Magnons – the collective waves of spin that ripple through magnetic materials – are central to the emerging field of magnonics, where information could one day be transmitted not by electrons but by spin waves. Until now, capturing their faint signatures at the nanoscale has proved almost impossible: the signal is vanishingly weak and easily drowned out by nearby phonon excitations. That barrier has now been broken.
In a study published in Nature, an international team of researchers report the first direct detection of magnons in a scanning transmission electron microscope, using monochromated STEM-EELS and a state-of-the-art electron-counting detector to tease out signals just a thousandth the strength of nearby phonon peaks.
The findings not only confirm that magnons can be probed at sub-nanometer resolution, but also opens the door to correlating spin-wave behavior with atomic and electronic structures in real materials – potentially enabling faster, smaller, and more energy-efficient information technologies.
Here, Demie Kepaptsoglou, lead author, Deputy Director of the SuperSTEM National Research Facility for Advanced Electron Microcopy – and Senior Lecturer at the University of York, UK, discusses the challenges of detecting such a faint signal, the “potato-n” moment, and where magnonics might go next.
What are magnons – and why is detecting them at the nanoscale a significant step forward?
Magnons are collective excitations of precessing spins in magnetic materials, effectively representing waves of spin alignment propagating through a magnetic system.
The closest analogy I can think of is phonons, which are waves of atomic vibrations traveling through a solid. In essence, magnons are to magnetic spins what phonons are to atomic vibrations.
Magnons have some extremely fascinating nanoscale physics; as quasiparticles they can couple with other parts of a materials system, such as conduction electrons and phonons. From a more applied point of view, waves can be used to transmit information more efficiently and with less power than traditional electronics or spintronics. This opens up exciting possibilities for quantum computing and the development of faster, smaller, and more energy-efficient electronic systems.
Real materials are never perfect and systems for magnonics are often heterostructures, so consist of multiple materials. So understanding how magnons behave at the nanoscale – especially in the presence of structural and chemical imperfections, as well as interfaces – is crucial for optimising systems for future technologies.
Fundamental or applied, understanding the underlying physics at the nanoscale is key.
What were the biggest analytical challenges you faced during the research and how did you overcome them?
The technique we used to detect magnons is the now well established electron energy loss spectroscopy in a monochromated scanning transmission electron microscope (STEM-EELS).
The technique in its most advanced level has allowed us in the past to “see” the lattice vibrations of a single atom in a graphene lattice.
The main challenge in this work was the extremely weak nature of the magnon signal, which sits right next to a phonon signal roughly a thousand times stronger in the same material. In other words, it was like trying to spot something tiny overshadowed by something vast and dominating.
The detection was made possible by the use of the state-of-the-art Monoctromated Scanning Transmission Electron Microscope (Nion UltraSTEM100MC-Hermes) at SuperSTEM, equiped with a precise electron-counting detector (Dectris ELA) for spectroscopy, which allowed us to accumulate enough signal from an area just one nanometer across.
Over the course of the experiments, we estimated that only one to two electrons corresponding to the magnon signal were detected every two seconds. In this case, “every electron counts” – and we tried hard to count every single one of them.
Were there any surprising results or pivotal moments during the research?
Well, the somewhat surprising result was that we actually succeeded! I must admit that I was hopeful but not necessarily optimistic. We knew that the signal was there, but detecting it was a completely different story.
Our approach was a little different to our other STEM-EELS experiments from the start – they typically lasted only a few minutes at most. Here, the experiments took hours in order to accumulate sufficient signal.
The pivotal moment came when something “promising” appeared at the right position – a potential magnon. For a while, however, and until confirmation from simulations arrived, we did not dare call the peak a magnon. Because the signal had a somewhat broad, potato-like shape, we jokingly referred to it as a “potato-n.” Although the theoretical calculations from our colleagues at the University of Uppsala matched the experiment perfectly, most of our discussions stayed potato-themed – right up until the paper was accepted by Nature, marking the potato-n to magnon transition.
What new opportunities does your research open up for materials science, spintronics, or other areas?
This work opens a new avenue for studying spin waves and magnetism at length scales not accessible with traditional techniques. Because the measurements are performed in an electron microscope, they also allow direct correlation of magnons with the atomic and electronic structures of materials.
Our next step is to push the limits of resolution to see if we can reach the atomic scale – which is certainly very exciting. Now that we’ve demonstrated feasibility, we hope the technique will move beyond proof of concept toward understanding magnon behavior in real materials for magnonics applications.
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