Dynamic self-assembly of magnetic particles on the fluid interface: surface-wave-mediated effective magnetic exchange.

We report on studies of dynamic self-assembled structures induced by a vertical alternating magnetic field in an ensemble of magnetic particles suspended on a liquid surface. We find the formation of nontrivially ordered dynamic snakelike objects in a certain range of field magnitudes and frequencies. In order to probe the properties of the "snakes," we study their magnetic response to in-plane magnetic field applied at different angles with respect to its axis. The segments of the snake exhibit long-range antiferromagnetic ordering mediated by the surface waves, while each segment is composed of ferromagnetically aligned chains of microparticles. We propose a simple phenomenological model where the effect of surface waves is replaced by an effective exchange interaction to describe the observations. In the framework of the proposed model, the effective exchange constant corresponding to different regimes of magnetic driving was extracted from the experimental data.