  We report the optical detection of magnons with a broad range of wavevectors
in magnetic insulator Y3Fe5O12 thin films by proximate nitrogen-vacancy (NV)
single-spin sensors. Through multi-magnon scattering processes, the excited
magnons generate fluctuating magnetic fields at the NV electron spin resonance
frequencies, which accelerate the relaxation of NV spins. By measuring the
variation of the emitted spin-dependent photoluminescence of the NV centers,
magnons with variable wavevectors up to ~ 5 x 10^7 m-1 can be optically
accessed, providing an alternative perspective to reveal the underlying spin
behaviors in magnetic systems. Our results highlight the significant
opportunities offered by NV single-spin quantum sensors in exploring nanoscale
spin dynamics of emergent spintronic materials.
