How particle interactions and clustering affect the dynamic magnetic susceptibility of ferrofluids*

The effects of magnetic interparticle interactions on the frequency-dependent, dynamic magnetic susceptibility of ferrofluids are summarised with reference to recent theoretical and simulation studies. With weak to moderate interactions, the dynamic magnetic susceptibility qualitatively resembles the classic Debye prediction, but with features shifted to lower frequencies as the particle concentration and/or interaction strength are increased. This shows that the mutual spatial and orientational correlations arising from the interactions lead to slower collective motions. With strong interactions, and at low concentrations, the particles form chain-like and ring-like clusters, and this leads to distinct collective and intracluster motions being apparent in the dynamic magnetic susceptibility. The initial formation of chains increases the static susceptibility, while the subsequent formation of rings decreases it.