Browsing by Author "Namiki, T"

Now showing 1 - 6 of 6
  • Thumbnail Image
    Item
    Determination of the crystal field levels in TmV2Al20
    (Australian Institute of Physics, 2017-01-31) White, R; Hutchison, WD; Iles, GN; Mole, RA; Cadogan, JM; Namiki, T; Nishimura, K
    Recent interest in so called caged rare earth compounds of the RM2Al20-type (R = lanthanide, M = transition metal) follow from their fascinating physical and magnetic properties at low temperatures. Recent work on PrV2Al20 and PrTi2Al20 revealed unusual phenomena, including a quadrupolar Kondo effect and superconductivity, brought about by 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 for equivalent heavy Fermion behaviour at low temperatures. In previous work, specific heat and magnetisation data were modelled with the crystal field parameters W = 0.5 K and x = -0.6 based on the Lea, Leask and Wolf formalism. However, the experimental zero field specific heat near 0.5 K could only be matched in the modelled curves using an artificial ground state broadening. In this work inelastic neutron scattering data obtained from the PELICAN time of flight spectrometer located at the OPAL reactor, Lucas Heights has allowed further refinement of the values to W = 0.42(1) K and x = -0.63(1). In addition the CEF transitions are found to be very broad, as required for the specific heat, and suggestive of strong 4f-conduction electron coupling.
  • Thumbnail Image
    Item
    Determination of the crystal field levels in TmV2Al20
    (International Conference on Neutron Scattering, 2017-07-12) White, R; Hutchison, WD; Iles, GN; Mole, RA; Cadogan, JM; Namiki, T; Nishimura, K.
    So called caged rare earth compounds of the RM Al20-type (R = lanthanide, M = transition metal) exhibit interesting physical and magnetic properties at low temperatures. For example PrV Al20 and PrTi Al20 show a quadrupolar Kondo effect [1] and superconductivity [2] brought about by the non-magnetic ground state and the cubic symmetry of the Pr3+site. In this work the compound TmV Al20, a hole analogue of PrV Al20 has been investigated. Previous crystal field calculations based on specific heat and magnetisation [3] resulted in parameters of W = 0.5 K and x = -0.6 within the Lea, Leask and Wolf formalism [4]. However to match the experimental zero field specific heat near 0.5 K, an artificial broadening of the ground state was applied. To validate and clarify these results, we have carried out an inelastic neutron scattering experiment on the PELICAN time-of-flight spectrometer to determine the energy splitting between the crystal field levels. This has allowed a further refinement of the crystal field parameters to W = 0.42(1) K and x = -0.63(1). The very broad Lorentzian line shapes suggest strong 4f-conduction band electron coupling.
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    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.
  • No Thumbnail Available
    Item
    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
  • Thumbnail Image
    Item
    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.