The magnitude of the room temperature spin polarization of ferromagnetic materials is a key property for their application in spin transport-based electronics. Thus Heusler compounds, due to their predicted bulk half-metallic properties, i.e. 100% spin polarization at the Fermi energy, are in the focus of interest. However, for most applications it is not the bulk but the surface or interface of the material, which is relevant.

Investigating optimized thin films of the compound Co2MnSi by in situ spin-resolved UPS, we were able to demonstrate for the first time half-metallicity in combination with directly measured (93+7-11) % spin polarization at room temperature in the surface region of a Heusler thin film [Jou14]. Our novel band structure and photoemission calculations including all surface-related effects [Bra14, Bra15] show that the observation of a high spin polarization in a wide energy range below the Fermi energy is related to a stable surface resonance in the majority band of Co2MnSi extending deep into the bulk of the material. Spin-integrated ex situ HAXPES with a photon energy of 6 keV on Co2MnSi thin films and spin-integrated in situ UPS was carried out. The UPS and the HAXPES results fundamentally agree although the information depth of both experiments varies from less than 1 nm to 20 nm. Nearly quantitative agreement of the calculations with the experiments for both, UV and hard X-ray photon energies, was obtained.

As an outlook, results of HAXPES investigations of epitaxial Co2MnSi/ (Cr, Ag) interfaces will be shown, which deliver a first insight into the development of the Co2MnSi surface resonance into interface states.

[Jou14] M. Jourdan et al. Nat. Commun. 5, 3974 (2014).
[Bra14] J. Braun et al., New J. Phys. 16, 015005 (2014).
[Bra15] J. Braun et al., Phys. Rev. B 91, 195128 (2015).