Quantitative EMCD by use of a double aperture for simultaneous acquisition of EELS

Abstract

The weak signal strength in electron magnetic circular dichroism (EMCD) measurements remains one of the main challenges in the quantification of EMCD related EELS spectra. As a consequence, small variations in peak intensity caused by changes of background intervals, choice of method for extraction of signal intensity and equally differences in sample quality can cause strong changes in the EMCD signal. When aiming for high resolution quantitative EMCD, an additional difficulty consists in the fact that the two angular resolved EELS spectra needed to obtain the EMCD signal are taken at two different instances and it cannot be guaranteed that the acquisition conditions for these two spectra are identical. Here, we present an experimental setup where we use a double hole aperture in the transmission electron microscope to obtain the EMCD signal in a single acquisition. This geometry allows for the parallel acquisition of the two electron energy loss spectra (EELS) under exactly the same conditions. We also compare the double aperture acquisition mode with the qE acquisition mode which has been previously used for parallel acquisition of EMCD. We show that the double aperture mode not only offers better signal to noise ratio as compared to qE mode but also allows for much higher acquisition times to significantly improve the signal quality which is crucial for quantitative analysis of the magnetic moments.

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

“…The difference between these EELS spectra, typically referred to as “chiral plus” and “chiral minus” yields the EMCD signal, to which sum rules can be applied to calculate magnetic moments 21,22 . Related experiments have demonstrated the feasibility of this approach 23,24 .…”

Towards Quantitative Nanomagnetism in Transmission Electron Microscope by the Use of Patterned Apertures

“…First we built a double hole aperture (DA) to simultaneously acquire the two EELS spectra required for EMCD which removes all the complications produced due to the serial acquisition of the spectra. We show that we get a much better S/N ratio and the signal purity using the DA as compared to the previously used qE-mode for this purpose [6]. Second we built a special quadruple aperture (QA) to simultaneously acquire the EELS spectra and the sample orientation.…”

Simultaneous mapping of EMCD signals and crystal orientations in a transmission electron microscope

“…The EMCD technique was proposed in 2003 3 and experimentally demonstrated in 2006 4 . From the time of its discovery, EMCD has seen a continuous improvement in the signal to noise (S/N) ratio [5][6][7][8][9][10] , the spatial resolution [11][12][13][14][15][16] and the quantitative analysis. The technique has also been applied to explore material based questions such as interfacial magnetism 17,18 , magnetocrystalline anisotropy 19 , properties of dilute magnetic semiconductors 20 and rare earth magnets 21 .…”