Kenji Machida

Supercapacitor Performance of Hydrous Ruthenium Oxide Electrodes Prepared by Electrophoretic Deposition

Electrodes of hydrous ruthenium oxide were prepared by electrophoretic deposition (EPD) without binder and carbon additives. Through EPD and heat-treatment (250°C), a network of nanoparticles (ca. 10 nm) was developed with porous structure (packing density ca. 1.9 g/cm 3 ). As a result, high specific capacitance (734 F/g at I mV/s and 608 F/g at 50 mV/s) was obtained, where specific energy and power were as high as 25 Wh/kg (at 92 W/kg) and 21 kW/kg (at 12 Wh/kg), with loading weight of 0.19 mg/cm 2 . At higher loading weight (>0.4 mg/cm 2 ), specific capacitance decreased gradually. Also, volumetric energy was high because low-density additives, such as porous carbon and binder, were not used. From electrochemical impedance spectroscopy, specific energy and power were calculated, showing good agreement with values measured by CV. The decrease of rate capability with thicker electrode could be correlated by distribution of the time constant (τ), which was calculated from impedance spectra by using an equivalent circuit with multiple Warburg elements.

Hydrous RuO2/Carbon Black Nanocomposites with 3D Porous Structure by Novel Incipient Wetness Method for Supercapacitors

For supercapacitor electrode material, hydrous RuO 2 /carbon black nanocomposites were prepared by the novel incipient wetness method using a fumed silica nanoparticle. First, hydrous RuO 2 /fumed silica/Ketjen black (KB) was synthesized by the sol-gel-based method. After dissolving the fumed silica, the hydrous RuO 2 /KB nanocomposite, which is composed of RuO 2 nanoparticles (20-60 nm) dispersed on the high-surface-area KB (1180 m 2 g - 1 ), was formed with 3D porous structure at high loading of 60 wt % RuO 2 (Ru content is 46 wt %). The hydrous RuO 2 /KB nanocomposite electrode exhibited a specific capacitance of 647 F g - 1 with high charge utilization of RuO 2 (72%). which is significantly higher than reported values by other workers at similar loading of RuO 2 . The high capacitance and the high charge utilization were probably due to enhanced proton paths within the 3D porous structure of the nanocomposite materials.

Submicron Magneto-Optical Spatial Light Modulation Device for Holographic Displays Driven by Spin-Polarized Electrons

We have proposed a spin transfer switching (STS MO-SLM) device, based on Magneto-optical (MO) spatial light-modulation and driven by spin-polarized current flow, and confirmed its basic operation and characteristics experimentally. The proposed SLM device can be operated without active-matrix devices, has a spatial resolution as small as several hundred nanometers and possesses the potential for being driven at ultra-high speed of several tens of nanoseconds. Unlike existing SLM devices, this device satisfies both the size and speed requirements of SLMs for use in displaying holographic three-dimensional (3D) moving images. To improve the light modulation characteristics of SLM device, we carried out studies on magnetic films with perpendicular magnetic anisotropy to obtain large magneto-optical signals, which enabled us to realize enhanced light modulation performance. In addition, we measured a MO signal that was about twenty-times larger than that possible with in-plane anisotropy. We conclude that the MO-SLM device that we are developing is suitable for displaying future super-high definition, holographic three-dimensional moving images.

High-Voltage Asymmetric Electrochemical Capacitor Based on Polyfluorene Nanocomposite and Activated Carbon

An asymmetric electrochemical capacitor based on polyfluorene/carbon nanocomposite as positive electrode and activated carbon as negative electrode with nonaqueous electrolyte of 1 M tetraethylammonium tetrafluoroborate/propylene carbonate was investigated. From a galvanostatic charge-discharge test, the asymmetric capacitor test cell exhibited cell voltage as high as 3.2 V because of the high redox potential of the positive electrode (∼ 1.1 V vs Ag/Ag + ). Specific capacitance obtained for the test cell was 34 F g -1 (per electrode mass). The maximum energy density of the test cell was 47 Wh kg -1 (per electrode mass), which was two times higher than that of typical double-layer capacitors based on activated carbon/activated carbon electrodes.

A Magneto-Optical Spatial Light Modulator Driven by Spin Transfer Switching for 3D Holography Applications

We have successfully fabricated a basic magneto-optical spatial light modulator (MO-SLM) device driven by spin-transfer-switching (STS), which we call Spin-SLM technology. The SLM device is comprised of a one dimensional ten-pixel light modulation array (1 ×10 pixels) with a fine pitch of 1 μm. The light modulation pixels were composed of Gd-Fe based giant magneto resistance (GMR) devices, with a free layer of Gd-Fe, an Ag spacer and a Co-Fe/Tb-Fe-Co pinned layer. The GMR devices were sandwiched by a Cu bottom and Indium zinc oxide top electrodes in order to inject current perpendicular to the film plane for switching. Incident light penetrating the transparent top electrode can be modulated due to its magneto-optic Kerr effect with the Gd-Fe free layer. The fabricated device cell size is 220×300 nm2. We confirmed successful switching of the individual free layers, which was controlled by STS. We have also fabricated magnetic hologram patterns with the same magnetic materials used in the light modulation layer of the Spin-SLM device in order to determine its feasibility in display applications. The pixel pitch of the pattern was one micron and a reconstructed image by laser light was successfully observed with a wide viewing zone angle as 38 deg, which is nearly the same value expected from calculations. Although these patterns do not have an electrode to switch the magnetization direction, we confirmed the potential of Spin-SLM technology as a display device for 3D holography applications.

Spin transfer switching of closely arranged multiple pillars with current-perpendicular-to-plane spin valves

Spin transfer switching (STS) characteristics of two closely arranged spin valve (SV) pillars sharing a pair of top and bottom electrodes were investigated. Each pillar had a 300×100nm2 rectangular shape, which was fabricated by electron beam lithography. The separation between the pillars was 300nm or 1μm. The STS curves clearly show the two-step switching of the free layer for the device with a separation of 300nm. The first switching occurred at a switching current density of a single SV pillar or below. The second switching occurred at a switching current density approximately 1.2 times the first one. Furthermore, the STS characteristics of the paired free layers were estimated by a micromagnetic simulation using the Landau–Lifshitz–Gilbert–Slonczewski equation, which showed similar switching behavior to the experimental result of the free layers switched first.

Low-current-density spin-transfer switching in Gd22Fe78-MgO magnetic tunnel junction

Magnetization switching of a relatively thick (9 nm) Gd-Fe free layer was achieved with a low spin injection current density of 1.0 × 106 A/cm2 using MgO based magnetic tunnel junction devices, fabricated for light modulators. At about 560 × 560 nm2 in size, the devices exhibited a tunneling magnetoresistance ratio of 7%. This low-current switching is mainly attributed to thermally assisted spin-transfer switching in consequence of its thermal magnetic behavior arising from Joule heating.

Spin Transfer Switching and MR Properties of Co/Pt Multilayered Free Layers for Submicron Sized Magneto-Optical Light Modulation Device

Co/Pt multilayered films show strong perpendicular magnetic anisotropy and have a large magneto-optical Kerr effect in the short wavelength side. To use these films with submicron spatial light modulators driven by spin transfer switching (STS), we fabricated current perpendicular to plane giant magnetoresistance (CPP-GMR) and tunnel magnetoresistance (TMR) devices with Co/Pt multilayers for free layers and investigated the MR properties, the STS characteristics, and the Kerr effects. A Kerr hysteresis loop was clearly observed in the CPP-GMR device, which was 125 × 180 nm2. Full magnetization reversal of the Co/Pt multilayered film by spin transfer torque was demonstrated for a CPP-GMR device with a Cu-based top electrode. On the other hand, there was hardly any resistance change in a CPP-GMR with a transparent top electrode due to the low MR ratio of the device. A TMR stack with a Ag buffer layer showed a strong perpendicular magnetic anisotropy. An MR curve was detected for a TMR device with a transparent top electrode.

Spin transfer switching of current-perpendicular-to-plane giant magnetoresistance with various Gd-Fe free-layer compositions

We have investigated the spin transfer switching (STS) properties of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with various compositions of Gd-Fe free-layers for magneto-optical light modulator applications, whereby iron (Fe) concentrations are varied from 72.5 to 80.3 at. %. Switching current densities decreased dramatically with increases in Fe concentration within the Gd-Fe compound. The switching current density (Jc0) for a device with composition Gd Fe80.3 at.% was 20.5 MA/cm2, which is more than eight times smaller than that for devices with composition Gd Fe72.5 at. %. This reduction in switching current can be attributed to a decrease in the effective perpendicular anisotropy in Fe-richer Gd-Fe compounds.

Yoke-type TMR head with front-stacked gap for perpendicular magnetic recording

This paper describes the design and performance of yoke-type read heads with spin-dependent tunneling magnetoresistive (TMR) elements. Magnetic field calculations have been carried out for a yoke-type TMR head with a front-stacked gap (Front-SG). The head has the advantage of a higher signal-to-noise ratio than that of a shielded TMR head and produces Lorentzian waveforms without any distortion in combination with perpendicular magnetic recording media.