An oscillatory-like relaxation process in which there are two valleys in the T-t curve is observed when light is transmitted through binary ferrofluids composed of both ferrimagnetic CoFe_(2)O_(4) nanoparticles and paramagnetic p-MgFe_(2)O_(4) nanoparticles in the presence of a high magnetic field and through pure (single) CoFe_(2)O_(4) ferrofluids in a low magnetic field. This relaxation behavior is explained using a model of a bidispersed system based on both chained and unchained particles. In such a bidispersed system, the variation of the transmitted light results mainly from the motion of the chains, with the polarized unchained particles' gas producing the modulation effect. The oscillatory-like relaxation phenomenon depends on the features of both the chained and unchained particle systems. If either the particle volume fraction of chained particles or of unchained particles is very low, or the degree of polarization of the unchained particles gas is very weak, a simple nonlinear relaxation process, giving only a valley in the T-t curve, will appear for the transmitted light. For pure CoFe_(2)O_(4) ferrofluids, the number of chained and unchained particles does not remain constant under different values of the magnetic field. According to the analysis of the relaxation behavior of transmitted light, it is known that binary ferrofluids based on strong magnetic CoFe_(2)O_(4) particles and weak magnetic p-MgFe_(2)O_(4) particles can be much closer to the theoretical bidispersed system than single ferrofluids containing only strong magnetic particles.
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