An experimental study of the dynamic properties of nanoparticle colloids with identical magnetization but different particle size

Measurements of the frequency dependent complex magnetic susceptibility, χ(ω)=χ′(ω)−iχ″(ω), have been used to determine the dynamic properties of three specially prepared 400 G (0.04 T) magnetic fluids. The samples, denoted by sample 1, sample 2 and sample 3, consisted of magnetite particles of mean diameter 6.4 nm, 7.5 nm and 9 nm respectively and were identical in terms of carrier liquid, surfactant and particle material.From polarized ferromagnetic measurements, the anisotropy field, HA, the Landau–Lifshitz damping parameter, α, and the precessional decay time, τ0 of the particles were determined. The results show that the HA value for sample 3 was almost twice that of sample 1 and 2, thus confirming that the smaller the particle size, the lower the value of HA. The damping parameter, α, was found to be 0.174 (for sample 1), 0.18 (for sample 2) and 0.16 (for sample 3). The values determined for the precessional decay time, τ0 were 1.197×10−9 s, 1.157×10−9 s and 0.789×10−9 s, for samples 1, 2 and 3 respectively.