Centre d’Élaboration de Matériaux et d’Etudes Structurales (UPR 8011)


Accueil > Recherche > I3EM : Interférométrie, In situ et Instrumentation pour la Microscopie Electronique > Matériaux et dispositifs

Energy-Loss Magnetic Chiral Dichroism

The EMCD technique [1] can be compared to the XMCD (X-ray Magnetic Circular Dichroism) technique proposed in 1988 to measure the magnetic moments with synchrotron radiation. During the interaction with the sample, the incident beam (electron or X-rays) transfers energy to the material enabling transitions from core to excited energy levels. The exploration of the empty electronic band provides access to crystallographic information on the chemical bonds but also on the filling of bands and thus on magnetism. X-ray absorption spectra or energy loss of electrons (EELS) spectra in a TEM are recorded to determine this information. The spatial resolution available in a TEM allows EMCD to locally probe the magnetic moments.

To get access to magnetic information, some constraints have to be imposed to the incident beam. In XMCD, the beam is circularly polarized and the absorption difference between the two polarization vectors (right and left-handed) is exploited. In a TEM, the polarization of the electron beam has been shown but is not widely used [2].

An EELS transition induced by the electric field of an incident electron gives an electronic transition parallel to the scattering vector q. It is therefore possible to interpret this transition as the absorption of a virtual photon with a polarization vector parallel to the scattering vector . X ray polarization vectors and diffusion vectors in a TEM play comparable roles.

A virtual circular polarization with electrons is obtained by combining diffusion vectors perpendicular to each other and showing a fixed phase relationship . The specimen orientation set to get the transmitted beam and a diffracted beam is used to validate the phase condition and the spatial location of the vectors ensures that they are perpendicular.

EELS signals are recorded in two positions in the diffraction plane of the material, corresponding to the combinations of two different scattering vectors.

The polarization vectors of the X beam and scattering vectors in the TEM play therefore comparable roles. The quantitative data treatment was formalized through the sum rules for XMCD and these rules have been derived in the case of EMCD [3,4].

Various applications were proposed by the group using an original mode of acquisition, the image filtering. This mode of acquisition involves a specific treatment for image distortions, developed in the team [5]. Different materials have been studied : the magnetite [6], iron [7], the alloys [8] iron-cobalt and more recently MnAs [9,10] and DyFe2.

 

[1] P. Schattschneider et al., “Detection of Magnetic Circular Dichroism Using a Transmission Electron Microscope,” Nature 441,  486 (2006)

[2] J. Verbeeck, H. Tian, and P. Schattschneider, “Production and Application of Electron Vortex Beams,” Nature 467, 301 (2010)

[3] L. Calmels et al., “Experimental Application of Sum Rules for Electron Energy Loss Magnetic Chiral Dichroism,” Physical Review B 76, 060409 (2007)

[4] J. Rusz et al., “Sum Rules for Electron Energy Loss near Edge Spectra,” Physical Review B76, 060408 (2007)

[5] C. Gatel, B. Warot-Fonrose, and P. Schattschneider, “Distortion Corrections of ESI Data Cubes for Magnetic Studies,” Ultramicroscopy109, 1465–71, (2009)

[6] B. Warot-Fonrose et al., “Mapping Inelastic Intensities in Diffraction Patterns of Magnetic Samples Using the Energy Spectrum Imaging Technique,” Ultramicroscopy 108, 393–98 (2008)

[7] B. Warot-Fonrose et al., “Effect of Spatial and Energy Distortions on Energy-Loss Magnetic Chiral Dichroism Measurements : Application to an Iron Thin Film,” Ultramicroscopy 110, 1033–37 (2010)

[8] B. Warot-Fonrose et al., “Magnetic Properties of FeCo Alloys Measured by Energy-Loss Magnetic Chiral Dichroism,” Journal of Applied Physics107, 09D301 (2010)

[9] X. Fu, B. Warot-Fonrose et al., “Energy-loss magnetic chiral dichroism (EMCD) study of local ferromagnetic properties of epitaxial MnAs thin film on GaAs(001)”, Applied Physics Letters 107, 062402 (2015)

[10] X. Fu, B.Warot-Fonrose et al., “In Situ Observation of Ferromagnetic Order Breaking in MnAs/GaAs(001) and Magnetocrystalline Anisotropy of Alpha-MnAs by Electron Magnetic Chiral Dichroism,” Physical Review B 93, 104410 (2016)