  For the innovation of spintronic technologies, Dirac materials, in which the
low-energy excitation is described as relativistic Dirac fermions, are one of
the most promising systems, because of the fascinating magnetotransport
associated with the extremely high mobility. To incorporate Dirac fermions into
spintronic applications, their quantum transport phenomena are desired to be
manipulated to a large extent by magnetic order in a solid. We here report a
bulk half-integer quantum Hall effect in a layered antiferromagnet EuMnBi$_2$,
in which field-controllable Eu magnetic order significantly suppresses the
interlayer coupling between the Bi layers with Dirac fermions. In addition to
the high mobility more than 10,000 cm$^2$/Vs, Landau level splittings
presumably due to the lifting of spin and valley degeneracy are noticeable even
in a bulk magnet. These results will pave a route to the engineering of
magnetically functionalized Dirac materials.
