Browsing by Author "Mole, RA"

Now showing 1 - 20 of 78
  • No Thumbnail Available
    Item
    2nd Asia-Oceania Instrument Scientist Workshops, Manly, July 19, 2015
    (Taylor & Francis Online, 2015-11-17) Hester, JR; Holt, SA; Imperia, P; Piltz, RO; Rehm, C; Rule, KC; Mole, RA; McIntyre, GJ
    No abstract available
  • Thumbnail Image
    Item
    37th Annual Condensed Matter and Materials Meeting Wagga 2013
    (Australian Institute of Physics, 2013-02-05) McIntyre, GJ; Mole, RA
    The 37th Annual Condensed Matter and Materials Meeting was held at Charles Sturt University, Wagga Wagga, NSW from 5th – 8th February, 2013. There were 88 attendees, including international visitors from Singapore, Brazil, Turkey and Germany. A total of 11 invited and 18 contributed oral papers were presented during the two and one half days of scientific sessions. There were also two sessions with a total of 53 poster presentations. All presenters were invited to submit a manuscript for publication in the conference proceedings. Each manuscript was refereed by at least two anonymous reviewers who worked to a set of guidelines made available by the editors. Each accepted publication therefore satisfies the requirements for classification as a refereed conference publication (E1). The organizers would like to thank the 25 reviewers for their time and effort in reviewing manuscripts, which resulted in 14 papers being accepted for publication. The accepted manuscripts are available at the on-line publication section of the Australian Institute of Physics national web site (http://www.aip.org.au/).
  • Thumbnail Image
    Item
    Application of linear spin wave theory to the Cr8 antiferromagnetic Heisenberg Ring
    (Australian Institute of Physics, 2017-02-01) D'Adam, TM; Mole, RA; Stride, JA
    The investigation of single molecule magnets (SMMs) has proven to be a focal point of magnetism research for over three decades, leading to the discovery of structures which may find applications in data storage, quantum information processing (QIP) and spintronics. Though molecular magnetism is not a new field, there are still many complexes to investigate and understand, including a range of chains, rings, discs and cages. Amongst the considerable number of structures, particular interest has been shown to antiferromagnetic Heisenberg rings (AFHR) such as Cr8, CsFe8 and Fe18. These structures have been investigated due to their interesting magnetic behaviours which include quantum tunnelling of the Neel vector (QTNV) and a long magnetic relaxation time below their blocking temperature TB [1]. The Cr8 homometallic AFHR is one of the most well understood structures of its type having been extensively investigated since its initial synthesis using techniques including high-field EPR, cantilever torque magnetometry [2] and INS [3]. Through application of Linear Spin Wave Theory (LSWT) using the SpinW Matlab library [4] it has been possible to calculate the dynamic structure factor of the Cr8 ring; this agrees well with both the INS data collected for this structure as well as models produced using alternate methods [3]. This demonstrates that LSWT is applicable to the Cr8 ring and we plan to use this method to analyse more complex structures which also do not exhibit long range magnetic ordering.
  • Thumbnail Image
    Item
    Application of linear spin wave theory to the Cr8 antiferromagnetic Heisenberg Ring
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) D’Adam, TM; Mole, RA; Stride, JA
    The investigation of single molecule magnets (SMMs) has proven to be a focal point of magnetism research for over three decades, leading to the discovery of structures which may find applications in data storage, quantum information processing (QIP) and spintronics. Though molecular magnetism is not a new field, there are still many complexes to investigate and understand, including a range chains, rings, discs and cages. Amongst the considerable number of structures, particular interest has been shown to antiferromagnetic Heisenberg rings (AFHR) such as Cr8, CsFe8 and Fe18. These structures have been investigated due to their interesting magnetic behaviours which include quantum tunnelling of the Neel vector (QTNV) and a long magnetic relaxation time below their blocking temperature TB [1]. The Cr8 homometallic AFHR is one of the most well understood structures of its type having been extensively investigated since its initial synthesis using techniques including high-field EPR, cantilever torque magnetometry [2] and INS [3]. Through application of Linear Spin Wave Theory (LSWT) using the SpinW Matlab library [4] it has been possible to calculate the dynamic structure factor of the Cr8 ring; this agrees well with both the INS data collected for this structure as well as models produced using alternate methods [3]. This demonstrates that LSWT is applicable to the Cr8 ring and we plan to use this method to analyse more complex structures which also do not exhibit long range magnetic ordering.
  • Thumbnail Image
    Item
    Boson peak in ultrathin alumina layers investigated with neutron spectroscopy
    (American Physical Society, 2020-06-11) Cortie, DL; Cyster, MJ; Ablott, TA; Richardson, C; Smith, JS; Iles, GN; Wang, XL; Mitchell, DRG; Mole, RA; de Souza, NR; Yu, DH; Cole, JH
    Bulk glasses exhibit extra vibrational modes at low energies, collectively known as the boson peak. The vibrational dynamics in nanoscale alumina glasses have an impact on the performance of qubits and other superconducting devices; however, the frequency of the boson peak has not been previously measured. Here we report neutron spectroscopy experiments on Al/Al2O3 nanoparticles consisting of spherical metallic cores with a radii from 20 to 1000 nm surrounded by a 3.5-nm-thick alumina glass. A low-energy peak is observed at ωBP = 2.8 ± 0.6 meV for highly oxidized particles, indicating an excess in the density of states. The intensity of the peak scales inversely with particle size and oxide fraction, indicating a surface origin, and is redshifted by 3 meV with respect to the van Hove singularity of γ -phase Al2O3 nanocrystals. Molecular-dynamics simulations of α-Al2O3, γ -Al2O3 and α-Al2O3 show that the observed boson peak is a signature of the ultrathin glass surface and the characteristic frequency is reduced compared to the peak in the bulk glass. © 2020 The Authors. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license.
  • Thumbnail Image
    Item
    Chemical disorder in a frustrated J1/J2 quantum spin chain material
    (Australian Institute of Physics, 2018-01-30) Rule, KC; Mole, RA; Zanardo, J; Krause-Heuer, AM; Darwish, TA; Lerch, MLF
    Recently a new one-dimensional (1D) quantum spin chain system has been synthesised: catena-dichloro(2-Cl-3Mpy)copper(II), [where 2-Cl-3Mpy=2-chloro-3-methylpyridine]. We shall refer to this compound as cd-Cu. Preliminary calculations and bulk magnetic property measurements indicate that this system does not undergo magnetic ordering down to 1.8K and is a prime candidate for investigating frustration in a J1/J2 system (where the nearest neighbour interactions, J1, are ferromagnetic and the next nearest neighbour interactions, J2, are antiferromagnetic) [1]. Calculations predicted 3 possible magnetic interaction strengths for J1 below 6meV depending on the orientation of the ligand [2]. For one of the predicted J1values, the existence of a quantum critical point is implied. A deuterated sample of cd-Cu was produced at the National Deuteration Facility and the excitations measured using the PELICAN TOF spectrometer. Scattering was weak from this sample, but indicated the most likely scenario involves an average of the 3 possible magnetic excitations in this material, rather than the random array of exchange interactions as predicted by Herringer et al., [2]. This may indicate the possibility of tuning the chemical structure to favour a system which may exhibit a quantum critical point.
  • Thumbnail Image
    Item
    Comparing inelastic neutron scattering with theory to understand the complex magnetic interactions in a low-dimensional frustrated magnet
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Allen, J; Rule, KC; Mole, RA; Heinze, L; Süllow, S
    Frustrated magnetic systems have gained a lot of recent interest as they can exhibit exotic magnetic quantum states such as spin-liquid and spin-nematic states [1,2]. The natural mineral atacamite, Cu2Cl(OH)3, is one such material which has been reported to exhibit magnetic behaviour characteristic of a frustrated quantum magnet [3]. It also shares structural similarity with another cuprate, herbertsmithite, whose kagomé lattice is predicted to host a spin liquid ground state [4]. Little is understood about the magnetic properties of the natural orthorhombic structure of atacamite in contrast to its other structural polymorphs. This work studies this lesser understood orthorhombic atacamite, where Cu2+ ions form a pyrochlore lattice. The novel magnetic interactions and quantum states in materials like atacamite lends itself to spintronic applications where understanding spin interactions can unveil opportunities for intentionally manipulating these spins. Time-of-flight inelastic neutron scattering measurements on single crystalline atacamite have been performed at Pelican with an incident wavelength of 4.69Å. Excitations were observed at 1.5K, and weakened as temperature was increased beyond the antiferromagnetic transition temperature of TN = 9.0K [5] to 20K. The strongest dispersion was found along the H00 direction with two nested modes observed up to an energy transfer of 3meV. Relatively weak dispersion was seen along 00L, and a flat mode was observed in out-of-plane scattering covering a narrow section in 0K0. Ab initio band structure calculations have indicated a 1D sawtooth chain model for the dominant magnetic exchange paths in atacamite [6]. This chain model can be consolidated with our neutron scattering measurements and SpinW calculations. However, the relative magnitudes and directions of the predicted exchange couplings indicate the strongest interactions to be along 0K0, and this does not agree with the strongest dispersion measured experimentally in the H00 direction at zero field. To better understand the interactions in atacamite, in-field time-of-flight measurements have also been conducted in magnetic fields up to 6.5T, applied along the crystallographic b-axis. Data were collected using Pelican and the new open geometry, fully compensated, 7T vertical magnet. We will present the results of these inelastic neutron scattering measurements along with corresponding SpinW calculations to address the inconsistencies between theory and data when considering the exchange interactions in atacamite.
  • No Thumbnail Available
    Item
    Copper diffusion rates and hopping pathways in superionic Cu2Se
    (Elsevier, 2021-08-15) Nazrul Islam, SMK; Mayank, P; Ouyang, Y; Chen, J; Sagotra, AK; Li, M; Cortie, MB; Mole, RA; Cazorla, C; Yu, DH; Wang, XL; Robinson, RA; Cortie, DL
    The ultra-low thermal conductivity of Cu2Se is well established, but so far there is no consensus on the underlying mechanism. One proposal is that the fast-ionic diffusion of copper suppresses the acoustic phonons. The diffusion coefficients reported previously, however, differ by two orders of magnitude between the various studies and it remains unclear whether the diffusion is fast enough to impact the heat-bearing phonons. Here, a two-fold approach is used to accurately re-determine the diffusion rates. Ab-initio molecular dynamics simulations, incorporating landmark analysis techniques, were closely compared with experimental quasielastic/inelastic neutron scattering. Reasonable agreement was found between these approaches, consistent with a diffusion coefficient of 3.1 ± 1.3 x 10−5 cm2.s−1 at 675 K and an activation barrier of 140 ± 60 meV. The hopping mechanism includes short 2 Å hops between tetrahedral and interstitial octahedral sites. This process forms dynamic Frenkel defects. Despite the latter processes, there is no major loss of the phonon mode intensity in the superionic state, and there is no strong correlation between the phonon spectra and the increased diffusion rates. Instead, intrinsic anharmonic phonon interactions appear to dictate the thermal conductivity above and below the superionic transition, and there is only subtle mode broadening associated with the monoclinic-cubic structural transition point, with the phonon density-of-states remaining almost constant at higher temperatures. © 2021 Acta Materialia Inc.
  • No Thumbnail Available
    Item
    Coupled short-range ferroelectric and magnetic order in PbFe1/2Nb1/2O3
    (American Physical Society, 2013-09-13) Stock, C; Dunsiger, SR; Mole, RA; Li, XB; Luo, H
    A neutron scattering investigation of the magnetoelectric coupling in PbFe1/2Nb1/2O3 has been undertaken. Ferroelectric order occurs below 400 K, as evidenced by the softening with temperature and subsequent recovery of the zone center transverse optic phonon mode energy ((h) over bar Omega(0)). Over the same temperature range, magnetic correlations become resolution limited on a terahertz energy scale. In contrast to the behavior of nonmagnetic disordered ferroelectrics [namely Pb(Mg, Zn)(1/3)Nb2/3O3], we report the observation of a strong deviation from linearity in the temperature dependence of ((h) over bar Omega(0))(2). This deviation is compensated by a corresponding change in the energy scale of the magnetic excitations, as probed through the first moment of the inelastic response. The coupling between the short-range ferroelectric and antiferromagnetic correlations is consistent with calculations showing that the ferroelectricity is driven by the displacement of the body-centered iron site, illustrating the multiferroic nature of magnetic-lead-based relaxors in the dynamical regime. © 2013, American Physical Society.
  • Thumbnail Image
    Item
    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.
  • No Thumbnail Available
    Item
    Crystal field excitations of YbMn2Si2
    (Elsevier Science BV, 2013-12-01) Mole, RA; Hofmann, M; Adroja, DT; Moze, O; Campbell, SJ
    The crystal field excitations of the rare earth intermetallic compound YbMn2Si2 have been measured by inelastic neutron scattering over the temperature range 2.5-50 K. The YbMn2Si2 spectra exhibit three low energy excitations (similar to 3-7 meV) in the antiferromagnetic AFil region above the magnetic phase transition at T-N2 = 30(5) K. The crystal field parameters have been determined for YbMn2Si2 in the antiferromagnetic AFil region. A further two inelastic excitations (similar to 9 meV, 17 meV) are observed below T-N2=30(5) K, the temperature at which the high temperature antiferromagnetic structure is reported to exhibit doubling of the magnetic cell. Energy level diagrams have been determined for Yb3+ ions in the different sites above (single site) and below the magnetic transition temperature (two sites). The excitation energies for both sites are shown to be temperature independent with the temperature dependences of the transition intensities for the two sites described well by a simple Boltzmann model. The spectra below T-N2 cannot be described fully in terms of molecular field models based on either a single Yb3+ site or two Yb3+ sites. This indicates that the magnetic behaviour of YbMn(2)Si2 is more complicated than previously considered. The inability to account fully for excitations below the magnetic phase transition may be due to an, as yet, unresolved structural transition associated with the magnetic transition. © 2013, Elsevier Ltd.
  • Thumbnail Image
    Item
    Crystal growth and characterisation of a new J1-J2 spin-chain material
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) Zanardo, J; Rule, KC; Krause-Heuer, AM; Mole, RA; Lerch, MLF
    Recently a new one-dimensional (1D) quantum spin chain system has been synthesised: catena-dichloro(2-Cl-3Mpy)copper(II), [where 2-Cl-3Mpy=2-chloro-3-methylpyridine] [1]. Preliminary calculations and bulk magnetic property measurements indicate that this system does not undergo magnetic ordering down to 1.8K and is a prime candidate for investigating frustration in a J1/J2 system (where the next nearest neighbour interactions, J2, are antiferromagnetic and the nearest neighbour interactions, J1, are ferromagnetic) [2]. Calculations predict 3 possible magnetic excitations below 6meV which may reveal the nature of the random static structural disorder predicted in this material. One method for directly observing the magnetic excitations is neutron scattering and measurements have been performed on the neutron Time of Flight spectrometer PELICAN at ANSTO [3]. To a first approximation, linear spin-wave theory has been used to model the expected neutron excitations for this J2/J1 system using the Matlab package SpinW. The results of this project may provide valuable insight into the nature of magnetic frustration in materials. To optimise the observed magnetic signal via the reduction of incoherent neutron scattering, this compound was deuterated at the National Deuteration Facility at ANSTO. In this presentation we will outline our deuterated growth procedure as well as the characterisation methods performed to understand the material further. This work forms the Honours thesis project of Jack Zanardo from University of Wollongong.
  • 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
    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, 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
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) White, R; Hutchison, WD; Iles, GN; Mole, RA; Cadogan, JM; Nishimura, K
    There has been increasing interest in compounds of the RM2Al20-type (R = lanthanide, M = transition metal) in recent years due to the unique physical and magnetic properties many have been shown to display at low temperatures. Recent work carried out on PrV2Al20 and PrTi2Al20 has revealed a number of interesting phenomena, including a quadrupolar Kondo effect [1, 2] and superconductivity [3, 4] 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 to see whether it too displays such phenomena at low temperatures. Crystal field calculations based on specific heat and magnetisation have been carried out previously [5] with parameters W = 0.5 K and x = -0.6 determined based on the Lea, Leask and Wolf formalism [6]. These results have been further refined to W = 0.42(1) K and x = -0.63(1) using inelastic neutron scattering data obtained from the PELICAN time-of-flight spectrometer located at the OPAL reactor, Lucas Heights.
  • No Thumbnail Available
    Item
    Development of the magnetic excitations of charge-stripe ordered La2-x Sr (x) NiO4 on doping towards checkerboard charge order
    (Springer, 2013-05-01) Freeman, PG; Giblin, SR; Hradil, K; Mole, RA; Prabhakaran, D
    The magnetic excitation spectrums of charge stripe ordered La2-x Sr (x) NiO4 x = 0.45 and x = 0.4 were studied by inelastic neutron scattering. We found the magnetic excitation spectrum of x = 0.45 from the ordered Ni2+ S = 1 spins to match that of checkerboard charge ordered La1.5Sr0.5NiO4. The distinctive asymmetry in the magnetic excitations above 40meV was observed for both doping levels, but an additional ferromagnetic mode was observed in x = 0.45 and not in the x = 0.4. We discuss the origin of crossover in the excitation spectrum between x = 0.45 and x = 0.4 with respect to discommensurations in the charge stripe structure. © 2013, Springer.
  • Thumbnail Image
    Item
    Does the boson peak survive in an ultrathin oxide glass?
    (arXiv.org, 2019-07-29) Cortie, DL; Cyster, MJ; Smith, JS; Iles, GN; Wang, XL; Mitchell, DRG; Mole, RA; de Souza, NR; Yu, DH; Cole, JH
    Bulk glasses exhibit extra vibrational modes at low energies, known as the boson peak. The microscopic dynamics in nanoscale alumina impact the performance of qubits and other superconducting devices, however the existence of the boson peak in these glasses has not been previously measured. Here we report neutron spectroscopy on Al/Al2O3−x nanoparticles consisting of spherical metallic cores from 20 to 1000 nm surrounded by a 3.5 nm thick alumina glass. An intense low-energy peak is observed at ωBP = 2.8 ± 0.6 meV for highly oxidised particles, concurrent with an excess in the density of states. The intensity of the peak scales inversely with particle size and oxide fraction indicating a surface origin, and is red-shifted by 3 meV with respect to the van-Hove singularity of γ-phase Al2O3−x nanocrystals. Molecular dynamics simulations of α-Al2O3−x, γ-Al2O3−x and a-Al2O3−x show that the observed boson peak is a signature of the ultrathin glass surface, and the frequency is softened compared to that of the hypothetical bulk glass.
  • No Thumbnail Available
    Item
    Driving forces for the phase transition of CuQ2-TCNQ molecular crystals
    (Royal Society of Chemistry, 2016-05-23) Yu, DH; Kearley, GJ; Liu, G; Mole, RA; McIntyre, GJ; Tao, X
    The driving forces for the phase transition and relative stability of the two forms of CuQ2-TCNQ molecular crystals have been studied using inelastic neutron scattering (INS), density functional theory (DFT), and Hirshfeld surface analysis. DFT molecular dynamics (MD) simulations show that form-II has a lower enthalpy, but with increasing temperature form-I becomes thermodynamically stable due to the greater entropy. INS and MD simulations both show that the entropy of the hydrogen-bond network that holds molecules together within layers is higher in form-I. The interlayer π–π interactions are also weaker in form-I, leading to an overall “loosening” of the structure. The phase transition is kinetically hindered by the requirement to re-optimize the orientation of the layers. The strong H-bond interactions keep the in-plane atomic arrangement stable, while the weak interlayer π–π interactions provide the coupling between layers during the phase-transition. This subtle interplay of the two interactions maintains the integrity of the crystal upon phase transition even with dramatic physical dimension changes. © The Royal Society of Chemistry 2016
  • No Thumbnail Available
    Item
    Effect of temperature and compositional changes on the phonon properties of Ni-Mn-Ga shape memory alloys
    (Americal Physical Society, 2012-10-11) Ener, S; Neuhaus, J; Petry, W; Mole, RA; Hradil, K; Siewert, M; Gruner, ME; Entel, P; Titov, I; Acet, M
    We report on the vibrational properties of the ferromagnetic shape memory alloy system Ni-Mn-Ga in its stoichiometric Ni(2)MnGa and off-stoichiometric Ni(49)Mn(32)Ga(19) compositions. Elastic and inelastic neutron scattering measurements at different temperatures are presented with a focus on the austenite phase and compared to first-principles calculations. The overall behavior of the full phonon dispersion is similar for both compositions with remarkable exceptions for the TA(2)[xi xi 0] acoustic branch and optical phonon branches. Less dispersion is found in the optical phonons for Ni(49)Mn(32)Ga(19) in the whole reciprocal space when compared to Ni(2)MnGa and is explained by the occupation of regular Ga sites by excess Mn atoms. A pronounced softening in the TA(2)[xi xi 0] phonon branch within the austenite phase is observed in both samples when approaching the martensitic transition. Its location in reciprocal space reveals the martensitic transition mechanism. The austenite L2(1) structure transforms to the tetragonal modulated martensite structure by shuffling (110) planes in the [1 $(1) over bar $0] direction, similarly to what has been observed at the martensitic transitions of the d(1) and d(2) transition metals. Whereas the temperature dependence of the softening of the TA(2)[xi xi 0] phonons in the stoichiometric sample coincides perfectly with the magnetic and structural transitions, this is not the case for the off-stoichiometric sample. Here the relation between the magnetic ordering and the vibrational properties is still an open question. © 2012, American Physical Society.