Molecular dynamics study on the equilibrium magnetization properties and structure of ferrofluids

Abstract

We investigate in detail the initial susceptibility, magnetization curves, and microstructure of ferrofluids in various concentration and particle dipole moment ranges by means of molecular dynamics simulations. We use the Ewald summation for the long-range dipolar interactions, take explicitly into account the translational and rotational degrees of freedom, coupled to a Langevin thermostat. When the dipolar interaction energy is comparable with the thermal energy, the simulation results on the magnetization properties agree with the theoretical predictions very well.For stronger dipolar couplings, however, we find systematic deviations from the theoretical curves.We analyze in detail the observed microstructure of the fluids under different conditions. The formation of clusters is found to enhance the magnetization at weak fields and thus leads to a larger initial susceptibility. The influence of the particle aggregation is isolated by studying ferrosolids, which consist of magnetic dipoles frozen in at random locations but which are free to rotate.Due to the artificial suppression of clusters in ferro-solids the observed susceptibility is considerably lowered when compared to ferrofluids.

The Effect of Gelation on the Apparent Magnetism of ZnFe2O4 Sol-Gel Systems

“…This is similar to the adsorption of surfactant molecules on the surface of The results whereby the apparent magnetization or susceptibility of gels are larger than those of sols might appear to be contradictory because a computer simulation has shown that for a ferrosolid consisting of magnetic dipoles frozen at random locations but free to rotate, its susceptibility was considerably lower than for ferrofluids having fluidity (Wang, Holm, & Müller, 2002). This paradox can be explained in relation to hydrodynamic effects.…”

“…Experiments have shown that compared to a ferrofluids system without magnetic interaction (i.e. in the dilution), the reduced initial susceptibility of the system with magnetic interaction is less rather than larger as theory predicts (Wang et al, 2002;Taketomi et al, 2002). The difference may be understandable with the help of the "viscomagnetic effect" resulting from the hydrodynamic interaction.…”

Magnetic Properties of Colloidal Suspensions of Interacting Magnetic Particles

“…For example, the one by Ivanov and Kuznetsova [124] compares well with experimental data. However, recent numerical simulations by Wang et al [126,128] for χ and M(H) are not well described by this theory when the volume fraction of the magnetic particles is small and the dipolar coupling is strong: they are better explained by a model of small non-interacting chains of different…”

Branching points in the low-temperature dipolar hard sphere fluid

“…Indeed, s exhibits a maximum in density for both chains and rings. 39 The average ring and chain sizes in a system of non-interacting clusters are monotonically increasing functions of ρ, 40 and therefore we ascribe the presence of the maxima to either finite size effects or to intercluster interactions. 41,42 …”