  Optically active spin defects in wide band-gap semiconductors serve as a
local sensor of multiple degrees of freedom in a variety of "hard" and "soft"
condensed matter systems. Taking advantage of the recent progress on quantum
sensing using van der Waals (vdW) quantum materials, here we report direct
measurements of spin waves excited in magnetic insulator Y3Fe5O12 (YIG) by
boron vacancy $V_B^-$ spin defects contained in few-layer thick hexagonal boron
nitride nanoflakes. We show that the ferromagnetic resonance and parametric
spin excitations can be effectively detected by $V_B^-$ spin defects under
various experimental conditions through optically detected magnetic resonance
measurements. The off-resonant dipole interaction between YIG magnons and
$V_B^-$ spin defects is mediated by multi-magnon scattering processes, which
may find relevant applications in a range of emerging quantum sensing,
computing, and metrology technologies. Our results also highlight the
opportunities offered by quantum spin defects in layered two-dimensional vdW
materials for investigating local spin dynamic behaviors in magnetic
solid-state matters.
