Magnetic spectroscopy as an aide in understanding magnetic fluids

Accurate data on the frequency dependent, complex susceptibility, chi(omega) = chi"(omega) - ichi(omega), of magnetic fluids are vital for an understanding of the dynamic behavior of these colloidal suspensions. They enable relaxation mechanisms, both Brownian and Neel, as well as ferromagnetic resonance to be identified and investigated. They also provide a convenient means of determining the macroscopic and microscopic properties of the fluids, such as the mean particle radius, (r) over bar, the mean value of anisotropy field, (H) over bar (A), the gyromagnetic constant, gamma, and the damping constant, alpha. Also, through the medium of chi(omega) one can investigate the frequency dependence of the loss tangent (tan 5) and power factor (sin delta) together with the signal-to-noise ratio of such particulate systems. In this brief review, the above-mentioned topics together with the relevant theory are presented and by means of suitable examples it is demonstrated how the appropriate equations may be employed to determine such properties from experimental data. (C) 2002 Elsevier Science B.V. All rights reserved.