Comment on "Quantized Orbital Angular Momentum Transfer and Magnetic Dichroism in the Interaction of Electron Vortices with Matter"

TL;DR

This paper critiques a previous claim that electron vortex beams can be used for magnetic dichroism measurements, showing that the effect is limited to very small nanoparticles due to a key constraint.

Contribution

It clarifies the limitations of angular momentum transfer in electron vortex interactions, challenging prior assumptions about EMCD feasibility.

Findings

EMCD signal is only strong for nanoparticles 1-2 nm in size.

Angular momentum transfer selection rule applies only at the vortex center.

Previous claims overestimated the applicability of vortex beams for magnetic dichroism.

Abstract

It was claimed (Lloyd et al., PRL 108 (2012) 074802) that energy loss magnetic chiral dichroism (EMCD) with electron vortex beams is feasible, and has even advantages over the standard setup with Bragg diffracted waves. In this Comment, we show that Lloyd et al. ignored an important constraint on the proposed selection rule for the transfer of angular momentum in the interaction, namely that it is only valid for an atom located in the very center of the vortex. As an experimental consequence, the EMCD signal will only be strong for extremely small nanoparticles of 1 to 2 nm diameter.