  At high magnetic fields, monolayer graphene hosts competing phases
distinguished by their breaking of the approximate SU(4) isospin symmetry.
Recent experiments have observed an even denominator fractional quantum Hall
state thought to be associated with a transition in the underlying isospin
order from a spin-singlet charge density wave at low magnetic fields to an
antiferromagnet at high magnetic fields, implying that a similar transition
must occur at charge neutrality. However, this transition does not generate
contrast in typical electrical transport or thermodynamic measurements and no
direct evidence for it has been reported, despite theoretical interest arising
from its potentially unconventional nature. Here, we measure the transmission
of ferromagnetic magnons through the two dimensional bulk of clean monolayer
graphene. Using spin polarized fractional quantum Hall states as a benchmark,
we find that magnon transmission is controlled by the detailed properties of
the low-momentum spin waves in the intervening Hall fluid, which is highly
density dependent. Remarkably, as the system is driven into the
antiferromagnetic regime, robust magnon transmission is restored across a wide
range of filling factors consistent with Pauli blocking of fractional quantum
hall spin-wave excitations and their replacement by conventional ferromagnetic
magnons confined to the minority graphene sublattice. Finally, using devices in
which spin waves are launched directly into the insulating charge-neutral bulk,
we directly detect the hidden phase transition between bulk insulating charge
density wave and a canted antiferromagnetic phases at charge neutrality,
completing the experimental map of broken-symmetry phases in monolayer
graphene.
