Keishin Ota

Structure and random anisotropy in single-phase Ni nanocrystals

We prepared high-purity oxygen-free and single-phase Ni nanocrystals using the gas condensation and deposition method in order to examine the random anisotropy model (RAM). The grain size of an as-prepared sample was estimated to be 8–10nm. Thermal annealing increased grain sizes (D) up to 22nm. Clear D6 dependence of the coercive force on a grain size smaller than 13 nm was obtained for samples prepared on both plastic and KBr single-crystal substrates. For larger grain sizes, Hc decreased with D−1 and these results clearly conform to the RAM in single-element nanocrystal systems. A local anisotropy constant was estimated from the slope of the D6 relation and the peak of Hc at D=13nm.

Magnetic anisotropy of Ni nano-crystals prepared with gas-deposition method

Abstract Ni nano-crystals were prepared with Gas-deposition method. As deposited films were composed of fine particles with approximately 10 nm in diameter. Thermal annealing was applied to increase the grain size. Coercive forces of the nano-crystals vary in accordance with the 6 powers of grain size. This is explained by the Random Anisotropy Model. Effective local magnetic anisotropy is deduced to be 1.9∼3.5 × 10 4 J/m 3 and the magnitude is 2∼5 times larger than that of bulk Ni. From the field cool magnetization measurement, Hc shift due to exchange anisotropy was not observed. The oxygen atoms in nano-crystals are not responsible for the global anisotropy but may reduce the exchange stiffness.

Improvement of Field Emission Characteristics by Fabricating Aligned Open-Edged Particle-Free Carbon Nanotubes.

We have developed a process to thin the tips of the aligned carbon nanotubes grown by the microwave plasma enhanced chemical vapor deposition. In the process, an oxygen-plasma oxidation treatment was performed at first to reduce the thicknesses of the nanotubes, and a magnetic-force particle-drawing treatment was then performed to remove the catalyst particles capped on the tops of the nanotubes. Scanning and transmission electron microscopy images of thus treated nanotubes show that the average radius of curvature at the tips has been reduced by over 50% of that of the as-grown nanotubes and most of the catalyst particles have been successfully removed. The field emission characteristics of the open-edged particle-free nanotubes show that the turn-on voltage has been reduced by 23% of that of the as-grown nanotubes.

Electrical conduction and tunneling magnetoresistance of Fe, Co/MgF2 granular multilayers

Electrical transport was investigated in Fe/MgF2 and Co/MgF2 granular thin films prepared by alternating deposition in ultrahigh vacuum. Temperature dependence of the resistivity changed from metallic to semiconductor-like when Fe (Co) layer thickness was less than 45 A (40 A) with a fixed MgF2 layer of 30 A. Discontinuous ferromagnetic layers were formed in films with semiconductor-like conduction and were confirmed with transmission electron micrography. The largest room-temperature magnetoresistance of 8.5% at 15 kOe (11% at 50 kOe) was found in a [Fe(15 A)/MgF2(20 A)] film, and that of 5.4% was found in a [Co(15 A)/MgF2(30 A)] film. The resistivity of the Fe,Co/MgF2 films, which show large tunneling magnetoresistance (TMR), was one order higher than those of other systems prepared with co-deposition. Tunnelling activation energy, estimated to be from 200 to 1700 K, did not markedly differ from that of other systems.