Theory of magnon transport in quantum Hall ferromagnet

In two recent experiments, Stepanov et. al [Nat. Phys 14, 907–911 (2018)] and Wei. et al [Science 362, 6411, 229-233 (2018)] observed long distance, nonlocal magnon transport in quantum Hall magnetic states of graphene. In these experiments, bulk magnons are excited in close proximity to conducting spin polarized edge states that are under external voltage bias. While Stepanov et. al observed a nonlocal voltage linear in the bias voltage, Wei. et al observed a nonlocal voltage only when within a window of bias voltage that begins at Zeeman energy. Motivated by these observations, we present a simple theoretical model to study edge-current induced torque -- the coupled dynamics of itinerant spin-polarized edge states and insulating bulk magnons.