Comparison between a magneto-optical method and Fannin´s technique for the measurement of Brown´s relaxation frequency of ferrofluids

A magneto-optical method, conceived in our laboratory to measure Brown´s relaxation frequency of ferrofluids, is compared with Fannin´s technique based on impedance measurements (slit toroid). We have developed a complete theory and an experimental setup of the first method and have briefly described the second one. We present a study of Brown´s relaxation of cobalt ferrite Fe₂CoO₄ in different carrier liquids (glycerol, diethylene glycol, dibutyl phthalate, and water). The two methods show very close results, proving the high sensitivity of both. The relation between the viscosity η and Brown´s relaxation frequency f_B has been verified. Moreover the analysis of relaxation and viscosity versus temperature has enabled us to estimate the hydrodynamic diameter of particles