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

摘要

We investigate in detail the initial susceptibility, magnetization curves,and microstructure of ferrofluids in various concentration and particle dipolemoment ranges by means of molecular dynamics simulations. We use the Ewaldsummation for the long-range dipolar interactions, take explicitly into accountthe translational and rotational degrees of freedom, coupled to a Langevinthermostat. When the dipolar interaction energy is comparable with the thermalenergy, the simulation results on the magnetization properties agree with thetheoretical predictions very well. For stronger dipolar couplings, however, wefind systematic deviations from the theoretical curves. We analyze in detailthe observed microstructure of the fluids under different conditions. Theformation of clusters is found to enhance the magnetization at weak fields andthus leads to a larger initial susceptibility. The influence of the particleaggregation is isolated by studying ferro-solids, which consist of magneticdipoles frozen in at random locations but which are free to rotate. Due to theartificial suppression of clusters in ferro-solids the observed susceptibilityis considerably lowered when compared to ferrofluids.