Probing the localization of magnetic dichroism by atomic-size astigmatic and vortex electron beams

Abstract

We report localization of a magnetic dichroic signal on atomic columns in electron magnetic circular dichroism (EMCD), probed by beam distorted by four-fold astigmatism and electron vortex beam. With astigmatic probe, magnetic signal to noise ratio can be enhanced by blocking the intensity from the central part of probe. However, the simulations show that for atomic resolution magnetic measurements, vortex beam is a more effective probe, with much higher magnetic signal to noise ratio. For all considered beam shapes, the optimal SNR constrains the signal detection at low collection angles of approximately 6–8 mrad. Irrespective of the material thickness, the magnetic signal remains strongly localized within the probed atomic column with vortex beam, whereas for astigmatic probes, the magnetic signal originates mostly from the nearest neighbor atomic columns. Due to excellent signal localization at probing individual atomic columns, vortex beams are predicted to be a strong candidate for studying the crystal site specific magnetic properties, magnetic properties at interfaces, or magnetism arising from individual atomic impurities.

Elastic scattering of twisted electrons by an atomic target: Going beyond the Born approximation

“…This process can be used to better understand the effects induced by a complex interplay between the spin and orbital magnetic moments. These effects can be studied, as an example, by means of measuring magnetic circular dichroism in the elastic and inelastic scattering of the vortex electron beams [7,10,16,17],…”

Orbital angular momentum resolved electron magnetic chiral dichroism

“…In order to clarify this point, we evaluated the contributions to the magnetic signal (Figs 3a and 3c) and the non-magnetic signal (Fig. 3b and 3d) for for both the atoms in the column on which the probe is centered and those in the neighboring columns [28,32]. The magnitudes of the terms are represented as solid spheres centered on each atomic position.…”

“…A reasonable figure of merit is given by the expression [29,31] (11) where the ratio between the magnetic and non-magnetic terms is analogous to the SNR in Ref. [32], the magnetic contribution is the "signal" that should be optimized and is the "noise" to be decreased. The function describes the increase in SNR that we expect to observe on enlarging the collection semi-angle.…”

Understanding electron magnetic circular dichroism in a transition potential approach

“…For exact zone axis orientation of the specimen, the signal sensitively depends on the probe shape. While in STEM imaging with electron vortex beams the magnetic signal is well localized, for astigmatic probes it is the neighboring atomic columns which contribute the majority of the magnetic signal [48]. Electrons which have previously been thermally scattered (phonon excitation) may also provide a significant component to the signal and from atoms not directly illuminated by the probe, as discussed for EELS and energy-dispersive x-ray elemental mapping [49].…”