Motivated by recent nonlocal transport studies of quantum-Hall-magnet (QHM) states formed in monolayer graphene’s $N=0$ Landau level, we study the scattering of QHM magnons by gate-controlled junctions between states with different integer filling factors $\nu$. For the $\nu =1|-1|1$ geometry we find that magnons are weakly scattered by electric potential variation in the junction region, and that the scattering is chiral when the junction lacks a mirror symmetry. For the $\nu =1|0|1$ geometry, we find that kinematic constraints completely block magnon transmission if the incident angle exceeds a critical value. Our results explain the suppressed nonlocal–voltage signals observed in the $\nu =1|0|1$ case. We use our theory to propose that valley waves generated at $\nu =-1|1$ junctions and magnons can be used in combination to probe the spin or valley flavor structure of QHM states at integer and fractional filling factors.

• Received 25 August 2020
• Revised 11 January 2021
• Accepted 12 February 2021

DOI:https://doi.org/10.1103/PhysRevLett.126.117203