Triple-axis Neutron Spectrometer Research Articles

Inelastic neutron scattering study of the lattice dynamics of LaCoO3

The lattice dynamics of lanthanum cobaltite is investigated using the coherent inelastic neutron scattering technique in a temperature range from 10 to 536 K, in which two phase transitions are observed: a change in the Co spin state and a metal-insulator phase transition. The measurements were performed using an IN8 high flux thermal neutron triple-axis spectrometer (ILL, Grenoble). Temperature changes in the phonon dispersion curves caused by the rearrangement of the electron and spin subsystems are found.

Upgrade of the primary spectrometer of the cold triple-axis spectrometer FLEX at the BER II reactor

Monte Carlo simulation results for the upgrade of the FLEX/V2 cold neutron triple-axis spectrometer are presented. The main goals are increased flux and energy resolution, while keeping a low background. In the same time the energy transfer range is significantly increased, bridging the gap between cold and thermal neutrons. We aim towards a flexible instrument, by employing the virtual source concept defining a beam before a double-focusing monochromator. This enables fine-tuning according to energy resolution, intensity and sample size requirements. A velocity selector adds the extra flexibility of tuning the spectrometer to any desired fixed wavevector. Neutron ray-tracing programs McStas and VITESS were extensively used and tested in this process.

Parabolic versus elliptic focusing – Optimization of the focusing design of a cold triple-axis neutron spectrometer by Monte-Carlo simulations

We present Monte-Carlo simulations for the focusing design of a novel cold-neutron triple-axis spectrometer to be installed at the end position of the cold guide NL-1 of the research reactor FRM-II in Munich, Germany. Our simulations are of general relevance for the design of triple-axis spectrometers at end positions of neutron guides. Using the McStas program code we performed ray trajectories to compare parabolic and elliptic focusing concepts. In addition the design of the monochromator was optimized concerning crystal size and mosaic spread. The parabolic focusing concept is superior to the elliptic alternative in view of the neutron intensity distribution as a function of energy and divergence. In particular, the elliptical configuration leads to an inhomogeneous divergence distribution.

Incommensurate Magnetic Excitation in Spin-Glass Phase of Bi2201 Cuprate

Static and dynamic magnetic susceptibilities of lightly-doped Bi 2.4 Sr 1.6 CuO 6+ y were studied by a SQUID magnetometer and triple-axis neutron spectrometers. The static magnetic susceptibility showed a difference between field-cooled and zero-field-cooled processes at low temperature, showing a spin-glass-like behavior. The irreversibility temperature was determined to be T sg ∼3 K, which is slightly lower than T sg of 5–6 K in the lightly-doped LSCO with similar hole concentration. Furthermore, we have confirmed the existence of low-energy spin fluctuations with the spatially modulated spin correlation along diagonal to Cu–O bond direction in the Bi2201 system for the first time. This observation is consistent with the previously reported result for the spin-glass phase of LSCO. The magnetic intensity χ ′′ (ω) measured at the energy transfer of 1 meV drastically developed below 100 K, while that at 6 meV shows a gradual increase below 200 K with decreasing the temperature. Although the spin fluctuatio...

Instability of magnons in two-dimensional antiferromagnets at high magnetic fields

Spin dynamics of the square lattice Heisenberg antiferromagnet, \BaMnGeO, is studied by a combination of bulk measurements, neutron diffraction, and inelastic neutron scattering techniques. Easy plane type antiferromagnetic order is identified at $T \le 4.0$ K. The exchange interactions are estimated as $J_1$ = 27.8(3)${\mu}$eV and $J_2$ = 1.0(1) ${\mu}$eV, and the saturation field $H_{\rm C}$ is 9.75 T. Magnetic excitation measurements with high experimental resolution setup by triple axis neutron spectrometer reveals the instability of one magnon excitation in the field range of $0.7H_{\rm C} \lesssim H \lesssim 0.85H_{\rm C}$.

Magnetic-Field-Induced Martensitic Transformation of Off-Stoichiometric Single-Crystal Ni2MnGa

The effect of a magnetic field on the martensitic transformation of an off-stoichiometric Heusler type Ni 2.16 Mn 0.78 Ga 1.06 single crystal has been revealed by neutron diffraction. The alloy undergoes a martensitic transformation at room temperature, which is nearly coincident with its Curie temperature. Splitting of the cubic (020) peak on the reciprocal lattice cubic c * -plane was traced at 293 K by a triple-axis neutron spectrometer under an increasing magnetic field of up to 10 T. It was found that the magnetic field causes the martensitic transformation from the cubic structure to the orthorhombic structure, which is the same as that caused by decreasing the temperature without a magnetic field. The increase in the magnetic field to 10 T appears to correspond to a decrease in temperature of nearly 12 K, i.e., from 293 to 281 K. The present experiment suggests the possibility of realizing a magnetic-field-induced shape memory alloy.

Investigation of crystal structure formation under microwave heating

Recently, we have developed new methods to form nano-structures consisting of small magnetic domains of 5–20 nm with random orientations, when some ferromagnetic materials are exposed to the magnetic field with microwave frequencies. To investigate the consequences of the non-equilibrium reactions by electromagnetic wave energy, we performed polarized neutron diffraction measurements to characterize the magnetic nano-domain structure of the microwave heated magnetite powder by using the polarized neutron triple-axis spectrometer TAS-1 at JRR-3 of Japan Atomic Energy Agency. In the magnetite samples heated under magnetic field of microwave up to 1000 and 1200 °C, magnetic Bragg peaks arising from crystalline magnetite of at least 20 nm in size were observed. In the latter sample the formation of non-magnetic substance, mostly hematite, was observed. These findings are consistent with the results of the magnetization measurement and X-ray diffraction.

Double focusing Heusler monochromator and analyzer systems for the polarized neutron triple-axis spectrometer

The polarization analysis options on the thermal neutron triple-axis spectrometer TAS-1 at JRR-3 of Japan Atomic Energy Agency (JAEA) have been improved as a result of an upgrading program of the Advanced Science Research Center. In order to increase the polarized neutron intensity, we have replaced the previous flat polarizing Cu 2MnAl (Heusler) monochromator and analyzer with doubly focusing ones. Intensity gain factors of more than 30 have been achieved for 14.7 meV neutrons in measurements of incoherent neutron scattering of vanadium. Using the new monochromator and analyzer, elastic and inelastic polarized neutron scattering experiments with the conventional setup for one-dimensional polarization analysis and with CRYOPAD (Cryogenic Polarization Analysis Device) for three-dimensional polarization analysis are now routinely performed on TAS-1.

Versatility of advanced KINKEN triple-axis neutron spectrometer, AKANE

AKANE is a thermal-neutron triple-axis spectrometer, which has been upgraded recently from the former double-axis neutron diffractometer. One of characteristic features of AKANE is the coupling of a monochromator of Ge (3 1 1) and an analyzer of PG (0 0 2). The incident neutron energy is fixed to 20 meV, which is intermediate between typical PG-filtered energies of 14.7 meV and 30.5 meV. Below ω ∼ 5 meV , the energy resolution is calculated to be medium and intermediate between those from PG-monochromator configurations. Surprisingly, inelastic-scattering signals are observable with reasonable statistics up to ω ∼ 10 meV , indicating validity of Ge monochromators for research on low-energy excitations in condensed matter physics.

Monte Carlo simulations for instrumentation at SINQ

The Paul Scherrer Institut (PSI) operates a spallation source SINQ equipped with 11 different neutron scattering instruments. Beside the optimization of the existing instruments, the extension with new instruments and devices are continuously done at PSI. For design and performance studies different Monte Carlo packages are used. Presently two major projects are in an advanced stage of planning. These are the new thermal neutron triple-axis spectrometer Enhanced Intensity and Greater Energy Range (EIGER) and the ultra-cold neutron source (UCN-PSI). The EIGER instrument design is focused on an optimal signal-to-background ratio. A very important design part was to realize a monochromator shielding which covers best shielding characteristic, low background production and high instrument functionality. The Monte Carlo package MCNPX was used to find the best choice. Due to the sharp energy distribution of ultra-cold neutrons (UCN) which can be Doppler-shifted towards cold neutron energies, a UCN phase space transformation (PST) device could produce highly monochromatic cold and very cold neutrons (VCN). The UCN-PST instrumentation project running at PSI is very timely since a new-generation superthermal spallation source of UCN is under construction at PSI with a UCN density of 3000–4000 n cm −3. Detailed numerical simulations have been carried out to optimize the UCN density and flux. Recent results on numerical simulations of an UCN-PST-based source of highly monochromatic cold neutrons and VCN are presented.

Energy separation of neutrons scattered at small angles from silicon using time-of-flight techniques

The time-of-flight technique is used on a small-angle neutron scattering instrument to separate the energies of the scattered neutrons, in order to determine the origin of the temperature-dependent scattering observed from silicon atQ> ∼0.1 Å−1. A quantitative analysis of the results in comparison with the phonon dispersion curves, determined by Dolling using a triple-axis neutron spectrometer, shows that the temperature-dependent scattering can be understood in terms of Umklapp processes whereby neutrons gain energy from phonons.

Advanced KINKEN Triple-Axis Neutron Spectrometer, AKANE

AKANE is a triple-axis neutron spectrometer, which has been upgraded recently. Inelastic scattering is observable with combination of a monochromator of Ge (311) and an analyzer of PG (002) in AKANE. Surprisingly, inelastic-scattering signals are detectable with reasonable statistics up to ω=10 meV, indicating validity of Ge monochromators for research on low-energy excitations in condensed matter physics.

How to Use a Triple-Axis Spectrometer (Practice-I)

Neutron triple-axis spectrometer (TAS), which is installed at steady neutron sources, is used for various purposes in condensed matter science. This article is aimed to describe practical use of TAS for beginners. I begin with explaining each component of TAS. Then, I will show how to check quality of single crystals and align single crystals by using TAS.

Adaptation of triple axis neutron spectrometer for SANS measurements using alumina samples at TRIGA reactor of Bangladesh

Abstract Double crystal method known as Bonse and Hart's technique has been employed to develop small angle neutron scattering (SANS) facility on a triple axis neutron spectrometer at TRIGA Mark II (3 MW) research reactor of Atomic Energy Research Establishment (AERE), Savar, Dhaka, Bangladesh. Two Si(1 1 1) crystals with very small mosaic spread ∼1 min have been used for this purpose. At an incident neutron wavelength of 1.24 A, this device is useful for SANS in the Q range between 1.6×10 −3 and 10 −1 A −1 . This Q range allows investigating particle sizes and interparticle correlations on a length scale of ∼200 A. Results of SANS experiments on three alumina (Al 2 O 3 ) samples as performed using above setup are presented. It is seen that Al 2 O 3 particles, indeed, scatter neutrons in regions of small angles. It is also seen that scattering is different for different samples showing that it changes with a change in particle size.

Development of a thermal neutron focusing device using neutron supermirrors

Neutron focusing is an indispensable technique used to compensate for weak signals in experiments under extreme conditions with tiny samples. We describe the development of a thermal neutron focusing device for a typical triple-axis neutron spectrometer. The device has a length of 400mm and is placed between the second collimator and the sample position. The focal length is 150mm from the downstream end of the device. The optic has 18 pieces of the circular blade consisting of 3m NiC∕Ti neutron supermirrors. The extensions of the end of each blade meet at the sample position, namely the focal point. Neutrons undergo successive reflection through the same angle from the inner surface of each blade and arrive at the sample position. The test results show that the gain of the neutron intensity at the sample position increases by a factor of 2 or more for the neutron energy below 30.5meV.

Polarized neutron scattering studies at JRR-3M

The modified research reactor JRR-3M of the Japan Atomic Energy Research Institute reached critical power in November 1990. To construct the polarized neutron triple-axis (PONTA) spectrometer at JRR-3M, Heusler single crystals for use as a polarizer were characterized by one of us (MN) at the Institut Laue-Langevin in Grenoble during 1990. These Heusler crystals are of good quality, with unpolarized beam reflectivities of 14–21% and polarizabilities of 94–99%. They were used as the polarizer and analyzer crystals of PONTA, and the neutron polarizability of the instrument is about 90% as a whole. The intensity ratio of the unpolarized beam to the polarized beam mode of triple-axis operation is about 40 due to the differences in the reflectivity and focussing efficiencies of the pyrolytic graphite and the Heusler alloy monochromator and analyzer crystals. In this short article, a few examples of experiments using polarized neutrons on ISSP-PONTA are reviewed: studies of the paramagnetic scattering of β-Mn, the two-dimensional magnetic diffuse-scattering modulated by interlayer interaction of ErFe 2O 4, the singlet ground state to triplet excitation of spin-Peierls gap in CuGeO 3, the magnetic-polaron state of 4f-electron in CeP, the orbital ordering in YTiO 3, and the phase transition in KDCO 3 using spin-echo technique.

Roton lifetime measurement in superfluid 4He using the spin-echo option on the IN20 triple-axis spectrometer

High-resolution measurements of the temperature dependence of the roton lifetime and energy shift in superfluid 4He have been performed using the IN20 thermal neutron triple-axis spectrometer spin-echo option (TASSE). This experiment constitutes the first implementation of the spin-echo technique for inelastic neutron scattering measurements on IN20 using the optimized field-shape superconducting precession coils. An energy resolution of 0.7 μeV was achieved and the results are in agreement with previous studies. The use of additional gradient coils enables to substantially improve the energy and the momentum transfer resolution, and to match excitation dispersion slopes and curvatures. This setup is designed to reach the nano-eV energy resolution in the case of quasi-elastic scattering experiments.

Looking below the Brillouin line in a polymer melt

We report on high-resolution inelastic neutron-scattering studies in amorphous deuterated polybutadiene at low momentum transfer, taken on the cold neutron triple-axis spectrometer IN12 at the ILL in Grenoble. The measurements show a clearly non-zero contribution down to the lowest accessible Q-vectors. The results are interpreted in terms of an elastic distortion around low-frequency localized modes.

Spin dynamics of Er3+ in ErBa2Cu3O7−δ

Neutron scattering techniques are used to study the magnetic excitations of the Er ions in superconducting ErBa2Cu3O7, a system where the Er ions order antiferromagnetically at TN=0.62 K and the sublattice magnetization obeys the relation for a 2D S=1/2 Ising model. Our data, which were taken on a cold neutron triple axis spectrometer using a horizontally focusing analyzer, reveal that there is a large gap of about 0.20 meV in the excitation spectrum, about four times the thermal energy kTN, as expected for an Ising system. However, we also observe dispersion of the spin waves in the a-b plane. These excitations are found to renormalize with temperature and only diffuse paramagnetic scattering is observed well above the ordering temperature.

Thermal neutron spin echo three-axis spectrometer with μeV resolution

A successful combination of TAS+NSE=TASSE has been built on the Polarised Neutron Triple-Axis Spectrometer PONTA of the ISSP, University of Tokyo, at the JRR-3M reactor, based on optimized field shape (OFS) precession coils. With a pair of simple and short OFS coils, energy resolutions (FWHM) of 0.2μeV have been obtained on the thermal three-axis spectrometer PONTA. With respect to classical TAS, the relative resolution (ΔE/E) was improved from ∼ 10−1 to 10−5 at a cost representing less than 10 percent of the polarized TAS itself. No intensity loss as compared to the normal polarized mode has been observed. Critical slowing down of (DCO3)2 dimmers in KDCO3, with extremely large relaxation times 0.15μeV/K, has been observed near the T=353K order-disorder phase transition. Furthermore, an incomplete slowing down of the quasielastic scattering due to the dynamical Huang scattering at the phase transition has been observed for the first time, which can be related to the softening of the shear elastic constant c66 observed in the ultrasonic measurements.