Spin Wave Transport in a Graphene Quantum Hall Ferromagnet

We report the observation of neutral spin-excitations in a graphene quantum Hall (QH) ferromagnet. The integer quantum Hall effect is often considered to be a single-particle phenomenon in which electron-electron interactions can be ignored. However, for certain integer Landau level filling factors, these interactions cause electron spins to spontaneously polarize into a ferromagnetic state. In our experiments we focus on the ν = 1 QH ferromagnet in monolayer graphene at low temperatures and high magnetic fields. We create an imbalance in chemical potential between two edge states adjacent to the insulating ferromagnetic bulk, exceeding the minimum energy for a magnon to be generated (the Zeeman energy). This allows electrons to equilibrate to lower energies by flipping their spins, thereby launching magnons into the ferromagnet. A reverse process occurs at distant sites in the device where magnons are absorbed at a pair of oppositely spin polarized edge channels, resulting in a redistribution of chemical potential between the channels, and a change to the conductance. Using a multi-terminal device, we are able to measure a non-local signal that indicates chargeless spin transfer through the insulating bulk.