Browsing by Author "Ryan, DH"

Now showing 1 - 13 of 13
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    Crystal field excitations for Ho3+ in HoFeO3
    (Australian Institute of Physics, 2017-01-31) Stewart, GA; Iles, GN; Mole, RA; Yamani, Z; Ryan, DH
    The orthoferrites, RFeO3 (R = rare earth), are promising candidates for innovative spintronic applications. HoFeO3 is of particular interest because optical measurements indicate that the magnetic splitting of the Ho3+ ion’s crystal field (CF) ground state lies in the range of antiferromagnetic–resonance frequencies for the Fe subsystem [1]. Inelastic neutron scattering data recorded on the Australian Neutron Beam Centre’s PELICAN time-of-flight spectrometer are consistent with Ho3+ CF levels at about 10.5, 15.4 and 22.0 meV. Additional low energy transitions (< 1 meV) exhibit behaviour that groups into three distinct temperature ranges (Fig. 1). Given that the Fe sub-lattice undergoes magnetic reorientation over the temperature range of 35 K to 60 K, it is believed that these excitations are associated with magnetic splitting of the Ho3+ ground CF level due to an exchange field originating from the Fe sub-lattice.
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    Freudenbergite - a new example of electron hopping
    (Australian Institute of Physics, 2014-02-06) Cashion, JD; Lashtabeg, A; Vance, ER; Ryan, DH; Solano, J
    Mössbauer spectra of freudenbergite samples with different composition have showed that although the Fe and Ti populate the octahedra randomly, Fe prefers the M(1) site over the M(2) site by approximately 1.3:1. Ti was able to accommodate mixed valence more easily than Fe, but some samples showed dynamic electron hopping in the Fe ions, which also affected the diffuse reflectance in the 400-800 nm region.
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    Magnetic order and spin-reorientations in RGa (R = Dy, Ho and Er) intermetallic compounds
    (Australian Institute of Physics, 2013-02-06) Susilo, RA; Cadogan, JM; Ryan, DH; Lee-Horne, NR; Cobas, R; Muñoz-Pérez, S; Rosendahl-Hansen, B; Avdeev, M
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    Magnetic ordering in Er2Fe2Si2C and Tm2Fe2Si2C
    (Australian Institute of Physics, 2015-02) Susilo, RA; Cadogan, JM; Hutchison, WD; Campbell, SJ; Avdeev, M; Ryan, DH; Namiki, T
    The magnetic ordering of two members of the R2Fe2Si2C (R = rare-earth) series of compounds (monoclinic Dy2Fe2Si2C-type structure with the C2/m space group), Er2Fe2Si2C and Tm2Fe2Si2C, have been studied by neutron powder diffraction and 166Er Mössbauer spectroscopy, complemented by magnetisation and specific heat measurements. In both cases, antiferromagnetic ordering of the R sublattice is observed, with Neel temperatures of 4.8(2) K and 2.6(3) K for Er2Fe2Si2C and Tm2Fe2Si2C, respectively. The magnetic structures of the Erand Tm-based compounds are quite different from those found for the other members of the R2Fe2Si2C series. Previous studies show that the common magnetic structure of the heavy- R2Fe2Si2C compounds involves ordering of the R sublattice along the b-axis with a propagation vector k = [0, 0, ½]. However, the antiferromagnetic structure of the Er sublattice in Er2Fe2Si2C is described by k = [½, ½, 0] with the Er magnetic moments lying close to the ac-plane. Tm2Fe2Si2C is found to exhibit a more complex magnetic structure that is characterised by a square-wave modulation of the Tm magnetic moments along the a-axis and a cell-doubling along the b-axis with k = [0.403(1), ½, 0]. The differences in the magnetic structures of these compounds are interpreted in terms of the RKKY exchange interaction, which depends on the R-R interatomic distances, and crystal field effects acting on the R3+ ions.
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    Magnetic properties of Ln2CoGe4O12 and LnBCoGe4O12 (Ln = Gd, Tb, Dy, Ho, Er; B = Sc, Lu)
    (Royal Society of Chemistry, 2017-11-03) Xu, D; Avdeev, M; Battle, PD; Ryan, DH
    Polycrystalline samples of Ln2CoGe4O12 (Ln = Gd, Tb, Dy, Ho or Er) and LnBCoGe4O12 (B = Sc or Lu) have been prepared and characterised by a combination of magnetometry, 155Gd Mössbauer spectroscopy and, in the case of Tb2CoGe4O12 and TbScCoGe4O12, neutron diffraction. The holmium- and erbium-containing compositions remain paramagnetic down to 2 K, those containing dysprosium behave as spin glasses and the terbium and gadolinium-containing compounds show long-range magnetic order with transition temperatures below 4 K in all cases. The data can be rationalized qualitatively in terms of the interplay between magnetic anisotropy and crystal field effects. © Royal Society of Chemistry 2021
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    Magnetic structures of R2Fe2Si2C intermetallic compounds: Evolution to Er2Fe2Si2C and Tm2Fe2Si2C
    (American Physical Society, 2019-05-20) Susilo, RA; Rocquefelte, X; Cadogan, JM; Bruyer, E; Lafargue-Dit-Hauret, W; Hutchison, WD; Avdeev, M; Ryan, DH; Namiki, T; Campbell, SJ
    The magnetic structures of Er2Fe2Si2C and Tm2Fe2Si2C (monoclinic Dy2Fe2Si2C-type structure, C2/m space group) have been studied by neutron powder diffraction, complemented by magnetization, specific heat measurements, and 166Er Mössbauer spectroscopy, over the temperature range 0.5 to 300 K. Their magnetic structures are compared with those of other R2Fe2Si2C compounds. Antiferromagnetic ordering of the rare-earth sublattice is observed below the Néel temperatures of TN=4.8(2)K and TN=2.6(3)K for Er2Fe2Si2C and Tm2Fe2Si2C, respectively. While Er2Fe2Si2C and Tm2Fe2Si2C have the same crystal structure, they possess different magnetic structures compared with the other R2Fe2Si2C (R = Nd, Gd, Tb, Dy, and Ho) compounds. In particular, two different propagation vectors are observed below the Néel temperatures: k=[12,12,0] (for Er2Fe2Si2C) and k=[0.403(1),12,0] (for Tm2Fe2Si2C). For both compounds, the difference in propagation vectors is also accompanied by different orientations of the Er and Tm magnetic moments. Although the magnetic structures of Er2Fe2Si2C and Tm2Fe2Si2C differ from those of the other R2Fe2Si2C compounds, we have established that the two magnetic structures are closely related to each other. Our experimental and first-principles studies indicate that the evolution of the magnetic structures across the R2Fe2Si2C series is a consequence of the complex interplay between the indirect exchange interaction and crystal field effects. ©2019 American Physical Society
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    Mössbauer study of the temperature dependence of electron delocalization in mixed valence freudenbergite
    (John Wiley & Sons, Inc., 2020-05-04) Cashion, JD; Vance, ER; Ryan, DH
    The evolution of the electron delocalization in the ferrous subspectra in a sample of mixed valence ferrous‐ferric freudenbergite has been followed by Mössbauer spectroscopy from 6 K to 650 K. The spectral changes do not involve the ferric component, leading to the conclusion that it is due to a thermally driven delocalization of the sixth d‐electron on the ferrous ions. The phenomenon does not occur in samples of pure ferrous freudenbergite. © 1999-2020 John Wiley & Sons, Inc.
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    Neutron powder diffraction determination of the magnetic structure of Nd2Al
    (IOP Publishing LTD, 2011-07-06) Cadogan, JM; Avdeev, M; Kumar, P; Suresh, K; Ryan, DH
    We have determined the magnetic structure of Nd 2 Al by neutron powder diffraction. This orthorhombic intermetallic compound orders ferromagnetically below 36 K with the Nd moments aligned along the b-axis. Even at 1.7 K, the larger of the two Nd moments is only 2.3(2) μ B , about 70% of the 'free-ion' value of 3.27 μ B . This reduction is a consequence of the substantial crystal-field effects at the Nd 3+ sites.(c) 2011 IOP Publishing LTD
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    Phonon mode softening at the ferroelectric transition in Eu0.5Ba0.5TiO3
    (Springer Nature, 2010-11-10) Rowan-Weetaluktuk, WN; Ryan, DH; Sushkov, AO; Eckel, S; Lamoreaux, SK; Sushkov, OP; Cadogan, JM; Yethiraj, M; Studer, AJ
    We have observed the effects of phonon mode softening at the ferroelectric transition in Eu0.5Ba0.5TiO3 by 151Eu Mössbauer spectroscopy. Both Eu2+ and Eu3+ spectral components are observed in the relative area ratio of 90% : 10% and both show a decrease in subspectral area at the transition, centred at 175 K, due to phonon mode softening. Surprisingly, the temperature dependence of the f-factor shows a much stronger response in the Eu3+ component than in the Eu2+ one. Preliminary analysis of neutron powder diffraction data rules out the possibility that some of the europium might be located on titanium sites. © 2010 Springer Science Business Media B.V.
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    Striped magnetic ground state of the ideal kagomé lattice compound Fe4Si2Sn7O16
    (Society of Crystallographers in Australia and New Zealand, 2017-12-03) Ling, CD; Allison, MC; Schmid, S; Avdeev, M; Gardner, JS; Ryan, DH; Soehnel, T
    We have used representational symmetry analysis of neutron powder diffraction data to determine the magnetic ground state of Fe4Si2Sn7O16. We recently reported a long-range antiferromagnetic (AFM) Néel ordering transition in this compound at TN = 3.0 K, based on magnetisation measurements [1]. The only magnetic ions present are layers of high-spin Fe2+ (d6, S = 2) arranged on a perfect kagomé lattice (trigonal space group P-3m1). Below TN = 3.0 K, the spins on 2/3 of these magnetic ions order into canted antiferromagnetic chains, separated by the remaining 1/3 which are geometrically frustrated and show no long-range order down to at least T = 0.1 K [2]. Moessbauer spectroscopy shows that there is no static order on the latter 1/3 of the magnetic ions — i.e., they are in a liquid-like rather than a frozen state – down to at least 1.65 K. A heavily Mn-doped sample Fe1.45Mn2.55Si2Sn7O16 has the same ground state. Although the magnetic propagation vector k = (0, ½, ½) breaks hexagonal symmetry, we see no evidence for magnetostriction in the form of a lattice distortion within the resolution of our data. To the best of our knowledge, this type of magnetic order on a kagomé lattice has no precedent experimentally and has not been explicitly predicted theoretically. We will discuss the relationship between our experimental result and a number of theoretical models that predict symmetry-breaking ground states for perfect kagomé lattices.
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    Striped magnetic ground state of the kagome lattice in Fe4Si2Sn7O16
    (American Physical Society, 2017-11-15) Ling, CD; Allison, MC; Schmid, S; Avdeev, M; Gardner, JS; Wang, CW; Ryan, DH; Zbiri, M; Söhnel, T
    We have experimentally identified a different magnetic ground state for the kagome lattice, in the perfectly hexagonal Fe2+ (3d6,S=2) compound Fe4Si2Sn7O16. A representational symmetry analysis of neutron diffraction data shows that below TN=3.5 K, the spins on 23 of the magnetic ions order into canted antiferromagnetic chains, separated by the remaining 13 which are geometrically frustrated and show no long-range order down to at least T=0.1 K. Mössbauer spectroscopy confirms that there is no static order on the latter 13 of the magnetic ions—i.e., they are in a liquidlike rather than a frozen state—down to at least 1.65 K. A heavily Mn-doped sample Fe1.45Mn2.55Si2Sn7O16 has the same magnetic structure. Although the propagation vector q=(0,12,12) breaks hexagonal symmetry, we see no evidence for magnetostriction in the form of a lattice distortion within the resolution of our data. We discuss the relationship to partially frustrated magnetic order on the pyrochlore lattice of Gd2Ti2O7, and to theoretical models that predict symmetry breaking ground states for perfect kagome lattices. ©2017 American Physical Society
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    Striped magnetic ground state on an ideal S = 2 Kagomé lattice
    (International Union of Crystallography, 2017) Ling, CD; Allison, MC; Schmid, S; Avdeev, M; Ryan, DH; Soehnel, T
    We have used representational symmetry analysis of neutron powder diffraction data to determine the magnetic ground state of Fe4Si2Sn7O16. We recently reported a long-range antiferromagnetic (AFM) Néel ordering transition in this compound at TN = 3.0 K, based on magnetization measurements. [1] The only magnetic ions present are layers of high-spin Fe2+ (d6, S = 2) arranged on a perfect kagomé lattice (trigonal space group P-3m1). [2] Below TN = 3.0 K, the spins on 2/3 of these magnetic ions order into canted antiferromagnetic chains, separated by the remaining 1/3 which are geometrically frustrated and show no long-range ordered down to at least T = 0.1 K. Moessbauer spectroscopy shows that there is no static order on the latter 1/3 of the magnetic ions – i.e., they are in a liquid-like rather than a frozen state – down to at least 1.65 K. A heavily Mn-doped sample Fe1.45Mn2.55Si2Sn7O16 has the same ground state. Although the magnetic propagation vector k = (0, 1/2, 1/2) breaks hexagonal symmetry, we see no evidence for magnetostriction in the form of a lattice distortion within the resolution of our data. To the best of our knowledge, this type of magnetic order on a kagomé lattice has no precedent experimentally and has not been explicitly predicted theoretically. We will discuss the relationship between our experimental result and a number of theoretical models that predict symmetry breaking ground states for perfect kagomé lattices. © International Union of Crystallography
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    Temperature dependence of electron delocalization in mixed balance freudenbergite
    (Australian Institute of Physics, 2020-02-04) Cashion, JD; Vance, ER; Ryan, DH
    Since our last reports, we have carried out further experiments on the mixed valence sodium iron-titanate freudenbergite [1, 2]. Freudenbergite has the nominal formula Na2FexTi8-xO16, with x = 1 being ferrous and x = 2 being ferric. However, a ferrous composition sample, calcined in air, became mixed valence with a closely 50:50 valence split. It is normally considered that the Fe and Ti ions are randomly distributed in the two (Fe,Ti)O6 octahedra. However neutron diffraction showed that the Ti:Fe ratio was 0.82:0.18 in the larger M(1) site and 0.93:0.07 in the smaller M(2) site compared to the average 0.875:0.125. It is expected that all the iron in the smaller M(2) site will be ferric. The sample turned out to have very unusual Mössbauer spectra as the temperature was varied. At low temperatures, well resolved ferric and ferrous doublets were observed. But as the temperature was increased, the ferrous doublet slowly collapsed and had to be fitted with up to three doublets to match the envelope. The ferric doublet remained unchanged in intensity and hyperfine parameters. The collapse of the ferrous spectrum is due to a thermally driven electron delocalization of the sixth d-electron. The electric field gradient in ferrous materials is mainly due to this electron and its removal causes the quadrupole splitting to more closely resemble that of the ferric ions, which is due entirely to the lattice. We have tried unsuccessfully to manufacture more of these electron mobile samples with various compositions and calcining in air and argon. However, pure ferrous and pure ferric samples do not display any dynamic behaviour, and even a ferrous sample with 3% ferric iron did not display any dynamics [2]. Spectra of the present sample taken at 6K and 10 K showed evidence of broadening, presumably due to the inset of magnetic ordering. However, it was not clear whether both ions were ordering or only one. There is no record of a magnetic ordering temperature for freudenbergite in the literature, and any such observation will undoubtedly be strongly sample dependent. The electron dynamics can be caused by intervalence charge transfer between ions or by crystal field effects or electron delocalisation in single ions, and are the primary cause of the colour in popular minerals. Other Fe-Ti minerals which exhibit such behaviour in the Mössbauer spectra include sapphire, kyanite, fassaite, omphacite, aenigmatite, and Ti andradite.