There is an ever-increasing interest in the study and application of magnetic fluids and thus it is important to be able to accurately characterise such fluids. One method is by the measurement of the frequency dependent, complex susceptibility, chi(omega) = chi'(omega) - ichi"(omega), which, when measured over a frequency range of Hz to GHz, enables the convenient determination of the macroscopic and microscopic properties of the fluids. Ferromagnetic resonance, identified by the chi'(omega) component passing from a positive to a negative value at a frequency f(res), together with relaxation mechanisms, both Brownian and Neel, can be readily investigated. The latter components can be determined from maxima occurring in a plot of chi"(omega) against frequency and enable the effective particle radius of the suspension to be estimated. From measurements of chi(omega) one can also investigate magnetic losses arising in the fluids. An alternative method for determining the effective particle size is the principal or differential susceptibility technique. This fixed frequency technique consists of applying an external DC polarizing magnetic field, Ho, to the ferrofluid sample contained within a cylindrical coil and measuring the parallel, chi(parallel to)(H-0), and perpendicular, chi(perpendicular to)(H-0), incremental susceptibility components. In this paper, a brief review is given of the above-mentioned topics and examples of results obtained, presented. (C) 2002 Elsevier Science B.V. All rights reserved.
