  Cavity spintronics explores light matter interactions at the interface
between spintronic and quantum phenomena. Until now, studies have focused on
the hybridization between ferromagnets and cavity photons.In this article, we
realize antiferromagnetic cavity-magnon polaritons. The collective spin motion
in single hematite crystals ({\alpha}-Fe2O3) hybridizes with 18 - 45 GHz
microwave cavity photons with required specific symmetries. We show
theoretically and experimentally that the photon-magnon coupling in the
collinear phase is mediated by the dynamical Neel vector and the weak magnetic
moment in the canted phase by measuring across the Morin transition. The
coupling strength g is shown to scale with the anisotropy field in the
collinear phase and with the Dzyaloshinskii-Moriya field in the canted phase.
We achieve a strong coupling regime both in canted (C > 25 at 300 K) and
noncolinear phases (C > 4 at 150 K) and thus coherent information exchange with
antiferromagnets These results evidence a generic strategy to achieve
cavity-magnon polaritons in antiferromagnets for different symmetries, opening
the field of cavity spintronics to antiferromagnetic materials.

Mesoscale and Nanoscale Physics;Materials Science