Application of the effective medium theories to interpret the optical properties of Mn discontinuous films

Two groups of Mn films with different mass thicknesses d(m) from 3 to 13 nm were evaporated onto quartz-glass substrates under vacuum conditions (P = 10(-5) Pa). The Mn films of the first group were thermally annealed at a temperature of approximately 690 K for half an hour. The second group of the films was obtained without thermal annealing. The microstructures and crystallographic orientations were examined by electron microscope. It has been found that the films of the first group consist of Mn small grains and MnO grains, the films of the second one exhibit aggregate structure. Transmittance spectra from both groups of Mn films were measured in the wavelength range from 250 to 2500 nm. In the UV the transmittance decreases, in the visible and IR range it slightly increases or is independent of wavelength. To interpret the measured optical spectra the effective medium theories of Bruggeman and Maxwell-Garnett were used. In these theories a size effect connected with electrons scattering on the grain boundaries was incorporated. Comparison of experimental results with calculated ones shows that the size effect is not essential for thermally annealed as well as unannealed Mn films. (C) 1999 Elsevier Science Ltd. All rights reserved.