An international team lead by the Max Born Institute has developed a new type of momentum microscopy to image magnons — the quanta of collectively excited spins — directly in two-dimensional reciprocal space using soft X-rays. Owing to its remarkable sensitivity, simplicity, and access to nanometer-scale wavelengths, this novel technique establishes a powerful and versatile platform for exploring nonlinear magnon interactions, which are promising for future computing schemes.
Highlights
- Magnon momentum microscopy (MMM) uses soft X-rays to image spin waves directly in momentum space
- Access to sub-100-nm magnon wavelengths with high sensitivity
- Direct observation of four-magnon scattering from a parametric instability of propagating magnons forming omnidirectional magnon populations in momentum space
- Enables research on nonlinear magnonics, nanoscale magnetism, and future wave-based technologies
Spins—the building blocks of magnets—are not rigid. Because of their strong coupling over comparatively long distances, they can be easily excited and exhibit wave-like dynamics. The study of these spin waves has provided deep insights into the fundamental physics of magnetic materials for decades. More recently, magnons—the quanta of these collective excitations—have attracted growing interest for next-generation computing concepts, as they could enable information processing based on waves rather than the flow of electrons, potentially reducing energy losses.
Researchers are now pushing magnon wavelengths into the nanometer regime, corresponding to frequencies in the terahertz range—around 100 times faster than today's CPU clock speeds. On the one hand, such short wavelengths are essential for integration into modern device architectures. On the other hand, accessing spin-wave properties and their interactions in this regime is mainly uncharted territory and has remained a major experimental challenge.
Now, a team of researchers from the Max Born Institute (MBI), in collaboration with the Helmholtz-Zentrum Berlin (HZB), the Università degli Studi di Napoli Federico II (UniNa), and the École Polytechnique Fédérale de Lausanne (EPFL), has developed a powerful new method to observe nanoscale spin waves. The technique, called magnon momentum microscopy (MMM), uses resonant soft X-rays to directly detect short-wavelength magnons.
In the experiment, magnons act like a dynamic diffraction grating for soft X-rays. Based on the resulting diffraction pattern, the researchers can determine the magnon wavelengths and amplitudes across the two-dimensional sample plane in a single measurement.
"We can now directly observe magnon properties and their full distribution in momentum space," says Steffen Wittrock, first author of the study. "This gives us a completely new level of access to magnon dynamics."
The method combines high sensitivity with rapid acquisition and does not rely on complex nanostructuring of the sample. It is compatible with a wide range of excitation schemes, making it broadly applicable to many magnetic systems.
Using MMM, the team investigated magnons in the prototypical magnetic material yttrium iron garnet (YIG). At high excitation strengths, they observed that magnons do not simply propagate in a single direction. Instead, they redistribute across momentum space, forming characteristic patterns that reveal strong nonlinear interactions.
Most strikingly, the experiments show an omnidirectional population of magnons forming an elliptical ring in momentum space—direct evidence of a four-magnon scattering process. In this mechanism, two magnons interact and generate two new magnons with different propagation directions.
"While such nonlinear interactions are well known for uniform spin-wave modes, we discovered a more general type of four-magnon scattering involving propagating magnons," explains Salvatore Perna, who developed the theoretical model. "Our analysis shows that it arises from a parametric instability of magnons at finite wave vectors, redistributing energy across many modes."
Beyond this first demonstration, MMM provides a versatile platform for studying spin-wave physics across a wide range of systems. Its unique combination of properties—high sensitivity, element specificity, and direct access to nanometre-scale wavelengths—sets it apart from existing techniques.
The researchers expect that MMM will enable new insights into nonlinear magnonics, mode coupling, and wave-based phenomena in magnetic materials. Future developments could extend the technique to ultrafast timescales and to systems operating at much higher frequencies, including antiferromagnets.
