EMCD: Electron Energy Loss Magnetic Chiral Dichroism

Circular dichroism is the dependence of the absorption cross section on the helicity of the incident X-rays; XMCD (X-ray Magnetic Circular Dichroism) is a powerful technique to analyse magnetic properties in ferro-magnetic or paramagnetic compounds. XMCD experiments carried out in synchrotron facilities have a strong sensitivity to the specimen surface and up to 30nm of lateral spatial resolution. Transmission electron microscopy is a powerful tool to investigate the chemistry, the crystal and the electronic structure of solids with sub-Angstrom spatial resolution. The similarity between Electron Energy Loss Spectroscopy (EELS) and X-Rays Absorption Spectroscopy (XAS) is well known, but it was also believed that the measurement of circular dichroic effects in the TEM would have required spin polarized electrons. A recent theoretical work suggested that it is possible to realize in a TEM scattering conditions equivalent to absorption of circular polarized photons in XMCD experiments, preparing the incident electron beam as a superposition of two plane waves with a relative phase shift of ?/2. The first direct comparison between EMCD measurements so obtained in a TEM and XMCD experiments in the synchrotron was recently achieved on Fe thin film deposited on GaAs substrate (1). GaAs(001) substrates suitable for TEM analysis were prepared at the Center for the Electron Microscopy of TASC laboratory in Trieste. A thin layer of crystalline Fe was epitaxially grown at Ape beamline at the ELETTRA synchrotron in Trieste. The crystal quality and the magnetic properties of the specimens were analysed together with XMCD experiments. EMCD experiments were performed to elucidate some of the unknown aspects of this new approach. Here we present the state of the art in EMCD experiments showing some interesting synergies between XMCD and EMCD.