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The frequency dependencies of the complex dielectric permittivity,  and of the complex magnetic permeability, , over the range 100 Hz – 6 GHz, of a kerosene-based magnetic fluid are presented.

Using the Sillars theory for heterogeneous dielectrics, we have determined the permittivity, electric conductivity and shape factor of the colloidal particles within the magnetic fluid, in microwave range.

From the magnetic relaxation measurements, the Brownian relaxation time , the hydrodynamic radius , the Nèel relaxation time , the effective anisotropy field  and the effective anisotropy constant  of the magnetic colloidal particles were determined. In addition, these measurements give useful information about the colloidal stabilization of the particles within the magnetic fluid in zero polarizing field.

Based on the ferromagnetic resonance measurements the effective anisotropy constant, anisotropy field, gyromagnetic ratio, , damping parameter,  and spectroscopic splitting factor,  of the magnetite colloidal particles are determined.

The paper demonstrates how the investigation of the dielectric and magnetic behaviour of magnetic fluids in electromagnetic field can be employed to determine theirs microscopic and macroscopic properties.

Keywords: Magnetic fluid, Complex dielectric permittivity, Complex magnetic permeability, Relaxation processes, Ferromagnetic resonance