Browsing by Author "O'Brien, J"

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    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.
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    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
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    Investigations into the magnetic and crystal field excitations of the orthorhombically distorted perovskites TbVO3 and CeVO3
    (Australian Institute of Physics, 2018-01-30) O'Brien, J; Reynolds, N; Rovillain, P; Danilkin, SA; Schmalzl, K; Reehuis, M; Mole, RA; Miyasaka, S; Fujioka, F; Tokura, Y; Keimer, B; McIntyre, GJ; Ulrich, C
    Inelastic neutron scattering experiments have been performed on a series of vanadates, in particular TbVO3 and CeVO3, to categorise the crystal field and magnetic excitations. The vanadates possess a configuration with corner sharing, distorted VO6 octahedra (space group Pbnm) with a collinear C-type antiferromagnetic structure occurring below Néel temperatures of TN = 110 K and 124 K respectively. Data from neutron scattering experiments reveal a hitherto unobserved shift of crystal field excitation energy in TbVO3 and CeVO3. Point-charge model calculations have confirmed this shift by theoretically calculating the crystal field excitation spectrum. We propose that the mechanism behind the effect is the onset of local magnetism caused by the ordering of the vanadium sublattice at the magnetic phase transition. This magnetic exchange field from the vanadium ions polarises the spins of the rare-earth ions located at the centre of the unit cell. This results in a Zeeman-like splitting of crystal field energy levels. As a result, crystal field transition energies demonstrate a linear shift as a function of internal magnetic field strength.
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    Investigations of the magnetic and crystal field excitations in orthorhombically distorted perovskites RVO3 (R=Dy, Tb, Pr, Ce)
    (Australian Institute of Physics, 2017-01-31) O'Brien, J; Reynolds, NM; Mole, RA; Rovillain, P; Danilkin, SA; Schmalzl, K; Reehuis, M; Miyasaka, S; Fujioka, F; Tokura, Y; Keimer, B; McIntyre, GJ; Ulrich, C
    Inelastic neutron scattering experiments have been performed on a series of vanadates, in particular DyVO3, TbVO3, PrVO3, and CeVO3, to categorise the crystal field and magnetic excitations. The vanadates are isostructural to the multiferroic manganites TbMnO3 and DyMnO3, with corner sharing, Jahn-Teller distorted VO6 octahedra (orthorhombic space group Pbnm). However, they posses a collinear C-type antiferromagnetic structure, instead of an incommensurate spin arrangement as in the manganites. In the vanadates, the antiferromagnetic order sets in below Neel temperatures of TN = 110 K to 124 K [1-5]. Using inelastic neutron scattering on single crystals we were able to determine the crystal field spectrum and spin wave dispersion relations independently. In order to determine the nature of the crystal field excitations of these materials and in order to understand how the magnetic and crystal field excitations influence one another, we have theoretically calculated the crystal field excitation spectrum. The results are compared to the crystal field and spin wave excitations in the multiferroic maganites [6], in order to obtain a deeper understanding of the coupling mechanism between the rare earth elements and the transition metals in RVO3 and RMnO3, respectively.