Description of the magnetisation decay in ferrofluids with a narrow particle size distribution

D. Eberbeck - S. Hartwig - U. Steinhoff - L. Trahms

Physikalisch Technische Bundesanstalt Berlin, Fachlabor für Bioelektrizitat und Biomagnetismus, Berlin, Germany

Abstract
The time decay of the magnetization of a system of magnetic nanoparticles (MNP) with uniaxial anisotropy was modeled by the superposition of the contribution of non moments of the particles with different sizes. Relaxation curves were modeled by the so called Moment Superposition Model (MSM) and then fitted with the well known modified logarithmic function M_r (t) = A ln(1 + t_c /t). The goodness of fit decreases with narrowing of the energy barrier distribution. Also we have analyzed the experimental data by fitting with the modified logarithm and MSM. The particles have a broad energy barrier distribution (σ ≈ 0.5). Even in this case MSM shows slightly better results. The parameters of MSM have a clear physical meaning and could be determined by other measurements. We recommend the application of the MSM to analyze the relaxation curves of samples with a narrow particle size distribution. Figs 3, Refs 16.