Dynamic magnetogranulometry of ferrofluids

Magnetogranulometry involves analysing the magnetic properties of a material in order to determine the microscopic composition. For ferrofluids, this means determining the number of magnetic particles of particular size and magnetic dipole moment. Previous work has focused on analysing the static initial magnetic susceptibility, χ, using an accurate theory for how χ depends on the Langevin magnetic susceptibility χ_L, which is a function of the concentration and dipole moment of each particle fraction. Herein, the application of similar techniques to the frequency-dependent magnetic susceptibility, χ(ω), is examined with the assumption of the Brownian rotation mechanism. The usefulness of the analysis relies on the accuracy of the underlying theory. Ignoring interparticle interactions gives the Debye theory. Interactions are taken into account using a modified mean-field theory and a modified-Weiss theory. Using computer-simulation results for known compositions as model ‘experimental’ data, it is shown that it is essential to take interactions into account, and that the modified-Weiss theory provides the most accurate results.