XAS-PEEM

X-ray absorption spectroscopy

The PEEM is often used to implement laterally resolved applications of x-ray absorption spectroscopy (XAS) and its related methods. In XAS, the PEEM microscope is used to image the secondary electron emission at fixed kinetic energy, while the photon energy hν is scanned. When the photon energy matches a core level energy, a resonance in the secondary emission intensity is observed, originating from electronic transitions from core levels into unoccupied valence states via excitation processes occurring during the filling of the core holes. Such resonances are unique fingerprints that enable us to get precious information about the emitter chemical state, site location and valence state (x-ray absorption near edge spectroscopy). Due to the very low energy of the secondary electrons (less than 10 eV) and the increase of the inelastic mean free path of electrons at very low energy, XAS and XANES are used to probe buried interfaces or films up to a depth of ~10nm. The resolving power of the monochromator determines the attainable energy resolution so that, in principle, no energy filter is required.

Magnetic imaging

Magnetic circular dichroism. X-ray circular magnetic dichroism (XMCD) is a well-established technique for the characterisation of the magnetic state of ferromagnetic 3d transition metals and their compounds. Excitation of 2p electrons with circularly polarized radiation transfers the photon angular momentum to the spin of the excited photoelectron, which is used to probe the unfilled 3d states in the spin-split valence band. At resonance, the secondary electron yield is proportional to the dot product between the magnetization direction and the photon helicity vector, which is parallel or anti-parallel to the beam propagation direction according to the handedness of the circular polarisation. Thus, local differences in the orientation of the magnetization produce differences in the secondary electron yield. This principle is exploited to image domains and domain walls in thin magnetic films and surfaces. The XMCD image is obtained by subtraction of two PEEM images obtained with opposite helicity, i.e.:
I_XMCD = (I_minus-I_plus)/(I_minus+I_plus).
Domains with opposite orientation along the photon beam direction appear as bright and dark regions; domains aligned normal to beam appear in neutral greyscale.

Magnetic linear dichroism. XMLD-PEEM is used in the characterization of antiferromagnetic materials, and is sensitive to the alignment of the magnetic axis A. In XMLD the secondary electron intensity depends upon the charge distribution and magnetism. The magnetic contrast is proportional to the cosine of the angle between A and electric field vector E. XMLD-PEEM can thus image the AFM order, e.g. the alignment of the axis A and the domains. Data are acquired using horizontal and vertical linear polarisation of the photons. The XMLD images are usually obtained by subtraction of PEEM images obtained at two energies around the resonances, in order to maximize LMD contrast.

References