Four ferrofluids, distinct in size distribution and aggregate
structure, were investigated. The relaxation time tm(T),
related to the temperature of susceptibility maximum, was fitted
to a Vogel-Fulcher law. A mean ordering temperature, T0, was
calculated using magnetic particle parameters derived from the
structure. It is assumed that at T0 the particle moments of
particle clusters correlate, leading to a spin glass-like
transition. Hence, then dynamic slows down considerably, as
indicated by a strong broadening of relaxation-time distribution.
T0 roughly agrees with the energy of competing interaction
between particle moments, as calculated from the structure of
particle aggregates.
Differences between particle arrangements clearly influence the
dispersion and absorption, particularly within the cluster phase
