The electrons’ wavefront can be arbitrarily shaped by placing holographic masks (HMs) in the condenser system of a TEM. Using HMs with dislocation gratings, it is possible to impart quantized orbital angular momentum (OAM), as well as quantized magnetic moment onto the imaging electrons [1]. Due to their OAM, some peculiar effects can be observed for these so-called vortex electrons or electron vortex beams (EVBs), e.g. topological protection [2], peculiar rotation dynamics in magnetic fields [3] and intrinsic chirality. Owing to the latter of these properties EVBs have become a promising candidate for atomic scale energy-loss magnetic chiral dichroism (EMCD) measurements. However, it soon became clear that atom-sized EVBs are needed to achieve this goal [4,5].

In magnetic materials, the outgoing inelastically scattered probe electrons carry OAM, so they are EVBs. This fact can be utilised to detect spin polarized transitions in an alternative manner by placing a HM in the selected-area-aperture (SAA) holder and using it as a vorticity filter after the specimen, see Fig. 1. This approach does not rely on the standard EMCD geometry and the specimen’s role as a beam splitter and thus would not need a precise alignment of the crystal. The scattering geometry is chosen such that the SAA HM is in the far-field of the scattering centres, which is realized by lifting the specimen in the z-direction. Additionally, the electron probe is focused onto the lifted specimen in order to reduce the effective source size the SAA HM eventually “sees”. Nevertheless, the incident electron wave has a flat phase surface (i.e., behaves similar to a plane wave) all over the illuminated area, provided that the Rayleigh range of the probe beam is much larger than the sample thickness. Therefore, all the scattering ”light cones” point in the same direction towards the vortex filter HM. As the scattered probe electrons are of atomic-size their focused image could not be resolved, thus the imaging plane is defocused by 4 µm to observe broader vortices. A proof-of-principle experiment is shown in Fig. 2a. The azimuthally averaged radial intensity profiles of the upper and lower vortex orders (red and green full dots in Fig. 2b) are in good agreement with the simulation (blue and orange full lines in Fig. 2b). Curiously, the experimental radial profiles show stronger differences in the central region than is expected from the simulation, compare the experimental EMCD signal (magenta open circles) to the theoretical one (green dot-dashed curve) in Fig. 2b. This is probably due to skew optic axes giving rise to slight differences in apparent defocus for the positive and negative vortex orders. Also, artefacts from the mask production and OAM impurities could deteriorate the signal.

The experiment shows that the RMS error (magenta shaded region in Fig.2b) is still too high, such that the faint EMCD signal cannot be discerned under present experimental conditions. To improve the SNR we propose to incorporate larger SAA HMs, e.g. at least 30 to 50 µm in diameter, as the collected signal scales with the mask area, lowering the acquisition times. Also, increasing the coherence of the probe while still keeping the probe current high, which is possible in state-of-the-art aberration corrected microscopes, would enhance the EMCD signal strength by an order of magnitude. If successful, this technique could be applied to study magnetic properties of amorphous or nanocrystalline materials.

Acknowledgements: The financial support by the Austrian Science Fund (I543-N20, J3732-N27) and the European research council (ERC-StG-306447) is gratefully acknowledged.

References:

[1] J. Verbeeck et al., Nature 467 (2010): 301-304

[2] A. Lubk et al., Physical Review A 87 (2013): 033834

[3] T. Schachinger et al., Ultramicroscopy 158 (2015): 17–25

[4] J. Rusz and S. Bhowmick, Physical Review Letters 111 (2013): 105504

[5] P. Schattschneider et al., Ultramicroscopy 136 (2014): 81–85

Figures:

Fig. 1: EMCD vortex filter setup: A Co atom (red dot) scatters the incident probe electron into 3 transition channels (µ=-1,0,1) with different OAM. The outgoing probe electron (red profile) is incident on a HM in the SAA plane. Due to the beam's OAM, the radial intensity profiles of the diffracted vortex orders are modified (blue profiles). The EMCD signal is the difference in the central region of the radial intensity profiles between m=±1 vortices.

Fig. 2: Proof-of-principle experiment compared to simulations. (a) Energy filtered vortices of nanocrystalline Co (energy: 780 eV, slit: 15 eV, defocus: 4 µm) produced by a 10 µm HM in the SAA holder. (b) Azimuthally averaged radial intensity profiles of m=±1 vortices in (a) (red and green full dots) compared to the simulation (full lines). The RMS error in the EMCD signal (magenta shaded area) indicates that the faint EMCD signal (3%, green dot-dashed curve) cannot be detected under present experimental conditions.

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