Keywords: Transition Metal, Energy Characteristics, Cluster, Density Functional Theory (DFT)
Interest in the properties of nanoparticles of transition metal and their oxides has grown recently, partly because of the advances in synthesis of nanofunctional materials and partly because of unusual magnetic behavior of ferromagnetic and superparamagnetic nanoparticles. The finite-size effect results in about 70% of atoms belonging to the surface in a particle of 3 nm radius. The surface atom ratio creates misalignment of spins on the surface and induces weakening of exchange interaction of surface atoms with the surrounding ones. In spatially confined configurations, a reduction of Fermi surfaces and/or strong electron interaction had been suggested for that group of materials in the absence of external field. In the present study, we want to find how the basic electronic structure and energetic characteristics of the clusters of transition metals, such as Ni, will be affected by finite-size confinements for different spatial configuration. It would help us to assess the contribution of d electrons into the surface atoms interaction and as a consequence the response of electron spins to the external magnetic field and subsequent magnetization. In order to calculate ab initio energies, we use the density functional (DFT) method of calculation of energy characteristics such as total, exchange -correlation and Fermi energy, as well as individual electronic energy levels forming the band gaps, and apply it to the clusters of transition metals.
Our preceding investigation revealed that the DFT/LDA pseudopotential model can provide good initial platform for comparison of different structures. The detailed estimation of energy states and their density for the relaxed model of bcc 9 atom Ti cluster and its extension to the 1D wire structure showed order of the states reversal due to the formation of the dangling bonds on the surface that can contribute to the electronic flux in the external field whether its electric, magnetic, or thermal field. In the present work, we continue to study transition metal clusters of Ni in which the number of valent s and d electrons is much larger than in Ti atoms. As a consequence, the further reduction in the Fermi level of the single free bcc and fcc cells is confirmed compared with Ti bcc free cell cluster. The Fermi energies for the Ni bcc and fcc cells respectively are -3.2643 a. u. and -10.58402 a. u., while for the Ti bcc cell it is -2.1894 a. u. Presence of a large number of d electrons in the Ni clusters relative to Ti one further reduces the Fermi energies, especially in the fcc cluster configuration. The effect of the electron-electron interaction on the individual energy levels and contribution of exchange coupling is currently studied.
Tatiana N. Zolotoukhina
Research Center for Advanced Manufacturing on Nanoscale Science and Engineering,
National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba East,
1-2-1 Namiki, Tsukuba, Ibaraki-ken 305-8564, Japan
Phone: +81 (29) 861-7870 Fax: +81 (29) 861-7871