Collective spin-wave excitations-magnons-in a quantum Hall ferromagnet are
promising quasi-particles for next-generation spintronics devices, including
platforms for information transfer. Detection of these charge-neutral
excitations relies on the conversion of magnons into electrical signals in the
form of excess electrons and holes, but if these signals are equal the magnon
detection remains elusive. In this work, we overcome this shortcoming by
measuring the electrical noise generated by magnons. We use the symmetry-broken
quantum Hall ferromagnet of the zeroth Landau level in graphene to launch
magnons. Absorption of these magnons creates excess noise above the Zeeman
energy and remains finite even when the average electrical signal is zero.
Moreover, we formulate a theoretical model in which the noise is generated by
equilibration (partial or full, depending on the bias voltage) between edge
channels and propagating magnons. Our model, which agrees with experimental
observations, also allows us to pinpoint the regime of ballistic magnon
transport in our device