Ac-Susceptibility Studies of the Energy Barrier to Magnetization Reversal in Frozen Magnetic Nanofluids of Different Concentrations

We used ac-susceptibility to measure the blocking temperature, TB, and energy barrier to the magnetization reversal, EB, of nanomagnetic fluids of different concentrations, c. We collected data on five samples synthesized by dispersing Fe3O4 nanoparticles of average diameter D = 8 nm in different volumes of carrier fluid (hexane). We found that TB increases with the increase in c, a behavior predicted by the Dormann–Bessais–Fiorani (DBF) theory. In addition, our observed TB vs. c dependence is excellently described by a power law TB = A·c^γ, with A = 64 K and γ = 0.056. Our data also show that a Néel–Brown activation law τ(T) = τ0 exp(EB/(kBT)) describes the superspin dynamics in the most diluted sample, whereas an additional energy barrier term, Ead, is needed at higher concentrations, according to the DBF model: τ(T) = τr exp ((EB + Ead)/(kBT)). We found EB/kB = 366 K and additional energy barriers Ead/kB that increase linearly with the common logarithm of the volume concentration, from 138 K at c = 8.3 × 10^(−4)% to 745 K at c = 4 × 10^(−2)%. These results add to our understanding of the contributions by different factors to the superspin dynamics. In addition, the quantitative relations that we established between the TB, Ead, and c support the current efforts towards the rational design of functional nanomaterials.