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PREPARATION AND PROPERTIES OF Co/SiO2 AND Co 3 0 4 /SiO2 COMPOSITES W. WIN*, E. M. LOGOTHETIS*, R. E. SOLTIS*, H. K. PLUMMER*, AND L. E. *Physics Department, Ford Motor Company, Dearborn MI 48121 **Department of Physics, Wayne State University, Detroit MI 48202

WENGER**

ABSTRACT Films of Co/SiO2 composites were prepared by sequential sputtering from Co and Si0 2 targets in Ar. As-grown films as well as films annealed in H2/N2 atmospheres consist of hcp Co particles of uniform size embedded in the SiO2 matrix. The measured optical absorption spectra of these composites are consistent with the Maxwell-Garnett theory except for the absence of the predicted structure that arises from the interband transitions of cobalt. Composites consisting of semiconducting Co 3 0 4 particles embedded in Si0 2 were prepared by post-growth annealing of Co/SiO2 films in air. The optical properties of these composites were found to be comparable to those of Co 3 0 4 . INTRODUCTION Composite materials consisting of finely dispersed mixtures of a metal and an insulator have been the subject of continuing scientific and technological interest because of the unique properties of these materials. In previous publications [1-3], the preparation and properties of Au/SiO2 composites prepared by the tandem deposition technique [3] were reported. This technique produces metal particles with a narrow size distribution uniformly dispersed in the insulator and allows control of the particle size independent of the metal volume fraction, f. Electron energy loss spectroscopy (EELS) and optical studies on well-characterized Au/SiO2 composite films prepared by the tandem deposition technique permitted a detailed comparison between experiment and theories [1,2]. In particular, these studies showed the following: 1) For small metal volume fractions, the Maxwell-Garnett theory (MGT) [4] accurately predicts the dielectric properties of composites having a microstructure consistent with model assumptions. 2) For larger volume fractions, the MC theory predicts the position and width of the dipole resonance with reasonable accuracy, but this theory as well as other effective dielectric function (EDF) theories [5-7] grossly underestimate the intensity of the dipole resonance. 3) There is no evidence for the red shift of the dipole resonance predicted by more advanced EDF theories which include the effect of disorder. 4) The structures in the composite EDF arising from the interband transitions in Au are much weaker in the experimental spectra than predicted by all the EDF theories. In this paper, we describe some initial results of our studies on the preparation and properties of Co/SiO2 and Co 3 0 4 /SiO 2 composites prepared by the tandem deposition technique. The objectives of these studies are : to investigate the applicability of the tandem deposition method to metal/insulator systems other than Au/Si0 2 , to further compare theory with experiment, and to explore the possibility of preparing composites films where the particles embedded in an insulator a