Ten years of <scp>EMCD</scp> : what has been achieved

Energy loss magnetic chiral dichroism (EMCD), established in 2006 [1] celebrates its 10 th anniversary. EMCD is the TEM equivalent of the X‐ray magnetic circular dichroism (XMCD) technique routinely applied on synchrotron beam lines for the study of magnetic moments. The EMCD signal is detected as an asymmetry in the energy filtered diffraction pattern, or alternatively as a slight difference in the fine structure of energy loss spectra for particular momentum transfers in the inelastic interaction (Fig. 1). The extremely high spatial resolution of modern TEMs makes EMCD interesting for spintronic and micromagnetic applications. In the last decade, the technique has evolved into a reliable tool demonstrating nm‐resolution, site selectivity and separation of spin and orbital moments via sum rules. One of the consequences of EMCD is that the outgoing inelastically scattered probe electrons have topological charge. Structurally, they are identical with vortex electrons that can be routinely created in the electron microscope [2]. Such vortices are characterized by a spiraling wavefront and a phase singularity at the center, similar to optical vortices that were first described by Nye & Berry [3]. Owing to their short wavelength, these matter waves can be focused to atomic size. Another novel aspect is their magnetic moment, quantized in multiples of the Bohr magneton, independent of the electron spin. These features make electron vortices extremely attractive as a nanoscale probe for magnetic materials. The discovery of vortex electron beams has spurred efforts to use them for EMCD because of their intrinsic chirality. It became soon clear that atom‐sized vortices are needed to achieve this goal [4], as shown in Fig. 2. At the time of writing, the closest successful approach to such beams is the shaping of the incident wave front with a Cs corrector such that it matches the point group symmetry of the selected atomic column, revealing EMCD signals with atomic resolution [5]. A promising alternative is the use of holographic vortex filters in the outgoing beam to detect spin polarized transitions [6], as sketched in Fig. 3. Other than in the standard EMCD geometry, this approach does not require a precise alignment of the crystal, and would thus allow the study of nanocrystalline and amorphous materials.

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