Detection of magnetic circular dichroism on the two-nanometer scale

Magnetic circular dichroism (MCD) is a standard technique for the study of magnetic properties of materials in synchrotron beamlines. We present here a scattering geometry in the transmission electron microscope through which MCD can be observed with unprecedented spatial resolution. A convergent electron beam is used to scan a cross sectional preparation of a Fe/Au multilayer sample. Differences in the energy-loss spectra induced by the magnetic moments of the Fe atoms can be resolved with a resolution of better than 2 nm. This is a breakthrough achievement when compared both to the previous energy-loss MCD resolution (200 nm) or the best x-ray MCD experiments (approximately 20 nm).

Figure 1

(Color online) Principle of EMCD. (A) Two coherent incident plane waves, dephased by $\pi /2$, produce a rotating electric perturbation $\vec{F}$ during the atomic excitation. (B) When crystal diffraction is used, the detector position with respect to the 0 and $g$ beam determines the final scattering direction ${\vec{k}}_f$ and thus $\vec{q}$ and ${\vec{q}}^{\prime }$. (C) A convergent electron beam is scanned across the Au/Fe multilayer sample. The detector is alternatively placed at positions $+$ and $-$ in the diffraction plane selecting two scattering vectors $q\perp q^{\prime }$. The specimen image is a high-angle annular dark field (HAADF) map of the multilayer taken with the 1.7 nm electron probe.

Figure 2

(Color online) (A) $\text{Fe}{L}_3$ signal in the diffraction plane. Symmetric three-beam case $g=\left(200\right)$, $t=22\text{nm}$, for parallel illumination, which is similar to the LACDIF condition described in the text. The three diffraction spots $-g$, (000), and $g$ are marked by small circles (blue). The detector positions are marked with large circles. (B) The dichroic signal in the diffraction plane is obtained by subtracting the signal at a point in the left half plane from the signal at the mirror point in the right half plane. Maximum values are $\pm 15\%$ relative. The dashed vertical line in both panels is a mirror plane for the cubic crystal, yet the symmetry across the plane is broken by the presence of magnetism.

Figure 3

(Color online) (A) High-resolution TEM image of the investigated area; the shape and position of the beam during the scan are indicated by the superimposed circles. (B) Spectra from the middle of the 3 nm Fe layer. The difference is the dichroic signal.

Figure 4

(A) Profile of the line scanned in Fig. 3. The experimental points are the integrated Fe signal (sum of the $+$ and $-$ spectra) at the $L_3$ edge (707.9–713.9 eV). The best fit with a Gaussian spot shape gives a FWHM of 1.66 nm for the spot. The error bars are $3\sigma =855\text{counts}$. (B) Corresponding line profile of the dichroic signal (difference of the $+$ and $-$ spectra) integrated at the $L_3$ edge.