  Magnetocrystalline anisotropy is essential in the physics of antiferromagnets
and commonly treated as a constant, not depending on an external magnetic
field. However, we demonstrate that in CoO the anisotropy should necessarily
depend on the magnetic field, which is shown by the spin Hall magnetoresistance
of the CoO $|$ Pt device. Below the N\'eel temperature CoO reveals a spin-flop
transition at 240 K at 7.0 T, above which a hysteresis in the angular
dependence of magnetoresistance unexpectedly persists up to 30 T. This behavior
is shown to agree with the presence of the unquenched orbital momentum, which
can play an important role in antiferromagnetic spintronics.
