AC susceptibility measurements in the investigation of aggregation in magnetic fluids

Measurements of the complex, frequency dependent susceptibility, chi(omega) = chi'(omega) - i chi "(omega) of two water based magnetic fluids, over th e frequency range 10 HE to 1 MHz are reported. Plots of the susceptibility components against log[f(Hz)] are found to have approximate Debye-type prof iles with the presence of Brownian relaxation being indicated by the freque ncy, f(max), of the maximum of the loss-peaks in the chi "(omega) profiles. Corresponding calculations of particle hydrodynamic radius indicate the pr esence of aggregation. Frohlich, normal and lognormal distribution function s are used to obtain an estimate of the aggregate size distribution in the samples.