  Time-reversal symmetry breaking is the basic physics concept underpinning
many magnetic topological phenomena such as the anomalous Hall effect (AHE) and
its quantized variant. The AHE has been primarily accompanied by a
ferromagnetic dipole moment, which hinders the topological quantum states and
limits data density in memory devices, or by a delicate noncollinear magnetic
order with strong spin decoherence, both limiting their applicability. A
potential breakthrough is the recent theoretical prediction of the AHE arising
from collinear antiferromagnetism in an anisotropic crystal environment. This
new mechanism does not require magnetic dipolar or noncollinear fields.
However, it has not been experimentally observed to date. Here we demonstrate
this unconventional mechanism by measuring the AHE in an epilayer of a rutile
collinear antiferromagnet RuO$_2$. The observed anomalous Hall conductivity is
large, exceeding 300 S/cm, and is in agreement with the Berry phase topological
transport contribution. Our results open a new unexplored chapter of
time-reversal symmetry breaking phenomena in the abundant class of collinear
antiferromagnetic materials.
