Abstract
The effective magnetic permeability mu and effective dielectric constant epsilon are two equally important quantities for the characterization of electromagnetic properties of composites. For granular ferromagnetic metals, however, the presence of permanent magnetic moments means that the Maxwell-Garnett theory is inapplicable to the calculation of effective mu . By extending the approach of Onsager, the authors derive a simple equation for mu whose numerical solution in the case of granular Ni shows significant deviation from unity in far-infrared and microwave frequencies. The behaviour of mu is predicted to be a sensitive function of particle size and temperature, thus making the material potentially usable as an absorber of far-infrared or microwave radiation.