Atomic site sensitivity of the energy loss magnetic chiral dichroic spectra of complex oxides

The quantitative analysis of magnetic oxide core level spectra can become complicated when the magnetic atoms are located at several nonequivalent atomic sites in the crystal. This is, for instance, the case for Fe atoms in magnetite, which are located in tetrahedral and octahedral atomic sites; in this case, the x-ray magnetic circular dichroic (XMCD) spectra recorded at the L2,3 edge of Fe contain contributions from the different nonequivalent atomic sites, which unfortunately cannot be separated. Energy loss magnetic chiral dichroic (EMCD) spectra are the transmission electron microscope analogies of the XMCD spectra. One of the important differences between these two techniques of magnetic analysis is that EMCD uses a fast electron beam instead of polarized light. The fast electrons behave like Bloch states in the sample, and the fine structure of the EMCD spectra is strongly influenced by channeling and dynamical diffraction effects. These effects can be adjusted by changing the experimental configuration. We use theoretical calculations, which include dynamical diffraction effects and in which electronic transitions are treated in the atomic multiplet formalism, to show that the relative weight of the Fe atoms in different nonequivalent atomic sites can be changed by a proper choice of the position of the detector and of the magnetite sample orientation and thickness. We conclude that EMCD spectra could be used to isolate the magnetic contribution of atoms in each of the nonequivalent atomic sites, which would not be possible with XMCD techniques.