Glass like magnetic alternating field susceptibility behavior of structurally frozen ferrofluids

Four ferrofluids, distinct in size distribution and aggregate structure, were investigated. The relaxation time ,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. Received 15 July 1998