  We report results from a computer simulation study on the rotational ratchet
effect in systems of magnetic particles interacting via dipolar interactions.
The ratchet effect consists of directed rotations of the particles in an
oscillating magnetic field, which lacks a net rotating component. Our
investigations are based on Brownian dynamics simulations of such many-particle
systems. We investigate the influence of both, the random and deterministic
contributions to the equations of motion on the ratchet effect. As a main
result, we show that dipolar interactions can have an enhancing as well as a
dampening effect on the ratchet behavior depending on the dipolar coupling
strength of the system under consideration. The enhancement is shown to be
caused by an increase in the effective field on a particle generated by
neighboring magnetic particles, while the dampening is due to restricted
rotational motion in the effective field. Moreover, we find a non-trivial
influence of the short-range, repulsive interaction between the particles.

Soft Condensed Matter