Crystallization of quasiantiferromagnetic colloids

The microscopic interactions between atomic magnetic moments determine the macroscopic magnetic properties of matter. For strongly correlated magnetic systems the local spin configuration plays a key role. High relaxation times, however, make direct investigations of dynamic processes such as crystallization very difficult. Here, we present an artificial spin system of magnetic colloids, which are often discussed as potential mesoscopic model systems for condensed matter. The very low relaxation rates of interacting colloids enable the visualization of phase transitions or crystallization processes by video microscopy. The used colloids have a predened net magnetic moment, as analogue to the atomic spin. These micromagnets show a quasiantiferromagnetic interaction. They form two-dimensional hexagonal clusters with a spin configuration similar to the 120° antiferromagnetic Neel state in the cluster center and strong deviations along the edges. The cluster size emerged as critical parameter for the occurrence of spin defects. Dur-ing the cluster growth the total magnetization of the system increased in discrete steps. Further we obtained a linear increase of the inverse spin pair correlation for particles in the center. The influence of an external constant or uctuating magnetic field is investigated as control tool for cluster growth and defect formation.