Magnetism of palladium clusters in nanoporous carbon
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SORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Magnetism of Palladium Clusters in Nanoporous Carbon B. D. Shaninaa, A. M. Danishevskiib, *, A. I. Veingerb, A. A. Sitnikovab, R. N. Kyuttb, A. V. Shchukarevc, and S. K. Gordeevd a
Lashkarev Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, pr. Nauki 45, Kiev, 03028 Ukraine b Ioffe Physicotechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia c Department of Chemistry, Environmental and Biogeochemistry, Umee University, Umee, S90187 Sweden d Central Research Institute of Materials, St. Petersburg, 191014 Russia *email: [email protected] Received April 13, 2009
Abstract—To determine the electronic properties of metal–carbon clusters, palladium clusters are intro duced into nanoporous carbon samples. The structural and magnetic properties of the nanocomposite thus obtained are investigated. A ferromagnetic resonance (FMR) component is observed in the electron spin res onance (ESR) spectra measured in a wide temperature range. The electron density of states (DOS) of clusters consisting of a palladium atom surrounded by carbon atoms is calculated, and it is shown that, due to the interaction between electronic terms of carbon and metal, such a cluster should exhibit magnetism. The investigation of the structures of the porous skeleton of nanoporous carbon and of palladium clusters has allowed us to reveal how carbon and palladium atoms are brought into close contact so that to give rise to the observed ferromagnetism. PACS numbers: 76.30.He, 81.05.Rm, 81.05.Uw, 62.23.Pq, 68.37.Lp DOI: 10.1134/S1063776109100070
1. INTRODUCTION In recent years, there has been considerable inter est in the properties of small clusters of transition met als. In the group of 4d elements, ruthenium, rhodium, and palladium are isoelectronic analogs of iron, cobalt, and nickel, respectively. However, none of these elements is ferromagnetic in the bulk phase. Therefore, a number of papers have discussed the problem of the possible appearance of magnetic prop erties in small clusters or in thin films of the above mentioned metals. For instance, palladium is a strong paramagnet, and it does not satisfy the Stoner criterion for ferro magnetism. Nevertheless, in a large number of theo retical studies [1–5], it was argued that small free clus ters of palladium can exhibit ferromagnetism. For this purpose, one should somehow increase the density of states (DOS) at the Fermi level, N(EF). Various meth ods have been considered that allow one to reach this goal: (1) to increase the localization of electrons by reducing the coordination number, (2) to increase the lattice parameter, and (3) to change the local symmetry. In [2, 3], the authors discussed how to obtain ferro magnetism in monolayers and multilayer films of pal ladium atoms on various substrates, which is possible due to the decrease of the coordination number.
In bulk facecentered cubic (fcc) palladium struc tures that contain thousands of atom
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