B11-O-13A New Atomic Scale EMCD Measurement Scheme by STEM-EELS under 3-beam Diffraction Condition

Abstract

Electron magnetic circular dichroism (EMCD) in the transmission electron microscope (TEM) is a relatively immature technique, though it has already demonstrated quantitative results with a spatial resolution superior to what can be achieved with X-rays [ 1 ]. The main obstacles for widely sharing the method for routine use are (i) low SNR because of off-axis EELS detection, (ii) requiring a very thin sample for quantitative analysis and (iii) unstable pre- and post-edge backgrounds by subsequent measurements in changing the aperture positions. There have been a number of attempts to overcome those difficulties, such as collecting hundreds of data and applying statistical/information data processing methods [ 2 , 3 ]. In the present study we found a novel experimental condition that allowed us to solve almost all these difficulties and even applicable to atomic resolution EMCD measurements. The present scheme proposed is based on a symmetrical three-beam diffraction condition in a Fe film at STEM mode with the EELS aperture placed slightly off-axis on either side of the systematic row of the reflections. A focused electron probe is scanned on the sample to find 1-dimensional lattice fringes as the ADF-STEM image. The chiral ± EMCD signals appear at the both sides of the atomic column fringes, with the EMCD signal of the opposite sign appearing on the opposite sides of a lattice fringe. This method requires only a single scan to obtain the chiral ± EMCD signals and also exhibits stable signal fractions with the change in sample thickness. We show the first experimental results based on this concept, which can solve the most of difficulties existing and be a novel breakthrough in the quantitative atomic scale EMCD measurements.