We obtain good agreement between the calculated and measured ratio of harmonics only when the model includes nanoparticles which have a distribution in the hydrodynamic diameter - that is polydisperse. We are unable to find good agreement if the diameter of the nanoparticles is constrained to only one value - that is monodisperse. In Fig. 1 we plot the measured [3] ratios of 5th/3rd harmonics of the magnetization for samples using “100 nm” iron oxide particles (top plot) and “40 nm” iron oxide particles (bottom plot). The measurements were collected with the nanoparticles in ferrofluid solutions with a range of water/glycerol ratios corresponding to different viscosities (and therefore different Brownian relaxation times.) As described in [3] the data are plotted as a function of ωτB. Also shown in Fig. 1 are the calculations of the ratios of 5th/3rd harmonics assuming both monodisperse and polydisperse ferrofluids. Details of the calculations and parameters used in the models can be found in [5].
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