  Antiferromagnetic insulators (AFMI) are robust against stray fields, and
their intrinsic dynamics could enable ultrafast magneto-optics and ultrascaled
magnetic information processing. Low dissipation, long distance spin transport
and electrical manipulation of antiferromagnetic order are much sought-after
goals of spintronics research. Here, we report the first experimental evidence
of robust long-distance spin transport through an AFMI, in our case the
gate-controlled, canted antiferromagnetic (CAF) state that appears at the
charge neutrality point of graphene in the presence of an external magnetic
field. Utilizing gate-controlled quantum Hall (QH) edge states as
spin-dependent injectors and detectors, we observe large, non-local electrical
signals across a 5 micron-long, insulating channel only when it is biased into
the nu=0 CAF state. Among possible transport mechanisms, spin superfluidity in
an antiferromagnetic state gives the most consistent interpretation of the
non-local signal's dependence on magnetic field, temperature and filling
factors. This work also demonstrates that graphene in the QH regime is a
powerful model system for fundamental studies of antiferromagnetic, and in the
case of a large in-plane field, ferromagnetic spintronics.
