"Pushing the limits of detection in STEM EELS: magnon spectroscopy in an electron microscope"

Abstract:

Nearly a  decade since first demonstration, vibrational electron-energy-loss spectroscopy has pushed the capabilities of analytical in a scanning transmission electron microscope (STEM). Phonon eigen modes can now be detected at atomic resolution, along with their dispersion in momentum space, and related to local atomic structure and chemistry. Magnons are quasiparticles representing the collective excitation of spins in magnetic materials. They, along with hybrid magnon-phonon quasiparticles (magnon polarons), are the basis for the operation of new spin wave transfer logic devices. They occupy the same  energy loss windows as phonon modes, ranging from a few to a few hundred meV in solid-state materials, suggesting that STEM-EELS may offer the ability to detect them at the nanoscale.  

Here, we show that bulk THz magnons can be excited and detected at the nanoscale using high-energy-resolution STEM EELS with the help of hybrid-pixel electron detectors. Momentum-resolved (ω-q) vibrational EELS measurements on antiferromagnetic and ferromagnetic material systems (NiO and yttrium iron garnet, YIG, respectively) reveal the unambiguous dispersion behaviour of the magnon signal in NiO, and magnon-polaron bands in YIG. The experimental findings are shown to be in excellent agreement with theoretical momentum-resolved magnon EELS dispersion curves, calculated using  theoretical methodologies to electron inelastic scattering of magnons and phonon-magnon coupling in an electron microscope.  Finally we explore the limits of spatial resolution, by performing atomically resolve measurements and discuss of the contrast formation in atomically resolved magnon maps.