September 29, 2026
10:00 a.m. - 11:30 a.m.
N-201 Millennium Science Complex
University Park, PA
Progress in momentum- and position-resolved magnon spectroscopy in the STEM
Dr. Quentin Ramasse
SuperSTEM Laboratory and University of Leeds, UK
ABSTRACT:
Magnons, the collective excitations of the spin lattice in materials such as ferro- and antiferro-magnets, occupy the same meV-range spectral window as phonons. The promise of detecting magnons spectroscopically in a STEM, and to follow the phonon spectroscopy blueprint towards studies of their dispersion and their atomic-scale modifications, is tantalizing. With spin-based architectures becoming a cornerstone of quantum technology, studying magnons at the nanoscale is a key challenge for spintronics.
We demonstrate here for the first time how bulk THz magnons can be excited and detected at the nanoscale using STEM. We carried out nanoscale momentum-resolved measurements on (anti-)ferromagnetic materials (NiO and yttrium iron garnet, YIG). The experiments reveal the unambiguous signature of magnons in NiO, and magnon-polaron bands in YIG. The experiments are in excellent agreement with theoretical EELS calculations using parameters that reflect the experimental conditions, particularly the sample temperature, and magnetic environment. We also show that the magnon signal can be mapped spatially with nanoscale precision, while preliminary results suggest some atomic-level information is present in the magnon maps, in agreement with early numerical predictions using a dark-field EELS geometry. Finally, to overcome the challenges of low scattering cross-section of magnons compared to other excitations, the different dependence on temperature of magnons compared to phonons can be exploited to engineer better signal separation or affect dispersive behaviour.
BIO:
Quentin is the Director of the SuperSTEM Laboratory, the UK National Research Facility for Advanced Electron Microscopy, and holds the Chair of Advanced Electron Microscopy jointly at the Schools of Chemical and Process Engineering & Physics and Astronomy, University of Leeds, U.K.
After a MEng in France and a MMaths at the University of Cambridge, he obtained his Ph.D. in Physics from the University of Cambridge working on optical aberration measurements methodologies for aberration-corrected STEM. Before taking up his post at SuperSTEM he held a Staff Scientist position at the National Center for Electron Microscopy (NCEM) in Berkeley, a U.S. Department of Energy-funded user facility where he took part in the TEAM project. As the director of SuperSTEM for the last 15 years, Quentin has masterminded the facility’s adoption of emerging technologies for the benefit or its user community, commissioning one of the first meV-capable instrument in the world. He has pioneered single-atom core-loss and vibrational spectroscopy, nanoscale momentum-resolved EELS and real-space orbital mapping in the STEM. He was recently awarded the Royal Microscopical Society Mid-Career Scientific Achievement Award, as well as the European Microscopy Society EM Award for the Physical Sciences, which highlighted his contributions to the development of advanced electron microscopy.