[1] Institute of Solid State Physics, Research Center Juelich, D-52425 Juelich, Germany
[2] Institute for Advanced Materials Processing, Tohoku University, 1-1 Katahira, Aoba-ku, Sendai 980-77, Japan
[3] Centre d'Etudes de Chimie Metallurgique, 15, rue Georges, F-94407 Vitry-sur-Seine Cedex, France
[4] Institute of Solid State Physics, Research Center Juelich, Juelich, Germany