Triple-axis Spectrometer Research Articles

Magnetic excitations of ordered Pd 3Fe and disordered [formula omitted]Fe alloys

Neutron scattering studies of the magnetic excitations of single crystal specimens of Pd 3Fe and some Pd Fe alloys have been made, using several triple axis spectrometers (TAS) at the ILL, Grenoble. For Pd 3Fe, the renormalisation with temperature of the damped excitations in the “Stoner Continuum” has been observed. The influence of stoichiometry and chemical order has been examined by studies of partially disordered Pd 3Fe and Pd 81Fe 19. For low concentrations of Fe in Pd heavily damped excitations with unusual dispersion have been measured.

Low energy magnetic fluctuations in an ordered invar alloy of Fe 3Pt

The low energy magnetic response has been studied for a perfectly ordered Fe 3Pt alloy with high resolution triple axis spectrometers. In addition to the magnon excitations which exhibit no softening effect down to q = 0.03 A −1, the central mode starts to develop below q = 0.03 A −1. The intensity of this mode is significantly reduced in a magnetic field of 5 kOe.

Resolution effects for systems with strong dispersion

The effects of instrumental resolution are considered for systems where the dispersion relations vary rapidly with wave vector q . In such cases the observed profile of scattering in a “scan” is determined primarily by the wave vector resolution of the instrument rather than the more customary situation where the energy resolution is most important. We show that when the energy transfer varies substantially during the course of a constant- q measurement the observed scattering can be strongly distorted from that expected. To demonstrate these effects explicitly, calculations of the low temperature spin-wave lineshapes and integrated intensities in iron are presented for the case of a triple-axis spectrometer operated in both constant- q and constant-E modes, but the general results are also applicable to time-of-flight and constant- Q spectrometers.

Cellular proliferation at the vitreo-retinal junction--an animal model

The dispersion relation E(q) for spin waves in Cr2O3 has been measured at 78 K by means of inelastic neutron scattering using a triple-axis spectrometer. Thespin-wave energy is found to vary between 7.9 K (0.68 meV) at the Brillouin zone centre to 635 K (54.7 meV) at the zone boundary near (111). The data were fitted to a theoretical E(q) derived from a Heisenberg Hamiltonian involving neighbours out to about 6.1 Å distance. It is found that the interactions to the first and second neighbours are strong, but weak interactions exist out to at least the fifth neighbours (4.1 Å). The sublattice magnetization, the Néel point, the Curie-Weiss temperature and the perpendicular susceptibility are calculated using the spin-wave data and are found to agree well with observations. The density of spin-wave states is also calculated.

Antiferroelectric surface layers in a liquid crystal as observed by synchrotron X-ray scattering

The X-ray reflectivity form the surface of a liquid crystal with terminally polar (cyano substituted) molecules has been studied using a high-resolution triple-axis X-ray spectrometer in combination with a synchrotron source. It is demonstrated that at the surface of the smectic A1 phase a few antiferroelectric double layers develop that can be distinguished from the bulk single layer structure. A model is developed that separates the electron density in a contribution from the molecular form factor, and from the structure factor of the mono- and the bilayers, respectively. With only a few adjustable parameters it accounts for the rather complex observed reflection curve. It shows that (i) the first molecular layer has tails up rather than heads up, (ii) the smectic order parameter of the first mono- and bilayer is saturated, (iii) the antiferroelectric bilayering does decay rather abruptly and not exponentially Etude RX de la surface d'un cristal liquide pres de la transition nematique-smectique A1. Developpement en surface de quelques doubles couches antiferroelectriques qui peuvent etre distinguees de la structure en simples couches du materiau massif

Neutron scattering studies of hidden magnetic excitations in invar alloys

The paper summarizes recent neutron scattering studies of hidden magnetic excitations in the Invar alloys (Fe 65 Ni 35 , Fe 3 Pt) performed with three different spectrometers; D5(ILL), SAN(KENS) and high resolution triple axis spectrometers (BNL). The results suggests that thece exist dispersed magnetic excitations developed along magnon dispersions which would be the hidden excitations responsible for the Invar effect.

Resolution effects for systems with strong dispersion

Abstract The effects of instrumental resolution are considered for systems where the dispersion relations vary rapidly with wave vector q. In such cases the observed profile of scattering in a “scan” is determined primarily by the wave vector resolution of the instrument rather than the more customary situation where the energy resolution is most important. We show that when the energy transfer varies substantially during the course of a constant-q measurement the observed scattering can be strongly distorted from that expected. To demonstrate these effects explicitly, calculations of the low temperature spin-wave lineshapes and integrated intensities in iron are presented for the case of a triple-axis spectrometer operated in both constant-q and constant-E modes, but the general results are also applicable to time-of-flight and constant-Q spectrometers.

Experimental study of lattice dynamics in LaB6 and YbB6

Lattice dynamics of La11B6 was studied on the neutron triple axis spectrometers at HFIR. The acoustic and optical modes were measured in the high symmetry directions. The rotary motion of the B6 molecule was measured only near the zone boundaries but shows no softening. Instead, very flat low frequency La modes were observed. The agreement with the elastic constants and the temperature dependence of specific heat is satisfactory. In YbB6, the elastic constants and specific heat were measured. Both results give the quite different feature from those of LaB6, no softening of C12 and more softened flat low frequency Yb modes, due to the lack of the metallic bonds.

Magnetic excitations in amorphous isotropic ferromagnets

Neutron-scattering techniques have been used to investigate the magnetic excitations in the structurally amorphous ideal isotropic ferromagnet (${T}_{c}=513$ K) ${\mathrm{Fe}}_{40}$${\mathrm{Ni}}_{40}$${\mathrm{P}}_{14}$${\mathrm{B}}_{6}$. Small-angle inelastic-scattering measurements were taken with a triple-axis spectrometer to determine the temperature dependence of the spin-wave dispersion relation at long wavelengths. Most of the measurements, however, were concentrated in the momentum region near the first peak (${Q}_{0}=3.1$ ${\mathrm{\AA{}}}^{\ensuremath{-}1}$) in the static structure factor, and were made at 295 and 17 K with a pulsed polarized-beam time-of-flight spectrometer that uses the cross-correlation technique of data collection to obtain a high signal-to-noise ratio. It is found that there is a "cone" of scattering as a function of energy with its apex at ${Q}_{0}$, in general agreement with the powder-averaged model proposed by Shirane et al. However, not all of the features of the data can be explained by this model; in particular at ${Q}_{0}$ there is a broad distribution of scattering as a function of energy with a maximum at $E\ensuremath{\sim}12$ meV. Time-of-flight results are also presented for a single crystal and a powdered crystalline material in order to gain a better understanding of the corrections needed to obtain accurate data, and to establish that the instrument performs properly. The advantages as well as the limitations of this polarized-beam technique are discussed.

Lattice dynamics of pure barium titanate and barium strontium titanate solid solutions

The lattice dynamics of cubic BaTiO3 and Ba0.8Sr0.2TiO3 has been extensively investigated by inelastic neutron scattering using various triple-axis spectrometers. Measurements were carried out to obtain dispersion curves in the Gamma M, Gamma R and Gamma X directions of the Brillouin zone. A rigid-shell model has been developed for the pure BaTiO3, and for the mixed compound within the scope of a pseudo-unit-cell model. The branches measured are in good agreement with the theoretical calculations.

The resolution function of triple-axis neutron spectrometers in the limit of small scattering angles

The Cooper-Nathans formulation of the resolution function of a triple-axis crystal spectrometer for neutron-scattering experiments gives a singular resolution matrix when the scattering angle is small. The origin of this singularity is discussed and an alternative derivation, of the resolution matrix given which avoids this difficulty. The results are illustrated by numerical calculations for several typical experiments showing that resolution corrections may be large and very significant for experiments at small scattering angles.

Evidence for ID coupling in the neutron spectroscopy study of uranium trihalides

Abstract Published data of magnetic susceptibilities for UBr3 and UCl3 show apparent but rather broad maxima occurring at 15 and 22 K respectively. They were interpreted as due to cooperative magnetic transitions [1]. However, recent neutron scattering experiments performed at low temperatures on polycrystalline samples of UBr3 and UCl3 have proved unambiguously that the long range 3D antiferromagnetic ordering occurs at temperatures (UBr3: TN This highly unusual behaviour can be explained by the suggestion that the magnetic exchange interaction is strong within linear chains of uranium ions lying parallel to the c axis of the crystal, and much weaker between uranium ions lying in different chains. The above hypothesis has interesting consequences in the case when intrachain coupling between nearest neighbours is antiferromagnetic, since the dominance of the nearest-neighbour magnetic interaction will result in the ions ordering in antiferromagnetic linear chains along c axis. This by itself does not lead directly to a long-range-ordered structure, and by symmetry argument appears to preclude any simple magnetic ordering. Consequently, we may expect these compounds to behave as antiferromagnetic linear chains over a considerable range of temperature. The magnetic susceptibility maxima at 15 K and 22 K observed for UBr3 may be interpreted as due to exchange interactions within the chains, whereas at sufficiently low temperature, interchain interactions become important. They are expected to be largely, if not entirely dipolar in character, and then to align the ions which lie in planes perpendicular to the c axis, into what is equivalent to the highly degenerate antiferromagnetic triangular net arrangement [3]. In fact, neutron experiments performed at T In spite of the fact that both substances appear to be a good example of ID system, the hypothesis of the existence of linear antiferromagnetic chains in UBr3 and UCl3 needs direct experimental confirmation. Of course, for powdered sample the diffraction method is useless but the neutron spectroscopy can provide some valuable informations even if they are masked due to polycrystalline nature of the sample. The measurements to be presented here were performed on a triple axis spectrometer at Wurenlingen working in the constant Q mode and scanning the incident neutron energy. The outgoing energy was fixed at 15 meV. Various energy loss configurations at different temperatures and with different incoming neutron energies were realized to cover energy transfers ℏω up to 65 meV. To a first sight all observed lines of magnetic origin might be attributed to crystal-field transitions from the ground state to the excited states since the intensities of ground state transitions decrease with increasing temperature. On the other hand, recently published optical crystal spectra for U3: in LaCl3 preclude up to ∼25 meV and CF excitation from the ground state [4]. Therefore the inelastic scattering of neutrons corresponding to energy transfers of ∼5 meV and ∼4 meV in UCl3 and UBr3 respectively, appears to have now a very simple explanation: most likely it corresponds to the excitation of ID system. Such excitation are quasi ID spin waves which propagate along individual chains. Theory predicts that for the momentum components perpendicular to the chains these excitations are dispersionless, whereas along the chain direction, dispersion relation should follow a perfect sine curve. In powder measurements possible dispersion effects are most likely smeared out due to the polycrystalline nature of the sample. From Fig. 1 it is seen that as the temperature is raised, the 5 meV excitation in UCl3 decreases rapidly, but there is no appreciable change either in peak position or in its linewidth. The same feature was noticed in earlier measurements on UBr3 where damping of the 4 meV excitation was even more severe, but also no apparent change in its halfwidth was detected even at 100 K. This result is not quite surprising as might first appear, since as a consequence of the low dimensionality, the length over which the spins are correlated will be larger than in 3D systems at the same temperature relative to TN or Tc. As a result, in Ising systems well defined excitations were detected at temperatures as high as 10 TN [5]. The spin wave linewidth in ID system is mainly due to the finite correlation length and the excitations which are very sharp for T

X-rays from synchrotron: New challenge for neutron scattering

A brief review is given of current developments in X-ray scattering techniques at synchrotron radiation facilities. Highly collimated, intense sources of white radiation open up new areas of research in condensed matter physics and challenge the traditional domains of neutron scattering. These include energy dispersive scattering, the use of anomalous dispersion, magnetic diffraction and direct energy analysis by backscattering. The relative merits of X-ray and neutron scattering techniques will be discussed. The unique advantage of neutron scattering is the capability of performing polarization analysis. We will discuss in some detail the current developments at Brookhaven in inelastic scattering of polarized neutrons. In addition, we will also discuss special technical problems associated with the search for phasons utilizing a high-resolution triple axis spectrometer.

A neutron spin echo triple axis spectrometer

We describe a neutron spin echo set-up for the ILL D10 triple axis spectrometer. This set-up consists of vertically focussing Heusler alloy polariser/analyser systems, iron yoke spin precession electromagnets mounted on air-cushion tables similar to the classical spectrometer tables and air-wound flipper and field correction coils. The polarized neutron intensity in the detector is reduced by a factor of five as compared to the classical TAS with P.G. crystals. The energy resolution, on the other hand, is improved by two orders of magnitude. This has been demonstrated for quasielastic scattering where 1 μeV resolution at 15 meV incident energy has been reached. Experiments involving finite energy transfers for phonon lifetime measurements will also be reported on.

Neutron Scattering Study on Distribution of Cations in α-AgI-Type Superionic Conductors

Neutron diffuse scattering from powder samples of α-AgI type superionic conductors AgI, Ag 3 SI and Ag 2 S was measured using a triple-axis spectrometer with a fixed position of the energy analyzer so as to measure only the elastic scattering caused by a disordered distribution of cations. The disorder scattering curves thus obtained were analyzed under the assumption that Ag atoms distribute over crystallographic 12(d) sites among the bcc lattice formed by anions, and probability functions α n for a simultaneous occupation in the n-th neighbor sites were obtained. The results are discussed comparing with the pair correlation function g Ag–Ag ( r ) being obtained theoretically as well as by the analysis of X-ray diffuse scattering based on a liquid-like distribution model. The liquid phase of AgI was also studied by the same neutron technique and the results are discussed together with those for the solid phase.

Use of a position sensitive detector for data acquisition of synchrotron X-ray diffraction from adsorbed gas monolayers on graphite

The structure of monolayers of gases adsorbed on graphite has been studied by synchrotron X-ray diffraction [1–3]. Exfoliated Union Carbide ZYX graphite was used as a substrate. A spectrometer using a position sensitive detector and a slit configuration increased the data acquisition efficiency compared to that of the triple axis spectrometer on which the measurement was initiated. The origin of this comes from the finite coherence length of the graphite flakes, which is [4] 1600 Å. This is much less than the 10 000 Å which can be studied by the triple axis spectometer using Ge(111) both as monochromator and analyser [4]. So the resolution of the triple axis spectrometer is far too good for the object studied. The reason why this is bothering us is that we are studying monolayers of atoms, thus having very few atoms to scatter from, and even though we are working at the storage ring DORIS some of the experiments are on the limit of the doable with the intensity available. As no monochromator and analyser crystal exists which can fill out the gap between 1600 and 10 000 Å the slits configuration was invented. Although the introduction of slits limited the part of the synchrotron beam we utilized, this was well compensated for by the use of a position sensitive detector. However, this clearly illustrates the need for focusing elements. In some of the experiments the intensity need will also require wiggler lines.

Lattice dynamics ofFe0.5Ti0.5

The phonon dispersion curves for the intermetallic compound ${\mathrm{Fe}}_{0.5}$${\mathrm{Ti}}_{0.5}$ at room temperature have been determined by inelastic neutron scattering. The initial slopes of the dispersion curves at low frequencies were measured with a triple-axis spectrometer with the use of a small single crystal. The resulting elastic constants are ${c}_{11}=325\ifmmode\pm\else\textpm\fi{}10$ GPa, ${c}_{12}=121\ifmmode\pm\else\textpm\fi{}10$ GPa, and ${c}_{44}=69\ifmmode\pm\else\textpm\fi{}1$ GPa. The phonons at higher frequencies were determined by fitting Born-von K\'arm\'an force constants to time-of-flight spectra obtained from a polycrystalline sample. A good reproduction of the measured intensities could be obtained with tensor force constants between atoms up to the fourth neighbors. A striking feature of the result is the strong Ti-Ti nearest-neighbor longitudinal force constant which is about twice as strong as the corresponding Fe-Fe one. The averaged force constants correspond closely to the values found in chromium, which has very similar electronic structure.

Excitations in an amorphous ferromagnet Fe83B17

The scattering of neutrons from the magnetic excitations of an amorphous ferromagnet, Fe83B17, has been measured by using a neutron spin-polarised triple-axis crystal spectrometer to eliminate the nuclear scattering. The theory of the resultant scattering is discussed and it is shown that the scattering cannot be simply related to the magnetic susceptibility. The experimental results for wave vector transfers close to the first peak in the structure factor show that the scattering is similar to that expected from a powdered ferromagnet although the specimen is truly amorphous. Finally, there is no evidence of the gap in the spin-wave density of states as found in Co4P and Fe75P15C10.

Magnetic excitations in amorphous ferromagnets

The magnon excitations near the first $\mathcal{Y}(Q)$ maximum in amorphous ferromagnets have been reexamined by the use of a triple-axis polarized-beam spectrometer. This technique permits us to separate inelastic magnetic cross sections from phonons and all elastic components. We point out that this technique is not useful once the magnon excitation energy becomes smaller than the instrumental width. We have examined three different Fe-based alloy compositions at room temperature. In each case our results are consistent with a mechanism involving "umklapp scattered" long-wavelength spin waves with no energy gap. We compare our results on ${\mathrm{Fe}}_{75}$${\mathrm{P}}_{15}$${\mathrm{C}}_{10}$ with those of Mook and Tsuei who interpreted their results in terms of a 19-meV energy gap in ${\mathrm{Fe}}_{75}$${\mathrm{P}}_{15}$${\mathrm{C}}_{10}$.

Magnetic structure of single-domain antiferromagnetic FCT γ-Mn-Cu and γ-Mn-Fe alloys

Detailed neutron diffraction studies both by double axis and triple axis spectrometer are reported for the first time for fct single-domain single crystals of γ-Mn-Cu and γ-Mn-Fe. The main purpose of these studies was to elucidate further the nature of the diffuse (001) peak, forbidden by the AF1 magnetic structure, established earlier in this group of γ-Mn alloys. Line shapes and their temperature dependence were measured for (001), (100) and (010) reflections in Mn-Fe and in Mn—Cu, in the latter case both for homogeneous (quenched) and chemically decomposed (aged) samples. Only (001) reflection was found in Mn-Fe, while in Mn-Cu samples diffuse peaks were also observed at (100) and (010). Together with their temperature dependence and the results of energy analysis the data suggest a new model of the magnetic structure of these alloys. In Mn-Fe the spins deviate by a fixed angle from the c axis and this perturbation has an infinite correlation range and life time. In Mn-Cu a more complex set of perturbations is required to interpret the data.