On the broadband measurement of the permittivity and magnetic susceptibility of ferrofluids

Transmission line techniques have been used to determine both, the complete magnetic susceptibility chi(omega) = chi'(omega) - i chi ''(omega), and complex permittivity epsilon(omega) = epsilon'(omega) - i epsilon ''(omega), over the frequency range 30 MHz-6 GHz, of a number of test specimens including four ferrofluid samples of ferrite particles in a hydrocarbon carrier. All four fluids display a resonance effect indicated by the transition of the chi'(omega) components from a positive to a negative value. The corresponding epsilon'(omega) components are found to be approximately constant up to a frequency of 1 GHz and it is shown that a good estimate of the measured values can be determined by the use of the magnetic analogue of the Gunther and Heinrich formula for the complex dielectric constant of a composite dielectric. Automated measurements are obtained by means of a Hewlett-Packard Network Analyser which operates over the frequency range concerned and appropriate equations for the calculation of the complex components of chi(omega) and epsilon(omega) are presented.