Percolation effects in the optical properties of Ni-MgO composites

We prepared Ni-MgO granular metal composites by coprecipitation of NiO-MgO solid solutions and their preferential reduction in a hydrogen atmosphere. These composites have a different topology from that of the granular metal system prepared by cosputtering or coevaporation. The percolation threshold $f_c$ of these Ni-Mgo composites was estimated as 0.32±0.02, which is in good agreement with the prediction of the effective-medium theory (EMT) but much lower than $f_c$≊0.5 of conventional granular metal films. For each composite with a metal-volume fraction between 0 and 0.4, the reflection spectrum was measured between 20 and 36 000 ${\mathrm{cm}}^{\mathrm{-}1}$ at room temperature. The detailed comparison of these spectra with the Maxwell-Garnett theory (MGT) and the EMT showed that the Ni-MgO composites were better described by the EMT than by the MGT. The conductivities and the dielectric constants of the composites were derived by a Kramers-Kronig transformation, and they showed a qualitative agreement with the EMT. In the low-frequency region, especially in the far-infrared region, the percolation transition was observed in the reflection spectrum, and also seen in the derived conductivity and the dielectric constant.