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 Fe/sub 2/CoO/sub 4/ 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 /spl eta/ and Brown's relaxation frequency f/sub B/ has been verified. Moreover the analysis of relaxation and viscosity versus temperature has enabled us to estimate the hydrodynamic diameter of particles.