Brownian dynamics analysis of fractal growth of ferromagnetic colloldal particles

摘要

We studied cluster structures and cluster aggregations in a twodimensional ferromagnetic colloidal system numerically. We developed a Brownian dynamics calculaton method in which both the translational and rotational motions of ferromagnetic particles were taken into account. The phase transition for the cluster formations and the fractal dimensions of clusters are analysed and the dependence of the cluster structures on the nondimensional parameters #lambda# and #xi# is investigated, where #lambda# is the ratio of the magnetic dipole energy of a particle to thermal energy and #xi# is the ratio fo the magnetic energy to thermal energy. The fractal dimension was 1.3 in the absence of a magnetic field irrespective of #lambda#. On the other hand, the fractal dimension was very close to 1.0 when the system was subjected to a magnetic field. The clustercluster aggregations are also investigated and the validity of the dynamic scaling law is examined. It has been found that the fractal dimensions obtained by the dynamic analysis coincide with those obtained by the analysis of the cluster structures. The critical exponent in the dynamic scaling law was obtained as a function of #lambda#. It has been found that the critical exponent increases with #lambda# and becomes constant when #lambda# is larger than 12. The critical exponent was larger in the mgnetic field than in the absence of a magnetic field.