  The external field control of antiferromagnetism is a significant subject
both for basic science and technological applications. As a useful macroscopic
response to detect magnetic states, the anomalous Hall effect (AHE) is known
for ferromagnets, but it has never been observed in antiferromagnets until the
recent discovery in Mn3Sn. Here we report another example of the AHE in a
related antiferromagnet, namely, in the hexagonal chiral antiferromagnet Mn3Ge.
Our single-crystal study reveals that Mn3Ge exhibits a giant anomalous Hall
conductivity $|\sigma_{xz}|$ ~ 60 per ohm per cm at room temperature and
approximately 380 per ohm per cm at 5 K in zero field, reaching nearly half of
the value expected for the quantum Hall effect per atomic layer with Chern
number of unity. Our detailed analyses on the anisotropic Hall conductivity
indicate that in comparison with the in-plane-field components $|\sigma_{xz}|$
and $|\sigma_{zy}|$, which are very large and nearly comparable in size, we
find $|\sigma_{yx}|$ obtained in the field along the c axis is found to be much
smaller. The anomalous Hall effect shows a sign reversal with the rotation of a
small magnetic field less than 0.1 T. The soft response of the AHE to magnetic
field should be useful for applications, for example, to develop switching and
memory devices based on antiferromagnets.
