Rotational diffusion in iron ferrofluids

The dynamic magnetic susceptibility of relatively monodisperse iron ferrofluids was measured from 1 Hz to 100 kHz for different sizes of the iron particles (all with a 7-nm-thick organic surface layer, dispersed in Decalin). In the case of particles with an iron core of 6-nm radius, the orientation of the magnetic dipole moment thermally rotated inside the particles (Neel rotation). In the case of particles with a slightly larger iron core, the orientation of the magnetic dipole moment was blocked inside the particles but could still change by rotational diffusion of the particles themselves (Brownian rotation). With even larger particles (above 7-nm iron core radius), aggregates were formed: the rotational diffusion rate was lower than that of single particles by more than 1 order of magnitude. This sudden appearance of aggregates above a certain size of the iron particles agrees with previous observations in two dimensions, by cryogenic transmission electron microscopy of ultrathin ferrofluid films. Here, it is found that the threshold for aggregation is practically the same in three dimensions. Moreover, the rotational diffusion rate of the aggregates is seen to increase upon dilution, indicating a decrease in aggregate size. This suggests that a dynamic equilibrium exists between the sticking of particles to each other and unsticking, especially when the particles are sufficiently small so that the sticking energy is not more than a few times the thermal energy.