M. Itou

Structural studies of disordered materials using high-energy x-ray diffraction from ambient to extreme conditions

High-energy x-rays from a synchrotron radiation source allow us to obtain high-quality diffraction data for disordered materials from ambient to extreme conditions, which is necessary for revealing the detailed structures of glass, liquid and amorphous materials. We introduced high-energy x-ray diffraction beamlines and a dedicated diffractometer for glass, liquid and amorphous materials at SPring-8 and report the recent developments of ancillary equipment. Furthermore, the structures of liquid and amorphous materials determined from the high-energy x-ray diffraction data obtained at SPring-8 are discussed.

Understanding Spin Structure in Metallacrown Single-Molecule Magnets using Magnetic Compton Scattering

The 3d-4f mixed metallacrowns frequently show single-molecule magnetic behavior. We have used magnetic Compton scattering to characterize the spin structure and orbital interactions in three isostructural metallacrowns: Gd2Mn4, Dy2Mn4, and Y2Mn4. These data allow the direct determination of the spin only contribution to the overall magnetic moment. We find that the lanthanide 4f spin in Gd2Mn4 and Dy2Mn4 is aligned parallel to the Mn 3d spin. For Y2Mn4 (manganese-only spin) we find evidence for spin delocalization into the O 2p orbitals. Comparing the magnetic Compton scattering data with SQUID studies that measure the total magnetic moment suggests that Gd2Mn4 and Y2Mn4 have only a small orbital contribution to the moment. In contrast, the total magnetic moment for Dy2Mn4 MCs is much larger than the spin-only moment, demonstrating a significant orbital contribution to the overall magnetic moment. Overall, these data provide direct insight into the correlation of molecular design with molecular magnetic properties.

Persistence of Covalent Bonding in Liquid Silicon Probed by Inelastic X-Ray Scattering

Metallic liquid silicon at 1787 K is investigated using x-ray Compton scattering. An excellent agreement is found between the measurements and the corresponding Car-Parrinello molecular dynamics simulations. Our results show persistence of covalent bonding in liquid silicon and provide support for the occurrence of theoretically predicted liquid-liquid phase transition in supercooled liquid states. The population of covalent bond pairs in liquid silicon is estimated to be 17% via a maximally localized Wannier function analysis. Compton scattering is shown to be a sensitive probe of bonding effects in the liquid state.

A new X-ray spectrometer for high-resolution Compton profile measurements at SPring-8

An X-ray spectrometer for high-resolution Compton profile measurements using 90-120 keV X-rays has been designed and constructed at SPring-8. A Cauchois-type triply layered bent-crystal analyzer was employed for the energy analysis. A novel use of a solid-state detector with a large active area was devised as a position-sensitive detector. A resolution of 0.10 atomic units in electron momentum has been achieved at an incident X-ray energy of 115 keV. A Compton profile of a single crystal of Nb was measured with a counting rate of 30 counts s-1 at the Compton peak, which demonstrates that the spectrometer is capable of measuring Compton profiles of heavy-element materials.

High-resolution Compton scattering study of the electron momentum density in Al

We report high-resolution Compton profiles (CPs) of Al along the three principal symmetry directions at a photon energy of 59.38 keV, together with corresponding highly accurate theoretical profiles obtained within the local-density approximation (LDA) based band-theory framework. A good accord between theory and experiment is found with respect to the overall shapes of the CPs, their first and second derivatives, as well as the anisotropies in the CPs defined as differences between pairs of various CPs. There are however discrepancies in that, in comparison to the LDA predictions, the measured profiles are lower at low momenta, show a Fermi cutoff which is broader, and display a tail which is higher at momenta above the Fermi momentum. A number of simple model calculations are carried out in order to gain insight into the nature of the underlying 3D momentum density in Al, and the role of the Fermi surface in inducing fine structure in the CPs. The present results when compared with those on Li show clearly that the size of discrepancies between theoretical and experimental CPs is markedly smaller in Al than in Li. This indicates that, with increasing electron density, the conventional picture of the electron gas becomes more representative of the momentum density and that shortcomings of the LDA framework in describing the electron correlation effects become less important.

Evidence of itinerant magnetism in a metallic nanoglass

Metallic nanoglasses are amorphous alloys which are prepared by consolidation of amorphous nanoparticles. The result is a unique two-component structure with comparable volume fractions, an interfacial component of nanometer-sized regions of reduced density, and a component in the cores of the nanoparticles. The magnetic properties of a Fe90Sc10 nanoglass and a rapidly quenched metallic glass as a reference have been investigated by means of Mossbauer spectroscopy and magnetic Compton scattering. The contribution of itinerant electrons to the magnetic moment in the interfaces is higher than in crystalline Fe and all known amorphous transition metal alloys. Consequently, the FeSc nanoglass represents a material with magnetic properties different from any known amorphous alloys.

Perpendicular magnetic anisotropy in Co/Pt multilayers studied from a view point of anisotropy of magnetic Compton profiles

Magnetic Compton profiles (MCPs) of Co/Pt multilayers have been measured from a view point of perpendicular magnetic anisotropy (PMA). The PMA and the MCPs are discussed in the present Co/Pt results together with the previous Co/Pd results. The anisotropies of the MCPs have been decomposed into each magnetic quantum number |m|=0, 1, and 2. The decomposition analysis has suggested that the |m|=1 state contributes to the PMA when the multilayer films change in-plane magnetic anisotropy to the PMA, and that the |m|=2 state contributes to the PMA when the multilayer films have large PMA energy.

Use of highly energetic (116 keV) synchrotron radiation for X-ray fluorescence analysis of trace rare-earth and heavy elements.

This study has revealed the advantages of the use of 116 keV X-rays as an excitation source of X-ray fluorescence (XRF) analyses. This technique is suitable for nondestructive multielemental analyses of heavy elements such as rare-earth elements. The lowest MDL value evaluated for the bulk analysis of a JG-1 standard reference sample (granite rock) was 0.1 ppm for W for a 500 s measurement. The spectrum of standard glass samples of SRM612 demonstrated clearly resolved K-line peaks of more than 30 elements, including all the existing rare-earth elements, at 50 ppm levels. The calibration curve for the determination of a rare-earth element shows a linear relation between the XRF intensity and concentrations from 10 to 0.03 ng. This powerful technique should be useful for nondestructive analyses of rare-earth and heavy elements in geological, geochemical and archaeological samples as well as industrial materials.

Fermi surface signatures in the compton profile of Be

We report high-resolution Compton profiles of two single crystals of Be with surface normals oriented along the [0001] and [1120] directions. The results are compared and contrasted with corresponding, highly accurate all-electron computations based on the band theory framework within the local-density approximation (LDA). A reasonable overall level of agreement is seen between theory and experiment with respect to the shapes of the Compton profiles, as well as the Fermi-surface-induced finer structure in the first and second spectral derivatives. However, the measured profiles are lower than theory at low momenta, clearly displaying broadened Fermi surface features compared with the theoretical predictions, and point to the importance of electron correlation effects beyond the LDA for a proper description of the momentum density. The aforementioned similarities and differences between theory and experiment in Be are rather like the situation that has been observed in recent high-resolution Compton studies of other materials.

Influence of interface on structure and magnetic properties of Fe50B50 nanoglass

In contrast to rapidly quenched metallic glasses, nanoglasses consist of two components, namely amorphous nanograins and interfacial regions with distinctively different properties. Various physical methods have been employed to obtain information on the atomistic and magnetic properties of such materials. For the case of a Fe50B50 nanoglass, using high-energy X-ray diffraction, it was found that the short-range order of the nanograins is similar to that of a crystalline FeB alloy. Magnetic Compton scattering shows that the total magnetic moment is the sum of the magnetic moment of the nanograins and the weak magnetic moment of the interfacial regions (μInterface = +0.08 μB). The measured moment of boron agrees (μBoron = −0.08 μB) with linear Muffin-Tin calculations. From the results of Mossbauer and magnetic Compton scattering, it can be concluded that the boron atoms segregate in the interfacial regions, resulting in a reduced boron concentration in the nanograins.