Detection of Magnetic Circular Dichroism in the TEM

Linear Dichroism has been known to be detectable in the Transmission Electron Microscope (TEM). However, the possibility to measure circular dichroism in the TEM was only predicted in 2003 [1] and experimentally verified recently with TEM and synchrotron measurements on the same specimen [2]. Several experimental setups based on the principle of angle resolved Electron Energy Loss Spectrometry allows to record a dichroic signal in the TEM. Measurements can be done in image mode or in diffraction mode and using the GATAN imaging filter as an energy filter or as a spectrometer. The choice of the experimental setup influences the achievable spatial resolution as well as the signal to noise ratio. 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 ?/2. The inelastic interference term carries the dichroic signature. Experimental details and recent experimental results on Ni, Fe and Co will be presented, as well as simulations. Calculation were done with a full-potential, fully-relativistic Augmented Plane Wave code based on Density Functional Theory. A good approach to the understanding of Energy Loss Magnetic Chiral Dichroism (EMCD) is the mixed dynamic form factor. Chiral dichroism shows up as an imaginary part of the MDFF whereas linear dichroism is equivalent to the anisotropy of the dynamic form factor. Of particular interest are L2,3 or M4,5 ionisation edges of atoms with magnetic moments. The XANES signal in X-ray absorption spectra depends on the orientation of the atomic magnetic moment relative to the photon's wave vector, and on its chirality. Similarly, the fine structure (ELNES) in an EMCD experiment depends on the orientation of the atomic magnetic moment relative to the incident electron's wave vector (for small energy losses), and on the above mentioned phase shift. The similarities and differences to synchrotron experiments are discussed. The EMCD technique provides a new analytical tool for the element specific study of local magnetic moments. Applications cover magnetic ordering, spin and orbital magnetization, and electronic correlation, e.g. in heavy fermion systems. The TEM may thus complement the synchrotron for the study of the magnetic properties.