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, T_B, and energy barrier to magnetization reversal, E_B, of nanomagnetic fluids of different concentrations, c. We collected data on five samples synthesized by dispersing Fe₃O₄ nanoparticles of average diameter áDñ=8 nm in different volumes of carrier fluid (hexane). We found that T_B increases with the increase of 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∙c ^g, with A=64 K and g=0.056. Our data also show that a Néel-Brown activation law describes the superspin dynamics in the most diluted sample, whereas an additional energy barrier term, E_ad, is needed at higher concentrations, according the DBF model: We found E_B/k_B=366K and additional energy barriers E_ad/k_B that increase linearly with the common logarithm of the volume concentration, from 138 K at c= 8.3×10⁻⁴ % 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 T_B, E_ad, and c support the current efforts towards the rational design of functional nanomaterials.