Shiro Kawachi

We developed a resistance measurement using radio frequency reflection to investigate the electrical transport characteristics under destructive pulsed magnetic fields above 100 T. A sample stage consisting of a homemade flexible printed circuit reduced the noise caused by the induced voltage from the pulsed magnetic fields, improving the accuracy of the measurements of the reflected waves. From the obtained reflectance data, the absolute value of the magnetoresistance was successfully determined by analyzing the phase with admittance charts. These developments enable more accurate and comprehensive measurements of electrical resistance in pulsed magnetic fields.

L1(0)-ordered ferromagnetic nanowires with large coercivity are essential for realizing next-generation spintronic devices. Ferromagnetic nanowires have been commonly fabricated by first L1(0)-ordering of initially disordered ferromagnetic films by annealing and then etching them into nanowire structures using lithography. If the L1(0)-ordered nanowires can be fabricated using only lithography and subsequent annealing, the etching process can be omitted, which leads to an improvement in the fabrication process for spintronic devices. However, when nanowires are subjected to annealing, they easily transform into droplets, which is well-known as Plateau-Rayleigh instability. Here, we propose a concept of "nanostructure-induced L1(0)-ordering" of twinned single-crystals in CoPt ferromagnetic nanowires with a 30 nm scale ultrafine linewidth on Si/SiO2 substrates. The driving forces for nanostructure-induced L1(0)-ordering during annealing are atomic surface diffusion and extremely large internal stress at ultrasmall 10 nm scale curvature radii of the nanowires. (Co/Pt)(6) multilayer nanowires are fabricated by a lift-off process combining electron-beam lithography and electron-beam evaporation, followed by annealing. Cross-sectional scanning transmission electron microscope images and nano-beam electron diffraction patterns clearly indicate nanostructure-induced L1(0)-ordering of twinned single-crystals in the CoPt ferromagnetic nanowires, which exhibit a large coercivity of 10 kOe for perpendicular, longitudinal, and transversal directions of the nanowires. Two-dimensional grazing incidence X-ray diffraction shows superlattice peaks with Debye-Scherrer ring shapes, which also supports the nanostructure-induced L1(0)-ordering. The fabrication method for nanostructure-induced L1(0)-ordered CoPt ferromagnetic nanowires with twinned single-crystals on Si/SiO2 substrates would be significant for future silicon-technology-compatible spintronic applications.

L1(0)-ordered CoPt with a large coercivity (H (c)) of 13 kOe was demonstrated on Si/SiO2 substrates by hydrogen annealing. Equiatomic 11.2 nm thick (Co/Pt)(4) multilayer thin films were fabricated by electron-beam evaporation and were annealed at 500 degrees C-900 degrees C for 10-90 min under an Ar/H-2 mixed gas atmosphere. The annealing temperature and time dependences of the crystal structures, magnetic properties, and surface morphologies of the films were systematically analyzed based on the experimental results obtained from grazing incidence X-ray diffraction (GI-XRD), vibrating sample magnetometer, and scanning electron microscope, respectively. Hydrogen annealing effectively promoted the out-of-plane c-axis orientation of L1(0)-ordered CoPt compared to the vacuum annealing according to the GI-XRD patterns. A maximum H (c) of 13.3 kOe was obtained in L1(0)-ordered CoPt with angular-outlined isolated grains by hydrogen annealing at 800 degrees C for 60 min, where the c-axis of L1(0)-ordered CoPt was randomly distributed.

We report the origins of the coloration of Bi2O3-SiO2, Bi2O3-B2O3, and Bi2O3-P2O5 glasses. The glasses were quantitatively analyzed to reveal the Bi valence state and network connectivity of silicate, borate, and phosphate. The electronic structure was correlated with Bi valence states, giving two types of signals: those related to optical bandgaps with Bi3+ s-p intra-transition states at 3.6-4.7 eV and visible absorption bands at 2.5-2.7 eV originating from the charge transfer between Bi3+ and Bi5+ and/or plasmonic Bi-0 clusters in the colored glasses. These Bi oxide glasses with tvisible transparent and high refractive index are attractive to replace toxic Pb-containing optical glasses.