10 K in cubic ZnFe2O4 which is strongly restrained by addition of slightly amount of Cu2+ ions. Upon doping, ferromagnetic order temperature was gradually reduced from 789 K. Collinear spin setting was observed and no indication of frustration was found even up to doping rate of x = 0.6. Highly frustrated Cu0.04Zn0.96Fe2O4 and ZnFe2O4 behave short-range antiferromagnetic order, induced by the competing between ferromagnetic interaction from first-nearest neighbor and antiferromagnetic interaction from the third-nearest neighbor in tetrahedron formed by Fe ions on B sites. © 2021 American Physical Society
Evolution of magnetic phase and cation distribution in Cu1-xZnxFe2O4 studied by neutron powder diffraction
(Australian Institute of Physics, 2015-02-06) Chang, FF; Deng, DC; Avdeev, M; Hester, JR; Bertinshaw, J; Ulrich, C
CuFe2O4 is a highly interesting material as it is a ferrimagnet with an unusual high magnetic ordering temperature of 780 K. ZnFe2O4, on the other hand, is a frustrated spin system with antiferromagnetic order below 10 K. By doping nonmagnetic Zn ions in CuFe2O4, frustration can be introduced and interesting properties might emerge. Given that, high resolution and high intensity neuron powder diffraction techniques have been applied to study the structural and magnetic phase transition in Cu1-xZnxFe2O4 from 4 K to 750 K. Coexistence of cubic and tetragonal structure in CuFe2O4 was observed in a wide temperature range, which indicates a second order phase transition nature. This transition is caused by Jahn-Teller distortion of the CuO6 octahedra. Although CuFe2O4 and ZnFe2O4 are inverse and normal spinels, respectively, mixed cation distribution was found in doped samples, with Cu and Zn ions sitting both either on the tetrahedral or the octahedral sites. All the doped Cu1-xZnxFe2O4 (x = 0.2 - 1) samples crystallise in the cubic structure and order in the ferrimagnetic spin configuration. Upon doping, the value of oxygen position parameter μ increases, indicating the compression of the octahedra with increasing Zn-composition. Short-range antiferromagnetic order was observed below 10 K in cubic ZnFe2O4. The spin frustration, which leads to the antiferromagnetic order in Cu0.04Zn0.96Fe2O4 and ZnFe2O4 is induced by the competing interaction between the first nearest neighbor and the third nearest neighbour tetrahedra formed by Fe ions on B sites.
Exploring the pyrophosphate series K2Cu1-xFexP2O7
(Australian Institute of Physics, 2020-02-04) Silk, R; Avdeev, M; Sauceda Flores, JA; Ulrich, C; Söhnel, T
Phosphate materials are of great interest to materials science. Metal phosphate compounds are known to exhibit conductive properties for battery use [1], photocatalytic abilities to decompose other compounds [2], the ability to conduct protons [3], etc. High temperature sintering techniques were used to produce the diphosphate series K2Cu1-xFexP2O7. The material forms two different modifications, a tetragonal (P-421m) and an orthorhombic modification (Pbnm) as shown in Figure 1. The crystal structure was determined through the use of powder X-ray diffraction (PXRD). PXRD patterns show a shift in peaks to lower angles with increasing iron content, which is consistent with an increase of lattice constant parameters. The presence of additional phases such as KPO3 can also be observed. UV-Vis measurements show how the transitions change which increasing amounts of iron and different modifications. Tauc plots revealed that the material has a direct band gap of approximately 3 eV. IR measurements revealed that water had been absorbed by the material and there a slight shift in peak position with increasing iron content. First magnetic measurements did not show any long-range ordering down to 2 K, the compounds remained paramagnetic. The K2CuP2O7 and K2Cu0.75Fe0.25P2O7 samples show short-range antiferromagnetic contribution across both modifications.
FeCr2S4 in magnetic fields: possible evidence for a multiferroic ground state
(Nature.com, 2014-08-15) Bertinshaw, J; Ulrich, C; Günther, A; Schrettle, F; Wohlauer, M; Krohns, S; Reehuis, M; Studer, AJ; Avdeev, M; Quach, DV; Groza, JR; Tsurkan, V; Loidl, A; Deisenhofer. J.
We report on neutron diffraction, thermal expansion, magnetostriction, dielectric, and specific heat measurements on polycrystalline FeCr2S4 in external magnetic fields. The ferrimagnetic ordering temperatures TC ≈ 170 K and the transition at TOO ≈ 10 K, which has been associated with orbital ordering, are only weakly shifted in magnetic fields up to 9 T. The cubic lattice parameter is found to decrease when entering the state below TOO. The magnetic moments of the Cr- and Fe-ions are reduced from the spin-only values throughout the magnetically ordered regime, but approach the spin-only values for fields >5.5 T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample below about 60 K. Below TOO this contraction is followed by a moderate expansion of the sample for fields larger than ~4.5 T. The transition at TOO is accompanied by an anomaly in the dielectric constant. The dielectric constant depends on both the strength and orientation of the external magnetic field with respect to the applied electric field for T < TOO. A linear correlation of the magnetic-field-induced change of the dielectric constant and the magnetic-field dependent magnetization is observed. This behaviour is consistent with the existence of a ferroelectric polarization and a multiferroic ground state below 10 K. © The Authors
Giant shifts of crystal field excitations with temperature as a consequence of internal magnetic exchange interactions
(Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) O'Brien, J; Schmalzl, K; Reehuis, M; Mole, RA; Miyasaka, S; Fuioka, J; Tokura, Y; McIntyre, GJ; Ulrich, C
Crystal field theory, invented in the 1930s by Hans Bethe, provides an explanation of the crystal field excitations (CFE) observed in inelastic neutron scattering (INS) spectra of rare-earth compounds [1]. However, some long withstanding problems remain. Our inelastic neutron scattering experiments on vanadates CeVO3 and TbVO3 did reveal an unexpected large shift of the energies of the crystal field excitations as a function of temperature. Thus far, only few publications on INS experiments mention shifts in crystal field excitation (CFE) energy in spectra above and below magnetic phase transition temperatures [2,3,4]. Recent IR transmission measurements also identified a CFE energy shift in hexagonal DyMnO3 with temperature and upon the application of an external magnetic field [5]. However, no studies report a detailed microscopic theory and to the best of our knowledge does not exist in literature. The vanadates CeVO3 and TbVO3 share the same orthorhombic Pbnm crystallographic structure featuring tilted, corner-sharing octahedra and possess a Cz-type antiferromagnetic structure below Néel temperatures 124 K and 110 K, respectively [6-9]. In both vanadates the CFE energies shift strongly below the magnetic phase transitions. We have used quantum-mechanical point-charge calculations to determine the energies of observed CFEs to model their large shift as a function of temperature. Two mechanisms have been simulated: (i) distortions of the crystallographic lattice due to magnetostriction, or (ii) internal magnetic exchange interactions with CF levels at the onset of the magnetic order. The effect of lattice distortions measured by neutron diffraction [7,8] produces a negligibly small shift of CFE energy, therefore cannot drive the shift. Results accounting for internal magnetic exchange fields arising from the ordered V3+ spins reveal a shift which agrees excellently with neutron data. The CFE energy shift is well reproduced with the same shift in CFE energy and intensity. Therefore, the unexpected large shift of CFE energies with temperature has been confirmed by point-charge model theoretical calculations and can be attributed to an internal magnetic exchange interaction. In addition to the CFEs, spin-wave excitations (magnons) are present in both vanadate materials below the magnetic phase transition. In TbVO3 there appears to be an anticrossing-like behaviour between magnon and CFE at 14 meV. Such an anticrossing has been reported in far-IR transmission investigations in Tb3Fe5O12 garnet [12]. In order to investigate this observation in TbVO3, magnon dispersion calculations have been performed to clarify the exact nature of the interaction. © The authors.
Giant shifts of crystal-field excitations in ErFeO3 driven by internal magnetic fields
(Cornell University, 2021-09-16) O'Brien, J; Deng, GC; Ma, XX; Feng, ZJ; Ren, W; Cao, SX; Yu, DH; McIntyre, GJ; Ulrich, C
Crystal-field excitations in transition-metal oxides where -rare-earth elements locate in the space between the transition-metal-oxide tetrahedra and octahedra, are assumed to be robust with respect to external perturbations such as temperature. Using inelastic neutron-scattering experiments, a giant shift of the energy of the lowest crystal-field excitation of Er3+ (4I15/2) in ErFeO3 from 0.35 meV to 0.75 meV was observed on cooling from 10K to 1.5K through the magnetic ordering temperature of Er3+ at 4.1 K. A crystal-field model was proposed to explain the observed crystal field excitations in this work. The model indicates the lowest-energy crystal-field excitation in ErFeO3 is the first Kramers doublet above the ground state. Its energy substantially shifts by the internal field induced by the ordered Er3+ magnetic moments. Further magnetic-field-dependent measurements provide strong supportive evidence for this scenario. By fitting the external magnetic-field dependency of the crystal-field excitation energy, the internal field generated by Er3+ magnetic moments was derived to be ~0.33meV. The result indicates that the internal field of Er3+ magnetic moments contribute to the energy shift of the crystal-field excitations. The giant energy shift under fields could be attributed to the anisotropy of the large effective g-factor. CC BY: Creative Commons Attribution
Highly anisotropic anomaly in the dispersion of the copper-oxygen bond-bending phonon in superconducting YBa2Cu3O7 from inelastic neutron scattering
(American Physical Society, 2011-10-19) Raichle, M; Reznik, D; Lamago, D; Heid, R; Li, Y; Bakr, M; Ulrich, C; Hinkov, V; Hradil, K; Lin, CT; Keimer, B
Motivated by predictions of a substantial contribution of the "buckling'' vibration of the CuO2 layers to d-wave superconductivity in the cuprates, we have performed an inelastic neutron scattering study of this phonon in an array of untwinned crystals of YBa2Cu3O7. The data reveal a pronounced softening of the phonon at the in-plane wave vector q = (0, 0.3) upon cooling below similar to 105 K, but no corresponding anomaly at q = (0.3,0). Based on the observed in-plane anisotropy, we argue that the electron-phonon interaction responsible for this anomaly supports an electronic instability associated with a uniaxial charge-density modulation and does not mediate d-wave superconductivity. © 2011, American Physical Society.

Browsing by Author "Ulrich, C"

Results Per Page

Sort Options