  Based on Brownian Dynamics (BD) simulations, we study the dynamical
self-assembly of active Brownian particles with dipole-dipole interactions,
stemming from a permanent point dipole at the particle center. The propulsion
direction of each particle is chosen to be parallel to its dipole moment. We
explore a wide range of motilities and dipolar coupling strengths and
characterize the corresponding behavior based on several order parameters. At
low densities and low motilities, the most important structural phenomenon is
the aggregation of the dipolar particles into chains. Upon increasing the
particle motility, these chain-like structures break, and the system transforms
into a weakly correlated isotropic fluid. At high densities, we observe that
the motility-induced phase separation is strongly suppressed by the dipolar
coupling. Once the dipolar coupling dominates the thermal energy, the phase
separation disappears, and the system rather displays a flocking state, where
particles form giant clusters and move collective along one direction. We
provide arguments for the emergence of the flocking behavior, which is absent
in the passive dipolar system.
