Magnons are quantised 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 quasiparticles. Yet, detecting such short-wavelength spin waves has remained a key experimental challenge. Here, we introduce Magnon Momentum Microscopy (MMM) -- 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, MMM can capture nonlinear magnon-magnon interactions over large regions of the dispersion plane. Applying MMM to the prototypical magnonic material yttrium iron garnet (YIG), we uncover a rich variety of previously unobserved nonlinear magnon interactions. With its element specificity, bulk sensitivity, as well as intrinsic access to nanometre-scale wavelengths without frequency limitation, soft-X-ray MMM establishes a powerful and versatile platform for exploring short-wavelength and nonlinear magnonics.
@article{arxiv.2504.20958,
title = {Soft-X-ray momentum microscopy of nonlinear magnon interactions below 100-nm wavelength},
author = {Steffen Wittrock and Christopher Klose and Salvatore Perna and Korbinian Baumgaertl and Andrea Mucchietto and Michael Schneider and Josefin Fuchs and Victor Deinhart and Tamer Karaman and Dirk Grundler and Stefan Eisebitt and Bastian Pfau and Daniel Schick},
journal= {arXiv preprint arXiv:2504.20958},
year = {2026}
}
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Ancillary files include videos presenting MMM images during (a) a frequency sweep and (b) a power sweep