Magnetic circular dichroism in the transmission electron microscope

Abstract

Dichroism is the property of certain materials that their photon absorption spectrum depends on the polarization of the incident radiation. In the case of X-Ray Magnetic Circular Dichroism (XMCD) the absorption cross section of a ferromagnet or a paramagnet in a magnetic field changes when the helicity of a circularly polarized probing photon is reversed relative to the magnetization. Although the similarities between X-ray absorption (XAS) and electron energy loss spectra (EELS) in the Transmission Electron Microscope (TEM) have long been recognized, it was presumed that extending such equivalence to circular dichroism would require a beam of spin polarized electrons. Recently, it was argued on theoretical grounds that this is probably wrong.<br />In this Thesis I report the first direct experimental proof of magnetic circular dichroism in the TEM by comparing what has been named electron Energy-loss Magnetic Chiral Dichroism (EMCD) with XMCD spectra from the same specimen together with theoretical calculations. The experiment shows that chiral atomic transitions in a specimen are accessible with inelastic electron scattering under particular scattering conditions.<br />The specimen itself is used as beam splitter and phase locker to obtain the equivalent of circularly polarized photons in the TEM.<br />A broad range of experimental conditions is explored and the effect of several experimental parameters is studied and compared with ab initio simulations. A few alternative scattering conditions are detailed, together with their advantages and disadvantages, and demonstrated on simple systems (Fe, Co and Ni single crystals).<br />A theoretical justification of the effect is provided within the Bloch theory framework with the use of the Mixed Dynamical Form Factors.<br />This result bears dramatic consequences for the study of magnetism at high resolution. Whereas circular dichroism of many magnetic materials has been studied with synchrotron radiation since twenty years, there is a number of technical limitations related to the spatial resolution and the signal depth with this technique. Circular dichroic experiments in the TEM, on the other hand, offer the potential of spatial resolutions down to the nanometre scale and provide depth information.<br />