Imaging Stimulated and Cascaded Four Magnon Scattering Using Nitrogen Vacancy Magnetometry

Nonlinear interactions among collective excitations underpin frequency conversion, gain, and wave control across many platforms. In magnets, these effects manifest as multi-magnon scattering, yet direct, quantitative visualization at the micron scale has been limited. We present a nitrogen-vacancy (NV) magnetometry approach that images both stimulated and cascaded four-magnon scattering in yttrium iron garnet (YIG) thin films with submicron spatial resolution. Using a two-tone microwave drive, we launch pump and signal magnons whose four-magnon mixing generates magnons at a third (idler) frequency, which we image with the NV. From the spatial maps we extract the growth - decay envelope and a mode-resolved four-magnon scattering matrix element. In a complementary regime near the ferromagnetic resonance of YIG, we observe discrete resonances and model them quantitatively as cascaded four-magnon processes augmented by a density-dependent mean-field shift. These results offer a novel window into the nonlinear wave dynamics of magnons and will help inform the design of future magnonic devices.