Dynamic susceptibility of a concentrated ferrofluid: The role of interparticle interactions.

摘要: The dynamic susceptibility of concentrated ferrofluids magnetite-kerosene type is studied experimentally to clarify the effect interparticle interactions on magnetization reversal dynamics and ferrofluid relaxation time spectrum. We synthesize six samples, four which have same wide particle size distribution with a high (more than 2kT) average energy magnetic dipole interactions. These samples differ in concentration viscosity. two remaining lower content large particles moderate For all we measure weak probing field at frequencies up 160 kHz amplitude dependence frequency 27 kHz. results show that dispersion 10 kHz due rotational diffusion colloidal aggregates. Steric hydrodynamic are main reason for strong viscosity so they also strongly influence susceptibility. van der Waals manifested indirectly, through formation multiparticle clusters. contribution clusters low-frequency reaches 80%. Their sizes (about 100 nm) shift region 1-100 Hz, depending temperature concentration. Experiments 27 kHz demonstrate increase increasing amplitude. This growth unexpected since spectral amplitudes Debye function expansion decrease monotonically field. To situation, auxiliary problem magnetodynamics uniaxial alternating solved numerically. Fokker-Planck-Brown equation used. It shown an reduces anisotropy barrier Neel increases by one orders magnitude compared weak-field limit. calculation qualitative agreement experimental data allow us propose consistent interpretation these data. find observed when necessary conditions met: (i) suspension enough cause blocking degrees freedom aggregates (ii) present ferrofluid.