Kazumichi Namikawa

Polarization tunability and analysis for observing magnetic effects on BL39XU at SPring-8

Polarization tunability and analysis of X-rays is one of the most advancing features of third-generation synchrotron radiation sources. In order to apply such developments to the observation of magnetic effects, a diffractometer for X-ray magnetic absorption and scattering experiments was constructed on BL39XU at SPring-8. The efficiency of the apparatus is clearly demonstrated by several observations of the magnetic effects. In particular, a diamond phase plate plays an essential role in regulating both circular and linear polarization states.

Magnetic XANES of circularly polarized X-ray absorption on Fe and Ni metals and Nd2Fe14B compound

Magnetic effects in the X-ray absorption process have been measured using circularly polarized X-rays emitted from an elliptical multipole wiggler. Magnetic XANES spectra on the K -edge of Fe and Ni taken by the transmission method show positive and negative spin polarizations, respectively, near the Fermi level which are consistent with the results in literature. A large positive magnetic XANES spectrum on the Nd L 2 -edge of Nd 2 Fe 14 B compound measured by the fluorescent method indicates that the unoccupied 5 d -bands are polarized due to a significant hybridization between 4 f - and 5 d -states. The magnetic XANES spectrum on the Nd L 3 -edge has a structure below the Fermi level, suggesting a quadrupole transition contribution of the partially vacant majority band of 4 f -states.

X-ray probe of the polar nanoregions in the relaxor ferroelectric 0.72Pb(Mg1∕3Nb2∕3)O3–0.28PbTiO3

The average, spontaneous (zero field) structure of the polar nanoregions (PNRs) in a single crystal specimen of the relaxor ferroelectric 0.72Pb(Mg1∕3Nb2∕3)O3–0.28PbTiO3 has been characterized above Tm by partially coherent x-ray diffraction. At these temperatures, the PNRs are randomly distributed within the crystal and exhibit dimensions of 17 and 10nm along the [001] and [100] directions, respectively. Based on the results of a previous study [Z. Guo et al., J. Appl. Phys. 101, 053505 (2007)], the long dimension of the PNR is observed to reorient preferentially along the four degenerate diagonal axes [1±1±1], which is believed to contribute to the high electromechanical response in these relaxor materials.

Design of a prototype split-and-delay unit for XFEL pulses, and their evaluation by synchrotron radiation X-rays

A prototype split-and-delay unit (SDU) for X-ray free-electron laser (XFEL) pulses is proposed based on the Graeff-Bonse four-Bragg-reflection interferometer by installing 12.5° slopes. The SDU can continuously provide a delay time from approximately -20 to 40 ps with a resolution of less than 26 fs. Because the SDU was constructed from a monolithic silicon crystal, alignment is straightforward. The obtained thoroughputs of the SDU reached 0.7% at 7.46 keV and 0.02% at 14.92 keV. The tunability of the delay time using the proposed SDU was demonstrated by finding the interference effects of the split X-rays, and the time resolution of the proposed SDU was evaluated using the width of the interference pattern recorded on the X-ray charge-coupled device camera by changing the energy, i.e. longitudinal coherence length, of the incident X-rays. It is expected that the proposed SDU will be applicable to XFEL experiments using delay times from tens of femtoseconds to tens of picoseconds, e.g. intensity correlation measurements.

Quantum Monte Carlo study on speckle variation due to photorelaxation of ferroelectric clusters in paraelectric barium titanate

Time-dependent speckle pattern of paraelectric barium titanate observed in a soft x-ray laser pump-probe measurement is theoretically investigated as a correlated optical response to the pump and probe pulses. The scattering probability is calculated based on a model with coupled soft x-ray photon and ferroelectric phonon mode. It is found that the speckle variation is related to the relaxation dynamics of ferroelectric clusters created by the pump pulse. Additionally, critical slowing down of cluster relaxation arises on decreasing temperature toward the paraelectric-ferroelectric transition temperature. The relation between critical slowing down, local dipole fluctuation, and crystal structure is revealed by quantum Monte Carlo simulation.

Polarization analyzing system for x‐ray magnetic Kerr rotation in x‐ray magnetic resonant scattering

We made a new polarization analyzing system for measuring a rotation of major axis of elliptical polarization in x‐ray magnetic resonant scattering. This system, based on a two‐axis diffractometer, is characterized by introducing a 45° linearly polarized x‐ray incident beam. Design and performance of the system are described. The rotation can be measured with precision of 0.2°–0.3° at the vicinity of Gd L3‐absorption edge where the maximum rotation is about −2°.

X-ray Resonance Exchange Scattering Study of Nd2Fe14B

The X-ray resonance exchange scattering (XRES) has been measured at the Nd L2,3-edges on the 330, 440 and 550 Bragg reflections in a ferromagnetic Nd2Fe14B single crystal. The XRES results at the L2-edge reflect the spin polarization of unoccupied 5d-bands of Nd due to electric dipole transitions. Those at the L3-edge show the possibility of electric quadrupole transitions to the highly localized 4f-states. The temperature dependence of XRES confirms the sensitivity to the local magnetic moment of specific site. The magnetic moments of Nd 4f and 4g sites rotate from the c-axis with different angles below spin reorientation temperature.

MAGNETIZATION PROCESS OF IRON SURFACE OBSERVED BY TRANSVERSE KERR MAGNETOMETRY

Transverse magneto-optical Kerr hysteresis loops were observed on pure iron single crystals. They exhibit a steplike behavior of magnetization different from that of the bulk. The hysteresis loop is compared with those magnetization curves which are obtained on the same sample by x-ray resonant magnetic scattering measurement and by the vibrating sample magnetometry. The magnetization process is discussed in terms of the components of magnetization parallel and perpendicular to magnetic field. The nature of the steplike magnetization was revealed to be characterized by the wall displacement of fracturized magnetic domains in the surface region.

X‐ray photon correlation spectroscopy study in valence fluctuation compound Eu3S4

X‐ray photon correlation spectroscopy (XPCS) is a powerful tool to perform direct measurements of the dynamics of fluctuations in condensed matter systems at atomic resolution. Here we have investigated spatial and time correlations of domains in the valence fluctuated phase in a typical valence fluctuation compound Eu3S4 near the charge ordering temperature (TCO). The spatial correlation was observed at the fundamental reflection (4 4 4) using coherent diffraction techniques, which indicated that the domain size is longer than 200 nm near TCO. The time correlation of the domain was measured by the XPCS technique, and we found that the slow fluctuation of the order of 10 second exists only near TCO.

X-ray Intensity Fluctuation Spectroscopy Using Nanofocused Hard X-rays: Its Application to Study of Relaxor Ferroelectrics

The use of a combination of coherent X-rays from a third-generation synchrotron light source and ultraprecise Kirkpatrick-Baez mirrors enables us to apply nanofocused hard X-rays in solid-state physics. We developed an apparatus for X-ray intensity fluctuation spectroscopy using the nanofocused hard X-rays and applied it to the study of relaxor ferroelectrics. We have successfully detected a large and slow intensity fluctuation of scattered X-rays above cubic-to-tetragonal phase transition temperature with a characteristic time scale on the order of 10 s. We speculated that the intensity fluctuation originates from domain number fluctuation, which is directly related to the dielectric response, particularly the frequency dispersion.