Brownian relaxation of magnetic colloids

Ferrofluids based on nanoscaled cobalt ferrites, stabilized either sterically or electrostatically, are prepared by controlled precipitation and subsequent surface modification. The translational and rotational dynamics are studied by means of photon correlation spectroscopy and frequency-dependent AC magnetic susceptibility measurements, respectively. From these data, the distribution of hydrodynamic diameters is determined. The agreement in the resulting diameters in the case of sterical stabilization indicates the absence of agglomerates, whereas the disagreement in the case of electrostatical stabilization points to week agglomerates. The proportionality of the magnetic relaxation times to the viscosity of the dispersion medium (different glycerol/water mixtures) is evidence for Brownian-type relaxation behavior.