Electron vortex beams prepared by a spiral aperture with the goal to measure EMCD on ferromagnetic films via STEM

Darius Pohl; Sebastian Schneider; Jan Rusz; Bernd Rellinghaus

doi:10.1016/j.ultramic.2014.11.025

Electron vortex beams prepared by a spiral aperture with the goal to measure EMCD on ferromagnetic films via STEM

Ultramicroscopy. 2015 Mar:150:16-22. doi: 10.1016/j.ultramic.2014.11.025. Epub 2014 Dec 2.

Authors

Darius Pohl¹, Sebastian Schneider², Jan Rusz³, Bernd Rellinghaus⁴

Affiliations

¹ IFW Dresden, Institute for Metallic Materials, P.O. Box 270116, d-01171 Dresden, Germany. Electronic address: d.pohl@ifw-dresden.de.
² IFW Dresden, Institute for Metallic Materials, P.O. Box 270116, d-01171 Dresden, Germany; TU Dresden, Institute for Solid State Physics, d-01069 Dresden, Germany.
³ Uppsala University, Department of Physics and Astronomy, P.O. Box 516, SE-75120 Uppsala, Sweden.
⁴ IFW Dresden, Institute for Metallic Materials, P.O. Box 270116, d-01171 Dresden, Germany.

PMID: 25497492
DOI: 10.1016/j.ultramic.2014.11.025

Abstract

X-ray magnetic circular dichroism is a well established method to study element specific magnetic properties of a material, while electron magnetic circular dichroism (EMCD), which is the electron wave analogue to XMCD, is scarcely used today. Recently discovered electron vortex beams, that carry a discrete orbital angular momentum (OAM) L, are also predicted to reveal dichroic signals. Since electron beams can be easily focused down to sub-nanometer diameters, this novel technique promises the possibility to quantitatively determine local magnetic properties with unrivalled lateral resolution. As the spiralling wave front of the electron vortex beam has an azimutally growing phase shift of up to 2π and a phase singularity in its axial center, specially designed apertures are needed to generate such non-planar electron waves. We report on the preparation and successful implementation of spiral apertures into the condenser lens system of an aberration-corrected FEI Titan(3) 80-300 transmission electron microscope (TEM). This setup allows to perform scanning TEM (STEM) with vortex beams carrying user-selected OAM. First experiments on the interaction of the vortex beam with a poly-crystalline sample are presented. Within the achieved signal to noise ratio no EMCD signal has been detected. This finding is supported by simulations of inelastic scattering of a beam generated by spiral aperture.

Keywords: Electron magnetic circular dichroism; Electron vortex beam; Orbital angular momentum; Scanning transmission electron microscopy.