2nd CHIRALTEM workshop

3rd CHIRALTEM workshop

NEW: get together at DPG conference


- Vienna - Regensburg -

- Dresden - Prag - Trieste -




The main mission of the center for electron microscopy of the TASC CNR INFM laboratory within CHIRALTEM project is to establish links between XMCD and EMCD taking the advantages of co-existence in the same group of expertise on TEM and synchrotron radiation based spectroscopies. So far our group designed and realized dedicated specimens and performed the relevant XMCD and EMCD experiments. To establish links between XMCD and EMCD it was necessary, at this initial level of knowledge, to perform the relevant experiments physically on the same specimens. Specimens specially designed to this purpose have been produced at TASC and studied by XMCD, on APE beamline at ELETTRA synchrotron in Trieste, and by EMCD by TUW in Vienna. The scheme of preparation of one typology of specimen is shown in figure 1:

Figure 1: design and preparation of specimens for both EMCD and XMCD experiments.

At the beginning three different specimens were prepared. Two of them were identical and prepared starting from a 3 mm in diameter disk cut from a [001] GaAs wafer 350 µm thick. The disks were then marked to recognize the crystallographic directions to study the specimen magnetisation as function of the crystal orientation. The specimens were grinded and then ion milled to electron transparency, so that to have a region of 90µm thickness on the edge of the specimen (that is therefore self supporting) and a hole in the center. The specimens were ion-milled only on one side in order to avoid miscut angle that could influence the epitaxial growth of the magnetic layer. To growth the magnetic layer the specimen were transferred in ultra-high vacuum better then 7x10-11mbar. After annealing and cleaning by ion-milling of the GaAs surface, 10 nm of Iron have been deposited by molecular beam epitaxy. The crystal quality of the substrate surface and of the Fe-layer were monitored in situ by low energy electron diffraction. The layer thickness was measured by quartz microbalance and by Auger spectroscopy. A first capping layer of 2.5 nm of Cu was then deposited. Measurements of transverse and longitudinal Magneto Optic Kerr Effect showed evidence of residual in-plane magnetisation. They also indicated that 80 Oersted are enough to completely magnetise the sample in the in-plane hard magnetisation direction. A secondary electrons image of the specimen was taken at 703 eV (Fe L3-edge), then XMCD measurements were performed by using the residual magnetisation of the sample (with the surface tilted of 45° with respect to the incident beam, to have a component of the magnetisation along the photon spin). The dichroic signal was obtained by scanning in energy over the Fe L2,3 edges and then either by flipping the circular polarisation or by rotating the sample of 180°. The measurements have been performed on different positions of each sample to be sure that it is magnetically active everywhere. The result, a representative spectrum is displayed in figure 2, showed a strong dichroic effect, with a 33% asymmetry at the L3 edge with 50 µm of spatial resolution (but the resolution can in principle be improved up to 200 nm).


Figure 2: representative XMCD measurement.

A further capping layer of 2 nm of Cu has been finally deposited allowing to safely remove the specimens from the ultra-high vacuum chamber and to transfer to Vienna where EELS spectra were taken on a FEI Tecnai F20-FEGTEM S-Twin equipped with a Gatan Image Filter. A flat region of 100 nm radius and uniform thickness was selected in a single grain of Fe. Chemical microanalysis revealed negligible traces of contaminants (C, O and Mo). The sample was immersed in the magnetic field of the TEM objective lens pole piece, which is 1.9 Tesla and oriented perpendicular to the surface. The magnetization of the iron film in the TEM experiment was therefore forced to be saturated in the out-of-plane direction by a field that is large with respect to the in-plane coercitivity. This is crystallographically identical to the in-plane magnetisation used in the XMCD experiment, providing two physically equivalent conditions. The experiment shows a clear dichroic signal, displayed in figure 3, even if the noise in the EMCD experiment is much higher then the noise in the XMCD experiment.