  Spin waves are collective excitations of magnetic systems. An attractive
setting for studying long-lived spin-wave physics is the quantum Hall (QH)
ferromagnet, which forms spontaneously in clean two-dimensional electron
systems at low temperature and in a perpendicular magnetic field. We used
out-of-equilibrium occupation of QH edge channels in graphene to excite and
detect spin waves in magnetically ordered QH states. Our experiments provide
direct evidence for long distance spin wave propagation through different
ferromagnetic phases in the N=0 Landau level, as well as across the insulating
canted antiferromagnetic phase. Our results will enable experimental
investigation of the fundamental magnetic properties of these exotic
two-dimensional electron systems.
