Temperature Dependence of Cluster-cluster Coalescence in Monodispersed Co Cluster Assemblies
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Temperature Dependence of Cluster-cluster Coalescence in Monodispersed Co Cluster Assemblies D. L. Peng1, T. Hihara and K. Sumiyama Department of Materials Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan 1 Electronic mail: [email protected] ABSTRACT Using a plasma-gas-condensation-type cluster beam deposition system, we deposited monodispersed Co clusters with mean diameter d = 8.5 nm on quartz and microgrid substrates. The cluster-cluster coalescence process of the Co cluster assemblies was investigated by in situ electrical conductivity measurements and ex situ transmission electron microscopy (TEM). The change of magnetic properties induced by the inter-cluster coalescence was also discussed. The electrical conductivity measurement indicated that, at temperatures T < 100°C, the Co clusters in the assemblies maintain their original size as deposited at room temperature, while the inter-cluster coalescence takes place at T > 100°C. The TEM observation showed that the size distribution and the interface morphology of the clusters do not change markedly at substrate temperatures Ts ≤ 200°C, while cluster-cluster coalescence starts at Ts > 200°C. Above Ts = 300°C, the interfacial area of coalesced clusters is crystalline, having its own orientation different from those of two connected cluster cores. INTRODUCTION The cluster-assembling method [1,2], in which nanometer-sized clusters are directly deposited on a substrate, is a promising alternative to fabricate ideally-controlled nano-scale materials. Throughout the assembling process, it is desirable to maintain the initial size and structure of the clusters. In practice, however, interaction among the deposited clusters takes place on a substrate, preventing one from achieving this goal. This interaction is known to depend on several factors: deposited materials, cluster size, substrate temperature and contamination. Recently, using a plasma-gas-condensation (PGC)-type cluster beam deposition apparatus, we have prepared monodispersed Co clusters with the mean sizes d = 6 to 13 nm and the standard deviation less than 10% of d [3]. In this size range, Co clusters reveal a characteristic percolation behavior during the assembling process [4]. With increasing assembly thickness, the superparamagnetic to ferromagnetic transition is detected for d = 6 nm [5]. In addition, the core-shell type Co/CoO monodispersed cluster assemblies thus obtained exhibited characteristic tunnel-type conductivity and enhanced magnetoresistance, arising from the uniform Co core size and CoO shell thickness [6]. For the same system, we have also reported enhancement of magnetic coercivity and macroscopic quantum tunneling of magnetization [7]. On the basis of the aforementioned studies, and considering the potential of the application of the Co cluster assembly, it is necessary to explore the clusters-cluster coalescence process above the room temperature. So far, however, there are few reports on the experimental study of the clusters-cluster coalescence b
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