Imaging spin filter for photoelectron microscopy
M. Kolbe, P. Lushchyk, G. Schönhense; Institut für Physik
In cooperation with:
Specular reflection from a high-Z single crystal surface can lead to high spin asymmetries in the reflected beam. Owing to the k-parallel conservation, an electron-optical image is preserved in the SPLEED process when the achromatic (0,0)-beam is exploited . Spinfiltering specular reflection has been exploited in a PEEM setup  and in a momentum microscope (Nano ESCA) . The scheme of image transmission in the spin-filtering reflection is sketched in Fig.1.
Figure 1: Schematic ray trajectories in the imaging spin filter setup. Without spin filter the domains of a ferromagnetic sample cannot be seen (right image). With spin filter (here a W(100) single crystal surface, the spin asymmetry of the specular reflection leads to a strong spin contrast (top image) (from ).
Figure 2: Compact design with imaging spin filter being placed in a telescopic beam behind the objective lens (from ).
In the first prototype we implemented the imaging spin filter right behind the electrostatic objective lens, Fig. 2. Later, the cooperation partners at MPI Halle integrated the filter into the projective column of a momentum microscope  (type NanoESCA). That instrument uses an aberration-corrected double-hemispherical energy analyzer for energy filtering. A transfer lens decelerates the electrons from the pass energy of the analyzer to the scattering energy of typically 27 eV. In that instrument a reciprocal image is formed at the W(100) scattering target, such that electrons originating from the same point in the spatial image arrive as a parallel beam at the crystal surface. Thus, the spatial information is encoded in the angle of incidence and is conserved upon specular reflection. After a mirror-like 90° reflection, the spatial image is recovered by a second, symmetrical, transfer lens. The spin-integrated direct image still can be obtained by retracting the crystal from the optical path, see Fig. 1. In both branches, identical electron optics project the image onto the multichannel plate (MCP) of a position-sensitive detector.
Fig. 3 shows PEEM images that were recorded from 8 monolayer (ML) thick cobalt films using two-photon photoemission (2PPE) by 3.1 eV p-polarized light from the second harmonic of a pulsed Ti:Sa laser (pulse length 20 fs and repetition rate 80 MHz). We compare the observed domain pattern in the spin-filtered images (a,c) to images obtained by magnetic circular dichroism (MCD) in 2PPE (b). For the latter contrast mechanism, see  and our summary. The scale bar in (a) is the intensity behind the spin filter, whereas the scale bar in (b) represents the MCD asymmetry, given by A = [N+ -N-]/[N+ + N-] with the intensities N+ and N- for left- and right-circularly polarized light.
Figure 3: Comparison of magnetic contrast in 2PPE-PEEM of an 8 ML Co film.
 German patent DE 2005 045 622 B4
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