  Observations of the anomalous Hall effect in RuO$_2$ and MnTe have
demonstrated unconventional time-reversal symmetry breaking in the electronic
structure of a recently identified new class of compensated collinear magnets,
dubbed altermagnets. While in MnTe the unconventional anomalous Hall signal
accompanied by a vanishing magnetization is observable at remanence, the
anomalous Hall effect in RuO$_2$ is excluded by symmetry for the N\'eel vector
pointing along the zero-field [001] easy-axis. Guided by a symmetry analysis
and ab initio calculations, a field-induced reorientation of the N\'eel vector
from the easy-axis towards the [110] hard-axis was used to demonstrate the
anomalous Hall signal in this altermagnet. We confirm the existence of an
anomalous Hall effect in our RuO$_2$ thin-film samples whose set of magnetic
and magneto-transport characteristics is consistent with the earlier report. By
performing our measurements at extreme magnetic fields up to 68 T, we reach
saturation of the anomalous Hall signal at a field $H_{\rm c} \simeq$ 55 T that
was inaccessible in earlier studies, but is consistent with the expected
N\'eel-vector reorientation field.

Mesoscale and Nanoscale Physics