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

www.nature.com/scientificreports OPEN Simultaneous mapping of EMCD signals and crystal orientations in a transmission electron microscope Hasan Ali1,3,4, Jan Rusz2, Tobias Warnatz2, Björgvin Hjörvarsson2 & Klaus Leifer1* When magnetic properties are analysed in a transmission electron microscope using the technique of electron magnetic circular dichroism (EMCD), one of the critical parameters is the sample orientation. Since small orientation changes can have a strong impact on the measurement of the EMCD signal and such measurements need two separate measurements of conjugate EELS spectra, it is experimentally non-trivial to measure the EMCD signal as a function of sample orientation. Here, we have developed a methodology to simultaneously map the quantitative EMCD signals and the local orientation of the crystal. We analyse, both experimentally and by simulations, how the measured magnetic signals evolve with a change in the crystal tilt. Based on this analysis, we establish an accurate relationship between the crystal orientations and the EMCD signals. Our results demonstrate that a small variation in crystal tilt can significantly alter the strength of the EMCD signal. From an optimisation of the crystal orientation, we obtain quantitative EMCD measurements. Electron magnetic circular dichroism (EMCD)1, a transmission electron microscope (TEM) based technique, has emerged as an important technique to determine the magnetic moments of the materials with much higher spatial resolution as compared to its X-ray counterpart XMCD2. The EMCD technique was proposed in 20033 and experimentally demonstrated in 2 0064. From the time of its discovery, EMCD has seen a continuous improvement in the signal to noise (S/N) ratio5–10, the spatial resolution11–16 and the quantitative analysis. The technique has also been applied to explore material based questions such as interfacial m agnetism17,18, magnetocrystalline anisotropy19, properties of dilute magnetic s emiconductors20 and rare earth m agnets21. Recently EMCD signals with single atomic plane resolution were achieved22,23. One of the apparently simple findings in the experimental evolution of the EMCD technique since its discovery is that the sample orientation and the electron beam position must be very well defined in order to obtain a quantitative EMCD signal. The accurate knowledge of sample orientation is thus of uttermost importance when aiming for quantitative atomic resolution EMCD. With this in mind, it is suprising that hitherto, the orientation dependence of the EMCD signal has not been analysed systematically in the experimental situation. Furthermore, when aiming for atomically resolved EMCD work, not only the orientation must be precisely known, but also the position of the electron beam with respect to the exposed atom must be well defined and strictly identical for both EELS spectra that are needed for obtaining the EMCD signal. Having acquired the STEM image of the atomic lattice of the magnetic material, the place of the EMCD analysis can be accurately determined. But, it is non-trivial to obtain all information needed for highly accurate EMCD, i.e. orientation and both EELS spectra simultaneously. In the classical EMCD experimental setup, the TEM sample is tilted to a 2-beam condition (2BC) and two electron energy loss (EELS) spectra are acquired at two conjugate scattering angles. From an experimental point of view, tilting the TEM sample to a perfect 2BC is not always trivial especially for thin samples and nanoparticles where the Kikuchi lines are not visible. Depending on the thickness and the extinction length of the material, the intensity of the diffracted beam can be lower or even higher than the direct beam. Another difficulty arises when acquiring the spatial maps of EMCD in scanning TEM (STEM) mode. Even if the magnetic materials are nearly 1 Applied Materials Science, Department of Materials Science and Engineering, Uppsala University, Box 534, 75121 Uppsala, Sweden. 2Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden. 3Department of Electrical Engineering, Mirpur University of Science and Technology (MUST), Mirpur 10250, AJK, Pakistan. 4Present address: Department of Materials and Environmental Chemistry, Stockholm University, 10691 Stockholm, Sweden. *email: Scientific Reports | (2021) 11:2180 | https://doi.org/10.1038/s41598-021-81071-4 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1.  Results from elastic scattering calculations of the intensities of the transmitted beam and the Bragg scattered beams g = (200) and 2g as a function of tilt from three-beam orientation and sample thickness for bcc iron crystal at 300 kV acceleration voltage. perfect single crystals, the crystal orientation of the TEM sample might locally change within the measured area, thus producing different orientation conditions at different scan points. In fact, most magnetic materials have small misorientations between different regions of the sample resulting in texture angles of the order of a few mrad so the precise orientation alignment remains a challenge. It has been shown in simulations that a deviation from a 2BC can produce a change in the strength and distribution of the EMCD signals in the reciprocal space24 but no systematic experimental study has been carried out in this context. One of the major difficulties in such work consists in the serial acquisition of the two EELS spectra needed for the EMCD and the diffraction patterns at each beam position which make it hard to ensure the spatial registration among these measurements. Here, we have developed a technique to simultaneously map both conjugate EELS spectra and the local orientation of the sample in a single acquisition and thus can obtain all signals, EMCD signal and sample orientation simultaneously at each point of a scanned map. This enables the determination of the effect of crystal tilt on the measured EMCD signal with high accuracy. By inserting a custom-made quadruple aperture (QA)25 in the reciprocal space plane of the electron beam trajectory, we simultaneously obtain four angle-resolved spectroscopic signals in a single acquisition. The four signals include the two conjugate EELS spectra required for the EMCD measurements and the inelastic intensities of the 0 and the g beams (in 2-beam orientation). The ′ approach is based on the hypothesis that a ratio of the inelastic intensities of the diffracted ( I ) and the direct g ′ ( I0) beams quantitatively represent the local orientation of the crystal. In fact, we show by our simulations how ′ ′ the values of Ig /I0 are related to a tilt of the crystal from exact 2-beam orientation. We establish an experimental relationship between the crystal orientation and the EMCD signals and demonstrate that a change in crystal orientation significantly affects the measured magnetic signals. We find the exact tilt direction in the experiment (...truncated)