Modeling the Brownian Relaxation of Nanoparticle Ferrofluids: Comparison with Experiment

Michael A. Martens¹, Robert J. Deissler¹, Yong Wu¹, Lisa Bauer¹, Zhen Yao¹, Mark A. Griswold², Robert W. Brown¹

We investigate the ability of current models for magnetic nanoparticles immersed in dilute ferrofluids and external sinusoidal magnetic fields to explain recent experiments in which the relaxation effects are dominated by viscous damping. The Fokker-Planck (FP) equation, appropriate for the nanoparticle magnetic moment distribution corresponding to the underlying stochastic Langevin model, is numerically studied and solutions compared to experimental results. By incorporating the polydisperse properties of the particles into the analysis we obtain good agreement with recent experimental results using ferrofluids containing nanoparticles with average hydrodynamic diameters in the 40-120 nm range.