A molecular dynamics study on the equilibrium magnetization properties and structure of ferrofluids

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 ferro-solids, 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.Comment: 33 pages including 12 figures, requires RevTex

Computer simulation of the structure and properties of a particulate dispersion

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Computer simulation of the structure and properties of a particulate dispersion

A dispersion of 0.25p elongated iron particles has been simulated by a 3-D forcebias Monte-Carlo computation employing an ensemble of 1000 sphero-cylinders with aspect ratio 10 1. The particles exhibit a strong magnetostatic interaction, derived from a bulk magnetisation of 1700emu/cc, which is modelled as a pole-pole interaction. They also have a surface coating of surfactant which is modelled as a short range surface-surface repulsive potential. The energetic behaviour of the particle ensemble is determined by the interactions derived from these two potentials. A primitive magnetisation reversal algorithm is employed to lessen the effect of any artificially high energy interactions between particles, therefore investigations are limited to the cases of zero field and saturating field. An initial state of small particles is simulated by randon placements within a cubic cell with 3-D periodic boundary conditions. A secondary computation scheme is employed to expand slowly the particle lengths, including a corresponding scaling of the the magnetic moment. During the computation small groups of particles may become mutually bound by the strong magnetostatic interactions and exhibit co-operative behaviour. The simulation therefore includes a cluster-allocating algorithm and a cluster moving algorithm in order to take account of this behaviour. Analysis of the equilibrium configuration indicates that clusters, i.e. small groups of strongly bound particles, are an important characteristic of the dispersion microstructure. The interactions between clusters are predominantly negative in zero field and although they appear to be relatively weakly bound, they may give rise to extended networks of connected clusters. These considerations imply that the clustering of particles is a significant factor in determining the physical and magnetic properties of a dispersion