  We report on the formation of topological defects emerging from the cycloidal
antiferromagnetic order at the surface of bulk BiFeO$_3$ crystals. Combining
reciprocal and real-space magnetic imaging techniques, we first observe, in a
single ferroelectric domain, the coexistence of antiferromagnetic domains in
which the antiferromagnetic cycloid propagates along different wavevectors. We
then show that the direction of these wavevectors is not strictly locked to the
preferred crystallographic axes as continuous rotations bridge different
wavevectors. At the junctions between the magnetic domains, we observe
topological line defects identical to those found in a broad variety of
lamellar physical systems with rotational symmetries. Our work establishes the
presence of these magnetic objects at room temperature in the multiferroic
antiferromagnet BiFeO$_3$, offering new possibilities for their use in
spintronics.
