Two relaxation peaks were found in the complex susceptibility of ferrofluids. Both can be described by the Vogel-Fulcher law $\tau ={\tau }_{0}f\left(T\right)\exp \left[E/k\right({T-T}_0\left)\right]$. Nevertheless, the physical origins for these two relaxations are quite different. We found that Néel relaxation strongly depends on the dipole-dipole interaction. The dramatic dependence can be described by a surprisingly simple scaling relation: $\tau ={\tau }_0\exp \left[E/k\right(T-a{\phi }^{0.8}\left)\right]$, 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.

• Received 1 February 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.390