Two mechanisms and a scaling relation for dynamics in ferrofluids

Two relaxation peaks were found in the complex susceptibility of ferrofluids. Both can be described by the Vogel-Fulcher law tau = tau(0)f(T) exp[E/k(T - T-0)]. Nevertheless, the physical origins for these two relaxations are quite different. We found that Neel relaxation strongly depends on the dipole-dipole interaction. The dramatic dependence can be described by a surprisingly simple scaling relation: tau = tau(0) exp[E/k(T - a phi(0.8))], where phi is the volume fraction of the dipoles. In contrast, Brownian relaxation is much less sensitive to the concentration of magnetic moments because the interparticle force is mainly hydrodynamic in nature.