Direct Observation of Antimagnons with Inverted Dispersion.

We report direct spectroscopic evidence of antimagnons, i.e., negative-energy spin waves identified by their signature inverted dispersion and left-handed precession with Brillouin light scattering (BLS) spectroscopy. We investigate an ultrathin BiYIG film with a perpendicular magnetic anisotropy that compensates the demagnetizing field. By injecting a spin-orbit torque, the magnetization is driven into auto-oscillation and eventually into a nonequilibrium reversed state above a secondary current threshold (∼1.2×10^{7}  A/cm^{2}). The dispersion is measured by wave-vector-resolved BLS and exhibits a sharp change from an upward dispersion to a downward one, in agreement with theoretical predictions and micromagnetic simulations. Around the threshold current, we observe the coexistence of conventional magnons and antimagnons. Our Letter establishes antimagnons with inverted dispersion and is a first step toward exploring novel phenomena and applications due to magnon-antimagnon coupling, such as magnon amplification and magnon-antimagnon entanglement, which are part of the emerging field of antimagnonics.