Studies of the effect of polarising field and particle interactions on the resonant frequency of magnetic nano-particles in colloidal suspensions

Measurements are presented of the frequency-dependent (10 MHz-18 GHz) complex magnetic susceptibility of colloidal suspensions of different concentrations of nano-particles of magnetite in a hydrocarbon. Studies of three colloids, fluids, 1, 2 and 3, with packing fractions of magnetite particles of 0.22, 0.1 and 0.05, were made at different magnetic polarising fields in the range 0-100 kA m-1. It is shown that the fluid with the highest packing fraction exhibits the highest resonant frequency. Using a local field approximation it has been shown that this observation can be explained by simply considering magnetic interactions between individual particles in the fluid. It is not necessary to invoke the presence of clustering as a function of increased packing fraction