Hayato Miyagawa

Direct Observation of Ferromagnetic Spin Polarization in Gold Nanoparticles

We report the first direct observation of ferromagnetic spin polarization of Au nanoparticles with a mean diameter of 1.9 nm using x-ray magnetic circular dichroism (XMCD). Owing to the element selectivity of XMCD, only the gold magnetization is explored. Magnetization of gold atoms as estimated by XMCD shows a good agreement with results obtained by conventional magnetometry. This evidences intrinsic spin polarization in nanosized gold.

Hard X-ray Photoelectron Emission Microscopy as Tool for Studying Buried Layers

We have performed photoelectron emission microscopy measurements with hard X-rays (HX-PEEM) and have shown that HX-PEEM has a spatial resolution higher than 106 nm for observing the sample surface and can detect signals from a Au layer buried under a 50-nm-thick Co layer. These results mean that HX-PEEM has a probing depth one order of magnitude deeper than that of PEEM with soft X-rays. This deeper probing depth with HX-PEEM provides a new opportunity to investigate electronic and/or magnetic structures of buried layers under a thick overlayer. The notable advantages and potential applications of HX-PEEM are discussed.

Fabrication of transparent antifouling thin films with fractal structure by atmospheric pressure cold plasma deposition.

Antifouling surface with both superhydrophobicity and oil-repellency has been fabricated on glass substrate by forming fractal microstructure(s). The fractal microstructure was constituted by transparent silica particles of 100 nm diameter and transparent zinc-oxide columns grown on silica particles by atmospheric pressure cold plasma deposition. The sample surface was coated with a chemically adsorbed monomolecular layer. We found that one sample has the superhydrophobic ability with a water droplet contact angle of more than 150°, while another sample has a high transmittance of more than 85% in a wavelength range from 400 to 800 nm.

Enhancement and suppression of terahertz emission by a Fabry-Perot cavity structure with a nonlinear optical crystal

We have fabricated Fabry-Perot (FP) cavities in the THz region with a ZnTe crystal as a cavity layer by a simple stacking method. We observed more than a three times enhancement of the THz emission intensity in the FP cavities compared with the bare ZnTe crystal at the frequencies of the resonant modes and stopband edges. On the other hand, suppression of the THz emission occurs at frequencies in the stopband. The enhancement and suppression of the THz emission are caused by the modification of the optical density of state in the FP cavities compared to the vacuum.

Structural Analyses of Fractional Monolayer (GaAs)m/(AlAs)n Superlattices by X-ray Resonant/Off-Resonant Scattering

The structure of (GaAs)m/(AlAs)n superlattices with a period length of a fractional number in monolayer (ML) units, which is fabricated by shutter control in molecular beam epitaxy (MBE) growth, is studied by X-ray diffraction (XRD) analysis using synchrotron radiation. Satellite peaks originating from the fractional superlattice (SL) periods, 1.69 monolayers (ML), are clearly observed in XRD profiles. XRD simulation, which is based on the model of modulated composition distribution, can reproduce the satellite peak positions. We also measure X-ray resonant scattering at the Ga K-edge and estimate the atomic composition in the SL region.

Growth of GaNAs/GaAs Multiple Quantum Well by Molecular Beam Epitaxy Using Modulated N Radical Beam Source

GaNAs/GaAs multiple quantum well (MQW) structures have been grown on GaAs(001) substrates by molecular beam epitaxy (MBE) using modulated N radical beam source under optimized conditions, wherein the amount of N2 gas flow, RF-power and shutter sequence are systematically controlled. Clear and flat GaNAs/GaAs interfaces were observed in the cross-sectional transmission electron microscopy (TEM) measurements. Fine MQW structures originating from the precise control of the modulated N radical beam have been demonstrated as clear satellite peaks from the X-ray diffraction (XRD) measurements and sharp photoluminescence (PL) peaks. The step-like behaviors in the absorption spectra which reflect the density of state in two-dimensional systems, were clearly observed for all MQW samples.

Orbital and spin moments in SmFe2 studied by means of white-x-ray magnetic diffraction

A magnetic structure of SmFe2 with a Laves phase crystalline structure was investigated by means of white-x-ray magnetic diffraction. The orbital and the spin magnetic form factors of Sm and Fe ions have been obtained as a function of momentum transfer. Their extrapolation to zero momentum transfer, according to the dipole approximation based on an atomic model, has given the values of the spin and the orbital magnetic moments of Sm ions as −2.54 +/- 0.55μB and 2.79 +/- 0.61μB, respectively. This result confirms that the small magnetic moment of Sm ions arises from antiparallel coupling between the spin and the orbital magnetic moments. The spin and the orbital magnetic moments of Fe ions have been also estimated, as 1.13 +/- 0.25μB and 0.19 +/- 0.04μB, respectively.

Development of a two-dimensional scanning micro-mirror utilizing magnetic polymer composite

In this study, we propose a magnetically driven micro-mirror, constructed using negative photoresist SU-8 containing magnetic particles, as a magnetic actuator and torsion bar structure. Because the magnetic polymer composite uses thick negative photoresist SU-8 as the main material, the micro-mirror is simply fabricated in just a few steps by conventional photolithography and deep reactive ion etching. A fabricated prototype of the micro-mirror, which is magnetically driven by using an external magnetic field, is shown to deflect with two-dimensional optical deflection angles of 6.5 and 12.5°. Moreover, Lissajous scanning motion of the fabricated mirror is achieved.

Enhancement of optical rectification for THz amplification in one-dimensional photonic crystals

We fabricated cavity type THz one-dimensional photonic crystal with a nonlinear material and demonstrated the amplification of THz light using these structures.

Optimization of Well Width and N Content for Optical Properties of GaNAs/GaAs Multiple Quantum Well Grown by RF-Molecular Beam Epitaxy

We have investigated the optical properties in GaNAs/GaAs multiple quantum wells (MQWs) with various GaNAs layer widths and N contents, which were fabricated on GaAs(001) substrates by molecular beam epitaxy using an RF-plasma source. In particular, when the GaNAs layer thickness, LW, was 3 monolayers (ML), the photoluminescence (PL) peak intensity around 1.31 eV was approximately 70 times stronger than that in the case of LW=10 ML. Moreover, the full width at half maximum of the dominant PL peak became narrow as GaNAs layer thickness decreased, even though N content was increased.