Takeshi Ohgai

Bridging the gap between template synthesis and microelectronics: spin-valves and multilayers in self-organized anodized aluminium nanopores

Current perpendicular to the plane, giant magnetoresistance (GMR) and the spin-valve effect were observed in Co/Cu multilayered nanowires electrodeposited into self-organized nanoporous anodized aluminium templates grown at the surface of bulk aluminium. Pores as short as 2000 nm could be used. The pore bottom oxide layer was removed or thinned by chemical etching. Alternating Co and Cu layers of 10 nm in thickness were synthesized by pulse plating. 20% of GMR was observed in 100 Co/Cu bilayer nanowires at room temperature. Co/Cu/Co tri-layered nanowires clearly showed the typical resistance switching of spin-valves.

Template synthesis and magnetoresistance property of Ni and Co single nanowires electrodeposited into nanopores with a wide range of aspect ratios

Template synthesis and magnetoresistance property of Ni and Co single nanowires electrodeposited into nano-pores with wide range of aspect ratios

Electrochemical synthesis and magnetoresistance properties of Ni, Co and Co/Cu nanowires in a nanoporous anodic oxide layer on metallic aluminium

The layer thickness of anodized aluminium oxide was controlled by anodization time and voltage to obtain templates with the desired pore-length . The barrier layer at the pore bottom of anodized aluminium templates was chemically etched to make good electrical contacts for nanowires electrodeposited in the pores thus formed on metallic aluminium substrates. Ni and Co homogeneous nanowires and Co/Cu multilayered nanowires were fabricated in the anodized aluminium templates using the electrodeposition technique and also in ion-track etched polycarbonate templates for comparison. The thicknesses of the Cu- and Co-rich layers were controlled in the range of 5 to 15 nm by varying each deposition time. Ni and Co homogeneous nanowires in aluminium templates showed typical anisotropic magnetoresistance (AMR) of around 1.0%. Co/Cu multilayered nanowires in aluminium templates reached a giant magnetoresistance (GMR) of 20% in current perpendicular to the plane geometry.

Role of polyethylene glycol in electrodeposition of zinc-chromium alloys

The role of polyethylene glycol (PEG) as an additive in the electrodeposition of zinc–chromium alloys was investigated in sulfate baths containing trivalent chromium. PEG with high molecular weight enabled the codeposition of metallic chromium with zinc, while chromium(III) was present in the deposits obtained from the baths containing PEG with lower molecular weight as well as the PEG-free bath. The polarization curves for the alloy deposition revealed that PEG with high molecular weight polarized the deposition potential of zinc to the reduction potential of chromium to permit the codeposition of chromium with zinc.

Electrochemical Surface Modification of Aluminium Sheets for Application to Nano-electronic Devices: Anodization Aluminium and Electrodeposition of Cobalt-Copper

A nano-porous anodized aluminium oxide layer was synthesized on the surface of bulk aluminium at a wide range of anodization voltages. The barrier layer at the pore bottom of anodized aluminium oxide layer was chemically etched to make good electrical contact for nanowires electrodeposited in the pores thus formed on metallic aluminium substrates. Cathodic polarization was examined at a wide range of cathode potentials to investigate the electrodeposition behaviour of Cu and Co into the pores. Co81Cu19/Cu multilayered nanowires were fabricated using a pulse-plating technique into the templates. Co-alloy layer and Cu layer thicknesses were adjusted to 10 nm, by controlling the deposition times. The temperature dependence of the resistance of Co81Cu19/Cu multilayered nanowires grown on the template presented clean metallic characteristics and a giant magnetoresistance (GMR) of 23% was reached at 4 K.

Ferromagnetic nickel silicide nanowires for isolating primary CD4+ T lymphocytes

Direct CD4+ T lymphocytes were separated from whole mouse splenocytes using 1-dimensional ferromagnetic nickel silicide nanowires (NiSi NWs). NiSi NWs were prepared by silver-assisted wet chemical etching of silicon and subsequent deposition and annealing of Ni. This method exhibits a separation efficiency of ∼93.5%, which is comparable to that of the state-of-the-art superparamagnetic bead-based cell capture (∼96.8%). Furthermore, this research shows potential for separation of other lymphocytes, B, natural killer and natural killer T cells, and even rare tumor cells simply by changing the biotin-conjugated antibodies.

Improvement in current efficiency of electroplated Fe-Ni films prepared in citric-acid-based baths

Fe-Ni films were electroplated in a citric-acid-based plating bath, focusing on the current efficiency of the plating process. We prepared the plating baths with various citric acid contents, and evaluated the magnetic properties of the films and the current efficiency. The film with Fe content of approximately 22 at. % was obtained by adjusting the iron sulfate content in the plating bath, and we found that the Fe-Ni films with low coercivity (<30 A/m) could be obtained in the baths with various citric acid contents. For the current efficiency, we found that the baths with low citric acid content are effective to obtain high efficiency. The bath with the citric acid content of 10 g/l showed high current efficiency (85%), and the high efficiency enables us to increase the plating rate. The maximum plating rate was 186 μm/h, and we obtained 1.3 times higher rate compared to that for our previous study. Therefore, we concluded that the bath with low citric acid content is a suitable plating bath to obtain t...

Electroplated Fe films prepared from a deep eutectic solvent

Electroplating of Fe films was carried out from choline chloride-ethylene glycol types of DES (Deep Eutectic Solvent). We investigated magnetic properties of the plated Fe films and evaluated the productivity for the electroplating process. Consequently, we found that surface morphology and current efficiency of the plated films were affected by the bath temperature. We obtained the Fe films with relatively smooth surface and high current efficiency in the bath temperature range from 70 to 110 °C. The deposition rate for our process depended on the current density, and we obtained high deposition rate value of approximately 120 μm/h. We also obtained high current efficiency values of approximately 90% in the wide range of plating time. These results indicate that the DES-based bath has industrial advantages for mass-producing Fe films. Therefore, we conclude that the DES-based bath is an attractive plating bath for Fe films.

Electroplated Fe–Ni Films Prepared From Deep Eutectic Solvents

Fe-Ni alloy films were prepared by electroplating in a plating bath containing a choline chloride and ethylene glycol-based deep eutectic solvent (DES). The Fe content of the electroplated films was found to vary from 0 at.% to 100 at.% and was dependent on the amount of Fe reagent in the plating bath. The composition of the electroplated films could be controlled easily by changing the composition of the bath. We observed bcc or fcc Fe-Ni crystalline phase in the electroplated films by X-ray diffraction and confirmed that magnetically soft Fe-Ni alloy films could be obtained from the DES-based bath. The current efficiency for the plating process was as high as >88% over a wide range of the Fe reagent concentrations. Therefore, we conclude that the DES is an attractive solvent for preparing the Fe-Ni alloy films.

Development of new additive for grain refinement of austenitic stainless steel

Abstract Recently, a new additive for grain refinement of Ni based superalloys has been developed. In those studies, it was considered that niobium carbide in Ni–Nb–C alloy additives would act as nuclei on the solidification of Ni based superalloys. It is known that the crystallographic characteristics of iron are quite similar to those of nickel. Thus, it was expected that niobium carbide would act as a grain refiner for iron base alloys, especially for austenitic steels. In this study, the effect of the additives on the microstructure of SUS316 steel was examined in various experimental conditions. The grain size of SUS316 specimens without inoculation was ∼2700 μm. On the other hand, when NbC containing alloy additives were added into the SUS316 melt, fine equiaxed grains were observed and the grain size of the specimen was significantly reduced to ∼200 μm.