Atomic-resolution spectroscopic analysis of magnons in 2D materials

Antiferromagnetic materials have a naturally occurring magnetic structure, but the direction of their ordered microscopic moments alternate between individual atomic sites in a crystal. This makes the characterization of magnetic materials difficult as their moments are invisible to common magnetic probes, and the lack of fine spatial resolution inhibits atomby- atom analysis of magnetic domains. Accordingly, it becomes imperative to have knowledge of the atomic arrangement of the respective film, and still retain the energy resolution to decouple magnon modes in the infrared regime. Hereby, the disordered phase of antiferromagnetic Fe2O3 (Hematene) is used; with the structure having an internal magnetic moment that can be picked up with spectroscopic means. Monochromated electron energy loss spectroscopy (EELS) inside an aberration-corrected scanning transmission electron microscope (STEM) is used to interpret the local spin contributions in Hematene. The use of an electron beam is important because the probe allows us to overcome the customary use of a diffraction-limited probe and localize our spectroscopic collection on the order of a unit cell. Next, by acquiring both our EELS spectrum in the on-axis and off-axis configuration in the same region, we make a direct correlation between spin signatures in the film and the localized vibrations. Interestingly, we find that long-range spin-momentum components can be suppressed by collecting electrons that scatter only at high angles, and thereby changing our selection rules from optical to neutron selection rules. Future outlook will be focused on mapping magnon-phonon interactions using in situ STEM techniques at variable temperatures.