EMCD in the TEM - Optimization of signal acquisition and data evaluation

• The method of electron magnetic circular dichroism (EMCD) was recently proposed anddemonstrated by Peter Schattschneider et al. [I]. The EMCD signal consists in differences of L3 andL2 edge intensities of ferromagnetic materials at specific diffraction vectors. EMCD is thereforeelement specific. Furthermore. it was shown recently that sum rules apply to the EMCD methodwhich is an essential progress to obtain quantitative magnetic information from this method [2-3].Though. the theoretical derivations of sum rule suppose that the obtained spin- to orbital magneticmoments do not depend on the choice of scattering vector. This is a simplification which must beverified both by simulations and experiments to make EMCD a quantitative method for the study ofmagnetic moments.In the experiment. the dichroic signal. i.e. the change ofLJ and L2 edges at different diffractionvectors is small. i.e. of the order of 5-15% of the total signal intensity. Therefore. the acquisitionconditions must be optimized to both approach a di ffraction geometry where sum rules can beapplied and to maximize the signal to noise ratio.In this work. we optimize both. the signal and the signal/noise ratio. The experiments were carriedout on a FEI Tecnai F30 equipped with a Gatan GIF2002 energy filter. Instead of recording k-vectordependent single spectra as in the original work [I], we acquire energy filtered diffraction patternsin the energy interval around the transition metal L-edge. This allows for extraction of the EMCDsignal at k-vectors where the signal contains quantitative magnetic information. All measurementsof2 dimensional k-space maps of the EMCD signal are compared with simulations of the EM CDsignal. We find very good agreement between theoretical predictions and experimental values forboth. the k-space evolution and quantitative agreement the EMCD signal. The data evaluationincludes a careful normalization procedure. a statistical optimization of the signal to noise ratio aswell as the consideration of the entire edge intensity. Finally. we demonstrate the quantitativeprecision of the proposed method at the example ofa slightly textured iron layer (figures I and 2).References[I] P Schattschneider et aJ.. Nature 44 I (2006) 486.[2] J. Rusz, O. Eriksson, P. Novak, P. M. Oppeneer. Phys. Rev. B 76 (2007) 060408.[3] L. Calmels et aJ.. Phys. Rev. B 76 (2007) 060409.

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NATURVETENSKAP  -- Fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences (hsv//eng)

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Physics

Fysik

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