CHIRALTEM: Circular Dichroism in the Electron Microscope

Whereas the similarities between XANES and energy loss near edge structures (ELNES) in the Transmission Electron Microscope (TEM) allow to "export" Linear Dichroism to the TEM, a similar task for Circular Dichroism was considered to be impossible with present technology. The possibility to detect chiral electronic transitions in the transmission electron microscope (TEM) was demonstrated recently [1, 2]. This makes ELNES in the TEM a strong competitor of the synchrotron for X-ray Magnetic Circular Dichroism (XMCD) studies. In analogy to XMCD we introduce the term EMCD: Energy loss Magnetic Chiral Dichroism. In the TEM, there is no such thing as a circularly polarized photon; rather, the chirality of allowed transitions is detected when the scattering geometry is chosen properly. In the experiment, a coherent superposition of two momentum transfer vectors perpendicular to each other is set up, tuning the phase difference between the two interactions to ?????The inelastic interference term carries the dichroic signature. The proper tool for description of EMCD is the mixed dynamic form factor (MDFF). More precisely, it turns out that chiral transitions show up in the imaginary part of the MDFF. In the dipole approximation, a particularly simple expression is obtained. Experiments on Ni, Fe and Co show a clear dichroic signal. Differences to XMCD are discussed. We report on the optimization of the scattering geometry in the TEM, and on possible future applications of the CHIRALTEM method. This research was supported by the European Commission, contract nr. 508971 (CHIRALTEM).