Excitonic nature of magnons in a quantum Hall ferromagnet

Magnons enable the transfer of a magnetic moment or spin over macroscopic distances. In quantum Hall ferromagnets, it has been predicted¹ that spin and charge are entangled, meaning that any change in the spin texture modifies the charge distribution. As a direct consequence of this entanglement, magnons should carry an electric dipole moment. Here we report evidence of this electric dipole moment in a graphene quantum Hall ferromagnet^2,3 using a Mach–Zehnder interferometer. As magnons propagate across the insulating bulk, their electric dipole moment modifies the Aharonov–Bohm flux through the interferometer, affecting both phase and visibility of the interference pattern. In particular, we relate the phase shift to the sign of this electric dipole moment and the loss of visibility to the flux of emitted magnons, and we show that the magnon emission is a Poissonian process. Finally, we probe the emission energy threshold of the magnons for transient states, between ν = 0 and ν = 1, and link them to the emergence of the gapless mode predicted in the canted-antiferromagnetic phase at charge neutrality^4,5. The ability to couple the spin degree of freedom to an electrostatic potential is a property of quantum Hall ferromagnets that could be promising for spintronics.