Published papers

Selective deposition of SiO2 nanoparticles was demonstrated on a soda-lime glass surface with a periodic sodium deficient pattern formed using the electrical nanoimprint. Positively charged SiO2 particles generated using corona discharge in a cyclic siloxane vapor, were selectively deposited depending on the sodium pattern. For such phenomena to occur, the sodium ion migration to the cathode side was indispensable to the electrical charge compensation on the glass surface. Therefore, the deposition proceeded preferentially outside the alkali-deficient area. Periodic SiO2 structures with 424 nm and 180 nm heights were obtained using one-dimensional (6 mu m period) and two-dimensional (500 nm period) imprinted patterns.

One-dimensional gratings of 700-nm period were imprinted on a sodaaluminosilicate glass (NAS) and a soda-lime silicate glass (NCS) using a platinum-coated SiO2 mold with application of DC voltage. The migration of network modifier cations below the anode side surface to the cathode side is a necessary condition for grating formation. Glass surfaces were chemically etched using a 55% KOH solution at 70°C. The top area of the NAS grating ridge, where the non-contacted area of the mold is located, was etched preferentially. Finally, the reverse concavo-convex grating appeared by etching. Localized stress corrosion in the grating ridge is expected to be an origin of the anisotropic etching and the grating pattern formation. In contrast, such anomalous etching behavior was not observed for the NCS. The bottom of the grating groove, the mold contacted area, was etched monotonously, maintaining the initial sinusoidal grating shape.

We have studied nanopatterns induced by nanosecond pulsed laser irradiation on (111) plane surfaces of a polycrystalline iron-aluminum alloy and evaluated their magnetic properties. Multiple nanosecond pulsed laser irradiation induces a wavelength-dependent surface transformation of the lattice structure from a B2-type to a supersaturated body centered cubic lattice. The selective formation of surface nanopatterns consisting of holes, stripes, polygonal networks, and dot-like nanoprotrusions can be observed. Furthermore, focused magneto-optical Kerr effect measurements reveal that the magnetic properties of the resultant nanostructured region changes from a paramagnetic to a ferromagnetic phase in accordance with the number of laser pulses. (C) 2013 AIP Publishing LLC.

We study the structural properties of the surface roughness, the surface mound size and the interfacial structure in Ni thin films vacuum-deposited on polyethylene naphthalate (PEN) organic substrates with and without the application of magnetic field and discuss its feasibility of fabricating quantum cross (QC) devices. For Ni/PEN evaporated without the magnetic field, the surface roughness decreases from 1.3 nm to 0.69 nm and the surface mound size increases from 32 nm to 80 nm with the thickness increased to 41 nm. In contrast, for Ni/PEN evaporated in the magnetic field of 360 Oe, the surface roughness tends to slightly decrease from 1.3 nm to 1.1 nm and the surface mound size shows the almost constant value of 28-30 nm with the thickness increased to 35 nm. It can be also confirmed for each sample that there is no diffusion of Ni into the PEN layer, resulting in clear Ni/PEN interface and smooth Ni surface. Therefore, these experimental results indicate that Ni/PEN films can be expected as metal/insulator hybridmaterials in QC devices, leading to novel high-density memory devices. (C) 2008 Elsevier B.V. All rights reserved.

As a platform for nano-science and technology, cleanliness in a compact and local clean environment, i.e. a portable clean-unit-box (CUP), is reported for establishing a large-scale network of an ultra-high clean environment platform towards cross-disciplinary research. Analyses of experimental results indicate that the CUP has cleanliness of ISO class similar to 2, which is one order of magnitude better than the conventional wafer transportation cleanbox.

The dielectric constant in annealing coercive differential spin tunneling junctions Co/Al-oxide/Co has been investigated in order to clear the influence of thermal annealing on oxidation states. Al oxidized naturally in pure O-2 shows the relative dielectric constant of 10-40 before annealing. It indicates that Al oxidized in pure O-2 can be AlO2 because the dielectric constant of AlO2 is approximately 22.7. This dielectric measurement result is in good agreement with XPS analysis results that O/Al ratio is 1.9-2.0. After annealing at temperature ranging from room temperature up to 250 degrees C, the relative dielectric constant approaches approximately 8.0, which is equal to that of Al2O3. This indicates that the phase of Al-oxide is changing from AlO2 to Al2O3 by annealing. This dielectric measurement will help investigate the oxidation state, such as chemical composition for junctions themselves. (C) 2005 Elsevier B.V. All rights reserved.

Magnetocapacitance (MC) effect, observed in a wide range of materials and devices, such as multiferroic materials and spintronic devices, has received considerable attention due to its interesting physical properties and practical applications. A normal MC effect exhibits a higher capacitance when spins in the electrodes are parallel to each other and a lower capacitance when spins are antiparallel. Here we report an inverse tunnel magnetocapacitance (TMC) effect for the first time in Fe/AlOx/Fe3O4 magnetic tunnel junctions (MTJs). The inverse TMC reaches up to 11.4% at room temperature and the robustness of spin polarization is revealed in the bias dependence of the inverse TMC. Excellent agreement between theory and experiment is achieved for the entire applied frequency range and the wide bipolar bias regions using Debye-Frohlich model (combined with the Zhang formula and parabolic barrier approximation) and spin-dependent drift-diffusion model. Furthermore, our theoretical calculations predict that the inverse TMC effect could potentially reach 150% in MTJs with a positive and negative spin polarization of 65% and -42%, respectively. These theoretical and experimental findings provide a new insight into both static and dynamic spin-dependent transports. They will open up broader opportunities for device applications, such as magnetic logic circuits and multi-valued memory devices.

We investigate magnetic properties of FeAl stripes and dots formed on the surface of Fe52Al48 induced by nanosecond pulsed laser irradiation. According to the in-plane magneto-optical Kerr effect (MOKE) measurements, the magnetization reversal in FeAl stripes can be explained by the domain-wall (DW) motion model, indicating that the magnetization rotates incoherently. In contrast, the in-plane and polar MOKE measurements reveal that the magnetic easy axis of the FeAl dots exists along the out-of-plane direction and the coercive force Hc shows as large as 1 kOe. These results indicate that the nanosecond pulsed laser irradiation can be expected to serve as a useful tool for obtaining magnetic nanostructures and creating high-Hc materials.

We demonstrate the formation of magnetic nanostripes on the surface of Fe52Al48 induced by nanosecond pulsed laser irradiation and investigate their magnetic properties. The magnetic stripe consists of a disordered A2 phase of Fe-Al alloys with Al-oxide along the [110] direction on the (111)-oriented plane. According to the focused magneto-optical Kerr effect measurement, the coercive force of the magnetic stripe obeys the 1/cos theta law, where theta is the field rotation angle estimated from the stripe direction. Also, the jump field can be observed in the magnetic hysteresis loop. These results indicate that the magnetization reversal in the magnetic stripe originates from the domain pinning, showing that the magnetization rotates incoherently. (C) 2014 AIP Publishing LLC.

A novel superconductor, LaPt5As, which exhibits a new crystal structure was discovered by high-pressure synthesis using a Kawai-type apparatus. A superconducting transition temperature was observed at 2.6 K. Depending on the sintering pressure, LaPt5As has superconducting and non superconducting phases with different crystal structures. A sintering pressure of around 10 GPa is effective to form single-phase superconducting LaPt5As. This material has a very unique crystal structure with an extremely long c lattice parameter of over 60 A and corner-sharing tetrahedrons composed of network like Pt layers. Density functional theory calculations have suggested that the superconducting current flows through these Pt layers. Also, this unique layered structure characteristic of LaPt5As is thought to play a key role in the emergence of superconductivity. Furthermore, due to a stacking structure which makes up layers, various structural modifications for the LaPt5As family are conceivable. Since such a high-pressure synthesis using a Kawai-type apparatus is not common in the field of materials science, there is large room for further exploration of unknown phases which are induced by high pressure in various materials.

YBa2Cu3O7-x (YBCO) thin films (with thickness of 220-230 nm) and thick films (with thickness of 1.1-1.2 mu m) were prepared using low fluorine solutions with different F/Ba ratios (F/Ba=1-4.5). The effect of F/Ba ratio on the properties of the films was systematically investigated. The results showed that films derived from solutions with F/Ba < 2, regardless of whether they were thin or thick, showed poor superconductivity due to the formation of BaCO3 intermediate phase. When solutions with F/Ba >= 2 were used, the superconductivity of as-prepared YBCO films was related to not only the film thickness, but also the F/Ba ratio in the solution. It was found that, for thick films derived from F/Ba > 2 solutions, the formation of BaF2/BaOF phase was retarded by the decomposition of Ba1-xYxF2+x (BYF) intermediate phase. We also found that extra fluorine in the solution resulted in a large number of impurities and a-axis grains in the thick films, degrading the film superconductivity. F/Ba=2 was the optimal ratio in the solution to fabricate high-J(c) YBCO thick films.

The magneto-resistance effect of Co/Al-oxide/Co tunneling junction is evaluated by a first-principles band calculation. To clarify the effect of unoxidized Al in the Al-oxide layer on the Co layer, we calculated the electronic structures of the junction with a local density approximation using the LMTO-ASA method. Periodic superlattices which consist of layers separated by vacant layers were used for the conventional calculation method. The magneto-resistance ratio, Delta R/R, was estimated by the equation Delta R/R = 2P(1)P(2)/(P-1 + P-2), where P-1 is the polarization of a boundary Co atom on one side of a layer and P2 is the polarization of a Co or Al atom with a vacant layer on the other side. The polarization of Co atoms near the boundary between the Co and Al layers decreases greatly due to the paramagnetic Al, although the Al was polarized by the ferromagnetic Co. These changes of polarization affect the magneto-resistance ratio with calculations indicating that the ratio is reduced by half for two atomic layers of residual Al. (c) 2005 Elsevier B.V. All rights reserved.

Metal/organic-molecule/metal nanoscale junctions, which consist of poly(3-hexylthiophene):6,6-phenyl C61-butyric acid methyl ester (P3HT:PCBM) organic molecules sandwiched between two Ni thin films whose edges are crossed, which are called quantum cross (QC) devices, have been fabricated and their structural and electrical properties have been investigated. The area of the crossed section, which was obtained without using electron-beam or optical lithography, can be as small as 16 x 16 nm(2). We have obtained ohmic current-voltage characteristics, which show quantitative agreement with the theoretical calculation results performed within the framework of the Anderson model under the strong coupling limit. Calculation results also predict that a high on-off ratio beyond 100000 : 1 can be obtained in Ni/P3HT:PCBM/Ni QC devices under the weak coupling condition. These results indicate that our method utilizing thin-film edges is useful for creating nanoscale junctions and Ni/P3HT:PCBM/Ni QC devices can be expected to have potential application in next-generation switching devices with high on-off ratios. (c) 2012 The Japan Society of Applied Physics

The coercivity of magnetic nanoparticles is enhanced by the exchange coupling effect at the interface of ferrimagnetic and antiferromagnetic self-assembled monolayers.

The oxidation states of At-oxide layer and the leakage current density in coercive differential spin tunneling junctions Co/Al-oxide/Co have been investigated in order to clear the mechanism of the increasing resistance change. X-ray photoelectron spectroscopy analysis shows that the resistance change increases with decreasing unoxidized At, which can be qualitatively explained by using first-principle band calculation based on linear-muffin-tin-orbital atomic-sphere-approximation method. The resistance change decreases with increasing leakage current density, which originates from Schottky effect. Reduction of unoxidized At and leakage current density originating from Schottky effect is required to obtain the large resistance change in spin tunneling junctions. (C) 2006 Elsevier B.V. All rights reserved.