Shigeo Sato

Structural study of amorphous Fe70M10B20(M = Zr, Nb and Cr) alloys by X-ray diffraction

The local atomic structures in the amorphous Fe70Zr10B20 ,F e 70Nb10B20 and Fe70Cr10B20 alloys with different Tx were studied by the ordinary X-ray diffraction, anomalous X-ray scattering (AXS) measurements and EXAFS analyses. The essential features of the atomic structures resemble in these amorphous alloys. Only the difference in the local atomic structure is a distorted shape of the triangular prism due to the size difference between Fe and M. From the correspondence of the size difference with Tx and the study of the crystallization process by the DSC measurements and the X-ray diffraction method, we conclude that the difference of their thermal stability is closely related with the difficulty of rearrangements of the prisms during crystallization.

Implementation of a new neurochip using stochastic logic

Even though many neurochips have been developed and investigated, the best suitable way for implementation has not been known clearly. Our approach is to exploit stochastic logic for various operations required for neural functions. The advantage of stochastic logic is that complex operations can be implemented with a few ordinary logic gates. On the other hand, the operation speed is not so fast since stochastic logic requires certain accumulation time for averaging. However, a huge integration can be achieved and its reliability is high because all of operations are done on digital circuits. Furthermore, we propose a nonmonotonic neuron realized by stochastic logic, since the nonmonotonic property is efficient for the performance enhancement in association and learning. In this paper, we show the circuit design and measurement results of a neurochip comprising 50 neurons are shown. The advantages of nonmonotonic and stochastic properties are shown clearly.

Analogue spin–orbit torque device for artificial-neural-network-based associative memory operation

We demonstrate associative memory operations reminiscent of the brain using nonvolatile spintronics devices. Antiferromagnet–ferromagnet bilayer-based Hall devices, which show analogue-like spin–orbit torque switching under zero magnetic fields and behave as artificial synapses, are used. An artificial neural network is used to associate memorized patterns from their noisy versions. We develop a network consisting of a field-programmable gate array and 36 spin–orbit torque devices. An effect of learning on associative memory operations is successfully confirmed for several 3 × 3-block patterns. A discussion on the present approach for realizing spintronics-based artificial intelligence is given.

Structural study of Fe90−xNb10Bx (x=10, 20 and 30) glassy alloys

Abstract The local atomic structures of Fe90−xNb10Bx (x=10, 20 and 30) glassy alloys have been investigated by anomalous X-ray scattering measurements. A trigonal prism-like chemical short range ordered (CSRO) structure of (Fe,Nb)6B was found in Fe60Nb10B30 and Fe70Nb10B20 alloys exhibiting a large supercooled liquid region defined by the temperature interval, ΔTx between the glass transition temperature, Tg and the crystallization temperature, Tx, (ΔTx=67 and 53 K, respectively), whereas this kind of local structure was not identified for Fe80Nb10B10 having no glass transition. The variation in thermal stability of the Fe–Nb–B alloys originates in a difference in the stability of the local atomic structures with particular compositions.

Surface‐sensitive x‐ray fluorescence analysis at glancing incident and takeoff angles

The possibility of the glancing incidence and takeoff x‐ray fluorescence (GIT‐XRF) method for surface analysis has been investigated. In this method the observation depth can be restricted by both the incident angle of primary x rays and the takeoff angle of observed fluorescent x rays. As a result, it has been found that the surface‐sensitive analysis is achieved when the incident angle is small below the critical angle for total reflection of the primary x rays and the takeoff angle is set slightly below the critical angle for total reflection of the fluorescent x rays from the element on the substrate.

Bidirectional thermal expansion measurement for evaluating Poisson’s ratio of thin films

An original and practical method is demonstrated for determining Poisson’s ratio of thin films by detecting thermal expansion in two directions perpendicular to each other. In the direction within the film, the temperature gradient of the biaxial thermal stress Δσf∕ΔT was obtained by substrate curvature measurements; in the direction perpendicular to the film, the temperature gradient of the whole thermal expansion strain Δd∕dΔT along the film thickness d was measured by x-ray reflectivity. It was found that Poisson’s ratio of thin films with a thickness of several hundred nanometers can be determined from Δσf∕ΔT, Δd∕dΔT, reduced modulus Er of the film, and from the thermal expansion coefficient αs of the substrate.

Nano quasicrystal formation and local atomic structure in Zr–Pd and Zr–Pt binary metallic glasses

Abstract Formation of the nanoscale icosahedral quasicrystalline phase (I-phase) in the melt-spun Zr70Pd30 and Zr80Pt20 binary metallic glasses were reported. Local atomic structure in the glassy and quasicrystal (QC)-formed states were also analyzed by XRD and EXAFS measurements in order to investigate the formation mechanism of QC phase. The distorted icosahedral-like local structure can be identified around Zr atom in the Zr70Pd30 metallic glass. In the QC formation process, a change of local environment around Zr is detected, in which the approximately one Zr atom substitutes for one Pd atom. In contrast, since the local environment around Pt atom is remaining during the QC precipitation, it is suggested that the stable icosahedral local structure is mainly formed around a center Pt atom in the glassy state in Zr80Pt20. We also found that the local environment around Zr atom significantly changes during the quasicrystallization in the alloy. These results differ from those in the Zr70Pd30 metallic glass.

Local atomic structure of Fe–Co–Ln–B(Ln=Sm,Tb or Dy) amorphous alloys with supercooled liquid region

Abstract The local ordering structures of Fe70Co10B20 and Fe 67 Co 10 Ln 3 B 20 ( Ln = Sm , Tb or Dy ) alloys have been studied by X-ray diffraction. The analyses of interference functions and radial distribution functions revealed that the local atomic ordering becomes weaker and the coordination number of transition metals around Fe decreases while that of transition metals around B remains almost unchanged by the addition of Ln element to Fe–Co–B alloy. These results indicate that the amorphous phase consists of (Fe,Co)–B-based rigid local structure units and the Ln atom is inserted between them. The unique structure may cause the appearance of supercooled liquid region with the difficulty of re-arrangement of atoms during annealing.

Effects of interfacial chemical states on the performance of perovskite solar cells

We showed that the widely used solvent molecule, N,N-dimethyl-formamide (DMF), readily adsorbs on the surface of TiO2 electrodes of perovskite solar cells (PSCs), and that the adsorbed DMF molecules remain intact on the TiO2 surface even after long-term annealing of the perovskite layer, resulting in an increase in the contact resistance of the PSCs. We found that the absorption of DMF is significantly suppressed by modifying the TiO2 electrode surface with a fullerene derivative, [6,6]-phenyl-C61-butyric acid (PCBA). We also suggested that the high electron affinity of PCBA enhances the charge transportation at the perovskite/TiO2 interface and reduces the contact resistance.

Local structure characterization in quasicrystal-forming Zr80Pt20 binary amorphous alloy

The local structure of melt-spun Zr80Pt20 alloy was investigated in the amorphous and icosahedral quasicrystal (QC)-formed states by x-ray diffraction and extended x-ray absorption fine structure measurements. While the local environment around the Zr atom in the amorphous state is considerably different from that in the QC-formed state, it remains during the quasicrystallization around the Pt atom. It is suggested that the stable icosahedral local structure is mainly formed in the center of the Pt atom in the amorphous state.