| 26.10.2006 |
Magnetic Linear and Circular Dichroism in Tarnsmission Electron Microscopy
PD Dr. Peter A. van Aken, Max-Planck-Institute for Metals Research, Stuttgart |
The study of microstructural magnetism has been experiencing a renaissance over the last
decades following the discovery of a variety of new scientific phenomena of which a large
number is spectroscopy based. Due to similarities between inelastic electron scattering and X-ray
absorption, magnetic linear and magnetic circular or chiral dichroism can be observed in electron
energy-loss spectroscopy (EELS) through the transmission electron microscope (TEM). These
experiments yield information about the anisotropic electronic structure at nanoscale spatial
resolution when operated with a focussed electron probe. This opens the way to measure
the local nanomagnetism in thin films and multilayered material with chemical specificity.
In this lecture, I report on strong magnetic linear dichroism (MLD) at the Fe L23
of the anti-ferromagnetic compound hematite α-Fe2O3 in high-resolution
orientation- and temperature-dependent electron energy-loss spectroscopy (EELS). Large intensity
differences of correspondding spectral features are observed when the Fe L23 and
O K edges are measured with momentum transfer either parallel or perpendicular to the
magnetization. The resultant difference spectra for the Fe L23 edges are consistent
with the MLD observed in x-ray absorption spectroscopy. For the first time, intense MLD at the
O K edge of hematite is observed. The magnetic origin of this O K edge MLD is proved
by temperature-dependent investigations across the Morin transition temperature TM = 263 K,
at which the Fe electron spins, i.e. the magnetic moments rotate by 90°. The O K edge MLD
is interpreted in terms of super exchange between the spins of the Fe 3d and O 2p electrons
through overlapping Fe 3d and the O 2p orbitals.
The possibility to detect chiral electronic transitions and to measure circular dichroism in
the TEM, without using spin-polarized electrons, was predicted in 2003 and experimentally
verified this year by the group of Peter Schattschneider in Vienna. Several experimental setups
based on the principle of angle resolved EELS, e.g. in image mode or in diffraction mode,
allow recording a dichroic signal in the TEM. First results obtained from an iron single
crystal will be presented and discussed in terms of achievable spatial resolution and
in terms of the signal-to-noise ratio.
Einladender: Prof. Dr. H. Kohl
| Ort: |
Wilhelm-Klemm-Str. 10, IG I, HS 2 |
| Zeit: |
donerstags 16 Uhr c.t. |
|
Kolloquiums-Kaffee ab 15:45 Uhr vor dem Hörsaal |
Im Auftrag der Hochschullehrer des Fachbereichs Physik
Apl. Prof. Dr. H. Bracht
