Triple-axis Neutron Spectroscopy Research Articles

Spin dynamics in the antiferromagnetic phases of the Dirac metals AMnBi2 ( A=Sr, Ca )

The square Bi layers in $A$MnBi$_2$ ($A =$ Sr, Ca) host Dirac fermions which coexist with antiferromagnetic order on the Mn sublattice below $T_\mathrm{N} = 290\,$K (Sr) and $270\,$K (Ca). We have measured the spin-wave dispersion in these materials by triple-axis neutron spectroscopy. The spectra show pronounced spin gaps of 10.2(2)$\,$meV (Sr) and 8.3(8)$\,$meV (Ca) and extend to a maximum energy transfer of 61 - 63$\,$meV. The observed spectra can be accurately reproduced by linear spin-wave theory from an Heisenberg effective spin Hamiltonian. Detailed global fits of the full magnon dispersion are used to determine the in-plane and inter-layer exchange parameters as well as on the magnetocrystalline anisotropy constant. To within experimental error we find no evidence that the magnetic dynamics are influenced by the Dirac fermions.

Triplet and in-gap magnetic states in the ground state of the quantum frustrated fcc antiferromagnet Ba2YMoO6

The geometrically frustrated double perovskite Ba${}_{2}$YMoO${}_{6}$ is characterized by quantum $s=1/2$ spins at the Mo${}^{5+}$ sites of an undistorted fcc lattice. Previous low-temperature characterization revealed an absence of static long-range magnetic order and suggested a nonmagnetic spin-singlet ground state. We report unique time-of-flight and triple-axis neutron spectroscopy of Ba${}_{2}$YMoO${}_{6}$ that shows a 28 meV spin excitation with a bandwidth of $\ensuremath{\sim}$4 meV, which vanishes above $\ensuremath{\sim}$125 K. We identify this as the singlet-triplet excitation that arises out of a singlet ground state, and further identify a weaker continuum of magnetic states within the gap, reminiscent of spin-polaron states arising due to weak disorder.

Understanding magnetic interactions in the seriesA2FeX5⋅H2O(A=K, Rb;X=Cl, Br). II. Inelastic neutron scattering and DFT studies

The compounds ${A}_{2}\text{Fe}{X}_{5}\ensuremath{\cdot}{\text{H}}_{2}\text{O}$ ($A=\text{alkali}$ or ${\text{NH}}_{4}$, $X=\text{Cl}$, Br) form a series of easy-axis antiferromagnets with transition temperatures in the range from 6 to 23 K, temperatures considerably higher than those of other similar hydrated salts of transition-metal ions. A study combining inelastic triple axis neutron spectroscopy (TAS) neutron scattering experiments and theoretical density functional theory (DFT) ab initio calculations of the magnetic superexchange constants has been done. A general spin-wave theory for an antiferromagnetic system with several magnetic ions in the magnetic unit cell and both uniaxial and rhombic magnetic anisotropies was employed to fit the observed magnon dispersion curves. The results obtained with the two techniques (TAS and DFT) allow us to determine with accuracy the magnetic exchange constants and therefore explain the efficiency of the superexchange pathways containing hydrogen bonds in transmitting the magnetic interactions.

Paramagnetic fluctuations in Pr0.65Ca0.35MnO3 around the charge-ordering temperature

We have studied the ferromagnetic and antiferromagnetic fluctuations in the charge-orderedPr0.65Ca0.35MnO3 antiferromagnet by triple-axis neutron spectrometry. Whereas ferromagneticfluctuations are observed above and below the charge-ordering transition(TCO), the antiferromagnetic fluctuations develop only belowTCO. The dynamicalexponent z of both ferromagnetic and antiferromagnetic fluctuations are determined.The ferromagnetic fluctuations are not completely suppressed belowTCO and their correlation lengths are short-ranged at all temperatures. The results are discussed withrespect to the Zener polaron model recently introduced to describe the charge-ordered state ofPr0.6Ca0.40MnO3.

Multiple magnetic singlet-singlet excitations in intermetallic PrNiSn

Inelastic neutron-scattering experiments have been carried out on a polycrystalline sample of PrNiSn, and seven of the eight excited crystal-field singlets of the Pr ions were detected. The system stays paramagnetic, at least down to $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the three principal susceptibility components have been measured on a PrNiSn single crystal between room temperature and $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The crystal-field excitations of the single crystal at low temperatures have been studied by triple-axis neutron spectroscopy. Dispersive effects are observed for three different levels of singlet-singlet excitations. The results are analyzed in terms of a mean-field/random phase approximation model, and it is concluded that the exchange interaction is highly anisotropic and of long range. The critical ratio between the maximum of the exchange interaction and that required for inducing a magnetic ordering of the singlet ground-state system is derived to be 0.48, leading to a predicted ordering temperature of $6\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ for the combined electron-nuclear magnetic system.

Lattice dynamics in austenitic stainless steels Fe–18Cr–12Ni–2Mo and Fe–18Cr–16Ni–10Mn

Phonon dispersion curves of austenitic stainless steels Fe–18Cr–16Ni–10Mn andFe–18Cr–12Ni–2Mo have been measured by triple-axis neutron spectroscopy. The data wereanalysed using Born–von Karman interactions as well as calculations including thecontribution of conduction electrons on the lattice dynamics. An appropriate description ofthe experimental data was obtained by taking into account two-neighbour shells plus thecontribution of the electron gas. The elastic constants and moduli obtained are close toreported results by ultrasonic studies on polycrystalline samples. The phonon densities ofstates in both systems calculated from the dispersion curves agree well with results obtainedby time-of-flight neutron spectroscopy on polycrystalline samples. The Debye temperatureΘ(T) shows a minimum around 40 K, similar to copper and nickel.

Neutron scattering study of the electron-phonon interaction in superconducting niobium

We have measured the lifetime and frequency of acoustic phonons in niobium by triple-axis neutron spectroscopy over a range of temperatures spanning the superconducting transition temperature, and have extracted quantitative information on the q and polarisation dependence of the electron-phonon interaction. The behaviour of the transverse modes supports a simple calculation based on the BCS theory of superconductivity, but the damping of a longitudinal mode appears to be unusually severe.

Collective excitations in liquid methanol studied by coherent inelastic neutron scattering

Inelastic spectra of deuteromethanol measured by triple-axis neutron spectroscopy at several temperatures within the liquid range are reported. The observed scattering intensities are analysed in terms of reconstructions of the dynamical structure factor after deconvolution from instrumental effects, as well as model fits to the measured intensities. The data indicate the existence of a propagating high-frequency excitation, which at T=200 K becomes overdamped for wavevectors Q>0.6 AA-1. An insight into the nature of the observed excitation is given by the temperature behaviour of the damping factors. Such a trend gives additional support to the assignment of the excitation to closely packed patches of hydrogen-bonded molecules.

Time-resolved triple-axis spectroscopy — A new method for real-time neutron scattering

A new method for kinetic experiments with neutrons is described. This method combines the conventional triple-axis neutron spectroscopy with relaxation-type experiments. Whereas the former method yields information about the microscopic dynamics of solids, the latter provides macroscopic relaxation times. The combination of both methods allows to determine the microscopic mechanism of relaxation processes. The time-resolved triple-axis spectroscopy is limited to reversible processes and to time scales from 10 −3 to 10 4 s. Typical applications are the kinetics of phase transformations (structural and dynamical changes), kinetics of domain distributions, kinetics of (spinodal) decomposition, reversible solid state reactions, time-dependent occupation of phonon states or phonon lifetimes.

Observation of itinerant-electron effects on the magnetic excitations of iron.

Triple-axis neutron spectroscopy has been used to study high-energy magnetic excitations for iron. The scattering contours established for the [100] and [111] directions show effects not present in localized-electron models of ferromagnetism. Interactions between spin waves and single-particle excitations are clearly seen for both crystallographic directions while two parts to the spin-wave branch are observed for the [100] direction.

Inelastic neutron scattering study of quartz near the incommensurate phase transition

Abstract Inelastic neutron spectra of B quartz were recorded at various temperatures, using triple axis neutron spectrometry. Our observations confirm the existence of a very low-lying phonon branch along the ⟨1,0,0⟩ directions, which exhibits a marked temperature dependence, in particular near the Brillouin zone center. This branch is very anisotropic in the (0,0,1) plane. Within the B phase a maximum of quasi elastic scattering is observed for q ≃ 0.03a*, premonitory to the appearance of satellite reflexions in the modulated phase. In addition, a transverse acoustic mode exhibits an anisotropic behaviour which is attributed to a coupling with the soft-mode. The results are consistent with a phenomenological model of the incommensurate phase transition and with a lattice dynamical model involving rigid SiO4 tetrahedra motions.

Spin waves in Ni-Cu alloys by neutron inelastic scattering

Spin wave dispersion relations were measured in Ni-Cu alloys by triple axis neutron spectrometry. The results were compared with theoretical calculations of the composition dependence of the spin wave stiffness constant D. The estimates were made in the Heisenberg model on the effect of clustering in Ni-Cu alloys on the value of D.

Neutron scattering study of anion hopping in strontium chloride

Abstract High resolution triple-axis neutron spectroscopy has been used to make a detailed investigation of the incoherent scattering from mobile anions in the fast-ion phase of strontium chloride. The quasielastic energy broadening has been measured as a function of the scattering vector at temperatures of 1001, 1053 and 1087 K. The data have been analysed in terms of the jump diffusion model of Chudley and Elliott, and the encounter model of Wolf. A vacancy diffusion mechanism is found to predominate in the fast-ion phase, with anion hops mainly to nearest neighbour and next-nearest neighbour regular anion sites.

Spin waves in Fe-Cr alloy by neutron spectrometry

Spin waves in Fe(30% Cr) and Fe(50% Cr) alloys were investigated by triple axis neutron spectrometry. The values obtained for the spin wave stiffness constant D are: for Fe(50% Cr): (150 ± 20) meV Å 2 at 295 K and (200 ± 20) meV Å 2 at 104 K, and for Fe(30% Cr): (220 ± 20) meV Å 2 at both temperatures.

Inelastic neutron scattering from a NiCl 2 — heavy water solution

An investigation of the density-density correlation function for NiCl 2 soluted in heavy water has been done by means of triple-axis neutron spectrometry. Such correlation function shows well defined peaks having a meaningful dispersion relation, i.e. collective excitations are present.

Lattice vibrations in tungsten at 22 °C studied by neutron scattering

The phonon spectrum of tungsten at 22 °C has been reexamined along major symmetry directions using triple-axis neutron spectrometry. The purposes of the experiment were to search for a predicted Kohn effect near the H-point and to extend the previously published measurements. The results are in agreement with those already published, but the expected Kohn effect could not be unambiguously observed with the instrumental resolution available.

Vertically bent pyrolytic graphite crystals applied to triple-axis neutron spectrometry

This paper demonstrates the great value of verticallt bent pyrolytic graphic crystals used as monochromator or analyzer in triple-axis neutron spectrometers. A simple model is presented for estimating the flux gain to be achieved by bending the monochromator or analyzer, and the effects this bending has on instrumental resolution are also considered. Experiments with three monochromators bent to 50, 35, and 21 inch radii over a wide range of neutron energy (5 to 40 meV) gave a maximum flux gain factor of 3.5 as predicted by the model, with no measurable change in resolution. In addition, if vertical resolution is not crucial, a further intensity gain can be achieved by using a bent analyzer.

Spin-Wave Dispersion Relation in Rare-Earth Metals

Recent measurements of the spin-wave dispersion relations for Gd, Dy, and Ho metal by triple-axis neutron spectrometry will be reviewed. Measurements along the c axis have been carried out for each metal at several temperatures, and for Gd additional measurements along the a and b axes have been made at 78°K. Information about the Fourier-transformed exchange interaction J(q) has been obtained from data analyses, which are based on a simple Hamiltonian containing only exchange interactions for Gd and a Hamiltonian containing exchange interactions plus appropriate anisotropy interactions (the usual frozen lattice model) for Dy and Ho. For both Dy and Ho a satisfactory analysis of the data requires the use of anisotropy interaction parameters which are not in good agreement with the results of macroscopic measurements. The neutron data suggest the existence of other anisotropy interactions, perhaps magnetoelastic, in addition to those usually considered in the frozen lattice Hamiltonian. The dispersion relation for Dy in the ferromagnetic phase has a pronounced minimum, and hence J(q) has a maximum, for q in the vicinity of the wave vector that is appropriate for the spiral structure just above Tc.

Spin-Wave Dispersion Relation in Dysprosium Metal

The spin-wave dispersion relation for the $c$ direction of dysprosium metal in the ferro-magnetic phase at 78\ifmmode^\circ\else\textdegree\fi{}K and in the helical magnetic phase at 98\ifmmode^\circ\else\textdegree\fi{}K has been measured by triple-axis neutron spectrometry. The energy gap measured in the ferromagnetic phase at $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}=0$ is in poor agreement with that calculated from the macroscopic magnetostriction and crystal-field anisotropy constants. The Fourier-transformed exchange interaction $J(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})\ensuremath{-}J(0)$ in the ferromagnetic phase posses a peak near the wave vector that gives the periodicity of the helical structure just above the Curie temperature.

Spin Wave and Critical Fluctuations in Magnetite

The long-wavelength spin dynamics of the ferrimagnet magnetite (Fe3O4) have been investigated at and below the critical temperature Tc. The magnetic fluctuations were observed in zero magnetic field near the (111) Bragg peak by triple axis neutron spectrometry. Well below Tc the dynamics can be described in terms of spin-wave excitations whose energy is Dq2. Over the temperature range 0.6>1 - T/Tc>0.01, D is found to vary as (Tc - T) to the power 0.265±0.03. Within about 1% of Tc the spin-wave lines broaden rapidly and become over-critically damped. At T = Tc there are no signs of spin-wave excitations and the energy width of the neutron scattering function varies as q to the power 1.65±0.20 over the range 0.05≤q≤0.16 Å−1.