By combining the momentum microscope with an unique imaging spin filter (Spin resolved photoelectron microscopy), it becomes possible to measure the spin polarization of the photoelectrons emitted from the sample in the complete k_|| momentum image.

We studied cobalt films with a thickness of a few monolayers grown epitaxially on copper (100). Electrons in the cobalt are confined between the vacuum barrier on one side, and the projected bulk band gap of the Cu(100) substrate on the other side. Similar to the quantum mechanical model of a particle in a box potential, this leads to a series of states in the electronic band structure of the film. In particular, such quantum well states (QWS) that exist in magnetic materials are of high interest, both from a fundamental aspect as well as for future device applications.

QWS in this system are located between the Fermi energy and the vacuum level, in the unoccupied part of the electronic structure. As schematically shown in Fig. 1a, such states can be resonantly excited by two 3.1eV photons. Figure 1b shows the corresponding measurement of the spin polarization at the Fermi energy. The result shows that at those momentum points, where the resonant condition is fulfilled (big red arrow in Fig. 1a), an increased spin polarization is observed, as displayed by the circular shape with dark red color.

Fig. 1: a) Scheme of one- and two-photon excitations. The resonant (red) emission channel has a higher intensity than the non-resonant channel (blue). b) Measured spin-polarization map the photoelectrons at the Fermi energy for a 6 ML thick cobalt film. Resonant emission through the QWS shows up as a ring with enhanced spin polarization due to selective excitation of majority electrons.