Soft-X-ray momentum microscopy of nonlinear magnon interactions

Magnons are quantized collective excitations of long-range ordered spins. At nanometre wavelengths, exchange interactions increasingly govern their dynamics, giving rise to a largely unexplored regime of couplings between magnons and other quasi-particles; however, detecting such short-wavelength spin waves has remained a key experimental challenge. Here we introduce magnon momentum microscopy-a quasi-elastic, resonant magnetic soft-X-ray scattering technique that directly images magnon populations across two-dimensional momentum space. Owing to its remarkable sensitivity, it can capture nonlinear magnon interactions over large regions of the dispersion plane. We apply magnon momentum microscopy to the prototypical magnonic material yttrium iron garnet and reveal a rich variety of previously unobserved nonlinear magnon interactions. With its element specificity and bulk sensitivity, as well as intrinsic access to nanometre-scale wavelengths without frequency limitation, this technique establishes a powerful and versatile platform for exploring short-wavelength and nonlinear magnonics.