Non-Debye Behavior of the Néel and Brown Relaxation in Interacting Magnetic Nanoparticle Ensembles.

We used ac-susceptibility measurements to study the superspin relaxation in FeO/Isopar M nanomagnetic fluids of different concentrations. Temperature-resolved data collected at different frequencies, χ″ vs. T|, reveal magnetic events both below and above the freezing point of the carrier fluid (T = 197 K): χ″ shows peaks at temperatures T and T around 75 K and 225 K, respectively.