Browsing by Author "Stewart, GA"

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    Crystal and magnetic structures in perovskite-related La1-xCaxFeO3-δ (x=0.2, 0.33)
    (Elsevier, 2011-12) Hudspeth, JM; Stewart, GA; Studer, AJ; Goossens, DJ
    Using sol gel synthesis, single phase perovskite-related compounds in the family La1-xCaxFeO3-δ have been formed for x=0.2 and x=0.33, but not for x=0.5. The x=0.2 and x=0.33 compounds are isostructural with LaFeO3 (Pnma). The magnetic structure of La 0.8 Ca0.2FeO3-δ has been studied through Mössbauer spectroscopy and neutron powder diffraction. La 0.8 Ca 0.2FeO3-δ is a G-type antiferromagnet with a magnetic moment magnitude of 3.0±0.2μB at room temperature. The reduction in the magnitude of the antiferromagnetic moment compared to that published for LaFeO3 is explained by the measurement being taken at room temperature rather than 4 K and by the presence of Fe4 ions which have weaker exchange interactions than Fe3, causing a strong reduction in TN. Room temperature Mössbauer shows a broad magnetic hyperfine field distribution on the Fe sites in both La0.8 Ca0.2FeO3-δ and La0.67 Ca 0.33FeO3-δ. On cooling, disproportionation of Fe14 into Fe3 and Fe5 is apparent, and the resulting Fe5 sextet measured at low temperature gives a reliable measure of the Fe4 fraction. This in turn shows that creation of high-oxidation-state Fe is the dominant charge balance mechanism on doping Ca2 into the Ln3 site indicating a disordered distribution of Fe3 and Fe4. The lack of broadening of the Fe 5 sextet suggests that there may be ordering in the distribution of Fe5. © 2011 Elsevier Ltd.
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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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    Crystal structure and magnetic modulation in β−Ce2O2FeSe2
    (American Physical Society, 2017-08-11) Wang, CH; Ainsworth, CM; Champion, SD; Stewart, GA; Worsdale, MC; Lancaster, T; Blundell, SJ; Brand, HEA; Evans, JSO
    We report a combination of x-ray and neutron diffraction studies, Mössbauer spectroscopy, and muon spin relaxation (μ+SR) measurements to probe the structure and magnetic properties of the semiconducting β-Ce2O2FeSe2 oxychalcogenide. We report a structural description in space group Pna21 which is consistent with diffraction data and second harmonic generation measurements and reveal an order-disorder transition on one Fe site at TOD≈330K. Susceptibility measurements, Mössbauer, and μ+SR reveal antiferromagnetic ordering below TN=86K and more complex short range order above this temperature. 12 K neutron diffraction data reveal a modulated magnetic structure with q=0.444bN∗. © 2017 American Physical Society.
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    Determination of the crystal field levels in TmV2Al20
    (Australian Institute of Physics, 2018-01-31) Hutchison, WD; White, R; Stewart, GA; Iles, GN; Mole, RA; Cadogan, JM; Namiki, T; Nishimura, K
    The interest in compounds of the RM2Al20-type (R = lanthanide, M = transition metal) in recent years reflects the fascinating physical and magnetic properties on display at low temperatures. For example, in PrV2Al20 and PrTi2Al20 the phenomena reported include a quadrupolar Kondo effect [1] and superconductivity [2]. Central to such systems is the cubic symmetry of the Pr3+ site inducing a non-magnetic ground state in the ion. As a hole analogue of the PrV2Al20 compound, TmV2Al20 has been investigated in the hope of observing similar phenomena at low temperatures. At last year’s ‘Wagga’ we reported that we had determined the Tm3+ crystal field parameters W = 0.42(1) and x = -0.63(1) [3] (based on the Lea, Leask and Wolf formalism [4]) for TmV2Al20 using inelastic neutron scattering on PELICAN at the OPAL reactor, Lucas Heights. However, the line shapes found were extremely broad Lorentzians, indicative of a coupling of crystal field states to conduction electrons, ‘smearing out’ the energy required for transitions. Here, we report more recent developments: Tm3+ electron spin resonance results together with modelling of physical properties lead to the conclusion that there is a small local distortion away from cubic symmetry.
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    An inelastic neutron scattering investigation of holmium orthoferrite
    (IOP Publishing, 2023-01-18) Stewart, GA; Iles, GN; Mole, RA; Yamani, Z
    The inelastic neutron scattering spectra recorded in this study and elsewhere provide a useful set of crystal-field (CF) energy levels for the ground J = 6 term of Ho3+ in HoFeO3. The resolution of the low-energy, temperature-dependent pseudo-quadrupole ground state splitting and magnon peaks is consistent with the self-ordering of the Ho3+ sublattice at T Ho ∼ 8–10 K and supports earlier electron spin resonance investigations of the Ho3+ magnon behaviour. Systematic analysis of the grouped singlet CF levels of Ho3: HoFeO3, in conjunction with the CF Kramers doublet levels of the neighbouring Er3+: ErFeO3, has yielded possible sets of CF parameters for the two systems. © 2022 IOP Publishing Ltd.
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    Low-energy crystal field excitations observed using inelastic neutron scattering
    (Australian Institute of Physics, 2016-02-02) Iles, GN; Stewart, GA; Mole, RA; Hutchison, WD; Cadogan, S
    The time-of-flight spectrometer, PELICAN, at ANSTO operates two choppers which provide a fixed initial energy of neutrons to the sample. Configuring the instrument to a wavelength of 4.75Å, sets this initial neutron energy to 3.6meV. By phasing the choppers, however, harmonic wavelengths can be obtained such as λ/2, etc. By measuring a powder sample of ErNiAl4 at λ/2 (λ = 4.75Å) we could observe a greater range of positive energy transfers (to the sample) and, after appropriate background removal, confirm the presence of an excitation at 7meV. This excitation represents the emission from the crystal field level at 7meV to 0meV confirming the result obtained in a previous experiment.
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    Magnetic and crystal field properties of thulium calcium manganite
    (Australian Institute of Physics, 2004-02-04) Stewart, GA; Edge, AVJ; Studer, AJ; Elcombe, MM; Hovat, J; Lewis, RA
    For sufficiently large average Ln/A radii, the hole-doped manganites Ln2/3A1/3MnO3 (Ln = lanthanide, A = divalent metal) exhibit “colossal” magneto-resistance (CMR), which is associated with a transition from paramagnetic insulator to a low temperature ferromagnetic metal phase. However, with decreasing radius, the transition temperature is lowered and the ordered phase eventually reverts to that of a magnetic insulator. In this work, 169Tm Mössbauer spectroscopy has been used to investigate Tm2/3Ca1/3MnO3, which falls into the latter category. From the temperature of the onset of line broadening, the magnetic ordering temperature is determined as Torder ≈ 40 K (compared with ≈ 32 K from a.c. and d.c. susceptibility). The local Tm3+ magnetisation is almost certainly that of an isolated pseudodoublet ground state driven by a weak (compared with the crystal field) Mn-Tm exchange interaction. To a good approximation, the low temperature 169Tm spectrum is the superposition of a relaxation-broadened sextet and a paramagnetic doublet. This is consistent with regions of large slowly fluctuating magnetic clusters and regions of smaller rapidly fluctuating clusters, as has been observed for the generic CMR manganite, La2/3Ca1/3MnO3. However, recent neutron diffraction measurements performed on the Bragg Institute's HRPD facility are insensitive to this weak magnetism, suggesting a complex (perhaps spin-glass in nature) Mn sub-lattice magnetisation. The refinement of the position parameters for the nearneighbour oxygen atoms has assisted with the interpretation of the temperature-dependent 169Tm quadrupole interaction data in terms of a crystal field scheme for the Tm3+ ion. The appropriateness of this scheme will be considered in terms of the above observations. This work was supported by separate grants for source irradiation and neutron diffraction from the Australian Institute of Nuclear Science and Engineering.
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    Magnetic order studies of ErNiAl4
    (Australian Institute of Physics, 2007-02-06) Hutchison, WD; Goossens, DJ; Saensunon, B; Stewart, GA; Avdeev, M; Nishimura, K
    Specific heat measurements on ErNiAl4 show a phase transition at 5.8(1) K. Magnetisation data confirms the low temperature phase is antiferromagnetic in nature, while neutron powder diffraction data suggests an incommensurate structure similar to the intermediate magnetic phase of TbNiAl4.
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    Magnetic ordering in TmGa
    (IOP Science, 2014-03-03) Cadogan, JM; Stewart, GA; Muñoz-Pérez, S; Cobas, R; Hansen, BR; Avdeev, M; Hutchison, WD
    We have determined the magnetic structure of the intermetallic compound TmGa by high-resolution neutron powder diffraction and 169Tm Mössbauer spectroscopy. This compound crystallizes in the orthorhombic (Cmcm) CrB-type structure and its magnetic structure is characterized by magnetic order of the Tm sublattice along the a-axis. The initial magnetic ordering occurs at 15(1) K and yields an incommensurate antiferromagnetic structure described by the propagation vector k1 = [0 0.275(2) 0]. At 12 K the dominant ferromagnetic ordering of the Tm sublattice along the a-axis develops in what appears to be a first-order transition. At 3 K the magnetic structure of TmGa is predominantly ferromagnetic but a weakened incommensurate component remains. The ferromagnetic Tm moment reaches 6.7(2) μB at 3 K and the amplitude of the remaining incommensurate component is 2.7(4) μB. The 169Tm hyperfine magnetic field at 5 K is 631(1) T. © Copyright IOP Publishing
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    Magnetic structure and spin reorientation of quaternary Dy2Fe2Si2C
    (IOP Publishing, 2017-02-07) Susilo, RA; Cadogan, JM; Hutchison, WD; Stewart, GA; Avdeev, M; Campbell, SJ
    We have investigated the low temperature magnetic properties of Dy2Fe2Si2C by using magnetisation, specific heat, x-ray diffraction, neutron powder diffraction and 57Fe Mössbauer spectroscopy measurements over the temperature range 1.5 K–300 K. Dy2Fe2Si2C exhibits two magnetic transitions at low temperatures: an antiferromagnetic transition at ${{T}_{\text{N}}}\sim 26$ K and a spin-reorientation transition at ${{T}_{t}}\sim 6$ K. The magnetic structure above Tt can be described with a propagation vector $\mathbf{k}~=~\left(0~0~\frac{1}{2}\right)$ with the ordering of the Dy magnetic moments along the monoclinic b-axis whereas on cooling below Tt the Dy moment tips away from the b-axis towards the ac-plane. We find that the spin-reorientation in Dy2Fe2Si2C is mainly driven by the competition between the second-order crystal field term B20 and the higher-order terms, in particular B40 and B64. © 2017 IOP Publishing Ltd
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    Reinterpretation of physical property data for TmV2Al20
    (Australian Institute of Physics, 2020-02-04) Hutchison, WD; Stewart, GA; White, R; Iles, GN; Cadogan, JM; Namiki, T; Nishiruma, K
    Compounds of the RM2Al20-type (R = rare earth, M = transition metal) are of interest for the study of fundamental low temperature physical and magnetic properties. Members of this series crystallise in the cubic CeCr2Al20 structure type with the space group 4d3̅m (#227). Given that the rare earth site (cubic4̅3m / Td site symmetry) is at the centre of a polyhedron of 16 Al ions [1], members of the series are referred to as ‘caged rare earth compounds’. The relatively large lattice parameter (typically of the order of 15 Å) results in a large separation of the rare earth nearest neighbours and leads to weak R-R exchange interactions. Consequently, the magnetic ordering temperature is suppressed, typically to less than 2 K. In some cases magnetic order has not yet been observed. Investigations of PrV2Al20 and PrTi2Al20 revealed interesting phenomena associated with the non-magnetic ground state of the cubic Pr3+ site. These included the quadrupolar Kondo effect [2] and superconductivity behaviour [3]. The compound TmV2Al20 is a hole analogue of PrV2Al20 and was subsequently investigated at low temperatures in search of similar or related phenomena. A key outcome of this later work [4] was that the high quality, single crystal, heat capacity data were interpreted in terms of a cubic crystal field (CF) interaction with just the two parameters, x and W, of the Lea, Leask and Wolf [5] formalism. However an additional arbitrary broadening of the CF ground state was necessary to better match the experimental data at low temperature. In order to improve on these CF results, we carried out inelastic neutron scattering and electron paramagnetic resonance measurements which better define x and W for Tm3+ in TmV2Al20 [6]. In this paper we show that in addition to this crystal field Hamiltonian, the single crystal magnetisation and specific heat data are better interpreted in terms of a model that involves partial Al flux substitution of an approximately 10% depleted Tm “cage” site; this interpretation allows inclusion of “rattling” contributions of caged Tm and Al ions in specific heat.
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    Spin-reorientation in quaternary Dy_2Fe_2Si_2C
    (Australian Institute of Nuclear Science and Engineering, 2016-11-30) Susilo, RA; Cadogan, JM; Hutchison, WD; Stewart, GA; Campbell, SJ; Avdeev, M
    The low temperature magnetic properties of Dy_2Fe_2Si_2C have been investigated by magnetisation, specific heat, neutron powder diffraction and "5"7Fe Mössbauer spectroscopy measurements. In contrast to other R_2Fe_2Si_2C compounds, we found that Dy_2Fe_2Si_2C undergoes two successive magnetic transitions at low temperatures. The first magnetic transition at T_N = 26(2) K is associated with the transition from paramagnetic to antiferromagnetic states, whereas our neutron diffraction and "5"7Fe Mössbauer spectroscopy studies reveal that the second magnetic transition at T_t = 6(2) K is likely related to a spin-reorientation of the Dy moments rather than the independent ordering of the Fe sublattice. The magnetic structure above T_t can be described with a propagation vector k = [0 0 1/2] with the ordering of the Dy magnetic moments along the monoclinic b-axis, whereas on cooling below T_t the Dy moment tips away from the b-axis towards the ac-plane. Magnetocrystalline anisotropy energy calculations show that a canted magnetic structure is more energetically favourable below T_t than b-axis order due to the important influence of higher-order crystal field terms at low temperatures, thus explaining the unique occurrence of spin reorientation in Dy_2Fe_2Si_2C compared with other R_2Fe_2Si_2C compounds.
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    Tributes to three "Wagga" scientists
    (Australian Institute of Physics, 2020-02-06) Finlayson, TR; Gregg, DJ; Stewart, GA
    Ralph Severin (Sev) Crisp, Eric Raymond (Lou) Vance and Geoffrey Victor Herbert Wilson AM, all of whom passed away during 2019, were strong supporters of the “Wagga” Conference. Therefore, it is fitting that these tributes should be presented to outline their respective contributions to the field of “Condensed Matter and Materials”, now the accepted name for the conference. In this presentation their respective contributions to this conference will be summarized and set in the context of their broader contributions to Australian and International Condensed Matter Physics. We shall not only celebrate their scientific contributions, but also their extremely giving natures which have seen them share their lifelong knowledge with our community.