Phase Diagrams and Functional Properties of Transition-Metal-Alloy Clusters

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In this research, we have elucidated the following points1) We have installed two sorts of metal targets in the plasma-gas-condensation cluster deposition apparatus, produced alloy clusters with 5-10 nm in diameter, and deposited on a substrate. The transmission electron microscope observation and EDX analysis indicate characteristic aspects in comparison with the equilibrium phase diagrams : (a) the compositional fluctuation is not so marked and a B2-type ordered phase is formed in Co-Al (a compound formation type), (b) the compositional fluctuation is slightly detectable and a disordered fcc phase in Co-Pt (an ordered phase formation type), (c) the compositional fluctuation is enhanced and a fcc disordered phase in Co-Pd (a homogeneous solid solution type), and (d) the compositional distribution curve has double peaks, but the primary solid solutions are formed in Nb-Ag (an immiscible type).2) Magnetization curves at 290 K as a function of the cluster size show ferromagnet-superparamagnet transition at around 8 nm in diameter.3) Co core-shell cluster assemblies, Co CSCA, prepared by slight oxidation and deposition on a substrate, show a marked tunneling type conductivity and magnetoresistance with a Coulomb-blockade effect. It also shows a crossover from a thermal type to a quantum tunneling type magnetic relaxation.4) The electrical resistivity measurement indicates that with increasing the degree of oxidation, Co, Ti and Cr CSCA changes from a metal (the temperature coefficient of resistivity, TCR, is positive) to a semiconductor (TCR is negative : an activation type), while Cu CSCA is always metallic. In the intemediate oxidation stage, TCR is positive at high temperature, while it become negative (-logT dependence) at low temperature, indicating a weak electron locarization effect. We are investigating their electronic states. These core-shell clusters will be useful for a tiny gas sensor and catalyses.