Concentration-dependent zero-field magnetic dynamic response of polydisperse ferrofluids

In this contribution, we discuss the experimental results obtained for seven ferrofluid samples with the same particle size distribution that differ only in concentration of magnetic material. The dynamic response to a weak linearly-polarised probing AC field is measured for each sample at five different temperatures. We investigate Cole-Cole diagrams and phase shifts in order to describe the impact of ferroparticle concentration on the initial magnetic susceptibility. Our findings show that the main contribution comes from the increasing effective viscosity of the ferrofluid that results in the growth of the Brownian relaxation time. This mechanism is very important for the systems containing even a small portion of large magnetic colloids. The influence of dipolar correlations was separately analysed using computer simulations.