Tsuneharu Koide

Elliptical-polarization analyses of synchrotron radiation in the 5–80-eV region with a reflection polarimeter

Abstract The Stokes parameters, which completely represent the polarization state of light, have been measured in the 5–80-eV energy region for synchrotron-radiation beams on the VUV to soft X-ray beamlines at the Photon Factory. Various states of polarization were produced for the emerging beam by deflecting the incoming beam vertically with respect to the entrance slit of the monochromator with a premirror of the beamline. The resultant beam was polarization-analyzed using a polarimeter comprising two triple-reflection polarizers. Increasing the beam-deflection angle was found to result in a change in the polarization state from predominantly horizontal linear polarization to elliptical polarization. This procedure could be used both to produce and to verify circular polarization with | P c | ≳ 80% in the energy range 60–80 eV, which could be immediately applied to MCD experiments. Unpolarized light could clearly be distinguished from circularly polarized light. The unpolarized component was found to become larger with increasing energy on a soft X-ray beamline; this was shown to be mainly due to scattering by beamline optical elements.

Indication of intrinsic room-temperature ferromagnetism in Ti1-xCoxO2-δ thin film : An x-ray magnetic circular dichroism study

Soft x-ray magnetic circular dichroism (XMCD) measurements at the Co L2,3 edges of Co-doped rutile TiO2 at room temperature revealed clear multiplet features characteristic of ferromagnetic Co2+ ions coordinated by O2− ions, being in sharp contrast to the featureless XMCD spectrum of Co metal or metallic clusters. The absorption and XMCD spectra agree well with a full atomic-multiplet calculation for the Co2+ high-spin state in the D2h-symmetry crystal field at the Ti site in rutile TiO2. The results indicate that the ferromagnetism arises from the Co2+ ions substituting the Ti4+ ions.

Investigation of carbon contamination of synchrotron radiation mirrors

Abstract Carbon-contaminated synchrotron radiation mirrors have been successfully cleaned by a dc glow discharge in oxygen. Reflectance and angle-resolved scattering have been measured before and after cleaning. The effect of carbon contamination on reflectance is severe at near normal incidence whereas it is small at grazing incidence. The decrease in reflectance is most noticeable for photons with an energy below about 10 eV. In the discharge arrangement in which the mirror surface did not face an electrode, the reflectance of contaminated mirrors was nearly restored to that of uncontaminated mirrors and the scattering level was reduced. An additional discharge, in which the mirror surface faced the electrode, degraded the reflectance but produced no appreciable change in the scattering level.

X-ray Absorption and X-ray Magnetic Circular Dichroism Studies of a Monatomic Fe(001) Layer Facing a Single-Crystalline MgO(001) Tunnel Barrier

We studied the electronic and magnetic states of a monatomic bcc Fe(001) layer facing a single crystalline MgO(001) tunnel barrier by using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at the Fe L2,3 core edges in order to clarify the origin of the huge tunnel magnetoresistance (TMR) effect observed in Fe/MgO/Fe magnetic tunnel junctions. Both the XAS and XMCD revealed that the 1 ML-Fe is not oxidized, which is crucial to the huge TMR effect. A sum-rule analysis of the XMCD shows that the 1 ML-Fe(001) has an enhanced total magnetic moment of about 2.6 µB per Fe atom compared with that of bulk Fe.

Front ends for the Photon Factory beam lines

Abstract The front end of each beam line at Photon Factory serves the experimental area with a synchrotron radiation beam of 30 mrad in horizontal aperture. Some specific aspects concerning an acoustic delay line, a fast closing valve, silicon carbide mirrors and a beryllium window assembly installed in the beam lines are presented.

Production and direct measurement of circularly polarized vacuum-ultraviolet light with multireflection optics

The conversion of linearly polarized synchrotron radiation to circular polarization has been successfully achieved beyond the LiF transmission cutoff in the vacuum ultraviolet by utilizing a triple‐reflection polarizer as a quarter‐wave retarder. The Stokes parameters of the emerging beam were directly measured at 30 eV with a reflection‐type polarimeter as a function of the rotation angle of the ‘‘circular polarizer’’ around the optical axis. The results show that a degree of circular polarization of up to ∼±82% was attained at ∼±25° rotation angles, respectively, between the incidence plane for the polarizer and the horizontal plane. This result is in good agreement with a calculation which took into account the independently measured Stokes parameters of the incoming beam.

Construction and performance of BL28 of the Photon Factory for circularly polarized synchrotron radiation

A branch beamline, BL28A, has been constructed for the application of circularly polarized vacuum ultraviolet radiation. The radiation can be obtained in the helical undulator operation mode of an insertion device, EMPW♯28, which is also cut for elliptically polarized hard x‐ray radiation. T first harmonic of the helical undulator radiation can be tuned from 40 to 350 eV with its corresponding K value from 3 to 0.2. A monochromator working basically with constant deviation optics was installed, and has started its operation. A circularly polarized flux of ∼1010 photons/s has been achieved with energy resolution of around 500–1000 at the first harmonic peak. The circular polarization after the monochromator was estimated to be higher than 70% by comparing theory and experiment on the magnetic circular dichroism of nickel films in the 3p‐3d excitation region. The design philosophy of the beamline and recent results on the performance tests are presented.

Optical Properties of CeO2 Crystal in the Photon Energy Range of 2.5–40 eV

Optical reflectance spectra of CeO 2 crystal have been measured in the photon energy range from 2.5 to 40 eV. A Kramers-Kronig analysis of the reflectance data has been performed to obtain the dielectric function and related functions. The observed spectral features can be explained in terms of the charge transfer and interband transitions and the atomic-like excitation of the Ce 5 p core electron. A peak regarded as a candidate for the “4 f 2 ” feature has been observed at an excitation energy of 13 eV.

Soft x-ray reflection from SiC, TiC, and WC mirrors.

The optical constants of SiC, TiC, and WC in the soft x-ray region (80–1200 eV) derived from the incidence-angle dependence of the specular reflectance are reported. The angular dependence of the specular intensity can be explained by simple scattering theory with a Gaussian distribution of the surface roughness in the soft x-ray wavelength range.

In situ dc oxygen‐discharge cleaning system for optical elements

In situ dc oxygen‐discharge cleaning arrangements have been developed at the Photon Factory for the removal of carbon contamination from optical surfaces. A high cleaning rate could be achieved by producing an oxygen plasma close to the optical elements with special care taken to avoid any harmful effects from the discharge; contaminant carbon was completely removed within a few hours, at most. This short exposure time and the use of dry oxygen gas resulted in a restoration of the original ultrahigh vacuum without a bakeout. Results with a Seya‐Namioka beamline for gas‐phase experiments showed a flux enhancement amounting to a factor of 50, and results with a grasshopper beamline showed a nearly complete recovery of the light intensity, even at the carbon K edge.