Towards Magnon Spectroscopy in an Electron Microscope

In the last decade, the advent of high-resolution vibrational EELS spectroscopy in an electron microscope has revolutionised materials science, enabling the detection of the spectroscopic signature of phonons down to the single atom level , a feat that was for a long time considered impossible. As the technique moves rapidly from proof-of-principle to established methodology, questions about the ‘next-quasiparticle’ that could be probed arise.<br/>Beyond phonons, the next obvious excitation to hunt for is arguably that of magnons, or spin waves, which arise from the collective excitation of the electrons’ spin in ferro- and antiferromagnets. The concept of using electrons as a probe for magnons is not new; they can be efficiently excited by electron scattering in reflection geometry using spin- and non-polarised electron sources (SPEELS, REELS respectively). It is therefore expected that, in direct analogy with phonons, the spectroscopic signature of magnons and their dispersion in momentum space should also be accessible within the remit of vibrational electron-microscopy-based EELS. Nevertheless, the challenges of magnon-EELS spectroscopy are significant; while they qualitatively occupy the same energy range as phonons, their relative intensity is several orders of magnitude lower than that of phonons making their detection extremely challenging.<br/>Here, we explore the prospects of using high-resolution EELS spectroscopy beyond the detection of phonons and propose a combined experimental and theoretical approach for magnon spectroscopy. We explore the conditions to excite and detect magnons in the electron microscope and their dispersion in energy and momentum, and present preliminary experiments in thin layers of ferromagnetic and antiferromagnetic materials grown by pulsed laser deposition. The experiments were performed using a monochromated Nion UltraSTEM MC, which is equipped with an IRIS spectrometer with a Dectris ELA direct electron detector for EELS and is capable of energy resolutions of ~6 meV. The experiments are guided and rationalized by theoretical calculations for the description of magnon excitation and momentum dispersion in an electron microscope and simulation of magnon energy loss spectra. The calculation of magnon diffuse scattering is analogous to thermal diffuse scattering due to atomic vibrations (phonons) and quantum excitation of phonons.