Specific Absorption Rates and Magnetic Properties of Ferrofluids with Interaction Effects at Low Concentrations.
The specific absorption rate (SAR) of a maghemite-based ferrofluid, measured at 315 K, 3 kA/m, and 109 kHz, was found to double as the ferrofluid concentration was decreased by a factor of 4. The ferrofluid contained nonagglomerated, highly crystalline, and monodisperse nanoparticles with an average size of 11.6 nm and an initial concentration of 8.14 mg/mL. The magnetic characterization of three different concentrations of this ferrofluid revealed several effects typical of the presence of magnetic interactions, such as the decrease of initial susceptibility values (liquid ferrofluid) and Neel relaxation times, tau(N) (frozen ferrofluid), with increasing concentration. The accurate SAR determination in adiabatic conditions allowed estimating the tau(N) values of the liquid ferrofluid, which displayed the same trend against concentration as those obtained in the frozen state. Such a trend allowed explaining qualitatively the degradation of the heating performance of the ferrofluid upon increasing concentration. Eventually, correlation between tau(N) values in both states was discussed in terms of several theoretical models described in the literature and developed to explain the properties of an assemble of nanoparticles with dipolar interactions.