Single-pass STEM-EMCD on a zone axis using a patterned aperture: progress in experimental and data treatment methods

Thomas Thersleff,Roman Adam,Cheuk-Wai Tai,Ján Rusz,Claus M Schneider,Shunsuke Muto,Linus Schönström,Daniel E Bürgler

doi:10.1038/s41598-019-53373-1

Thomas Thersleff, Roman Adam + Show 6 more

Open Access

https://doi.org/10.1038/s41598-019-53373-1

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Abstract

Measuring magnetic moments in ferromagnetic materials at atomic resolution is theoretically possible using the electron magnetic circular dichroism (EMCD) technique in a (scanning) transmission electron microscope ((S)TEM). However, experimental and data processing hurdles currently hamper the realization of this goal. Experimentally, the sample must be tilted to a zone-axis orientation, yielding a complex distribution of magnetic scattering intensity, and the same sample region must be scanned multiple times with sub-atomic spatial registration necessary at each pass. Furthermore, the weak nature of the EMCD signal requires advanced data processing techniques to reliably detect and quantify the result. In this manuscript, we detail our experimental and data processing progress towards achieving single-pass zone-axis EMCD using a patterned aperture. First, we provide a comprehensive data acquisition and analysis strategy for this and other EMCD experiments that should scale down to atomic resolution experiments. Second, we demonstrate that, at low spatial resolution, promising EMCD candidate signals can be extracted, and that these are sensitive to both crystallographic orientation and momentum transfer.

Highlights

Measuring magnetic moments in ferromagnetic materials at atomic resolution is theoretically possible using the electron magnetic circular dichroism (EMCD) technique in a transmission electron microscope ((S)TEM)
In the domain of high spatial resolution, EMCD has been detected using convergent beams of atomic size in a classical three-beam geometry[13], utilizing phase ramps introduced by beam shift[14], in zone axis orientation[15,16], and using atomic size beams distorted by four-fold astigmatism[17,18]
The first hypothesis is that an 8-blade patterned aperture can detect an EMCD signal on the[001] zone axis of bcc iron

Summary

Introduction

Measuring magnetic moments in ferromagnetic materials at atomic resolution is theoretically possible using the electron magnetic circular dichroism (EMCD) technique in a (scanning) transmission electron microscope ((S)TEM). It offers a path to a single-pass STEM acquisition of the spectra, if the data from the whole CCD camera can be recorded at each scan point This occurs because the geometry of the aperture filters electrons scattered with different transition matrix elements having conjugated orientations of q and q′ into the upper and lower portions of the post-prism CCD camera, resulting in their simultaneous acquisition. We report our experimental progress towards single-pass STEM-EMCD on a cubic metallic iron sample using a patterned aperture as well as advances in data processing that are necessary to search for and extract potential EMCD signals This second part proves to be challenging given the complex distribution of magnetic scattering on a zone-axis as well as the nature of geometric distortions present in 2D electron energy-loss spectroscopy (EELS) dispersion plane. A version of the script formatted for publication detailing the analysis is provided in the supplementary information

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