J. Z. Yu

First-principles studies on magnetism of Ni clusters coated and alloyed with Pd

Abstract First-principles electronic-structure calculations are performed for the Ni clusters coated and alloyed with Pd. For coated clusters, the magnetic moments of the Ni core are reduced as compared to the bared one. Especially, Ni1Ni12Pd6Pd24Pd12 cluster with Oh symmetry consists of the ferromagnetic Ni core, the ferromagnetic Pd and paramagnetic Pd shells, the induced magnetic moment of Pd in the interface is 0.1μB, qualitatively in agreement with experimental results found in Ni particles coated with Pd. The dependence of magnetic moment on Pd thickness displays an oscillatory behavior, similar to that found in Co clusters coated with Cu or Ag and in Ni/Pd multilayers. For the alloyed clusters, the enhanced magnetism is confirmed, and the mechanism is clarified: which is resulted not only due to the large atomic distance of Pd but also due to electron transfers. The explanations are given for the different magnetic behaviors between coated and alloyed clusters.

First-principles studies on pure and doped C32 clusters

On the basis of first-principles calculations, pure and doped C32 clusters are studied. Among the nine structural isomers, the fullerene structure with D3 symmetry is found to be the most stable. Due to the small size of the C32 cage, Li and Na atoms can be stably encapsulated, while K and Be atoms are not. On encapsulation, the bond length of the H2 molecule is reduced while the vibration frequency is increased. Substitutional doping is more favourable than endohedral doping for Si atoms. Because of the sp2-bonding features of C atoms, the Si atom is also threefold coordinated in substitutional doping; however, the existence of one dangling bond in the Si atom makes this doped heterofullerene reactive at the Si site, and H termination can produce substantial energy gain.

Geometry and energetics of Si60 isomers

Abstract The novel electronic, optical and super-conducting properties exhibited by C60 and its assembled materials have greatly stimulated the scientists all over the world racing to study Si60. In this paper we performed an extensive search from eighteen isomers including cages, wires, and stuffed structures of Si. It is found that the fullerene cage is not stable, and the tricapped-trigonal-prism unit is not the structural unit for Si60. While the spherical compact structure is the most stable energetically, in agreement with experiments which suggested a transition from the elongated geometry to a more spherical one for the medium sized Si clusters [J. Chem. Phys. 111 (1999) 7865, J. Chem. Phys. 117 (2002) 3219].

FIRST-PRINCIPLES STUDY ON THE LOCAL MAGNETISM OF FE/NB MULTILAYERS

Abstract The magnetic properties of Fe/Nb multilayers have been studied for the first time by means of the full-potential linearized augmented-plane-wave method. It has been found that a Nb(001) monolayer is magnetic with a moment of 1.89μB per atom, whereas 2Nb(001) and 3Nb(001) slabs are nearly nonmagnetic. The evolution of magnetism for Nb from monolayer to bulk is rapid. Fe/Nb(001) is nearly nonmagnetic, 3Nb/Fe/3Nb(001) is magnetically dead, 2Nb/3Fe/2Nb(001) is magnetically active with a large moment, and Nb layers are antiferromagnetically coupled with Fe layers. The tendency that the thinner Fe layers are magnetically dead, and the thicker Fe layers magnetically active, is found, in agreement with recent experiments.

First-principles studies on the adsorptive properties of hydrogenated Si cluster

Abstract The adsorption of Si, O and Al adatoms on the most stable hydrogenated Si cluster (Si 6 H 12 ) has been explored by using first principles. A special attention is paid to the comparisons of the similarity in adsorptions between the hydrogenated Si cluster and H-terminated Si surface: substitutional adsorption configuration is found for Si atom on Si 6 H 12 cluster, similar to that on H-terminated Si surface; Si–O–Si adsorption configuration is found for O atom on Si 6 H 12 cluster, similar to the case on H-terminated Si surface; nonadsorption of Al atom on Si 6 H 12 cluster is found, similar to the very weak adsorption of Al atom on H-terminated Si surface. This similarity in adsorption between hydrogenated Si 6 H 12 cluster and hydrogenated Si surface would be attributed to the fact: in hydrogenated Si 6 H 12 cluster, H atoms have terminated the dangling bonds, and the Si atoms favor the bulk-like sp 3 bonding even in this small cluster.

Nonmetal-metal transition in Ban clusters

Understanding the development of metallic behavior with increasing cluster size has been one of the important questions in cluster science. Based on Kubos criterion and optimized structures with ab initio calculations, the nonmetal to metal transition in Ba clusters is studied. It has been found that the critical size for this transition is around 32, in agreement with the mass spectra experiment.

Magnetic phase transition and hydrogen solubility in Fe, Co, and Ni

It was observed experimentally that hydrogen affects strongly the magnetic properties of metals and alloys. Recently, we performed a series of first-principles calculations to study the effect of hydrogen on the magnetism of magnetic metals, and we found that the strong hybridization among the orbitals of hydrogen and nickel atoms suppresses the ferromagnetism completely. On the basis of this first-principles calculation, we developed a statistical thermodynamic model and analyzed the experimental results of hydrogen solubility in transition metals Fe, Co, and Ni using the model. We found that our model can describe the temperature dependence of hydrogen solubility in a wide range of solid phase in the stated magnetic metals.

Effects of cluster–cluster interactions on the structure and magnetic properties in (Fe6)2

Abstract The structure and magnetic properties of the coupled Fe 6 :Fe 6 cluster have been systematically studied with the density functional formalism, and the Kohn–Sham equation is solved self-consistently with the discrete variational method (DVM). It is found that as the intercluster distance decreases the clusters are deformed, which results in the sensitive changes of magnetic moments. The cluster–cluster coupling energy indicates that the two clusters are weakly coupled, and the HOMO-LUMO gap of the coupled clusters is so small that the two coupled Fe 6 clusters are nearly metallic, and the metal–nonmetal transition can be realized by coupling and decoupling of clusters.

Multimedia database system KIND featuring amorphous factual database by object-oriented method

A large-scale materials database including literature and factual data, which is under construction at the Institute for Materials Research, Tohoku University is introduced featuring the collections of ternary amorphous formation phase diagrams. The database is arranged as a multimedia type, including both text and images, which is especially important in materials research. The object-oriented method is also applied to construct the database so it can predict new compositions to form an amorphous phase from existing data.