Miwako Takahashi

ABC6 type ordered structure in Pt-rich Pt-Mn alloys

In situ neutron diffraction experiments were performed on single crystals of Pt-12.5 and 14.4 at.% Mn alloys in order to elucidate the ordered structures of Pt-rich Pt–Mn alloys. The observed diffraction patterns at various temperatures up to 1050°C have revealed two ordered phases in these alloys. Below the order-disorder transition temperature, a Cu 3 Au type ordered structure appears and at low temperature, an ABC 6 type ordered structure with a unit cell 2 ×2 ×2 as large as that of the fundamental fcc cell was found to be stable. A secondary ordering in the alloys is discussed in terms of two order parameters in the ABC 6 type structure utilizing the method of static concentration waves originally proposed by Khachaturyan [Prog. Mater. Sci. 22 (1978) 24]. The ideal alloy composition for the structure is also discussed with the experimentally estimated order parameters.

Current status of a time-of-flight single crystal neutron diffractometer SENJU at J-PARC

SENJU is a state-of-the-art single crystal time-of-flight Laue diffractometer in Materials and Life Science Experimental Facility at Japan Proton Accelerator Research Complex (J-PARC). The diffractometer is designed for precise crystal and magnetic structure analyses under multiple extreme conditions, such as low temperature, high-pressure, high-magnetic field. Measurements using small sample less than 1.0 mm3 will be also realized, which allows us to study wide variety of materials. SENJU is using a poisoned decoupled moderator to obtain peak profiles of Bragg reflection, and intensity distributions of superlattice reflections and diffuse scatterings with good accuracy. At the beginning, the diffractometer will have 31 two-dimensional scintillator detectors to cover wide area of reciprocal lattice space by a single measurement. The instrument is currently under construction and is scheduled to start on-beam commissioning in February 2012.

Fracton excitations in a diluted Heisenberg antiferromagnet near the percolation threshold: RbMn0.39Mg0.61F3

Using neutron inelastic scattering techniques we investigated the magnetic excitations in a diluted Heisenberg antiferromagnet (RbMncMg1-cF3 with c=0.39) close to the percolation threshold of cp=0.312. No indication of propagating spin waves was observed throughout the Brillouin zone, but a broad peak superimposed on Ising-cluster excitations was observed. The origin of this broad peak is attributed to the excitation of fractons in a percolating network.

SENJU: a new time-of-flight single-crystal neutron diffractometer at J-PARC

SENJU, a time-of-flight Laue-type single-crystal neutron diffractometer, was developed at the Materials and Life Science Experimental Facility of the Japan Accelerator Research Complex (J-PARC). Molecular structure analysis of a sub-millimetre taurine crystal and magnetic structure analysis of an MnF2 crystal were performed to evaluate its performance.

New Ternary Ordered Structures in CuMPt6 (M = 3d elements) Alloys

X-ray and electron diffraction measurements were performed to investigate the structure and ordering behaviour of the ternary alloys CuMPt 6 (M=Ti, V, Cr, Mn, Fe, Co, and Ni). X-ray polycrystalline...

Secondary Ordering in Ternary Alloy CuMnPt6

Using the pulsed-neutron diffraction technique, we performed in situ measurements of structural ordering in the ternary alloy CuMnPt 6 . The diffraction patterns at various temperatures give a dire...

A Partial Phase Diagram of Pt-Rich Pt–Mn Alloys

We have performed the X-ray and electron diffraction studies to reconstruct a partial phase diagram of Pt-rich Pt–Mn alloys in the composition range of 10 to 35 at.% Mn. Electrical resistivity measurement was also used for determining the order–disorder transition temperature in Pt–14.2 at.% Mn alloy. The phase boundary between Cu 3 Au type and ABC 6 type ordered structures is established, in which the latter has been found recently by the present authors [J. Phys. Soc. Jpn. 71 (2002) 681]. In the ABC 6 type ordered phase, superlattice reflections both at \(\frac{1}{2}\frac{1}{2}\frac{1}{2}\) and its equivalent position (L-point) and at 100, 110 and their equivalent positions (X-point) appear in the composition range from 12.5 to 14.4 at.% Mn below 682°C. In the Cu 3 Au type ordered phase, diffuse maxima at L-point appear in the composition range from 15.9 to 19.7 at.% Mn in addition to the superlattice reflections at X-point. The Cu 3 Au type ordered structure is found to be stable in the composition ran...

A single-crystal time-of-flight neutron diffractometer at a spallation neutron source

A single-crystal diffractometer using the time-of-flight (TOF) Laue technique had been operated for studies of structural and magnetic disorder and phase transitions in alloys and magnetic materials at the pulsed spallation neutron source facility of the KEK Neutron Science Laboratory (KENS). In various sample environments, the diffractometer has demonstrated its usefulness in measurements of diffuse scattering for studying local structures, and in surveys of reciprocal space for studying phase transitions and incommensurate structures under the conditions of varying temperature and magnetic field. Particular emphasis is placed on the fact that though the instrument did not receive high-flux neutrons for long wavelengths suitable for the studies of magnetic scattering, it gave good results on the observations of magnetic diffuse scattering and superlattice reflections. Remarks and future tasks of the TOF Laue technique are discussed in relation to the asymmetric pulse shape, separation of the inelastic scattering, and dependence of data correction on the sample quality.

High-Resolution Inelastic Neutron Scattering from RbMncMg1-cF3 (c=0.10 and 0.15) under Magnetic Fields

Recently, a high-resolution inelastic neutron spectrometer equipped with a superconducting magnet has been installed at the pulsed spallation neutron source (KENS) at the National Laboratory for High Energy Physics (KEK). For the first experiment using this spectrometer system, we investigated inelastic neutron scattering from diluted Heisenberg magnetic systems, RbMn c Mg 1- c F 3 ( c =0.10 and 0.15) at 1.5 K with applied magnetic fields. Magnetic excitations in finite-size clusters were observed due to the low concentration of spins localized at the manganese ions. The observed spectra were compared with calculations which were performed by diagonalizing the Hamiltonian. Extending the number of spins in a cluster up to five, good agreement with calculations was obtained for both excitation energies and scattering intensities.

Instrument Design and Performance Evaluation of a New Single Crystal Neutron Diffractometer SENJU at J-PARC

A new single crystal time-of-flight neutron diffractometer has been installed at BL18 of the Materials and Life Science Experimental Facility of J-PARC. The diffractometer “SENJU” was designed for precise crystal and magnetic structure analyses using a small crystal less than 1 mm under multiple extreme environments such as low-temperature and high-magnetic field [1]. The first neutron beam was delivered to the sample position on March 5, 2012. Subsequently, the hardware and software of SENJU have been checked and confirmed that they worked fine. At the same time, diffraction experiments of several organic and inorganic crystals have been done using NaCl, 12CaO!7Al2O3 (Fig. 1), and so on. In these measurements, Bragg reflections in the high-Q region (d-spacing < 0.5 A) were clearly observed and analyzed successfully. In this presentation, we will show the instrument design and performance evaluation of SENJU in detail.