Spin resolved photoelectron microscopy

Photoelectron emission microscopy (PEEM) uses electrons, that were emitted from the surface of the investigated sample by illumination with UV light (photo effect), to form a strongly magnified spatial image of the surface. In the case of a ferromagnetic surface the spin of the photoelectrons is polarized ("up" or "down") along the local direction of magnetization. As a straight forward idea, spin resolved PEEM uses the electron spin polarization as contrast mechanism to image magnetic micro-structures.

As electrons cannot be separated by their spin in a Stern-Gerlach type experiment, an electron spin detector involves scattering at a suitable target where, for instance, the reflectivity depends on the spin. Most spin detectors are characterized by a fairly low efficiency, and are not suitable for use in electron microscopy. We overcome this efficiency problem by a unique imaging spin filter (see Fig. 1), that uses a tungsten single crystal as an electron mirror with spin dependent reflectivity.

Fig. 1: Electrons are scatted under parallel beam conditions at the W(100) crystal. Cylindrical electrostatic lenses are used to control the scattering conditions. Like the reflection at an optical mirror, the spatial image information is conserved. The spin-integrated image is obtained by retracting the W(100) crystal.

The advantage of this setup is that all pixels in the electron microscope image can be measured simultaneously, including the spin information at every pixel. An example is shown in Fig. 2, where the magnetic domains in a thin cobalt film are measured by spin resolved PEEM. The same imaging spin filter allows us to measure spin resolved photoelectron momentum distributions using our momentum microscope.

Fig. 2: a) Spin resolved PEEM image of the domain boundary in an ultra thin cobalt film on a copper (100) surface. The magnetic domains with opposite magnetization are observed as white and black areas in the image - a direct consequence of the spin dependent reflectivity of the electron beam at the W(100) crystal. b) The intensity profile across the domain boundary reveals the width of the domain wall of 500nm, very close to the natural domain wall width of 400nm in a 9ML thick cobalt film.