  In the Landau levels of a two-dimensional electron system or when flat bands
are present, e.g. in twisted van der Waals bilayers, strong electron-electron
interaction gives rise to quantum Hall ferromagnetism with spontaneously broken
symmetries in the spin and isospin sectors. Quantum Hall ferromagnets support a
rich variety of low-energy collective excitations that are instrumental to
understand the nature of the magnetic ground states and are also potentially
useful as carriers of quantum information. Probing such collective excitations,
especially their dispersion {\omega}(k), has been experimentally challenging
due to small sample size and measurement constraints. In this work, we
demonstrate an all-electrical approach that integrates a Fabry-P\'erot cavity
with non-equilibrium transport to achieve the excitation, wave vector selection
and detection of spin waves in graphene heterostructures. Our experiments
reveal gapless, linearly dispersed spin wave excitations in the E = 0 Landau
level of bilayer graphene, thus providing direct experimental evidence for a
predicted canted antiferromagnetic order. We show that the gapless spin wave
mode propagates with a high group velocity of several tens of km/s and
maintains phase coherence over a distance of many micrometers. Its dependence
on the magnetic field and temperature agree well with the hydrodynamic theory
of spin waves. These results lay the foundation for the quest of spin
superfluidity in this high-quality material. The resonant cavity technique we
developed offers a powerful and timely method to explore the collective
excitation of many spin and isospin-ordered many-body ground states in van der
Waals heterostructures and open the possibility of engineering magnonic
devices.
