New Paper: Quantitative Characterization of Nanoscale Polycrystalline Magnets with Electron Magnetic Circular Dichroism

Quantitative Characterization of Nanoscale Polycrystalline Magnets with Electron Magnetic Circular Dichroism


Complex materials with one or more magnetic species are of particular interest within the FemtoSpin project. As we stimulate these materials on faster timescales, it has been shown that the behavior of the individual species can be very different. To measure the different magnetic elements is a challenging field of experimental physics often requiring large-scale synchrotron facilities. Another challenge for the measurement of magnetic properties is their observation at high spatial resolution. In a recent publication in Nature Communications, a team involving Uppsala University have demonstrated a new technique to measure the spin and orbital angular momentum at resolutions of only a few nanometers.

 

Schematics of the proposed scanning-mode measurement of EMCD. (a) Schematic drawing of the experimental setup and the data obtained (ADF: annular dark field, PL: projector lens). The detector aperture is placed at the PL cross-over position. In the present STEM mode, the PL cross-over position is on the diffraction plane. (b) ATEM image of the investigated polycrystalline iron film. Scale bar, 50nm. (c) Calculated EMCD signal intensity distribution of a polycrystalline iron film in the diffraction plane. The highlighted area indicates the measured area covered by the detector entrance aperture. The detector entrance aperture (solid circle) is located at the position of 0.4 g(110) away from the origin, and its diameter is 0.5 g(110). The white broken circle represents the possible aperture centre positions in the diffraction plane and blue broken circle corresponds to g(110) ring position for comparison. Scale bar, 2nm-1. The minimum (black) and maximum (white) EMCD values range from -3 to +3%.

Schematics of the proposed scanning-mode measurement of EMCD. (a) Schematic drawing of the experimental setup and the data obtained (ADF: annular dark field, PL: projector lens). The detector aperture is placed at the PL cross-over position. In the present STEM mode, the PL cross-over position is on the diffraction plane. (b) ATEM image of the investigated polycrystalline iron film. Scale bar, 50nm. (c) Calculated EMCD signal intensity distribution of a polycrystalline iron film in the diffraction plane. The highlighted area indicates the measured area covered by the detector entrance aperture. The detector entrance aperture (solid circle) is located at the position of 0.4 g(110) away from the origin, and its diameter is 0.5 g(110). The white broken circle represents the possible aperture centre positions in the diffraction plane and blue broken circle corresponds to g(110) ring position for comparison. Scale bar, 2nm-1. The minimum (black) and maximum (white) EMCD values range from -3 to +3%.

 

The method is based on the electron magnetic circular dichroism (EMCD), in which electrons are transmitted though a magnetic sample in a transmission electron microscope (TEM). Electron energy loss spectroscopy (EELS) measured at core levels can then be employed to extract element-selective magnetic information. This method was outlined in 2003 by Hébert and Schattschneider (Ultramicropscopy 96, 463-468 (2003)) but until now quantitative information has not been possible to extract because of the inherently low signal strength. The new approach was stimulated from simulated distribution of dichroic signals (one such example was performed as part of a collaboration with Uppsala University) that suggested the EMCD is present almost everywhere in the diffraction plane. This means that by, rather than optimizing the signal-to-noise ration in a fixed geometry, the group was able to collect a large number of independent spectra and apply a new statistical technique to overcome the low signal strength restrictions of the technique. This significant step forward provides the ability to determine local magnetic moments on the nanometer scale, even for polycrystalline materials.

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