Synthesis and phase transformation of hcp-Co based alloy particles

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We have synthesized hcp-Co fine particles with the average diameter of less than 50 nm by sputtering Co in a somewhat high inert gas pressure. A nearly perfect hcp phase can be stabilized by increasing sputtering gas pressure and decreasing sputtering power in contrast to conventional preparation conditions. Magnetic measurements have revealed the clear relation of coercivity with the volume ratio of hcp-phase. That is, the increase in the hcp-phase considerably enhances the coercivity. It has been found that there is a close relationship between the particle size and the crystal phase ; that is, pure fcc (beta) phase for D*200*, mixture of hcp (alpha) and beta phases for D - 300*, and alpha phase with inclusion of very small amount of beta phase for D* 400*.Precise structural characterizations have revealed that the beta particles are multiply twinned icosahedrons and the alpha particles are perfect single crystals with external shape of a Wulff polyhedron. In order to explain the size effect on crystal phase of Co fine particles, we have performed theoretical calculations for total free energies of an alpha-single crystal, a beta-single crystal, and a multiply twinned beta icosahedron. The present calculations well explain the size dependence of the crystal phase of the Co fine particles, and have revealed that the stabilization of beta phase confirmed by enormous previous studies is the intrinsic effect caused by small dimensionality of fine particles. Moreover, the phase transformations occurred in annealing experiments can be also explained by the theory.