Magnon hydrodynamics in an atomically thin ferromagnet

Strong interactions between particles can lead to emergent collective excitations. Spin waves, known as magnons, have been predicted to reach a strongly interacting hydrodynamic regime, where they form a slow collective density mode. In this work, we isolate exfoliated sheets of chromium trichloride (CrCl 3 ), where magnon interactions are strong, and develop a technique to measure the collective magnon dynamics though nearby nitrogen-vacancy centers in diamond. Thermal magnetic fluctuations generated by monolayer CrCl 3 increase upon decreasing temperature; this anomalous trend may be a consequence of the damping rate of a low-energy magnon sound mode that sharpens as magnon interactions increase with increasing temperature. By measuring the magnetic fluctuations emitted by thin multilayer CrCl 3 in the presence of a variable-frequency drive field, we obtain spectroscopic evidence for this two-dimensional magnon sound mode.