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 hD i = 8 nm in different volumes of carrier fluid (hexane). We found that T-B increases with the increase in c, a behavior predicted by the Dormann-Bessais-Fiorani (DBF) theory. In addition, our observed T-B vs. c dependence is excellently described by a power law T-B = A center dot c(gamma), with A = 64 K and gamma = 0.056. Our data also show that a Neel-Brown activation law iota(T) = iota(0) exp (E(B|)k(B)T) describes the superspin dynamics in the most diluted sample, whereas an additional energy barrier term, E-ad, is needed at higher concentrations, according to the DBF model: iota(T) = iota(r) exp (EB + Ead| k(B)T). We found E-B/k(B) = 366 K and additional energy barriers E-ad/k(B) that increase linearly with the common logarithm of the volume concentration, from 138 K at c = 8.3 x 10(-4)% to 745 K at c = 4 x 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 T-B, E-ad, and c support the current efforts towards the rational design of functional nanomaterials.