T. Hihara

Formation and size control of a Ni cluster by plasma gas condensation

We have constructed a plasma-gas-condensation type cluster deposition apparatus and tried to find the optimum operation conditions for controlling the cluster size. Transmission electron microscope (TEM) observation has been done to evaluate sizes of Ni clusters produced when varying the volume of a cluster growth region, sputtering power, and inert gas pressure. The mean cluster size decreases by decreasing the volume of growth region and the sputtering power. The smallest cluster obtained in this work is about 2.3 nm in diameter. We have considered the following two models for the cluster growth: (1) a cluster–cluster collision growth and (2) an atomic vapor condensation growth. The cluster growth speed estimated from the former is too slow, while that from the latter is reasonable in comparison with the present experiments. When stable embryos are made from atom collisions, they grow up faster and the final cluster sizes estimated from the latter model are consistent with those observed by TEM.

Geometrical and electrical percolation in nanometre-sized Co-cluster assemblies

We deposited monodispersed Co-clusters in the size range of 6-13 nm on substrates using the plasma-gas-condensation cluster deposition system. The assembling process of the clusters from discontinuous to continuous networks was investigated by transmission electron microscopy (TEM) and in situ electrical conductivity measurement, and discussed in terms of the two-dimensional (2D) percolation concept. The electrical conductivity measurement indicates that the percolation process of Co clusters does not agree with a simple scaling-law: the critical conductivity exponent increases with increasing mean cluster diameter, d, although it is predicted to be independent of d in the ordinary 2D percolation theory. This anomaly is interpreted by the soft-percolation model, implying that there is distribution of electrical contacts between the clusters. The critical coverage of clusters (0.63) is much higher than the predicted one (0.45) irrespective of d, due mainly to the partial overlapping of deposited clusters, and partly to an attractive interaction between the clusters. Such cluster-overlapping also increases the critical thickness of electrical percolation with increasing d.

Enhancement of magnetic coercivity and macroscopic quantum tunneling in monodispersed Co/CoO cluster assemblies

Magnetic properties have been measured for monodisperse-sized Co/CoO cluster assemblies prepared by a plasma-gas-condensation-type cluster beam deposition technique. The clear correlation obtained between exchange bias field and coercivity suggests the enhancement of uniaxial anisotropy owing to the exchange coupling between the ferromagnetic Co core and antiferromagnetic CoO shell, and magnetic disorder at the core–shell interface. A nonthermal magnetic relaxation observed below 8 K, being referred to as macroscopic quantum tunneling of the magnetization, is ascribed to the enhanced uniaxial anisotropy.

Superparamagnetic Fe clusters in Ag matrix produced by sputter-gas aggregation

Experiments to generate a thin composite Fe‐Ag film employing a sputter‐gas‐aggregation process are described. The magnetic properties are studied at low temperatures. In addition, the morphology is determined by high‐resolution transmission electron microscopy and the structure by using x‐ray diffraction and XAFS. Small clusters of about 11 A in diameter are highly dispersed in an Ag matrix. They show the characteristics of interacting superparamagnets.

Metastable b.c.c. and amorphous Ni produced by mechanical alloying and chemical leaching

Abstract B.c.c. and amorphous Ni are obtained by a technique combining mechanical alloying of Ni and Al elemental powders, and chemical leaching of Al in basic solution. The as-milled and milled-and-leached specimens are characterized by X-ray diffraction, transmission electron microscopy, high pressure differential thermal analysis, differential scanning calorimetry and magnetization measurements. B.c.c. and amorphous Ni phases exhibit paramagnetic characters at low temperatures and transform to the ferromagnetic f.c.c. Ni phase at high temperature, specific heat measurements indicate the enhancement of its electronic coefficient in the paramagnetic b.c.c. and amorphous Ni in comparison with that of annealed f.c.c. Ni.

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.

Structural evolution and magnetic properties of nano-granular metallic alloys☆

Abstract Fe clusters have been dispersed in Ag matrices using cluster beam (CB) sources without any heat treatment. Their structure and morphology have been studied by comparison with those of the sputter-deposited FeAg alloys using small angle X-ray scattering, high resolution transmission electron microscopy and extended X-ray absorption fine structure measurements. In the granular alloys made by CB, bcc Fe clusters of a few nanometers dimension are detected, together with ten-nanometer scale geometrical and chemical fluctuations. In the sputter-deposited alloys, the Fe atoms are rather well mixed with the Ag atoms in the as-deposited state, and fcc Fe clusters are first formed and transform to larger bcc particles with increasing annealing temperature. The field-dependence of the magnetoresistance is well correlated with the growth mode of the Fe clusters in the granular alloys.

Kondo behaviour in Ce-Si amorphous alloys

Incoherent Kondo effects have been observed in CexSi100-x (x = 18, 53, 66 and 87) amorphous alloys produced by sputtering. They show large enhancement of the electronic specific-heat coefficient at low temperatures, being classified as heavy-fermion systems. The magnetic susceptibility reveals a Curie-Weiss law at T>100 K with an effective Bohr magneton number of above 2.45 mu B. However, the electrical resistivity is large, revealing a Kondo-type logarithmic temperature dependence. The atomic randomness prohibits the coherent Kondo state and the formation of a magnetically ordered state in the Ce-Si amorphous alloys.

Specific heat of MnAlGe, MnGaGe and MnZnSb

Abstract The specific heat of MnAlGe, MnGaGe and MnZnSb are measured. The values of the electronic specific heat coefficient are 8.9 mJ/mol K 2 for MnAlGe, 8.7 mJ/mol K 2 for MnGage and 11.3 mJ/mol K 2 for MnZnSb. The magnetic anomalies of the specific heat are discussed in connection with the pressure dependence of T C and magnetic thermal expansion anomalies.