Kimio Wakoh

Iron particles nesting in carbon cages grown by arc discharge

Fine particles of iron and iron carbide wrapped in multilayered graphitic sheets, which were synthesized by arc discharge of carbon rods containing iron oxide (Fe2O3), were studied by transmission electron microscopy. The size of the wrapped particles was typically in the range 20–200 nm. Two kinds of nested materials were found; one was α-Fe and the other, Fe3C (cementite).

Cobalt particles wrapped in graphitic carbon prepared by an arc discharge method

Fine particles of cobalt and cobalt carbide nesting in multilayered graphitic sheets, which were synthesized by an electric arc discharge of carbon rods containing cobalt oxide (CoO), were studied by transmission electron microscopy, including microdiffraction and energy dispersive x‐ray analysis. The size of the wrapped particles was typically in a range from 50 to 200 nm. Three phases of nested materials, hcp(α)‐Co, fcc(β)‐Co, and Co3C, were identified.

Giant magnetoresistance of Fe-cluster-dispersed Ag films

Abstract We report on giant magnetoresistance (GMR) of Fe-cluster-dispersed Ag films. Composite thin films with a thickness of about 2000 A and the compositional ratios Fe/Ag = 20/80, 33/67 and 45/55 have been produced by an ionized cluster beam (ICB) technique. Measurements of X-ray diffraction, transmission electron microscope, magnetization and magnetoresistance display the following salient features: (1) GMR is observed without any heat treatment of the samples, (2) GMR of about 25% is obtained for Fe/Ag = 33/67, and (3) MR is not saturated even in a magnetic field of 140 kOe for these specimens, although the magnetizations are saturated in a magnetic field of about 3 kOe.

Synthesis of Sc15C19 Crystallites Encapsulated in Carbon Nanocapsules by Arc Evaporation of Sc-C Composite

Crystallites of scandium carbides nesting in multilayered, polyhedral graphitic cages (nanocapsules) were produced by evaporating a scandium-graphite composite rod by electric arc discharge in helium gas. The composite particles were characterized by analytical electron microscopy and X-ray diffraction. The encapsulated scandium carbide was identified to be Sc15C19, instead of dicarbide RC2 (R represents rare-earth elements) which was the form of carbide commonly found for other rare-earth elements entrapped in nanocapsules. The size of the capsules ranged from about 10 to 100 nm. Morphological features of the outer graphitic carbon, multilayered and polyhedral, were quite similar to those previously discovered for the capsules protecting RC2 (R=Y, La, Ce,..., Lu).

Compositional partition in Ag-Nb alloy clusters produced by a plasma-gas-condensation cluster source

We have produced Ag-Nb clusters by a facing-target type plasma-gas-condensation cluster source as our first step toward alloy cluster formation. The Ag-Nb clusters have been deposited on substrates and examined by a transmission electron microscope with a nano-beam energy dispersive X-ray analysis. We have obtained Ag-Nb alloy clusters with the sizes range between 5 and 10 nm in diameter. Their chemical compositions are broadly dispersed and partitioned into Ag-rich and Nb-rich ones, being consistent with the immiscible type equilibrium phase diagram. This result suggests that alloy cluster formation is driven by the alloy phase stability.

Co cluster coalescence behavior observed by electrical conduction and transmission electron microscopy

We deposited monodispersed Co clusters with mean diameters d=6, 8.5, and 13 nm on quartz and microgrid substrates using a plasma-gas-condensation-type cluster beam deposition system. The cluster–cluster coalescence behavior of the Co cluster assemblies was investigated by in situ electrical conductivity measurements and ex situ transmission electron microscopy (TEM). The electrical conductivity measurement indicates that, below temperature T≈100 °C, the Co clusters with d=8.5 nm maintain their original size as deposited at room temperature, while the cluster–cluster coalescence takes place at their interface at T>100 °C. The TEM observation indicates that the morphology of the cluster distribution shows no marked change at substrate temperatures Ts<250 °C. Above Ts=300 °C, the interfacial area of coalesced clusters is crystalline, and has its own orientation, different from that of two connected cluster cores.

Characteristic High-Field Dependence and Composition Variation of Giant Magnetoresistance in Fe/Ag Granular Materials

Magnetoresistance (MR) measurements of Fe-cluster-dispersed Fe–Ag films fabricated by using the ion-cluster-beam technique display the following features. (1) The giant magnetoresistance (GMR) effect is observed in the Fe/Ag granular alloys with 12 to 45 at.% Fe, whereas it is suppressed in the Fe-rich region. (2) The MR curves display both saturation- and nonsaturation-type behavior even in a high magnetic field of 140 kOe, whereas the corresponding magnetization curves easily saturate at very low fields. The salient point here is that the GMR is optimized without any heat treatment of the specimens, and the results display some unique features of the GMR in this system.

Nanoscale structural evolution and associated changes in magnetoresistance in the granular FexAg100−x thin films

The microstructure of sputter‐deposited granular FexAg100−x thin films (0<x<60) has been investigated by using a high resolution transmission electron microscope. For x≤14 at. % Fe, two kinds of morphological regions exist, i.e., the crystallized and the highly disordered. As the x changes within 20–36 at. % Fe, small clusters with sizes around 1 nm are formed in the films. Further increase of x leads to the crystal grain fining and crystal frustration. The observed microstructural evolution in the films with the increase of Fe content can be correlated with the evolution of giant magnetoresistance at 4.2 K in the Fe‐Ag films: a linear relationship of the magnetoresistance ratio, Δρ/ρ, with H for x≤20 at. %, the saturation trend and the decrease of Δρ/ρ for x≥36 at. % Fe.

Comparative GMR study of Fe/Cu granular films deposited by co-evaporation and cluster beam techniques

Abstract We produced Fe/Cu thin films by co-evaporation and cluster-beam (CB) deposition, and compared their magnetoresistance (MR) and magnetic properties. Both co-evaporated and CB-deposited films exhibit giant magnetoresistance (GMR), which do not saturate even at high fields: conduction electrons suffer spin-disorder scattering. With increasing Fe concentration, MR of the co-evaporated films show a sharp maximum at around 23 at.% Fe owing to percolation of magnetic Fe atoms. MR of the CB-deposited films, on the other hand, does not show such a peak. The magnetization of co-evaporated films is smaller than that of CB-deposited films and they do not saturate easily. These observations suggests that the magnetic states of the former are spin glass, while the latter may contain small ferromagnetic clusters. These results demonstrate that the co-evaporated films are chemically and magnetically homogeneous, whereas the CB-deposited films are heterogeneous having granular natures.

Structural and magnetic evolution in granular FeAg alloys produced by the cluster beam technique

Abstract Using cluster beam sources, we have obtained FeAg granular films, which display giant magnetoresistance, GMR, without any heat treatment. The experimental results of small angle X-ray scattering and high-resolution transmission electron microscopy indicate that a few nanometer Fe clusters are embeded in Ag matrices with 10 nm order geometrical and chemical fluctuations. The extended X-ray absorption fine structure measurement indicates that the short range structure of Fe clusters is bcc but distorted in the as-deposited state. The magnetoresistance (MR) does not saturate at 140 kOe: conduction-electrons suffer spin-disorder scattering even in high fields, because small Fe clusters in Ag matrices reveal a spin-glass character and Fe atoms at the interface a superparamagnetic. After annealing above 570 K, MR saturates at high fields, which is ascribed to the ferromagnetism of grown Fe clusters. The concentration dependence of electrical resistivity and MR can be interpreted by geometrical and magnetic percolation of nanometric Fe clusters.