Comparison between a magneto-optical method and Fannin's technique for themeasurement of Brown's relaxation frequency of ferrofluids

A magneto-optical method, conceived in our laboratory to measure frown's re laxation frequency of ferrofluids, is compared with Fannin's technique base d on impedance measurements (slit toroid), We have developed a complete the ory and an experimental setup of the first method and have briefly describe d the second one. We present a study of Brown's relaxation of cobalt ferrit e Fe2CoO4 in different carrier liquids (glycerol, diethylene glycol, dibuty l phthalate, and water). The two methods show very close results, proving t he high sensitivity of both. The relation between the viscosity eta and Bro wn's relaxation frequency fs has been verified. Moreover the analysis of re laxation and viscosity versus temperature has enabled us to estimate the hy drodynamic diameter of particles.