FAR-INFRARED OPTICAL-PROPERTIES OF FERROMAGNETIC CERMETS

Presented is an extension of the Onsager theory, initially applicable to electric permanent dipoles, to the calculation of the effective magnetic permeability of cermets including monodomain grains of a ferromagnetic metal. Taking into account both the orientational alignment of the ferromagnetic moments of the metal grains and their induced magnetic moment, the magnetisation is calculated by considering the local magnetic field. The behaviour of the magnetic permeability mu is predicted, which is particle size and temperature dependent. The optical thickness beta=(2pid/lambda)square-root epsilonmu and the admittance g=square-root epsilon/mu are then calculated by introducing mu and the dielectric function epsilon given by an effective medium theory. It leads to a new expression of the reflection and transmission coefficients taking into account both the electrical and magnetical side of the optical properties of ferromagnetic cermet. As an example, the influence of the magnetic permeability on the absorption of a Ni-Al2O3 thin film is presented at near normal incidence, up to 1000 mum.