Papers by Author: Masaki Takata

Abstract: With the arrival of the third generation of synchrotron sources and/or the introduction of advanced insertion devices (wigglers and undulators), the high energy (E > 50 keV) x-ray diffraction technique has become feasible, leading to new approaches in the quantitative study of the structure of disordered materials than was hither to available. Since we built the SPring-8 bending magnet beamline BL04B2 and two-axis diffractometer for disordered materials in 1999, we have studied on disordered materials from ambient to extreme condition. In this article, the high-energy x-ray diffraction beamline BL04B2 of SPring-8 and recent developments of ancillary equipment (automatic sample changer, conventional high-temperature furnace, aerodynamic levitation furnace) are introduced. Furthermore the structural analysis on the basis of diffraction data with the aid of computer simulations, which we performed in the last 10 years is reviewed.

Abstract: Magnetic structure in a layered perovskite manganite, La_2-2xSr_1+2xMn₂O₇ (x = 0.307) has been investigated by neutron diffraction measurements. We found that the ground state (at 4 K) has a ferromagnetic structure with magnetic moment of Mn ions being aligned in a direction inclined by 10 degree from the c-axis. The magnetic structure changes to a canted antiferromagnetic one (CAFM-I) at about 20 K and then to another canted antiferromagnetic one (CAFM-II) at about 80 K. Here the magnetic moment lies in the ab-plane in CAFM-II but not in CAFM-I. The magnetic structure then changes to an antiferromagnetic one with magnetic moment lies in the ab-plane at about 90 K, and then to a paramagnetic one at about 100 K.

Abstract: In order to obtain an insight into the high glass-forming ability of bulk metallic glasses, we have analyzed liquid structures of the Zr-Cu and the Zr-Ni binary alloys with different compositions. High-energy (E = 113 keV) x-ray diffraction experiments were carried out for the liquid alloys levitated by a conical nozzle levitation (CNL) technique. While a peculiar shoulder on the second peak was observed in the structure factors of the Zr-Cu liquid alloys, those of the Zr70Ni30 and the Zr50Ni50 liquids exhibit an asymmetric shape of the second peak. In addition, it was found that the effect of concentration variation in the liquid Zr-Ni alloys was significantly different from that of the liquid Zr-Cu alloys. The liquid structure analyses using the reverse Monte Carlo (RMC) simulation have clarified that a degree of the short-range correlation between the constituents in the liquids affects substantially the glass-forming ability of the binary Zr alloys.

Abstract: Two topics are described for structure analyses of glass and liquid using a combination of conical nozzle levitation (CNL) technique and diffraction experiments. The structure of high-purity bulk forsterite (Mg2SiO4) glass synthesized by a CNL technique has been determined by a combination of high-energy x-ray, neutron diffraction, and reverse Monte Carlo (RMC) modeling technique. The 3-dimensional atomic configuration derived from RMC modeling revealed that unusual network structure. In order to study structures of high-temperature and undercooled liquids, a CNL system has been developed and integrated with the two-axis diffractometer for glass, liquid, and amorphous materials at SPring-8, which is one of the third-generation synchrotron source. High-energy x-ray diffraction experiments were performed to obtain reliable diffraction data for the liquid phase of metallic glass-forming Zr-Cu binary alloys.

Abstract: The crystal structures of barium titanate (BaTiO3) fine particles with a size around 140 nm were investigated using a synchrotron radiation X-ray diffraction method. The observed diffraction pattern was analyzed by Rietveld method assuming several models. As a result, it is found that the BaTiO3 fine particles have composite structures include (a) a tetragonal region with a constant c/a ratio, (b) a tetragonal region with gradient c/a ratios and (c) a cubic region. In order to estimate these structures, the partial profile relaxation technique was applied in the Rietveld refinement. It is considered that the composite structure is important for the size effect on BaTiO3 fine particles.