Synthesis and phase transformation of hcp-Co based alloy particles

我们在较高的惰性气体压力下溅射Co，合成了平均直径小于50 nm的hcp-Co细颗粒。与传统制备条件相比，提高溅射气体压力和降低溅射功率可以稳定接近完美的hcp相。磁测量结果表明，矫顽力与hcp相的体积比有明显的关系。也就是说，hcp相的增加大大提高了矫顽力。研究发现，晶粒尺寸与晶相有密切的关系；即D*200*为纯fcc （β）相，D - 300*为hcp （α）与β相的混合物，D* 400*为α相中含有极少量β相。精确的结构表征表明β粒子是多重孪生二十面体，α粒子是完美的单晶，外部形状为伍尔夫多面体。为了解释尺寸对Co细颗粒晶相的影响，我们对α -单晶、β -单晶和β -多重孪生二十面体的总自由能进行了理论计算。本文的计算很好地解释了Co细颗粒的晶相尺寸依赖性，并揭示了大量前人研究证实的β相的稳定是由细颗粒的小尺寸引起的内在效应。此外，在退火实验中发生的相变也可以用理论来解释。

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.